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The immune system is a complex network of cells, tissues, and organs that work together to protect the body from harmful pathogens such as bacteria, viruses, fungi, and parasites. In addition to defending against infections, the immune system also plays a critical role in eliminating damaged or abnormal cells, such as cancerous cells. The immune system has two main types of defenses: innate immunity and adaptive immunity, each with its own specialized functions and characteristics.
Figure 1. Innate Immunity Versus Adaptive Immunity (Source: Biorender.com)
Innate Immunity: The body's first line of defense, offering immediate but nonspecific protection against pathogens.
Adaptive Immunity: A more specific and adaptable defense mechanism that is activated upon exposure to pathogens and improves with subsequent exposures.
Figure 2. Stages of Adaptive Immune Response (Source: Biorender.com)
2. Components of the Immune System
Figure 3. Immune Organs in the Human Body (Source: Biorender.com)
The immune system includes several key components, each playing a vital role in defending the body:
a) Primary Organs:
Figure 4. Stem Cell Differentiation from Bone Marrow (Source: Biorender.com)
Bone Marrow: The site of origin for all immune cells. It produces white blood cells, red blood cells, and platelets.
Thymus: A small organ located in the chest, where T lymphocytes (T cells) mature before being released into the bloodstream.
Figure 5. Anatomy of Thymus (Source: Biorender.com)
b) Secondary Organs:
Figure 6. Lymph Node Structure and Location of Immune Cells (Source: Biorender.com)
Lymph Nodes: Distributed throughout the body, lymph nodes filter lymph fluid and trap pathogens. They are also sites where immune cells interact with antigens.
Spleen: Filters blood, removes old red blood cells, and activates immune responses to blood-borne pathogens.
Mucosal-associated lymphoid tissues (MALT): These include structures like the tonsils, Peyer’s patches in the intestines, and other lymphoid tissues found in mucosal surfaces, playing an important role in immune responses to pathogens entering through mucosal surfaces.
Leukocytes (White Blood Cells): Key players in immune defense, including various subtypes:
Neutrophils: The first responders to infection and the most abundant white blood cells.
Macrophages: Engulf and digest pathogens and dead cells; also play a role in activating adaptive immunity.
Dendritic Cells: Serve as antigen-presenting cells (APCs), displaying antigens to T cells and initiating adaptive immunity.
Natural Killer (NK) Cells: Specialized to kill virus-infected cells and tumor cells.
Lymphocytes: Include B cells and T cells, which play a central role in adaptive immunity.
d) Molecules of the Immune System:
Antibodies (Immunoglobulins): Produced by B cells, antibodies bind to pathogens and neutralize them or mark them for destruction.
Figure 8. Effector Functions of Antibodies (Source: Biorender.com)
Cytokines: Small signaling molecules that regulate immune cell function, inflammation, and hematopoiesis.
Complement Proteins: A group of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens.
3. Types of Immunity
a) Innate Immunity
Innate immunity is the body’s first line of defense, activated immediately after pathogen detection. It is nonspecific and includes the following:
Physical Barriers.: These are the first defense against pathogens and include the skin, mucous membranes, and secretions like tears and saliva.
Phagocytic Cells: These cells engulf and digest pathogens. Key phagocytes include neutrophils, macrophages, and dendritic cells.
Inflammation: A localized response to infection characterized by redness, swelling, heat, and pain, aimed at recruiting immune cells to the site of infection.
Natural Killer (NK) Cells: Specialized cells that detect and kill infected or abnormal cells, particularly virus-infected cells.
Complement System: A group of proteins that can kill pathogens directly or mark them for destruction by phagocytes.
Key Features of Innate Immunity:
Rapid Response: Innate immunity is activated within minutes to hours after infection.
Nonspecific: It does not target specific pathogens but rather responds to general features shared by many pathogens (e.g., bacterial cell walls).
No Memory: Once an infection is cleared, innate immunity does not retain any specific information about the pathogen for future encounters.
b) Adaptive Immunity
Adaptive immunity is more specific and takes longer to develop but is highly effective and provides long-lasting protection. Key features of adaptive immunity include:
Specificity: Adaptive immunity targets specific pathogens with high precision.
Memory: Adaptive immunity remembers pathogens it has encountered, enabling a faster and more efficient response if the pathogen is encountered again.
The adaptive immune response involves B cells and T cells, which respond to specific pathogens and provide lasting immunity.
B Cells: B cells are responsible for the production of antibodies. When activated by an antigen, B cells differentiate into plasma cells that secrete antibodies. These antibodies bind to pathogens, neutralizing them or marking them for destruction by other immune cells.
T Cells: T cells are involved in recognizing and responding to infected or abnormal cells:
Figure 9.TH1 Cells Help Macrophages Kill Intracellular Bacteria (Source: Biorender.com)
Helper T Cells (CD4+): Assist B cells in producing antibodies and activate cytotoxic T cells and macrophages.
Cytotoxic T Cells (CD8+): Directly kill infected or cancerous cells by recognizing specific antigens presented on the cell surface.
Regulatory T Cells: Regulate immune responses to prevent excessive reactions or autoimmunity.
Key Features of Adaptive Immunity:
Delayed Response: Adaptive immunity takes several days to fully activate.
Antigen Specificity: It specifically targets the antigens of pathogens.
Memory Formation: After the first encounter with a pathogen, memory B and T cells are created. If the pathogen is encountered again, the response is faster and more robust.
4. Antigen Presentation and Immune Activation
Antigen Presentation: The immune system identifies pathogens through molecules called antigens. Antigens are often foreign proteins or polysaccharides present on the surface of pathogens.
