Lab 3 - Sampling and the Detection of Pathogens

What are quantitative vs qualitative evaluation/results?

• Quantitative

  • Count colonies and calculate cell concentrations (log CFU/g or log CFU/mL)

• Qualitative

  • Positive/negative result or detected/non-detected

What is the LOD associated with any qualitative and quantitative detection method?

Lowest concentration/level of microorganisms present in a certain product that can be detected by one particular detection assay

• 50 CFU/mL for 0.1 mL spread plating method

• 5 CFU mL for 1 mL pour plating method

What are the important characteristics that can distinguish gastroenteritis-causing Salmonella from other bacteria of the same family (Enterobacteriaceae)?

1. They produce hydrogen sulfide from thiosulfate

2. Can’t ferment lactose or sucrose

^Used to differentiate Salmonella from E. coli

What is Universal Pre-enrichment Broth?

Used for recovering sub-lethally injured Salmonella and Listeria from food product

Why does Salmonella on Xylose Lysine Desoxycholate (XLD) agar look like black tops?

• Salmonella produces an enzyme, thiosulfate reductase, and the addition of SODIUM THIOSULFATE allows for sulfide production

• Can be fermented by all enterics except for Shigella

Why does Salmonella appear as black colonies on HE agar?

• Salmonella can’t ferment lactose, saccharose, and salicin, so agar remains blue-green color with black colonies

What are the three sugars in a triple sugar iron agar?

• Sucrose

• Dextrose

• Lactose