Culture media for common foodborne pathogens

Front

Culture media commonly used for the detection of Salmonella spp. in food, EXCEPT:

A) Mannitol Salt Agar

B) XLD Agar

C) Bismuth sulfite agar

D) Hektoen Agar

E) All of the above

Back

Front

Which culture medium is commonly used for the presumptive test of coliforms in water and food samples?

A) EMB Agar

B) Lauryl sulfate tryptose broth

C) VRBA

D) MacConkey Agar

Back

B) Lauryl sulfate tryptose broth (LSTB)

LST broth is used in the presumptive phase to detect gas-producing coliforms due to lactose fermentation under selective conditions.

Front

Which medium is used to differentiate E. coli O157:H7 based on non-sorbitol fermentation?

A) TSI Agar

B) EMB Agar

C) MacConkey Sorbitol Agar

D) Brilliant Green Agar

Back

C) MacConkey Sorbitol Agar

E. coli O157:H7 cannot ferment sorbitol and thus forms colorless colonies, unlike other E. coli strains.

Front

One of the culture media used in detecting Bacillus cereus in foods is Mannitol Yolk Polymyxin Agar or MYPA. What is the expected result when B. cereus is detected using this culture medium?

A) Pink colonies

B) Yellow colonies

C) Complete inhibition

D) Pink colonies w/ opaque zone

E) Colorless colonies

Back

D) Pink colonies w/ opaque zone

B. cereus does not ferment mannitol and produces pink colonies. Bacillus spp. other than B. cereus ferment mannitol and produce yellow colonies with phenol red as an indicator. B. cereus produces lecithinase, which degrades lecithin present in egg yolk and produces an insoluble compound.

Thus, pink colonies w/ opaque zone.

Front

What component in Oxford Agar and PALCAM Agar allows the differential screening and isolation of Listeria monocytogenes in food samples?

A.Acriflavine

B.Esculin

C.Sodium chloride

D.Phenol red

Back

Front

On MacConkey Agar, Escherichia coli O157:H7 typically forms _____ colonies.

A) Opaque pink

B) Opaque yellow

C) Colorless

D) Black

E. Bright pink

Back

C) Colorless

• MacConkey Agar contains lactose and a pH indicator (neutral red).

• Most E. coli strains ferment lactose, producing pink colonies due to acid production.

• However, E. coli O157:H7 is a non-lactose fermenter (or ferments very slowly), so it does not acidify the medium, resulting in colorless or pale colonies.

Front

Commonly used culture medium for the isolation and enumeration of Vibrio spp. from food and non-food samples:

A) EMB Agar

B) PALCAM

C) TCBS Agar

D) TSI Agar

E) MacConkey Agar

Back

C) TCBS Agar or Thiosulfate-citrate-bile-salts-agar

Front

Vibrio cholerae can be best recovered in TCBS Agar by increasing the ____ of the culture medium.

A) Salt concentration

B) Alkalinity

C) Bile salt concentration

D) Fermentable carbohydrate

E) Acidity

Back

B) Alkalinity

V. cholerae prefers an alkaline environment (optimal pH ~8.5–9.5), which:

• Enhances its growth, and suppresses the growth of competing intestinal flora.

Front

The most commonly used culture medium for the enumeration of Bacillus cereus in foods is:

A) Mannitol Polymyxin Agar

B) Mannitol Salt Agar

C) Malt Extract Agar

D) Dextrose-Bromcresol Purple Agar

E) Meuller Hinton Agar

Back

A) Mannitol Polymyxin Agar

MYP Agar is both selective and differential for Bacillus cereus:

• Mannitol: Used to test fermentation.

  • B. cereus does not ferment mannitol, so colonies remain pink (due to phenol red indicator).

• Egg yolk: Detects lecithinase activity (a virulence trait of B. cereus), producing a precipitate or halo around colonies.

• Polymyxin B: Suppresses the growth of most Gram-negative and some Gram-positive bacteria.

Front

What medium is recommended by ISO 4832 for the enumeration of coliforms in food and animal feed?

A) Plate Count Agar

B) EMB Agar

C) MacConkey Agar

D) SS Agar

E) VRBA

Back

E) VRBA (Violet Red Bile Agar)

VRBA is the standard medium recommended by ISO for coliform enumeration in foods, due to its selectivity and differential properties.

It is also a horizontal method - culturing microorganisms on a solid medium and counting the resulting colonies after incubation. Useful when dealing with large numbers of coliforms, potentially >100/mL or g of sample