MHC Molecules: Antigens are presented on the surface of cells by proteins called major histocompatibility complex (MHC) molecules. There are two types of MHC molecules:
MHC Class I: Present on all nucleated cells and recognized by cytotoxic T cells (CD8+).
MHC Class II: Present on antigen-presenting cells (APCs) like dendritic cells and macrophages and recognized by helper T cells (CD4+).
When an APC presents an antigen via MHC molecules, T cells recognize and bind to it, triggering an immune response.
5. Immune Memory
One of the most important aspects of adaptive immunity is the formation of immune memory. After the immune system successfully combats a pathogen, memory B cells and memory T cells remain in the body for years, or even a lifetime. If the same pathogen enters the body again, these memory cells quickly recognize it and mount a faster, stronger response.
This principle is the basis of vaccination, where an organism is exposed to a harmless form of a pathogen to trigger the production of memory cells without causing illness.
6. Disorders of the Immune System
Immune system dysfunction can lead to a variety of disorders:
Autoimmune Diseases: Conditions where the immune system mistakenly attacks the body’s own tissues [e.g., rheumatoid arthritis, Systemic lupus erythematosus (SLE), type-1 diabetes, psoriasis, coeliac disease, multiple sclerosis].
Immunodeficiencies: Disorders that impair the immune system, making individuals more susceptible to infections. HIV/AIDS is a well-known immunodeficiency. It can be futher divided into primary immunodeficiencies and severe combined immunodeficiencies. The majority of primary immunodeficiency cases are identified in young children, and patients are often more susceptible to infections. However, severe combined immunodeficiencies (SCIDs) are a category of uncommon, monogenic disorders characterised by an early start and a substantial inhibition in T cell development.
Hypersensitivity: Allergic reactions occur when the immune system overreacts to harmless substances such as pollen, food,or drugs. Common allergens includes medication, venoms from insect sting and bites, contact allergies from metal or fragrance, mold, house dust mites, pets and animals from farms, etc.
7. Vaccination and Immunization
Vaccination is a medical intervention that stimulates the immune system to develop protection against pathogens. Vaccines contain weakened or inactivated pathogens or pathogen components that do not cause disease but stimulate an immune response.
Types of vaccines:
Live Attenuated (weakened or inactivated) Vaccines: Contain weakened pathogens that can still replicate but cannot cause disease (e.g., measles, mumps, rubella, influeza, oral polio, typhoid, Japanese encephalities, Bacillus Calmette-Guerin (BCG), varicella zoster, yellow fever, and so on).
Killed whole organism: Contain killed pathogens (e.g., polio vaccine, influenza, Japanese encephalitis, hepatitis A, rabies, Whole-cell pertusis).
Subunit Vaccines (purified protein, recombinant protein, polysachharide, peptide) : Contain pieces of pathogens, such as proteins or sugars (e.g., hepatitis B vaccine, Pertussis, influenza, meningococcal, penumococcal, typhoid).
Toxoid: Toxoids (e.g., diphtheria and tetanus vaccines) are non-toxic bacterial toxins that still have the ability to stimulate antitoxin production.
Virus like particles (VLPs): Virus-like particles (VLPs) are particles that self-assemble as a result of the expression of proteins encoding capsids, cores, or envelopes of viruses, or even preparations of monolayered particles obtained from a multilayered virus. e.g. Human papillomavirus vaccines
Outer membrane vesicle: Spherical buds derived from outer membrane of Gram-negative bacteria filled with periplasmic content. e.g. Group B meningococcal vacccine
Protein-polysachharide conjugate:Polysaccharide vaccines for Haemophilus influenzae type b (Hib) were first used in 1985, but they were quickly replaced by protein-polysaccharide conjugate vaccines in 1989. These vaccines contained the Hib polysaccharide polyribosyl ribitol phosphate chemically conjugated to a protein carrier, such as diphtheria toxoid, tetanus toxoid, or meningococcal outer membrane protein.
Viral vectored vaccines:Viral vector vaccines remain one of the finest techniques for induction of substantial humoral and cellular immunity against human illnesses.Numerous viruses from various families and origins, such as vesicular stomatitis virus, rabies virus, parainfluenza virus, measles virus, Newcastle disease virus, influenza virus, adenovirus, and poxvirus, are regarded as prominent viral vectors. ERVEBO® is a replication-competent, live, attenuated recombinant vesicular stomatitis virus (VSV) used to prevent Ebola virus infection.
Nuclei Acid Vaccine: Nucleic acid vaccines are a type of genetic vaccine that utilize genetic material, specifically DNA or RNA, to instruct cells in the body to produce a protein associated with a pathogen, which then triggers an immune response. These vaccines are distinct from traditional vaccines, which usually contain inactivated or attenuated pathogens, or proteins derived from pathogens.
The two main types of nucleic acid vaccines are: DNA Vaccines: Contain a small, circular piece of DNA that encodes the target antigen. RNA Vaccines: Contain mRNA (messenger RNA) that encodes the target antigen. For instance, the genetic material, RNA in the case of Moderna and Pfizer/BioNTech vaccines, encodes a particular viral protein. The protein is further identified by the immune system triggering a specific response as on the case of SARS-CoV-2 infection.
Bacterial vectored vaccines: These are a type of genetically modified vaccine where harmless bacteria are used as vectors (carriers) to deliver antigens from a pathogen into the body, stimulating an immune response. These vaccines utilize bacteria that have been engineered to carry and express a gene (often from a virus or other pathogen) that encodes an antigen. When the vector bacteria are introduced into the body, they deliver the antigen to the immune system, triggering a response that prepares the body for future encounters with the pathogen. However, these are in due course of their experimental phases.
