Microorganisms in food Flashcards
how are strains of species classified in microbiology?
mainly based on their virulence
-types of toxins produced, type of disease they cause, how they spread disease or specific host sites
what is the cut off for bacterial taxonomy using DNA-DNA hybridization?
70% similarity
what is average nuclei identity used for?
a method to determine bacterial taxonomy that involves genome sequencing to determine genetic relatedness
what are marker genes?
a method for bacterial taxonomy that allow us to use conserved regions within gene sequence to map out bacterial diversity and similarities
what percent of average nuclei identity do organisms with genomic similarity share?
> 95%
- If two bacteria have an ANI score above 95-96%, they are considered to belong to the same species
when does a majority of contamination of foods occur?
during processing / handling
True or False: naturally, most muscle tissue, eggs and milk are sterile
True with some exceptions
- most of the contamination occurs after processing
what 3 microorganisms are common in plants and soil?
Bacillus spp. Pseudomonas and Lactiplantibacillus plantarum
what is a high nutrient surface?
spaces that are in contact with food
what are most bacteria associated with foods called? what do they do?
chemotrophs
-use organic compounds as a source to produce ATP
what nutritional requirements to chemotrophs have?
most are fastidious which means they have specific or demanding nutritional or environmental requirements in order to grow
- some can get by with water and few nutrients
what are the nutrients required for the growth of bacteria?
what considerations must be made in regards to growth vs survival?
in some environments bact
why is it important to understand the microbial growth curve?
because the rate of growth determines:
-how fast a food will spoil / ferment
-whether pathogenic bacteria will grow and produce toxin
what do control mechanisms of microbial growth aim to do?
-accelerate growth in fermentations
-stop growth of pathogens
-slow growth of spoilage organisms
why do some pathogens need to be killed and not just have their growth stopped? what are examples of these pathogens?
they may have a very low infectious dose
- salmonella and campylobacter
what is generation time?
the time required for a population of bacteria to DOUBLE
what is the equation to calculate generation time?
the difference between time A/B x log of 2 divided by the difference of the log of # bacteria at point A/B
how do you calculate number of generations?
time / generation time
what is the difference between gram positive and gram negative?
Gram-positive bacteria: Thick peptidoglycan layer, no outer membrane, purple Gram stain, susceptible to antibiotics targeting cell walls.
Gram-negative bacteria: Thin peptidoglycan layer, outer membrane with LPS, pink Gram stain, more resistant to many antibiotics due to their outer membrane.
what colour will a gram positive stain be?
purple
what colour will a gram negative stain be?
pink
why is gram strain important?
this gives taxonomists guidance for classification:
- outer membrane or not is the first step in classification
what effect does the presence of an outer membrane have on bacteria?
it increases resistance of bacteria to antibiotics, bile salts, bacteriocins as well as some preservatives and sanitation agents
what is the difference between spore forming bacteria and vegetative bacteria?
spore forming: form inactive spores in harsh environments in order to survive
- makes them highly resistant to many factors that usually kill bacteria
vegetative: typically in active growing state- do not grow spores making them less resistant to changes in environments
what bacteria commonly form spores?
Bacillus spp. and Clostridium spp.
what are yeasts? how can they affect food spoilage?
Eukaryotes- considered quite safe but can lead to spoilage
-fermentation
what type of yeast is key for brewing and baking?
Saccharomyces
what are fungi / molds? how can they lead to spoilage?
filamentous, multicellular fungi known for their role in decomposition and food production
-Grow in wide range of environments - T, pH , aW and produce mycotoxins
what is the most common type of fungi found on foods?
Septate hyphae
what are viruses and how are they important in food microbiology?
microscopic infectious agents that can only replicate inside the genetic material (cells) of a host organism
they are important because they cause diseases in plants, humans, animals and bacteria
why do viruses need to have a host in order to reproduce?
they don’t have metabolite and need the metabolism of the host cell to reproduce
-reproduction usually causes death of host cell
how are viruses transmitted?
through foods
how do viruses replicate inside foods?
they don’t!
What would Hepatitis A and Norovirus be classified as?
Viruses
what host do bacteriophages affect?
bacteria
what are parasites?
Eukaryotes that live on or inside a host, deriving nutrients at the host’s expense
what are the 3 parasitic groups relevant to food?
