Question

Describe the colony characteristics of E. coli, Bacillus cereus, Bacillus subtilis, Staphylococcus epidermidis

Answer 1

Cultural Characteristics of E. coli

• E. coli is a facultative anaerobes.
• The optimum growth temperature is 37°C and it ranges from 10°C to 40°C.

E. coli on Nutrient Agar (NA)

1. They appear in the form of large, circular, low convex, grayish, white, moist, smooth and opaque.
2. They are mainly of 2 forms: Smooth (S) form and Rough (R) form.
3. The smooth forms are emulsifiable in saline.
4. Because of repeated sub culture, there is smooth to rough variation (S-R variation).

E. coli on Blood Agar (BA)

1. The colonies are big, circular, gray and moist.
2. Here, Beta (β) hemolytic colonies are formed.

E. coli on MacConkey Agar (MAC)

1. The colonies are are circular, moist, smooth and of entire margin.
2. Colonies appear flat and pink.
3. They are also lactose fermenting colonies.

Cultural Characteristics of Bacillus cereus

1. Most Bacillus spp are readily grown on nutrient agar or peptone media.
2. Optimum temperature which is required for growth varies from 20°C to 40°C, mostly 37°C.
3. B. cereus is mesophilic and is capable of adapting to a wide range of environmental conditions.
4. On the Nutrient Agar at 37°C, it forms large (2-5 mm) grey-white, granular colonies with a less wavy edge and less membranous consistency.
5. When on 5% sheep blood agar at 37°C, the cereus colonies are large, feathery, dull, gray, granular, spreading colonies and opaque with a rough matted surface and irregular perimeters.
6. In blood agar, it is beta-hemolytic.
7. Colony perimeters are irregular and they represent the configuration of swarming from the site of initial inoculation, perhaps due to cereus swarming motility.
8. In a very few instances smooth colonies develop either alone or in the midst of rough colonies.

S.epidermidis colonies

• S. epidermidis forms white, raised, cohesive colonies approximately 1-2 millimeter in diameter after incubation for an overnight on tryptic soy agar.
• Colonies are  nonhemolytic on blood agar.
• It is catalase-positive, coagulase-negative, facultative anaerobe which can grow by aerobic respiration or by fermentation.
• Some strains may not even ferment.