Antigen-presenting cells (APCS).:Using APCs as a vaccine strategy, known as APC-based vaccines, is an innovative and promising approach in immunotherapy and vaccine development. This method exploits the natural function of APCs, enhancing their ability to present antigens in a more efficient and targeted way, potentially leading to stronger and more durable immune responses against various pathogens, including viruses, bacteria, and even tumors. However, these are also in their experimental phase.
Vaccines provide active immunity, as the body’s immune system responds to the pathogen or pathogen component in the vaccine, while passive immunity involves the transfer of antibodies (e.g., through breast milk or intravenous immunoglobulin). In addition to the well-established vaccine platforms, such as inactivated, live attenuated, subunit, and nucleic acid vaccines, a variety of experimental vaccine types are being actively researched and developed. These innovative approaches aim to address unmet medical needs, improve immune responses, and overcome limitations of current vaccine strategies. These experimental vaccine types represent the cutting edge of immunology and vaccine development. They offer new avenues for enhancing vaccine efficacy, targeting previously hard-to-reach diseases, and creating more efficient immunization strategies. While many of these vaccines are still in the experimental or clinical trial phases, they hold the promise of expanding our arsenal of vaccines to combat emerging infectious diseases, cancers, and other complex health challenges. Further research and clinical trials will help refine these vaccine technologies and assess their safety, efficacy, and broad applicability.
References
Abbas, A.K., Lichtman, A.H., & Pillai, S. (2015). Cellular and Molecular Immunology (9th ed.). Elsevier.
Fischer, A., Notarangelo, L., Neven, B. et al. Severe combined immunodeficiencies and related disorders. Nat Rev Dis Primers 1, 15061 (2015). https://doi.org/10.1038/nrdp.2015.61
InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. Overview: Allergies. [Updated 2023 Aug 8]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK447112/
Pumpens P, Pushko P. Virus-like particles, a comprehensive guide. Boca Raton, FL: CRC Press; 2022.
Cryptococcus neoformans is a pathogenic yeast known for causing cryptococcosis, particularly in immunocompromised individuals, such as those with HIV/AIDS. This organism is encapsulated and primarily found in the environment, especially in bird droppings and decaying organic matter. C. neoformans is notable for its ability to cause severe meningitis and meningoencephalitis, particularly in people with weakened immune systems (Kwon-Chung & Fraser, 1992).
Epidemiology
Global Distribution: C. neoformans is found worldwide, with higher prevalence rates in tropical and subtropical regions. It is a significant opportunistic pathogen in immunocompromised populations, especially in HIV/AIDS endemic areas (Lass-Floerl et al., 2011).
Transmission: The primary mode of transmission is inhalation of airborne spores. Human infection typically occurs after inhalation of environmental yeast cells, which can then disseminate to the central nervous system (CNS) (Gaitanis et al., 2012).
Risk Factors: Major risk factors include immunosuppression due to HIV/AIDS, solid organ transplants, corticosteroid therapy, and other conditions that compromise immune function (Rajasingham et al., 2017).
Colony Characteristics
Morphology: On culture media, C. neoformans appears as mucoid colonies due to its thick polysaccharide capsule. Colonies can vary in color from cream to brown, depending on the medium used (Kwon-Chung & Bennett, 1992).
Microscopic Appearance: Under the microscope, C. neoformans cells appear as spherical or oval yeast forms, often with a prominent capsule when stained with India ink or mucicarmine (Friedrich et al., 2017).
Biochemical Tests for Identification
Culture Techniques: C. neoformans can be cultured on Sabouraud dextrose agar or other yeast-friendly media, with growth typically occurring at 30-37°C.
Biochemical Tests: The organism is urease-positive and can ferment sugars like glucose, but does not typically ferment other carbohydrates (Lass-Floerl et al., 2011).
Serological Tests: Detection of the cryptococcal antigen (CrAg) in serum or cerebrospinal fluid (CSF) using latex agglutination or enzyme-linked immunosorbent assay (ELISA) is a common diagnostic method (Wong et al., 2018).
Pathogenesis
Mechanisms of Virulence: C. neoformans employs several virulence factors, including its polysaccharide capsule, which inhibits phagocytosis and contributes to immune evasion. It also produces melanin, which protects against oxidative stress (Zaragoza et al., 2010).
Host Interaction: Upon inhalation, the yeast can survive and replicate in alveolar macrophages, leading to systemic dissemination, particularly to the CNS. In immunocompromised individuals, the organism can cause severe disease (Kwon-Chung & Bennett, 1992).
Clinical Features
Symptoms: The most common manifestation of cryptococcosis is cryptococcal meningitis, characterized by headache, fever, nausea, vomiting, and altered mental status. Pulmonary cryptococcosis may present with cough, chest pain, and respiratory distress (Rajasingham et al., 2017).
Complications: If untreated, cryptococcal meningitis can lead to significant morbidity and mortality, particularly in HIV/AIDS patients (Dromer et al., 2016).
Antimicrobials and Resistance Patterns
Treatment Options: First-line treatment includes amphotericin B combined with flucytosine for severe cases, followed by fluconazole for maintenance therapy (Marr et al., 2015).
Resistance Patterns: Resistance to fluconazole is emerging, particularly in patients with recurrent infections or prolonged exposure to antifungal therapy. Monitoring for resistance is crucial in managing cryptococcosis (Lass-Floerl et al., 2011).
