INFECTIOUS AGENTS - Food Microbiology Flashcards
What is microbiological sampling of food?
Microbiological sampling of food is the process of collecting and analysing samples of food products to detect the presence of pathogens
What are the four purposes of food microbiological sampling?
Verification of hygienic food production and handling
Determine shelf life performance
Investigate food suspected to be the cause of food poisoning outbreaks
To verify the quality of imported food
What are the two main methods used to process and analyse food microbiological samples?
Culture
PCR
What are the six steps used to culture food microbiological samples?
Sample collection
Sample homogenising
Pre-enrichment
Selective enrichment
Isolation on selective agar
Biochemical or serological confirmation of colonies
How often should sampling collection be carried out within a meat plant?
At least weekly
How many carcasses should be sampled within a meat plant at least one a week?
Five carcasses
For poultry, how many carcasses should be sampled at least one a week?
Fifteen carcases
In a meat plant, what size samples should be taken using destructive sampling techniques (i.e. tissue sampling)?
Four 20cm2 tissue samples
In a meat plant, what size samples should be taken using non-destructive sampling techniques (i.e. swabs and sponge)?
100cm2 samples
What are the three main methods of microbiological sample collection?
Sponge
Swab
Tissue sample
What should be always used when sampling directly from the carcass?
10cm templates
What is homogenisation?
Homogenisation is the process of breaking down and mixing food samples to ensure that the pathogens present in the food are evenly distributed throughout the sample
What machine is used to carry out homogenisation?
Stomacher
What solution is used for pre-enrichment in food microbiology?
Buffered peptone water
What solution is used for selective enrichment in food microbiology?
Selective broth
How long are microbiological food samples left to incubate in pre-enrichment and selective enrichment solutions?
Overnight for both pre-enrichment and selective enrichment
Which microbiological test should always be carried out when culturing microbiological food samples?
Total viable counts (TVC)
Describe how to carry out a total viable count (TVC)
Carry out serial dilution and plate a portion of each dilution onto a sutiable selective agar plate. Count the number of colonies and calculate the total viable count through multiplying the number of colonies with the dilution factor
Which four factors can often cause damage to pathogens on food prior to microbiological sampling?
Heat
Storage
Chilling
Disinfectants
Give two examples of specialised media that can be used to restore damaged pathogens
Tryptone
Buffered peptone water
What are the seven main species of bacteria found in food?
Salmonella
Campylobacter
E.coli 0157
Clostridium perfringens
Staphylococcus aureus
Listeria monocytogenes
Clostridium botulinum
Which anatomical region of the chicken is used for sampling for salmonella and campylobacter?
Chicken neck
Why should Campylobacter be cultured in conditions with increased carbon dioxide (CO2) and decreased oxygen (O2)?
Campylobacter is microaerophillic
Which biochemical test should be used to confirm the presence of campylobacter?
Analytical profile index (API)
Which specific agar should be used to culture E.coli 0157?
Sorbitol MacConkey Agar (SMAC)
Why is sorbitol MacConkey agar (SMAC) used to culture E.coli 0157?
Sorbitol MacConkey agar (SMAC) contains sorbitol as a fermentable sugar and a pH indicator which helps to differentiate E.coli 0157 from other strains of E.coli as E.coli 0157 is unable to ferment sorbitol, resulting in translucent colonies
Other than culture, which other technique can be used to isolate E.coli 0157?
Immunomagnetic separation
Which serological test can be used to confirm the presence of clostridium perfringens?
ELISA to detect enterotoxin
Which specific agar should be used to culture staphylococcus aureus?
Mannitol salt agar
Which serological test can be used to confirm the presence of staphylococcus aureus?
ELISA to detect enterotoxin
Which two biochemical tests can be done to confirm the presence of staphylococcus aureus?
Coagulase test
DNAse test
How can a coagulase test be used to confirm the presence of staphylococcus aureus?
One of the characteristic features of staphylococcus aureus is the production of coagulase. A positive coagulase test is determined if the plasma used within the test forms a clot (image b and c) indicating that coagulase has been produced and converted fibrinogen to fibrin
How can a DNAse test be used to confirm the presence of staphylococcus aureus?
