Microbial detection

Question 1

Legal requirements for microbes

Answer 1

Standard 1.6.1 and schedule 27

Question 2

Recommendations for food standards

Answer 2

Compendium of Microbiological Criteria for Food
Monitors processes

Question 3

Sampling plans

Answer 3

2 class
- when a microbe is not permitted
- n, c, and m

3 class
- permittable level of microbes
- n, c, M, and m
n = number of sample units to be picked from a batch, c = maximum allowable number of sample units with positive results, M = separates good from unacceptable results, m = separates good from marginal results

Question 4

Testing for pathogens

Answer 4

1. Pre-enriched in non-selective broth > incubated cells will multiply
   
   2. Transfer to selective enrichment broth > only target pathogen will grow
   3. Streaked onto selective differential agar > colony characteristics suggest identity
   4. Isolated colonies sent off for further testing

Question 5

XLD agar

Answer 5

• like a combo of deoxycholate agar and MAC
• has sodium deoxycholate that inhibits the growth of gram +
• differentiates between H2S producers and non-producers

Question 6

Antibody-coated paramagnetic beads

Answer 6

Bead mixed with food sample, pulled out with a magnet
Detect attached bacterium

Question 7

Quantitative PCR

Answer 7

• Measures amount of DNA continuously ~ number of organisms in sample
  
  • DNA is attached to fluorescent dye > is detected
  • Signal increases exponentially until reagents run out
  • Time taken to get to positive signal that depends on the initial concentration of target DNA