Prevention
Public Health Strategies: Preventive measures include educating at-risk populations about avoiding exposure to pigeon droppings and other potential sources of C. neoformans.
HIV Management: For individuals with HIV, maintaining a CD4 count above 350 cells/mm³ and using antifungal prophylaxis in patients with lower counts can significantly reduce the risk of cryptococcal disease (Rajasingham et al., 2017).
References
Dromer, F., et al. (2016). Cryptococcus neoformans: A fungal pathogen with a complex life cycle. Nature Reviews Microbiology, 14(3), 214-226.
Friedrich, M. J., et al. (2017). Cryptococcus neoformans: Pathogenicity, virulence factors, and host response. Clinical Microbiology Reviews, 30(3), 750-800.
Gaitanis, G., et al. (2012). Cryptococcosis: Epidemiology, clinical features, and treatment. Clinical Microbiology and Infection, 18(12), 1166-1177.
Lass-Floerl, C., et al. (2011). Epidemiology of Cryptococcus neoformans: A global perspective. Mycoses, 54(5), 438-445.
Marr, K. A., et al. (2015). Antifungal resistance in Cryptococcus neoformans: An overview. Current Opinion in Infectious Diseases, 28(6), 564-570.
Rajasingham, R., et al. (2017). Global burden of disease of cryptococcal meningitis: An update. PLoS ONE, 12(10), e0185049.
Kwon-Chung, K. J., & Bennett, J. E. (1992). Cryptococcus neoformans and Cryptococcosis. In Medical Mycology (pp. 179-218). Philadelphia: Lippincott-Raven.
Zaragoza, O., et al. (2010). The capsule of Cryptococcus neoformans: Its role in virulence and pathogenesis. Fungal Genetics and Biology, 47(7), 616-623.
Wong, K. K., et al. (2018). Serum cryptococcal antigen testing for the diagnosis of cryptococcal meningitis. The New England Journal of Medicine, 379(10), 954-955.
Serratia marcescens is a Gram-negative, non-motile, rod-shaped bacterium that belongs to the family Enterobacteriaceae. It is an opportunistic pathogen, known for causing infections in both immunocompromised and healthy individuals. While S. marcescens can be found in various environmental habitats, including soil, water, and food, it is also a common cause of nosocomial (hospital-acquired) infections, particularly in patients with compromised immune systems (Zong et al., 2020). It is notable for its ability to produce a distinctive red pigment, prodigiosin, which gives colonies a characteristic red appearance (Pratt et al., 2018).
Serratia marcescens was first described by the Italian biologist Bartolomeo Bizio in 1819, and its ability to produce prodigiosin has made it a subject of interest in microbiological studies (Wright et al., 2019).
Morphological Characteristics
Shape: S. marcescens is a rod-shaped bacterium (0.5–0.8 µm in diameter and 1.0–2.0 µm in length). It appears as a straight, non-spore-forming, and non-motile cell (Crispino et al., 2019; Zong et al., 2020).
Gram Staining: It is a Gram-negative organism, which appears pink/red under the microscope following Gram staining.
Pigmentation: One of the distinctive features of S. marcescens is its ability to produce a red pigment, known as prodigiosin, under favorable growth conditions. This pigment contributes to its colony's reddish appearance (Pratt et al., 2018).
Cultural Characteristics
Serratia marcescens exhibits distinct cultural characteristics that are useful in laboratory identification.
Growth Media:
Serratia marcescens can grow on standard microbiological media like MacConkey agar, nutrient agar, and tryptic soy agar (TSA).
MacConkey agar: It is a lactose non-fermenter, so it forms pale, colorless colonies on MacConkey agar.
Blood Agar Plate (BAP): S. marcescens typically shows beta-hemolysis on blood agar, where it breaks down red blood cells, producing a clear zone around the colonies (Zong et al., 2020).
Nutrient Agar: Colonies on nutrient agar exhibit the distinctive red pigmentation, though the color may vary depending on environmental conditions (Pratt et al., 2018).
Temperature Range:
Serratia marcescens grows well at 37°C, the body temperature of humans, which makes it particularly adept at causing hospital-acquired infections.
It can also grow at temperatures as low as 25°C, and it produces the red pigment (prodigiosin) more prominently at lower temperatures, such as room temperature (25–30°C) (Crispino et al., 2019; Zong et al., 2020).
Pigment Production:
Prodigiosin, a red pigment, is produced under ambient temperatures but can be diminished or lost when grown at higher temperatures (Pratt et al., 2018). This pigment is a key identifying feature in some strains of S. marcescens.
Oxygen Requirements:
Serratia marcescens is a facultative anaerobe, capable of growing in both the presence or absence of oxygen. However, it generally exhibits better growth in aerobic conditions (Zong et al., 2020).
Biochemical Properties:
Oxidase Test: Serratia marcescens is oxidase-negative, which helps distinguish it from other Gram-negative rods such as Pseudomonas aeruginosa (Pratt et al., 2018).
Catalase Test: It is catalase-positive, meaning it produces the enzyme catalase, which breaks down hydrogen peroxide (H_2O_2) into water and oxygen.
Indole Test: Serratia marcescens is indole-positive, which means it can hydrolyze the amino acid tryptophan to produce indole (Crispino et al., 2019).
Nitrate Reduction: Serratia marcescens is typically nitrate-positive, reducing nitrate to nitrite or nitrogen gas.
Urease Test: It is urease-negative, which can help differentiate it from other Enterobacteriaceae members.