One of the characteristic features of staphylococcus aureus is the production of DNAse. A positive DNAase test is determined if there is a clear zone surrounding the bacterial growth on the DNA agar
What are psychotrophs?
Psychotrophs are bacteria that can survive at 4°C (i.e. fridge temperature)
Which two bacteria that can be found in food are psychotrophs?
Listeria monocytogenes
Clostridium botulinum
Which specific agar should be used to culture clostridium botulinum?
Clostridial agar
Which biochemical test should be carried out to confirm the presence of clostridium botulinum?
Analytical profile index (API)
Why should food manufacturers not solely rely on the results obtained from microbiological food samples?
Food manufacturers should not solely rely on results obtained from microbiological food samples as the absence of target pathogens in these samples is not always representative of the entire food product
Which two methods should food manufacturers use to in addition to food microbiological sampling?
Good manufacturing practice
Hazard analysis and critical control points (HACCP)
What are the six factors you should describe when examining a bacterial colony on an agar plate?
Shape
Size
Elevation
Surface
Colour
Opacity
How would you describe the shape of a bacterial colony on an agar plate?
Round or Irregular
How would you describe the size of a bacterial colony on an agar plate?
Describe the diameter of the colony in millimeters (mm)
How would you describe the elevation of a bacterial colony on an agar plate?
Low convexed or Domed
How would you describe the surface of a bacterial colony on an agar plate?
Bacterial colonies can be described as smooth; glistening; dull/dry or granular
How would you describe the opacity of a bacterial colony on an agar plate?
Transparent: when viewed from the bottom of the plate, none of the colonies can be seen
Translucent: when viewed from the bottom of the plate, the presence of a colony can be seen but light can pass through
Opaque: when viewed from the bottom, no light can be seen through the colony
Identify and describe this bacteria under the microscope
Campylobacter (curved, Gram negative rods)
Identify and describe this bacteria under the microscope
E.coli 0157 (Gram negative rods)
Identify and describe this bacteria under the microscope
Clostridium perfringens (large Gram positive rods)
Identify and describe this bacteria under the microscope
Staphylococcus aureus (Gram positive cocci)
Why is there microbiological criteria for food?
To provide guidance on the acceptabilily of food and the manufacturing process
What are the eight components of microbiological criteria for food?
- A statement on the microorganisms or their toxins of concern and reason for that concern
- Analytical methods
- Number and size of samples
- Micobiological limits considered appropriate for that food at specific points in the food chain
- Number of analytical units that should conform to these limits
- Food to which the criteria applies
- Point in the food chain where criteria arises
- Action to be taken when criteria not met
What is the main bacteria of concern in meat products?
Salmonella
What is the main bacteria of concern in ready-to-eat food products?
Listeria monocytogenes
What action should be taken when the microbiological criteria of food is not met?
If microbiological criteria are not met, the food business operator (FBO) must ensure that the individual batches which do not meet the criteria are disposed of safely or reprocessed in an approved fashion
What can you check to identify food spoilage?
State of packaging
Surface appearance of the food
Deep appearance of the food
Smell of the food
Taste of the food
What are the six mechanisms of food spoilage?
- Sugar fermentation with acid and gas production
- Production of proteases which break down proteins
- Production of pectinases which break down complex carbohydrates
- Production of lipolytic enzymes which break down lipids
- Oxidation of organic acids
- Surface growth of pathogens
Which fungal spores present as black spots of mould?
Cladosporium
Which fungal spores present as white spots of mould?
Sporotrichum
Which fungal spores present as whiskers of mould?
Mucor
Give two examples of fungi which produce bluish-green moulds
Aspergillus
Penicillium
What are the three spoilage manifestations of dry, shelf-stable foods?
Whiskers/spots
Unpleasant odour
Discolouration
What are the three spoilage manifestations of fermented foods?
Yeast growth
Sliminess
Greening
What are the three spoilage manifestations of cured foods?
Sliminess
Blackening/discolouration
Souring
What are the five spoilage manifestations of chilled, vacuum packaged foods?
Ropiness
Souring
Loss of vacuum
Sliminess
H2S odour
What are the two spoilage manifestations of canned foods?
Souring
H2 swell
