MICROBIOLOGY FOR STUDENTS (M4S)

Biochemical Test performed in routine laboratory work 1. Catalase Test All of the Enterobacteriaceae are catalase positive except Shigella dysenteriae . 2. Oxidase Test i) Used to demonstrate the ability of a bacterium to produce the enzyme cytochrome- C oxidase, capable of reducing oxygen. ii) Only aerobic bacteria have this enzyme. This test will distinguish aerobic vs. anaerobic metabolism. iii) A positive test will show a color change to blue, then to dark purple or black, within 10 to 30 seconds. iv) Assays for the presence of cytochrome oxidase, an enzyme sometimes called indophenol oxidase. v) In the presence of an organism that contains the cytochrome oxidase enzyme, the reduced colorless reagent becomes an oxidized colored product. vi) The Family Enterobacteriaceae are oxidase negative. vii) The lists of Enterobacteriaceae are given below. Salmonella Shigella Enterobacter Proteus Serrati Erwinia Yersinia viii) The members of the...

Colony characteristics of bateria in CLED (Cysteine Lactose Electrolyte Deficienct Media) Colony characteristics of bateria in CLED (Cysteine Lactose Electrolyte Deficienct Media) In CLED (Cysteine Lactose Electrolyte Deficient) agar, which is commonly used for the isolation and differentiation of urinary pathogens, bacteria grow in distinctive ways. The characteristics of bacterial colonies on CLED agar depend on the organism's ability to ferment lactose and produce certain by-products. CLED agar is deficient in cysteine, which inhibits the growth of certain fastidious organisms, especially some gram-negative rods. Therefore, bacteria that require cysteine for growth may not grow or may show slower growth. Lactose fermenters produce colonies that appear as yellow, whereas colonies of non-lactose fermenters appear blue on CLED agar. [1] Photograph 1. Plate A in the illustration does not contain any bacteria growing on it; instead, it dis...

Colony characteristics of bateria on Blood Agar (BA) 1. Streptococcus pyogenes The Gram positive streptococcus produces pinpoint colonies which are circular with entire margins and convex elevation. 2. Streptococcus pneumoniae The Gram positive streptococcus produces colonies which are circular with entire margins, often raised with depressed centres. This microbe exhibits alpha haemolysis; a partial breakdown of the red blood cells in the medium. 3. Enterococcus faecalis This Gram positive coccus forms pinpoint colonies with convex elevation. This strain exhibits gamma haemolysis; growth on Blood Agar Plate but no lysis of the red blood cells in the agar.

Colony characteristics of bateria in Trypticase Soy Agar (TSA) 1. Bacillus subtilis : Colonies which are dry, flat, and irregular, with lobate margins. 2. Pseudomonas aeruginosa This Gram negative rod forms mucoid colonies with umbonate elevation. Some strains produce a diffusible green pigment and a distinctive fruity odour. 3. Staphylococcus aureus Circular, pinhead colonies which are convex with entire margins. This gram positive coccus often produces colonies which have a golden-brown colour. 4. Mycobacterium smegmatis The waxy appearance of Acid-fast bacterium. 5. Escherichia coli This Gram negative rod forms shiny, mucoid colonies which have entire margins and are slightly raised. Older colonies have entire margins and are slightly raised. Older colonies often have a darker centre. 6. Serratia marcescens These gram negative rods produce mucoid colonies which have entire margins and umbonate elevation. There are both red and ...