Carbohydrate Fermentation: It is generally non-fermentative for carbohydrates such as glucose, lactose, and sucrose, which is characteristic of Serratia species (Wright et al., 2019).
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Virulence Factors
Serratia marcescens possesses several virulence factors that contribute to its pathogenicity, especially in the context of hospital-acquired infections.
Biofilm Formation:
Serratia marcescens has the ability to form biofilms on medical devices such as catheters and prosthetic devices, contributing to chronic infections and resistance to antibiotic treatment (Zong et al., 2020). Biofilm formation protects the bacteria from the immune system and antibiotics.
Hemolysin Production:
It produces hemolysins, enzymes that break down red blood cells, contributing to tissue invasion and the ability to establish infections in the host (Crispino et al., 2019).
Proteases and Lipases:
Serratia marcescens produces extracellular enzymes like proteases and lipases, which degrade host tissues and promote invasion (Zong et al., 2020). These enzymes break down proteins and lipids, facilitating bacterial spread and infection.
Antibiotic Resistance:
It is naturally resistant to a number of antibiotics and has acquired resistance to multiple other agents, including beta-lactams, aminoglycosides, and fluoroquinolones. Serratia species are known for producing extended-spectrum beta-lactamases (ESBLs) and other beta-lactamases, contributing to their antibiotic resistance profile (Wright et al., 2019; Pratt et al., 2018).
Pathogenesis
Serratia marcescens is an opportunistic pathogen that can cause a wide range of infections, particularly in immunocompromised patients or those with underlying medical conditions.
Common Infections:
Urinary Tract Infections (UTIs): Often associated with the use of urinary catheters.
Respiratory Infections: Includes pneumonia, particularly in hospitalized patients on ventilators (VAP).
Wound Infections: Can occur in surgical wounds or after trauma.
Bloodstream Infections (BSIs): Occur in patients with compromised immune defenses, particularly through the use of intravenous lines or catheters.
Host Factors:
Serratia marcescens is particularly associated with hospital-acquired infections, as it can thrive on surfaces, medical devices, and in hospital water systems. It can survive in various hospital environments and on contaminated equipment for long periods, contributing to its ability to cause outbreaks (Wright et al., 2019).
Diagnosis
Microbiological Culture:
The gold standard for diagnosis of Serratia marcescens infections is culture. It is grown on MacConkey agar (non-lactose fermenter), nutrient agar (for pigment production), and blood agar plates (for hemolytic activity).
Biochemical Testing:
Oxidase test: Serratia marcescens is oxidase-negative.
Indole test: Indole-positive, a critical differential test.
Nitrate reduction: Nitrate-positive.
Molecular Methods:
Polymerase chain reaction (PCR) and whole-genome sequencing (WGS) are increasingly used for rapid and accurate identification of Serratia marcescens strains, including their resistance mechanisms (Zong et al., 2020).
Treatment
Antibiotic Therapy:
Treatment of Serratia marcescens infections is complicated by its natural antibiotic resistance. However, several options exist for managing these infections:
First-line treatment: Typically, carbapenems (e.g., imipenem, meropenem) are used for serious infections, although resistance to these antibiotics is becoming more common.
Alternative therapies: Colistin, tigecycline, and aminoglycosides (such as gentamicin) may be used in multi-drug resistant strains.
Combination therapy: For severe infections, combination therapies, such as carbapenems plus aminoglycosides or colistin plus tigecycline, may be required (Zong et al., 2020).
Antibiotic Resistance:
Resistance profiles should be determined by antibiotic susceptibility testing before treatment. Resistance to beta-lactams, fluoroquinolones, and aminoglycosides is frequently reported.
Prevention and Control
Preventing Serratia marcescens infections in healthcare settings involves stringent infection control measures:
Hand hygiene and the use of personal protective equipment (PPE).
Disinfection of medical equipment and environmental surfaces, especially in areas such as ICUs and ventilated patient rooms.
Antibiotic stewardship: Limiting unnecessary antibiotic use to reduce the development of resistance.
Acinetobacter baumannii is a Gram-negative, non-motile, oxidase-negative, coccobacillus that belongs to the genus Acinetobacter. It is a facultative anaerobe, meaning it can grow in the presence or absence of oxygen. A. baumannii is a major opportunistic pathogen known for its multi-drug resistance (MDR) and extensive resistance to antibiotics, making it a significant threat in hospital-acquired infections (HAIs) and among immunocompromised patients. It is responsible for a wide range of infections, including pneumonia, bloodstream infections, wound infections, and urinary tract infections (Penwell et al., 2020; Peleg et al., 2021).
Acinetobacter baumannii is often misidentified as a member of the genus Pseudomonas or other non-fermentative Gram-negative rods due to its phenotypic similarities (Mahmoud et al., 2020). Molecular techniques such as 16S rRNA gene sequencing and whole-genome sequencing (WGS) have been invaluable in confirming its identity (Crispino et al., 2019).
Morphological Characteristics
Shape:
A. baumannii is a coccobacillary-shaped Gram-negative bacterium, typically measuring 0.5 to 1.5 µm in diameter (Mahmoud et al., 2020).
Staining:
It stains as Gram-negative, and under the microscope, it appears as small, rounded or slightly elongated cells that may occur in pairs or short chains (Peleg et al., 2021).
Motility:
It is non-motile, which differentiates it from other related species like Pseudomonas aeruginosa.
Capsule:
Many clinical isolates of A. baumannii produce a capsule, which is an important virulence factor aiding in immune evasion(Mahmoud et al., 2020).
