BIO 250 MICROBIOLOGY LABORATORY 1 2026
COMPREHENSIVE PRACTICAL QUESTIONS
AND SOLUTIONS VERIFIED
◉ What are EMB and McConkey Agar selective for. Answer: Gram
Negative Bacteria
◉ How does EMB agar select gram negative bacteria. Answer: it uses
eosin and methylene blue dyes to inhibit the growth of gram positive
bacteria
◉ How does McConkey agar select gram negative bacteria. Answer:
it uses blue salts and crystal violet dyes to inhibit the growth of gram
positive bacteria
◉ What are EMB and McConkey Agar differential for?. Answer:
lactose and non-lactose fermenters
◉ How does EMB agar differentiate between lactose and non-lactose
fermenters?. Answer: it uses lactose as its medium and when the
acid is fermented, the dyes react on the colony surface, coloring it
and eosin and methylene blue inhibit gram positive bacteria
, ◉ How does McConkey agar differentiate between lactose and non-
lactose fermenters?. Answer: the medium contains a natural pH red
dye and lactose and when lactose is fermented, the clear colonies
become pink on the agar and crystal violet and bile salts inhibit
gram positive bacteria
◉ How many colonies are on a countable plate?. Answer: between
30-300 CFUs
◉ NON-LACTOSE FERMENTERS on the EMB and McConkey agars
appear ____________________.. Answer: Colorless
◉ On EMB agar, LACTOSE FERMENTERS appear ____________________
(small and large acid production).. Answer: Metallic Green for large
acid production Purple for small acid production
◉ Beta-hemolysis (complete cell lysis). Answer: Clearing around
growth
◉ Alpha-hemolysis (partial cell lysis). Answer: Greening around
growth
◉ Gamma-hemolysis (no cell lysis). Answer: No change in the
medium
COMPREHENSIVE PRACTICAL QUESTIONS
AND SOLUTIONS VERIFIED
◉ What are EMB and McConkey Agar selective for. Answer: Gram
Negative Bacteria
◉ How does EMB agar select gram negative bacteria. Answer: it uses
eosin and methylene blue dyes to inhibit the growth of gram positive
bacteria
◉ How does McConkey agar select gram negative bacteria. Answer:
it uses blue salts and crystal violet dyes to inhibit the growth of gram
positive bacteria
◉ What are EMB and McConkey Agar differential for?. Answer:
lactose and non-lactose fermenters
◉ How does EMB agar differentiate between lactose and non-lactose
fermenters?. Answer: it uses lactose as its medium and when the
acid is fermented, the dyes react on the colony surface, coloring it
and eosin and methylene blue inhibit gram positive bacteria
, ◉ How does McConkey agar differentiate between lactose and non-
lactose fermenters?. Answer: the medium contains a natural pH red
dye and lactose and when lactose is fermented, the clear colonies
become pink on the agar and crystal violet and bile salts inhibit
gram positive bacteria
◉ How many colonies are on a countable plate?. Answer: between
30-300 CFUs
◉ NON-LACTOSE FERMENTERS on the EMB and McConkey agars
appear ____________________.. Answer: Colorless
◉ On EMB agar, LACTOSE FERMENTERS appear ____________________
(small and large acid production).. Answer: Metallic Green for large
acid production Purple for small acid production
◉ Beta-hemolysis (complete cell lysis). Answer: Clearing around
growth
◉ Alpha-hemolysis (partial cell lysis). Answer: Greening around
growth
◉ Gamma-hemolysis (no cell lysis). Answer: No change in the
medium
