Question

6. Refer to Tables 13.2 and 13.3 and classify Escherichia coli, Bacillus subtilis and Enterobacter aerogenes as Gram-positive, Gram-negative, coliform or non-coliform a. E. coli- b. B. subtilis - C. E. aerogenes- 7. What is the role of eosin and methylene blue in EMB agar? 8. Why do fermenters produce dark colonies on EMB agar? 4 Page

Answer 1

Ans.6- Escherichia coli is a gram-negative bacteria because they stain negative using the Gram stain. E.coli is a coliform bacteria that is directly linked to fecal contamination.

Bacillus subtilis is a gram- positive and non-coliform bacteria.

Enterobacter aerogenes is a gram-negative coliform bacteria.

And.7- Eosin methylene Blue agar (EMB) is a selective and differential medium. It is selective culture medium for Gram-negative bacteria and helps in differentiation of coliforms and fecal coliforms. Eosin and methylene blue are pH indicator dyes which combine to form a dark purple precipitate at low pH. Eosin and methylene blue both inhibits the growth of gram-positive organism's. Also helps in distinguish between organisms those ferment lactose and those that do not.

Ans. 8- Fermenters on EMB agar are Gram- negative bacteria that ferment the lactose produced acid which turn the colonies dark purple in color as the acids acts upon the dye. Hence, fermenters produce dark colonies on EMB agar.