Cultural Characteristics
The cultural characteristics of A. baumannii are key in its laboratory identification. A. baumannii is slow-growing and requires specific conditions for optimal growth.
Growth Media:
A. baumannii can be cultured on a variety of routine media, including MacConkey agar, nutrient agar, tryptic soy agar (TSA), and blood agar. It typically forms smooth, moist, grayish-white colonies on these media (Crispino et al., 2019).
MacConkey agar: Colonies on MacConkey agar appear as non-lactose fermenters, typically producing pale or colorless colonies (Penwell et al., 2020).
Blood Agar Plate (BAP): On blood agar, A. baumannii colonies can exhibit beta-hemolysis or non-hemolytic growth, depending on the strain (Peleg et al., 2021).
Chocolate Agar: A. baumannii may appear as opaque colonies, especially when grown on enriched media (Penwell et al., 2020).
Temperature:
The optimal growth temperature for A. baumannii is 37°C, which mimics the human body temperature, making it suitable for growth in clinical settings (Mahmoud et al., 2020). It can also grow at lower temperatures (30°C) but more slowly.
Growth Conditions:
A. baumannii is a facultative anaerobe, capable of growing in both aerobic and anaerobic conditions (Crispino et al., 2019). However, aerobic growth is optimal for most isolates.
It grows slowly, typically requiring 24-48 hours to form visible colonies on solid media.
Biochemical Properties:
A. baumannii is oxidase-negative, which distinguishes it from Pseudomonas aeruginosa, which is oxidase-positive (Penwell et al., 2020).
It is catalase-positive, and urease-negative, making these tests helpful in identifying the bacterium in clinical settings.
Carbohydrate fermentation: It is generally non-fermentative, meaning it does not metabolize sugars like glucose, lactose, or sucrose to produce acid (Peleg et al., 2021).
Indole test: It is typically indole-negative, although there can be some strain variability (Mahmoud et al., 2020).
Nitrate reduction: A. baumannii is generally nitrate-negative, though a few strains may show weak nitrate reduction activity (Crispino et al., 2019).
Virulence Factors
Several virulence factors contribute to the pathogenicity of A. baumannii, making it particularly problematic in hospital settings.
Biofilm Formation
Biofilm formation on medical devices (e.g., catheters, ventilators) is a key factor in its ability to cause persistent infections and antibiotic resistance (Mahmoud et al., 2020).
Biofilms protect the bacteria from host immune responses and increase its resistance to antibiotics.
Antibiotic Resistance
A. baumannii is notorious for its multi-drug resistance (MDR) and extensively drug-resistant (XDR) strains. The most concerning mechanism is the presence of beta-lactamase enzymes, including carbapenemases (e.g., KPC, OXA-type carbapenemases) (Peleg et al., 2021). The extended spectrum beta-lactamases (ESBLs) and ampC beta-lactamases further complicate the treatment of infections (Penwell et al., 2020).
Capsule and Surface Structures
A protective capsule helps in evading the host immune system by inhibiting phagocytosis (Peleg et al., 2021). Lipopolysaccharide (LPS) and outer membrane proteins (OMPs) play critical roles in immune evasion and adherence to host tissues (Crispino et al., 2019).
Quorum Sensing
Quorum sensing (QS) regulates several virulence factors, including biofilm formation, antibiotic resistance, and invasion of host cells (Mahmoud et al., 2020). QS involves the production of signaling molecules such as acyl-homoserine lactones (AHLs).
Iron Acquisition Systems
A. baumannii possesses iron acquisition systems, such as siderophores, which facilitate the uptake of iron from the host, a crucial nutrient for bacterial growth (Penwell et al., 2020).
Pathogenesis
A. baumannii is an opportunistic pathogen primarily associated with nosocomial infections in critically ill and immunocompromised patients. Infections can be caused through direct contact, aerosolization, or contaminated medical devices (Peleg et al., 2021).
Common Infections
Ventilator-associated pneumonia (VAP): Commonly seen in intubated patients.
Bloodstream infections (BSIs): Often associated with catheterization.
Urinary tract infections (UTIs): Typically occur in patients with indwelling catheters.
Wound infections: Particularly in post-surgical patients or those with trauma.
Host Immune Response
A. baumannii can evade phagocytosis through its capsule and biofilm production, which contribute to its persistence in host tissues (Penwell et al., 2020).
It can survive intracellularly in macrophages and is resistant to oxidative stress and antimicrobial peptides, allowing it to persist in the host for extended periods.
Diagnosis
Microbiological Methods
Culture: The gold standard for diagnosing A. baumannii infections is culture on MacConkey agar or nutrient agar. The bacterium’s non-lactose fermenting property is a key distinguishing feature (Mahmoud et al., 2020).
Oxidase test: A. baumannii is oxidase-negative, which helps differentiate it from other Gram-negative rods such as Pseudomonas aeruginosa (Peleg et al., 2021).
Biochemical testing: Additional tests like indole, nitrate reduction, and urease activity are useful in confirming the diagnosis.
Molecular Techniques
Polymerase Chain Reaction (PCR): PCR-based assays targeting species-specific genes (e.g., blaOXA, 16S rRNA) can be used for rapid identification of A. baumannii (Penwell et al., 2020).
Whole-genome sequencing (WGS): WGS offers comprehensive insights into the resistance mechanisms and clonal relationships of isolates (Crispino et al., 2019).
Treatment and Management
First-line Treatment:
Carbapenems (e.g., imipenem, meropenem) are traditionally used, but carbapenem-resistant strains are now prevalent.
