Escherichia coli (E. coli)

Escherichia coli (E. coli)

1. Introduction to Escherichia coli (E. coli)

Escherichia coli (E. coli) is a facultatively anaerobic, Gram-negative, rod-shaped bacterium that belongs to the family Enterobacteriaceae. It was first described by Theodor Escherich in 1885 and has since become one of the most well-known bacterial species due to its importance in human health, its use in biotechnology, and its prevalence in both environmental and clinical contexts. While the majority of E. coli strains are harmless and are part of the normal intestinal flora of humans and animals, some strains are pathogenic and can cause diseases such as urinary tract infections (UTIs), gastroenteritis, and neonatal meningitis.

2. Classification

• Domain: Bacteria
• Phylum: Proteobacteria
• Class: Gammaproteobacteria
• Order: Enterobacterales
• Family: Enterobacteriaceae
• Genus: Escherichia
• Species: Escherichia coli

3. Morphological Characteristics

• Shape: Gram-negative, straight rods with a size range of 0.5–1.0 µm in diameter and 1.0–3.0 µm in length.
• Arrangement: Single, in pairs, or in short chains.
• Motility: Most strains are motile due to the presence of flagella, but non-motile strains also exist.
• Capsule: Some E. coli strains possess a capsule, which may contribute to virulence by providing protection against host immune responses (Kaper et al., 2004).

4. Cultural Characteristics

E. coli grows well in a variety of laboratory media under both aerobic and anaerobic conditions. Key cultural characteristics include:

• Growth Temperature: E. coli is mesophilic, growing optimally between 37°C (body temperature) and 42°C, but it can grow within a range of 7°C to 50°C.
• Oxygen Requirement: Facultatively anaerobic, meaning it can grow in the presence or absence of oxygen.
• Colony Morphology: On nutrient agar, E. coli colonies are typically:
• Shape: Round
• Color: Off-white to pale cream
• Texture: Smooth, moist, and shiny
• Size: 2–3 mm in diameter after 24 hours of incubation (Madigan et al., 2015).

5. Biochemical Characteristics

E. coli displays a wide range of biochemical properties that are useful for identification, including:

• Catalase: Positive, producing bubbles when hydrogen peroxide is added.
• Oxidase: Negative, differentiating it from oxidase-positive organisms like Pseudomonas spp.
• Lactose Fermentation: Positive (ferments lactose to produce gas and acid, usually indicated by a change in color of indicator media such as MacConkey agar).
• Indole Test: Positive; E. coli produces indole from tryptophan.
• Methyl Red (MR) Test: Positive, indicating mixed acid fermentation.
• Voges-Proskauer (VP) Test: Negative.
• Citrate Utilization: Negative.
• H2S Production: Typically negative, distinguishing it from other enteric bacteria such as Salmonella.

These characteristics are essential for laboratory identification of E. coli and its differentiation from other enteric bacteria.

6. Pathogenic Strains and Mechanisms of Virulence

While E. coli is part of the normal flora, certain pathogenic strains cause significant health issues. Pathogenic strains of E. coli are classified based on their virulence factors into different pathotypes, such as:

1. Enteropathogenic E. coli (EPEC):
• Cause diarrhea, particularly in young children and infants.
• Attach to the intestinal mucosa and form characteristic “pedestals” by disrupting the host cell's cytoskeleton.
2. Enterotoxigenic E. coli (ETEC):
• Produce enterotoxins (heat-labile and heat-stable toxins) causing traveler’s diarrhea and other gastrointestinal infections.
• The toxins stimulate the release of water and electrolytes into the lumen of the gut, leading to diarrhea.
3. Enterohemorrhagic E. coli (EHEC):
• Responsible for severe cases of foodborne illness, including hemolytic-uremic syndrome (HUS).
• EHEC produces Shiga toxins (Stx1 and Stx2), which can cause cell damage in the intestines and kidney tissues.
• Most notorious strain: O157

.

4. Enteroinvasive E. coli (EIEC):
• Similar to Shigella in that they invade and destroy intestinal epithelial cells, leading to dysentery-like symptoms.
5. Enteroaggregative E. coli (EAEC):
• Cause persistent diarrhea, particularly in children.
• Characterized by the ability to form biofilms on the intestinal surface.
6. Uropathogenic E. coli (UPEC):
• The most common cause of urinary tract infections.
• Possess virulence factors like type 1 pili and other adhesins that allow them to adhere to the uroepithelial cells.

7. Antibiotic Resistance

Antibiotic resistance in E. coli is a significant public health issue, as strains of E. coli that are resistant to common antibiotics, including extended-spectrum beta-lactams (ESBLs) and fluoroquinolones, are increasingly being identified (Vahaboglu et al., 2016). The mechanisms of resistance can be attributed to:

• Plasmid-mediated resistance.
• Horizontal gene transfer between different bacterial species, facilitating the spread of resistant genes.
• Efflux pumps and alterations in target sites.

8. Environmental and Clinical Relevance

1. Normal Flora: E. coli is a common component of the normal gut flora in humans and animals, where it contributes to the synthesis of certain vitamins and plays a role in the competitive exclusion of pathogens.
2. Foodborne Infections: Pathogenic E. coli strains are often transmitted through contaminated food or water. The primary sources are undercooked ground beef, raw vegetables, and unpasteurized milk.
3. Urinary Tract Infections: UPEC strains are responsible for the majority of UTIs, often ascending from the urethra to the bladder and sometimes reaching the kidneys.
4. Neonatal Meningitis: Some strains of E. coli, particularly those with the K1 capsule, can cause neonatal meningitis. This is a serious condition in newborns.
5. Biotechnology: Non-pathogenic strains of E. coli are widely used in molecular biology and biotechnology due to their ability to replicate plasmids, making them useful for recombinant DNA technology (Sambrook et al., 2001).

9. Detection Methods

• Culture-based: Isolating E. coli from stool or urine samples using selective media (e.g., MacConkey agar, Eosin Methylene Blue (EMB) agar).
• PCR: Detection of specific virulence genes such as stx1, stx2, and eae for pathogenic strains.
• Immunoassays: Detection of E. coli O157

using ELISA and other antigen-detection methods.

10. Conclusion

Escherichia coli is a versatile bacterium with both beneficial and harmful strains. While most E. coli strains are part of the natural intestinal microbiota and have vital roles in human health, certain pathogenic strains pose significant threats through the production of toxins and the ability to cause infections. The study of E. coli continues to be essential for understanding microbial pathogenesis, antibiotic resistance, and the development of new diagnostic techniques and treatments.

---

References

• Kaper, J. B., Nataro, J. P., & Mobley, H. L. (2004). Pathogenic Escherichia coli. Nature Reviews Microbiology, 2(2), 123-140. https://doi.org/10.1038/nrmicro821
• Madigan, M. T., Martinko, J. M., & Parker, J. (2015). Brock Biology of Microorganisms (14th ed.). Pearson Education.
• Vahaboglu, H., et al. (2016). Antibiotic resistance in Escherichia coli: The global rise of resistant strains. Journal of Clinical Microbiology, 54(3), 529-537.
• Sambrook, J., & Russell, D. W. (2001). Molecular Cloning: A Laboratory Manual (3rd ed.). Cold Spring Harbor Laboratory Press.