GCU Bio 205 Lab Final Exam
Questions and Answers (updated
2025-2026) Set.
Selective medium- give examples - Answer Allows the growth of certain bacteria while
preventing or retarding the growth of other bacteria.
Ex: mannitol salt agar, eosin methylene blue (EMB) agar, HEA agar
Differential medium- give examples - Answer Allows for growth of all bacteria while
differentiating them from one another.
Ex: MacConkey Agar, blood agar
MacConkey agar selects for which group of bacteria? explain - Answer Gram negative because
the crystal violet inhibits the growth of gram positive.
MacConkey agar media differentiates between enteric bacteria based on what biochemical
reaction? Explain - Answer Lactose fermentation which is the mixture of carb lactose, bile
salts, and pH indicator neutral red which permits differentiation of enteric bacteria.
If the organism is E. coli, what would you see on your plate? explain. - Answer You would see
pink it is a coliform bacteria which is a lactose fermenter that produces lactic acid by fermenting
the lactose in the media
If the organism(s) is/are other enteric, what would you see on your plate? explain. - Answer
Coliform bacteria- pink/red colonies because the pH is lowered and they are lactose fermenters
Dysentery, typhoid, and paratyphoid would indicate beige and transparent colonies because
they are lactose non-fermenters
EMB agar selects for which group of bacteria? explain - Answer Gram negative bacteria.
The dyes eosin Y and methylene blue inhibit the growth of gram positive bacteria and allow the
gram negative to grow.
EMB agar media differentiates between gram negative enteric based on what biochemical
reaction? - Answer Lactose fermentation. Large amounts of acid are produced, metallic green.
small amounts will make the colonies pink/red. No fermentation= colorless colonies
, If the organism is/are other enterics, what would you see on your plate? explain. - Answer
Purple/pink colonies from no lactose fermentation
HEA agar selects for which group of enteric specifically which bacteria? - Answer Gram
negative. specifically salmonella and shigella. differentiates from lactose fermenters and lactose
non-fermenters.
HEA agar differentiates bacterial groups based on what biochemical reaction? explain - Answer
Lactose fermenetation. Acid produced from fermentation will make the agar yellow-orange.
Production of hydrogen sulfide gas turns the medium black. Non fermenters of not change
color.
If the organism is Salmonells, what would you see on your plates? explain - Answer
pink/orange because if the acid produced from fermentation of lactose.
If the organism(s) is/are other enteric, what would you see on your plate? explain - Answer No
color change or sometimes black
MSA agar selects for which group of bacteria? explain - Answer specific species of
staphylococci. contains high salt concentration 7.5% NaCl
MSA agar media differentiates staphylococcal species based on what biochemical reaction?
explain - Answer Mannitol fermentation and phenol red
If the organism is Staph. aureus, what would you see on your plates? explain - Answer Gold
yellow colonies because it ferments manatee and will turn it yellow golden.
If the organism is other Staph. epidermis, what would you see on your plate? - Answer no
color change because it does not ferment mannitol
Staph. and Strep. are both gram positive cocci. What test would you perform to differentiate
them? - Answer catalase test
Catalase is produced by which bacteria? - Answer Staph. positive
strep. negative
Questions and Answers (updated
2025-2026) Set.
Selective medium- give examples - Answer Allows the growth of certain bacteria while
preventing or retarding the growth of other bacteria.
Ex: mannitol salt agar, eosin methylene blue (EMB) agar, HEA agar
Differential medium- give examples - Answer Allows for growth of all bacteria while
differentiating them from one another.
Ex: MacConkey Agar, blood agar
MacConkey agar selects for which group of bacteria? explain - Answer Gram negative because
the crystal violet inhibits the growth of gram positive.
MacConkey agar media differentiates between enteric bacteria based on what biochemical
reaction? Explain - Answer Lactose fermentation which is the mixture of carb lactose, bile
salts, and pH indicator neutral red which permits differentiation of enteric bacteria.
If the organism is E. coli, what would you see on your plate? explain. - Answer You would see
pink it is a coliform bacteria which is a lactose fermenter that produces lactic acid by fermenting
the lactose in the media
If the organism(s) is/are other enteric, what would you see on your plate? explain. - Answer
Coliform bacteria- pink/red colonies because the pH is lowered and they are lactose fermenters
Dysentery, typhoid, and paratyphoid would indicate beige and transparent colonies because
they are lactose non-fermenters
EMB agar selects for which group of bacteria? explain - Answer Gram negative bacteria.
The dyes eosin Y and methylene blue inhibit the growth of gram positive bacteria and allow the
gram negative to grow.
EMB agar media differentiates between gram negative enteric based on what biochemical
reaction? - Answer Lactose fermentation. Large amounts of acid are produced, metallic green.
small amounts will make the colonies pink/red. No fermentation= colorless colonies
, If the organism is/are other enterics, what would you see on your plate? explain. - Answer
Purple/pink colonies from no lactose fermentation
HEA agar selects for which group of enteric specifically which bacteria? - Answer Gram
negative. specifically salmonella and shigella. differentiates from lactose fermenters and lactose
non-fermenters.
HEA agar differentiates bacterial groups based on what biochemical reaction? explain - Answer
Lactose fermenetation. Acid produced from fermentation will make the agar yellow-orange.
Production of hydrogen sulfide gas turns the medium black. Non fermenters of not change
color.
If the organism is Salmonells, what would you see on your plates? explain - Answer
pink/orange because if the acid produced from fermentation of lactose.
If the organism(s) is/are other enteric, what would you see on your plate? explain - Answer No
color change or sometimes black
MSA agar selects for which group of bacteria? explain - Answer specific species of
staphylococci. contains high salt concentration 7.5% NaCl
MSA agar media differentiates staphylococcal species based on what biochemical reaction?
explain - Answer Mannitol fermentation and phenol red
If the organism is Staph. aureus, what would you see on your plates? explain - Answer Gold
yellow colonies because it ferments manatee and will turn it yellow golden.
If the organism is other Staph. epidermis, what would you see on your plate? - Answer no
color change because it does not ferment mannitol
Staph. and Strep. are both gram positive cocci. What test would you perform to differentiate
them? - Answer catalase test
Catalase is produced by which bacteria? - Answer Staph. positive
strep. negative