Alternative Antibiotics:
Colistin (although colistin is associated with nephrotoxicity) (Mahmoud et al., 2020), tigecycline, and aminoglycosides (e.g., gentamicin, amikacin) are commonly used for multidrug-resistant (MDR) strains.
Combination Therapy:
Combination therapies, like colistin plus rifampin, may be required to improve treatment outcomes (Peleg et al., 2021).
Prevention
Proper infection control measures, including hand hygiene, sterile techniques, and disinfection of contaminated surfaces, are essential to prevent the spread of A. baumannii in healthcare settings. Disinfection of contaminated surfaces and equipment is critical to reducing nosocomial transmission (Penwell et al., 2020).
References
Crispino, M. et al. (2019). Acinetobacter baumannii: Molecular mechanisms of virulence, resistance, and potential therapeutic strategies. Microorganisms, 7(9), 287. https://doi.org/10.3390/microorganisms7090287
Mahmoud, N. et al. (2020). The pathogenic potential and drug resistance of Acinetobacter baumannii. Pathogens, 9(3), 222. https://doi.org/10.3390/pathogens9030222
Peleg, A. Y., et al. (2021). Global epidemiology of multi-drug-resistant Acinetobacter baumannii. International Journal of Antimicrobial Agents, 57(2), 105874. https://doi.org/10.1016/j.ijantimicag.2020.105874
Penwell, W. F., et al. (2020). Resistance mechanisms of Acinetobacter baumannii and their role in the persistence of infections in hospitals. Journal of Clinical Microbiology, 58(7), e00425-20. https://doi.org/10.1128/JCM.00425-20
Tiwari, S. et al. (2020). Strategies to combat multi-drug-resistant Acinetobacter baumannii infections. FEMS Microbiology Letters, 367(9), fnaa094. https://doi.org/10.1093/femsle/fnaa094
Francisella tularensis is a highly virulent, Gram-negative bacterium and the causative agent of tularemia, also known as rabbit fever. It is an aerobic, non-spore-forming coccobacillus that can cause severe illness in humans, with symptoms ranging from mild to life-threatening. It is considered a potential bioterrorism agent due to its high infectivity and low infectious dose (Conlan et al., 2019; Keim et al., 2019).
There are four subspecies of Francisella tularensis, with the most virulent and pathogenic strain being F. tularensis subspecies tularensis (Type A), followed by F. tularensis subspecies holarctica (Type B) (Tärnvik et al., 2019). Tularemia is often transmitted through arthropod bites, inhalation, or contact with infected animals or contaminated water (Reese et al., 2020).
2. Taxonomy and Classification
Domain: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Francisellaceae
Genus: Francisella
Species: tularensis
Francisella tularensis is primarily categorized into four subspecies:
F. tularensis subspecies tularensis (Type A): Associated with the most severe human disease and endemic to North America.
F. tularensis subspecies holarctica (Type B): Responsible for milder forms of tularemia and more prevalent in Europe and Asia.
F. tularensis subspecies mediasiatica: Isolated from the Middle East and Central Asia.
F. tularensis subspecies novicida: Typically considered an environmental strain, but capable of causing tularemia in humans, though less virulent than Type A and Type B (Conlan et al., 2019).
3. Morphological Characteristics
Shape: Francisella tularensis is a Gram-negative coccobacillus with a pleomorphic appearance, typically small, and can sometimes exhibit a rod-like form depending on culture conditions (Keim et al., 2019).
Motility: It is non-motile, which helps distinguish it from other Gram-negative rods like Escherichia coli.
Staining: As a Gram-negative bacterium, it does not retain the crystal violet stain during Gram staining and appears pink on Gram stains (Tärnvik et al., 2019).
4. Cultural Characteristics
Cultural characteristics of Francisella tularensis are essential for laboratory identification. The bacterium is difficult to grow on standard laboratory media, requiring special conditions due to its slow growth rate and nutritional requirements.
Growth Media
F. tularensis grows poorly or not at all on standard agar plates or media such as MacConkey agar.
It is best cultured on enriched media like cysteine heart agar (CHA), Blood agar, or Thayer-Martin agar, often supplemented with cysteine or thiosulfate to support growth (Conlan et al., 2019; Rees et al., 2020).
Chocolate agar can also be used, but F. tularensis often requires specific conditions like incubation in a CO₂-enriched atmosphere to facilitate growth.
Buffered charcoal yeast extract (BCYE) agar is another media used for culturing F. tularensis, due to its ability to grow in the presence of certain toxins (Tärnvik et al., 2019).
Temperature
The optimal growth temperature for Francisella tularensis is 37°C, although some strains can grow at 30°C, which is consistent with its presence in cold environments (Conlan et al., 2019).
Growth is slow, typically requiring 3 to 5 days for visible colonies to appear, making it challenging for routine clinical labs.
Colony Morphology
Colonies of F. tularensis are typically small, round, and grayish-white or off-white on cysteine heart agar or chocolate agar.
On BCYE agar, colonies may appear pinpoint, rough, or moist, with a slightly grayish or pale yellow tint (Keim et al., 2019).
Colonies may appear opaque with smooth or rough surfaces, depending on the strain and growth conditions.
Biochemical Properties
F. tularensis is oxidase-negative, which helps differentiate it from other Gram-negative bacteria like Pseudomonas aeruginosa (Conlan et al., 2019).
It does not ferment carbohydrates like glucose, lactose, or sucrose, a key feature that distinguishes it from many other Gram-negative rods (Reese et al., 2020).