Protozoa, flatworm and roundworms
how do parasites replicate in food?
they don’t!
what is a definitive host for parasites? what are the symptoms?
an animal in which an adult parasite carries out sexual cycle
-typically asymptomatic
what is a intermediate host for parasites? what are the symptoms?
an animal where larval or juvenile forms of a parasite develop
- associated with disease
what is a incidental host for parasites? what are the symptoms?
the parasite is unable to develop on host
- able to cause disease
what class of bacteria is Trichinella spiralis (trichinosis)? how is it controlled?
A parasite found in omnivores - rodents, humans, pigs and bears
-controlled by cooking meat
>65 degrees C or frozen
what are trophozoites?
the active, feeding, and growing stage of certain protozoan parasites
-can cause infection and have cysts that survive outside host
what are physical changes common in food spoilage?
drying, crystallization and flavour loss
what are chemical changes of food spoilage of lipids?
rancidity
oxidative - peroxides (chemical)
hydrolytic - free fatty acids (microbial )
what are chemical changes of food spoilage of proteins?
proteolysis (microbial)
-protein breakdown causing putrefaction (pungent odour production)
what are chemical changes of food spoilage of carbs?
fermentation (microbial)
- causes organic acids to sour
how does raw meat spoil under aerobic vs anaerobic conditions?
anaerobic - souring
aerobic - protein degradation (putrefaction)
what can be observed when bread has spoiled?
mould growth and starch crystallization
what can be observed when yogurt spoils?
acid production and growth of molds / yeast
what is the definition of a foods durable life?
the amount of time an unopened food product when stored in appropriate conditions will retain its freshness and nutritional values
when must a food have a best before date?
when its packaged at a place other than the retail store and has a durable life < 90 days
if a food is packaged by the retailer and has a durable life < 90 days what label must it contain?
A “packaged on” date
what are examples of foods that need an expiration date?
liquid diets, low energy prescription diets, meal replacements, nutritional supplements and instant formula
what can be assumed about a product if it can be consumed >90 days?
it is shelf stable
what do expiration dates indicate?
how long a food will maintain its nutritional value and freshness
are BBD and expiration dates an indication of food safety?
no
what ways are food infection possible?
bacteria/virus growth surviving in the food (few organisms)
bacteria after growth in food (many organisms)
how is food intoxication possible?
intoxication by bacterial or fungal toxins
how much cells is needed to produce toxin in bacterial vs fungal toxins?
bacterial - few to many
fungal - many
what is a toxicoinfection?
illness caused by ingestion of food containing pathogens which produces toxin in GI tract
what is classified as a food intoxication?
toxin produced in the food then the food is ingested and illness occurs
what do Bacillus cereus,
Clostridium botulinum,
Staphylococcus aureus and
Mycotoxins have in common?
they cause food intoxications
what is a food infection?
when an organism is consumed and it colonizes the GI tract causing illness
how many cells of Salmonella enterica are needed to cause a food infection?
10
how many cells of enterohemorrhagic E. coli are needed to cause a food infection?
10
how many cells of Salmonella enterica are needed to cause a food infection?
10
how many cells of enteropathogenic E. coli are needed to cause a food infection?
> 1 mil
what does blowing of vaccuum packaged beef indicate?
high volumes of gas production from organisms in food
is it common for organisms to release gas?
yes, in small quantities
-uncommon for them to produce sufficient enough quantities to blow package up
what organism is commonly the cause of blown meat vaccuum packages?
Clostridia
what macro in meat do organisms metabolize in order to produce gas an odour?
protein
-some carbohydrates
-fat doesn’t usually produce gas when metabolized
what is a strict anaerobe?
organism that will grow in the abscence of oxygen
what affect does heat shrinking have on clostridia when packaging meat?
heat shrinking meat packages can cause spores to acitivate
what T microorganism is most likely to cause blow packages?
Psychrophophiles
how does blown packaging occur if antimicrobial treatments are used in meat processing?
clostridia survives antimicrobial treatments
what types of meat are at risk for blown packages?
Beef, lamb, pork, venison
How does MAP blown pack soilage differ from meat blown packages? what type of macros would you expect to see in the package?
The lack of off odours and no change in eating quality
-the lack of protein source in the food will decrease the risk of off odours
- carb as main macro
For MAP blown packages:
what environment would you expect for the food? What kind of bacteria would you expect to grow as a result of this?
