PH4-5: Food Microbiology Flashcards
What are the main food-borne bacterial pathogens?
Campylobacter, Salmonella, E.coli, Listeria
What are the main food-borne viruses?
Hepatitis A, Heptatis E, Norovirus, Enterovirus, Adenovirus, Rotavirus
Define the microbiological criteria for foodstuffs?
Defined limits on presence or number of microbes or metabolites in food
Who is responsible for microbiological criteria for foodstuffs?
FBO is responsible to ensure product meets the requirements
Competent authority (ie OV) responsible for compliance
What are the three areas where microbiological criteria for foodstuffs are defined?
Process hygiene criteria, food safety criteria, meat and working surface testing
What is the purpose of testing against the process criteria?
It is not to assess the fitness of individual carcases or processed meat for human consumption but rather to provide an indication of performance and control of the slaughter, dressing, and production hygienic process at the time of sampling.
What are the requirements for process hygiene criteria?
- Aerobic colony count and Enterobacteriaceae on cattle, sheep, goats, horses, and pig carcases
- Salmonella on cattle, sheep, goats, horses, pig, broiler, and turkey carcases
- Aerobic colony count and E. coli in minced meat and mechanically separated meat
- E. coli in meat preparations
What is the purpose of the food safety criteria?
That the food safety criteria applicable throughout the shelf-life of the products can be met under reasonably foreseeable conditions of distribution, storage, and use
For what products have food safety criteria been set?
Poultry meat, minced meat, meat preparations, meat products, mechanically separated meat
What are the food safety criteria regarding Salmonella?
Salmonella must be absent in:
- Minced meat and meat preparations intended to be eaten raw
- Minced meat and meat preparations intended to be eaten cooked
- Mechanically separated meat (MSM)
- Meat products intended to be eaten raw
- Meat products made from poultry meat intended to be eaten cooked
- fresh poultry meat
What are the food safety criteria regarding Listeria?
The absence of L. monocytogenes in:
- RTE foods intended for infants
- RTE able to support the growth of L. monocytogenes other than those intended for infants
- RTE unable to support the growth of L. Monocytogenes other than those intended for infants
What are the criteria for selecting a test?
- Sensitivity (proportion of true-positives identified as positive by the test)
- Specificity (proportion of true-negatives identified as negative by the test)
- Limits of detection for enumeration tests
- Take taken to carry out the test
- Ease-of-use of the test
- Cost of test
What should you keep in mind when interpreting test results?
- The accuracy of the test: determined by sensitivity and specificity
- Result is only valid for one specific point in time
- Failure to detect specific organisms does not mean that metabolites or toxins of that organism are not present
- Results are only valid for the sample tested, bacteria may not be evenly distributed throughout a batch of product
What are the different methods of rapid detection?
IMS (Immunomagnetic separation)
HGMF (Hydrophobic grid membrane filter)
DEFT (Direct Epifluorescence Technique)
ELISA (Enzyme Linked Immunosorbent Assay)
PCR
Serum Agglutination
Whole Genome Sequencing
How does HGMF work? What is it used for?
- Filter sample to remove particular matter
- Filter through HGMF to trap organisms
- 1600 grid squares on membrane
- Transfer to agar plate for culture
Used for detection and enumeration of bacteria
How does IMS work? What is it used for?
Used to separate and concentrate target organism
- Specific antibody fixed on magnetic beads added to broth
- Binding of pathogen to antibody
- Magnet used to immobilise pathogen
- Pathogen then subject to other tests
How does DEFT work? What is it used for? Any drawbacks?
Useful for rapid counting of bacteria in samples
- Used in conjunction with membrane filter
- Stain filter membrane acridine orange, wash and dry
- Observe under epifluorescence microscope
- ssDNA and RNA fluoresces red with bacteria; dsDNA fluoresces green with eukaryotic cells
Test only indicated the general presence of bacteria but does not identify individual species?
How does an ELISA test work? What is it used for?
Rapid detection of specific organisms and toxins
- Test plate coated with specific primary antibody to organism of interest
- Organism in sample binds to antibody
- Washing to remove non-target organisms
- Secondary antibody then added - labelled with horseradish peroxidise
- Further wash step
- Substrate for enzyme then added
- If target organism is present it will be bound by primary antibody - secondary antibody with enzyme attached in turn will bind to target; addition of substrate will then result in colour change
How does PCR work? What is it used for? Any drawbacks?
ID for single pathogen strain
PCR identifies both viable and non-viable organisms as well as DNA in dead cells
Define thinning. How does it impact food safety?
When a producer is going to the poultry shed and pick out any birds that are not growing well and may have other problems
Going into the shed halfway through the production shed increases that chance that you are bringing in diseases
What are the two types of salmonellosis?
Enteric and typhoidal
What are the different pathotypes of E. coli?
Entertoxigenic - ETEC
Enteropathogenic - EPEC
Enteroaggregative - EAEC
Enteroinvasive - EIEC
Diffusely adherent - DAEC
Enterohaemorrhagic/Shiga-toxigenic/verotoxigenic - EHEC/STEC/VTEC
What is the most important control measure for STEC at the abattoir?
Only send clean cattle for slaughter.
Describe the relationship between viruses and food.
Viruses do not multiply and do not produce toxins in foods, food items merely act as vehicles for their transfer
As such, they are not zoonotic. Viral foodborne infections are limited to the recycling of human viruses back to humans.
What are the two separate entities of hepatitis? What is the difference between them?
Infectious: faecal-oral route transmission or “enteric” hepatitis
Serum: parenterally transmitted
What are the advantages and disadvantages of conventional culture?
Advantages
- Developed and validated over long period of time
- Considered gold standard tests
- Standardised methods
- Relatively cheap
- Easy to perform with minimal training
Disadvantages
- Slow
What are the different types of bacteriological agar?
Luria-Bertani Agar/Nutrient Agar
MacConkey Agar
Xylose Lysine Deoxycholate (XLD) Agar
Blood agar
Selective for E. coli
Charcoal-cefoperazone-desoxycholate agar (CCDA)
What grows on nutrient agar? What do bacteria look like on this agar?
It is non-selective, non-chromogenic
Most bacteria appear similar
What grows on MacConkey agar? What do bacteria look like on this agar?
Selective for enterobacteriacaea
Lactose fermenters appear pink/red (E. coli)
Non-fermenters appear white (Salmonella)
What grows on XLD agar? What do bacteria look like on this agar?
Selective for Salmonella and Shigella
Salmonella can metabolise thiosulphate to produce hydrogen sulphite and turn black in the center
What grows on Blood agar? What do bacteria look like on this agar?
Non-selective
Used for assessment of haemolysis, especially for staph and strep
What grows on CCDA agar? What do bacteria look like on this agar?
Selective for Campylobacter