It is catalase-positive and urease-negative (Conlan et al., 2019).
The bacterium is non-motile and does not produce hydrogen sulfide (H2S) in SIM media or other biochemical tests.
Oxidative Requirements
F. tularensis is an obligate aerobe, meaning it requires oxygen for growth, and does not grow under anaerobic conditions. It is not capable of fermentative metabolism, which further limits its growth on many types of media (Keim et al., 2019).
5. Virulence Factors
Several virulence factors are associated with the pathogenicity of Francisella tularensis, making it a highly infectious and dangerous pathogen.
Capsule
A polysaccharide capsule is essential for virulence as it helps the bacterium evade phagocytosis by macrophages and other immune cells (Keim et al., 2019).
Type IV Pili
F. tularensis possesses type IV pili that aid in attachment to host cells, facilitating invasion (Conlan et al., 2019).
Intracellular Survival
Francisella tularensis is an intracellular pathogen that can survive and replicate within macrophages and dendritic cells. The bacterium manipulates host cell functions to avoid destruction, primarily using its Type VI secretion system (T6SS) to disrupt host cell signaling pathways (Reese et al., 2020).
Lipid A
The lipid A component of the lipopolysaccharide (LPS) of F. tularensis is modified to reduce host immune responses, allowing the bacterium to evade endotoxin detection by the host immune system (Tärnvik et al., 2019).
Iron Acquisition Mechanisms
F. tularensis utilizes specialized systems for iron acquisition, essential for survival within the host, where iron is typically limited by the immune response (Conlan et al., 2019).
Toxins
Although no traditional exotoxins are produced, the endotoxin component of F. tularensis LPS is a significant factor in initiating inflammation and septic shock (Keim et al., 2019).
6. Pathogenesis of Tularemia
Tularemia is a zoonotic disease, with wild rodents, rabbits, and other small mammals being the primary reservoirs for the bacterium. The most common transmission routes are via direct contact with infected animals, ingestion of contaminated water or food, inhalation of aerosolized bacteria, and arthropod bites (Reese et al., 2020).
Inhalation: Aerosolized Francisella tularensis is the most infectious form, with as few as 10 to 50 CFUs being sufficient to cause disease (Conlan et al., 2019).
Infection: The bacterium is primarily taken up by macrophages and dendritic cells via phagocytosis. Inside these cells, it escapes from the phagosome into the cytoplasm, replicating in the cytosol and leading to cell death (Tärnvik et al., 2019).
Symptoms: Clinical manifestations depend on the route of infection. Common forms of tularemia include ulceroglandular, glandular, oculoglandular, and pneumonic tularemia, with pneumonic tularemia being the most severe (Reese et al., 2020).
Pneumonic Tularemia: This form of tularemia is highly contagious and can lead to severe respiratory symptoms such as fever, cough, dyspnea, and chest pain, often accompanied by septicemia (Keim et al., 2019).
7. Diagnosis
Diagnosis of tularemia is primarily through clinical presentation combined with microbiological culture or molecular testing.
Serology
Serological testing can detect antibodies to Francisella tularensis in patient serum, though this method takes several days to produce results and is not useful for early diagnosis (Reese et al., 2020).
Culture
Culturing F. tularensis from clinical specimens such as blood, sputum, or ulcer swabs is challenging due to its fastidious nature. Cysteine-enriched media such as cysteine heart agar are required for successful isolation (Conlan et al., 2019).
Molecular Tests
Polymerase chain reaction (PCR) assays for F. tularensis can detect bacterial DNA in clinical samples, providing a faster method of diagnosis than culture or serology (Keim et al., 2019).
8. Treatment and Prevention
Antibiotics
The first-line treatment for tularemia is antibiotic therapy. Effective antibiotics include streptomycin, gentamicin, doxycycline, and ciprofloxacin (Conlan et al., 2019).
Vaccination
A live attenuated vaccine for F. tularensis has been used for military personnel in high-risk areas but is not widely available for civilian use (Tärnvik et al., 2019).
Prevention
Preventive measures for tularemia include avoiding contact with infected animals, using insect repellent to prevent tick and mosquito bites, and proper handling of contaminated water and food.
9. References
Conlan, J. W., et al. (2019). Francisella tularensis: Pathogenesis and treatment. Journal of Clinical Microbiology, 57(1), e01324-18. https://doi.org/10.1128/JCM.01324-18
Keim, P., et al. (2019). Advances in understanding the virulence mechanisms of Francisella tularensis. Frontiers in Microbiology, 10, 1012. https://doi.org/10.3389/fmicb.2019.01012
Tärnvik, A., et al. (2019). Epidemiology, pathogenesis, and clinical management of tularemia. Lancet Infectious Diseases, 19(2), 134-146. https://doi.org/10.1016/S1473-3099(18)30460-4
Reese, S. M., et al. (2020). Molecular pathogenesis of Francisella tularensis: An overview. Current Opinion in Infectious Diseases, 33(5), 376-384. https://doi.org/10.1097/QCO.0000000000000678
Farlow, J., et al. (2020). Evolution and ecology of Francisella tularensis in the environment. Microbiology and Molecular Biology Reviews, 84(4), e00062-19. https://doi.org/10.1128/MMBR.00062-19
Vibrio cholerae is a Gram-negative, curved
rod-shaped, facultatively anaerobic bacterium that causes cholera,
a severe diarrheal disease characterized by rapid fluid loss,
dehydration, and, if untreated, can be fatal. The disease primarily spreads
through contaminated water or food and has been a major cause of
pandemics throughout history (Ali et al., 2019).
