Lect 2 : Microorganisms in foods pt 2 Flashcards
What are the 3 broad categories of factors affecting microbial growth in foods?
- Intrinsic factors
- Extrinsic factors
- Implicit factors
What is the definition of intrinsic factors? List 5 intrinsic factors.
Intrinsic factors are factors inherent to the food
1. nutrient availability
2. pH
3. Water activity
4. Oxidation reduction potential
5. Antimicrobial constituents
What is the definition of extrinsic factors? List 3 extrinsic factors.
Extrinsic factors are properties related to the storage / processing environment
- Temperature
- Relative humidity
- Presence of gasses
What is the definition of implicit factors?
Implicit factors are the interactions between microorganisms.
What nutrients do MOs require to grow, and what are the functions of these nutrients ?
- Source of energy : Carbs, proteins, lipids, vitamins, minerals
- source of nitrogen for growth : Amino acids and nucleotides
- For growth : Vitamins, minerals and growth factors
Which nutrient does MO mainly rely on as a source of energy?
Glucose (carbs)
If insufficient, proceeds with amino acids (protein) –> then lipids
Arrange these MOs in descending order in terms of the water activity they need to survive (highest to lowest water activity)
yeast, gram negative bacteria, mould, gram positive bacteria
- Gram negative bacteria (Aw : 0.95)
- Gram positive bacteria (0.91)
- Yeast (0.88)
- Mould (0.80)
Why do gram negative bacteria need a higher water activity to survive than gram positive bacteria? List 2 reasons.
- Gram negative bacteria have an outer membrane made of lipopolysaccharides, ( which acts as a barrier (lipids r hydrophobic) that restricts the passage of molecules and nutrients, including water, into the cell.
- Thus more water is needed to penetrate barrier - Gram-negative bacteria have a thinner peptidoglycan layer, making them more susceptible to osmotic stress –> cell collapse and die
- In low water activity, water travels from inside of bacteria cell (higher water activity) into the surrounding solution.
- When water leave cell, cell wall wont be as rigid cuz peptidoglycan layer thinner –> cell collape and die
What is the pH most MOs grow at?
Neutral pH (6.6-7.5)
Rank the MOs, from being able to survive in lowest pH to highest pH (ascending order)
- Moulds – pH 1.5-9.0 (most pH resistant)
- Yeasts – pH 2.0 - 8.5
Gram positive – pH 4.0 - 8.5
Gram negative – pH 4.5 - 9.0
What is meant by Oxidation-Reduction potential?
Ability of substrate to gain or lose electrons
What are differentnames for sub categories of bacteria (require free oxygen to carry out respiration vs dont require free oxygen)?
- Aerobic bacteria (obligate aerobes)
Anaerobic bacteria
2. Obligate anaerobes
3. Facultative anaerobes
- can livee in presence / absence of O2
What are antimicrobial constituents? Give some examples of foods with natural antimicrobial constituents
Naturally occurring substance found in food that kills or inhibits the growth of microorganisms.
Foods with natural antimicrobial constituents :
- Herbs and spices
- fruits
- egg white (contains lysozyme, a protein which is an antimicrobial constituent)
- milk (lactoferrin)
At what temperatures must food be at to inhibit / inactivate MOs in food?
below 4℃ or above 60℃
more detailed
- Refrigerated foods : <= 4℃
- Frozen foods : <= -18℃
- Hot foods : above 60℃
- Cooked foods : 72-82℃
What is the range of possible growth temperatures of MOs?
-18℃ to 70℃
What are the optimal temperature growth of psychropiles. mesophiles, thermophiles?
Psychrophiles (like cold) : 10 ℃, can grow at -18℃ - 20℃
Mesophiles (in btwn) : 30-37℃ , can grow at 5 to 50℃
Thermophiles : 55℃, can grow from 37-70℃
Psychrotrophs are able to grow at < __℃, but have optimum T℃ similar to mesophiles, which is __ - __ ℃
7
30
37
PSYCHOTROPHS
1. What are the names of the 2 most common psychrotophic bacteria?
- What are names of the 2 pathogens that are psychrotrophic?
- Pseudomonas
Enterococcus
(PE) - Listeria monocytogenes
Clostridium Botulinium (types B,E,F)
Mesophiles generally dont grow well. True or False?
