Food Microbiology Practical Exam

What organism is EMB (eoisin methylene blue) agar selective for?

EMB is a selective differential medium for COLIFORMS e.g., Escheria coli (E.coli). It inhibits gram positive organisms.

What medium is used to detect S.aureus? What is it’s appearance on this medium?

Baird Parker agar. S.Aureus appears on Baird Parker agar as shiny, convex metallic circles with a halo & a clear zone.

• Black colour: S.aureus reduces telluride → metallic tellurium, deposits in the colony & turns it black.

• Halo: lipases breakdown down egg yolk lipids → fatty acids forms cloudy ‘halo’.

What agar is used to detect L. monocytogenes? What is its appearance?

Oxford/PALCAM agar. L.monocytogenes appears on PALCAM agar as small grey green colonies with a black halo on a red background.

• Black halo: PALCAM contains esculin + ferric ammonium citrate → esculin + glucose, escultin reacts with Fe = black.

• Red background: L. monocytogenes DOESN’T ferment mannitol, no acid production so medium stays red.

What is the inoculation of Fraser Broth used to detect? What is the (presumptive) positive sign?

Detection of L.monocytogenes. Presumptive positive indicated with black broth as L.monocytogenes hydroluses esculin → reacts with Fe3+ for black colour.

What is the inoculation of Durham Tubes used to detect? What is the (presumptive) positive sign?

E.coli. As E.coli ferments lactose, the presence of gas in the Durham tubes indicates a presumptive positive as lactose fermentation → CO2, forming gas bubble. Needs to be confirmed with indole tryptone test.

What agar is used to detect E.coli? What is it’s appearance?

EMB agar. E.coli appears on EMB agar as small/medium circular dark blue/blackish/purple colonies with a green metallic sheen. EMB agar inhibits growth of gram positive organisms.

• Lactose fermentation: E.coli ferments lactose to mixed acids (i.e., lactic, acetic)

• Dark colours: low pH, 2 dyes bind + precipitate in the colony → dark colours.

• Green sheen: heavy acid production creates dense dye that reflects light.

Why is Salmonella used as a negative control when testing for E.coli?

Salmonella is used as a negative control when testing for E.coli as it does NOT ferment lactose (i.e., Durham tubes, Fraser Broth).

Why does L.monocytogenes need a second enrichment? (half Fraser broth → Fraser broth)

• Poor competitor (inhibit competing microbes)

• Often present in low doses

• Revive sublethally injured cells/cells in a VBNC state

What organism is shown on this stain? Is this a gram positive or gram negative organism?

S. aureus, gram positive organisms. Indicated by circular, grape like purple cocci and violet gram stain.

What organism is shown on this stain? Is this a gram positive or gram negative organism?

Listeria monocytogenes. Gram positive rods. Positive = purple.