1) low O2
2) High CO2
Lactic acid bacteria will grow and some may produce CO2 from carbs
what kind of microorgansims can you expect to be present in MAP blown packages with low protein availibility? why?
Enterobacteriaceae and Lactic acid bacteria
- these organisms can survive with lack of oxygen and they prefer carbohydrate-rich environments
what differneces might you see in the packages of products that have been blown by Enterobacteriaceae / LAB and Clostridia?
Packages blown by Clostridia are likely to produce more gas and cause more blowing than packages affected by LAB and enterobacteriaceae
what is the cause of blown packages by LAB and enterobacteriaceae?
-Temperature abuse (over 4 degrees C)
-Moderate distention
-more common towards the end of storage
what is a common natural habitat for clostridia?
soil
how can corrective action be taken to prevent blown packages?
1) determine the source of organism
- animal itself
- processing facility
- sampling
2) determine control measures
- LAB is highly resistant to sanitizers used in processing facility
- monthly control strategies
when is clostridia most likely to be transferred? why is this?
during slaughter
- this is because clostridia commonly comes from the animal itself
what are solutions to bread spoilage?
1) buy fresh bread when it’s needed
2) Bake dense/acidic bread such as sourdough (LAB inhibits growth of other microbes)
3) Add lots of sugar and fat
4) Add propionate and emulsifiers
what type of organism is resposible for the blue mold found on bread?
pennicilin
why is wrapped bread more succeptible to spoiling?
the wrapping causes condensation which can result in mold formation
what are control measures for bread storage?
1) preservatives
2) packaging bread under clean air
3) post-packaging heat treatment
what does a direct microscopic count measure? what is the limitation of this?
the number of cells
- we can’t tell if cells are dead or living
what is quantification of bacterial DNA? what is the limitation of this?
PCR amplification of 16S rRNA genes (measuring the amount of bacterial DNA present in a sample)
-everything, including mitochondria and chloroplasts are visible
- it is for the genus level only
- dead and living cells are counted
what is a standard plate count measure? what is the assumption with this?
measure of the # of live cells
-1 cell = one colony
what type of organisms spoil bread?
airborne conidiospores
what type of food do you expect to have a high cell count vs low cell count?
fermented or fresh plant food- high cell count
canned, cooked or pasterurized - low cell count
pasteurized / heated and stored in the fridge - starts low then goes high
how can cell count be used to determine spoilage?
It can’t, it is no indication of spoilage
what type of bacteria will grow on:
standard agar plate?
most bacteria
what type of bacteria will grow on:
Violet red bile agar?
mesophilic aerobic Gram-negatives
what type of bacteria will grow on:
APT?
meat spoilage organisms
what type of bacteria will grow on:
sourdough broth
sourdough lactobacilli
more acidic
what type of bacteria will grow on:
Baird-parker agar?
staphylococcous spp.
what type of bacteria will grow on:
Cetrimide agar?
Pseudomonas spp.
what is the difference between rich and defined laboratory media?
rich - nutrient dense
defined- has antibiotics to exclude some organsism and select for others
How many dilutions are needed for:
Fermented food or produce? why is this the case?
log 8-9 CFU/g
- more dilutions are needed due to their higher cell count
How many dilutions are needed for:
Heated food? why is this the case?
log 2 or lower
- fewer dilutions are needed do to lower cell count
How many dilutions are needed for:
MAP food (long-storage life food)
log 8-9 CFU/g
Explain how to inculate a food plate using the pour method?
1) add 1mL of sample to a steril petri dish
2) add sterile agar solution and mix well
- agar must be cooled to not kill the bacteria
Explain how to inculate a food plate using the pour method?
1) Spread plate- add 0.1 mL of the sample to a plate
2) spread sample evenly over the surface with a sterile glass spreader
Explain how to dilute a solid sample for a food plate
https://youtu.be/yYWFX4IXc5Y?si=yrMUGw1DeISSSSVT
1) blend solid with diluent (10mL per g)
2) make dilutions
3) pour onto plate
What is enrichment and why is it needed?
In regards to food plating
Enrichment is the use of a selective broth in order to select for growth of a specific type of organism
- this is needed because food pathogens typically have a low abundance so enrichment is needed
- helps to detect pathogens