True. They are often out-competed by psychrotrophs
Name 2 species of thermophiles
CB
Clostridium
Bacillus
Under what conditions do mesophiles grow more rapidly?
When temperature is abused
___ (of bacteria/mould) can survive heat processing conditions well
Spores
Give 2 examples where implicit factors can affect microbial growthj
- Symbiotic growth - one MO produce metabolic products that the other MO needs for proper growth, BUT cannot produce by itself
- e.g. SCOBY in kombucha (symbiotic culture of bacteria and yeast)
- Antagonistic effect – hostility that results in active resistance, opposition or contentiousness
What are the 2 kinds of antagonsim and how are they carried out?
- Contact-independent antagonsim
- MO depletes other MO of nutrients
- MO alter pH of envt, osmotic pressure and surface tension by producing metabolic debris
- MO produce toxic substances / antimicrobial products - Contact-dependent antagonsim
- One bacteria produces a structure called a needle, inserts it into another bacteria cell and transports effectors (proteins secreted by pathogenic bacteria into the cells of their host) through the needle
- See : type VI secretion system (T6SS)
What are microbiological indicators?
Microorganisms OR metabolic products, whose presence in a food or the environment at a given level is indicative of a potential quality, hygiene, and/or safety problem
Why do we use microbiological indicators instead of detecting the specific MO of interest? [4]
Detection of specific MO can be
1. Too expensive
2. Too long, unable to test in time considering that some products tested have a short shelf life
3. Technically complicated
4. Not widely available
What are the 4 different spoilage (quality) indicators of food products ?
- Total aerobic / anaerobic colony count
- Aerobic or anaerobic SPORE FORMING BACTERIA and/or spores
- Lactic acid bacteria
- Yeasts and moulds
When incubating a sample in an agar medium, the colony count that is determined is biased by ? [3]
- Time
- Temperature
- The agar medium used (different agar mediums have different types of nutrients, may support growth of some MO better)
For aerobic count, what kinds of conditions is the sample incubated in?
What kinds of bacteria will grow?
- incubated in air
- Strict aerobic and facultative anaerobic
For anaerobic count, what kinds of conditions is the sample incubated in?
What kinds of bacteria will grow?
- air-tight closed jar, O2 is removed or replaced by N2. Dedicated medium may contain redox potential reducing compounds (to reduce O2 by donating electrons)
- Strictly anaerobes and facultative anaerobes
What are the 3 kinds of dedicated medium and what are they used for?
- Plate Count Agar (PCA) : most commonly used agar
- Reinforced CLOSTRIDIAL agar (RCA) : anaerobic / facultative anaerobes
- Marine Agar (MA) : samples from marine origin
[PCM]
What are the steps involved in determining spore count in a food sample?
- Dilute sample once (if food is liquid, if not dont need to do dilution)
- Heat sample at 80℃ for 10 mins to ensure all vegetative bacteria / mould cells are killed
- Plating on PCA (aerobic) or RCA (anaerobic)
- Count the number of colonies (spores will germinate into colonies during incubation)
Lactic Acid Bacteria (LAB) are a large group of gram ____ bacteria and occur as ____ and ____ (shape)
positive ;; rod ;; cocci
LAB is a very competitive bacteria. Under what conditions are they very competitive?
- When sugars are present
- Low pH (minimum of pH 3.3 for some types of LAB)
LAB is a strictly aerobic bacteria. True or False?
False, they are facultative anaerobes, and are not sensitive for CO2
** LAB are able to grow at a _____ range of temperatures, and at ____ temperatures
wide ;; low
What is the name of the specific agar medium for LAB?
De Man, Rogosa Sharpe (MRS)
- However, do note that not all bacteria will grow
At what kinds of conidtions do yeast and moulds dominate?
Low pH and low water activity
[conditions that are less favourable for bacteria growth]
What kind of MO is mould?
1. Strict anaerobe
2. Strict aerobe
3. Facultative anaerobe
- Strict aerobe
Why are yeast less taken into account than mould?
As a spoilage organism, yeasts are less visual so people don’t really take yeast into account
(except on FRESH-CUT leafy greens, mixed salads, sprouted seeds)
I want to test a food sample for Salmonella. I used E.Coli as an indicator MO and incubated E.coli to test the food sample if it is positive / negative for the food sample.
State the idea behind using E.Coli as an indicator, instead of trying to isolate Salmonella.
E.Coli grows under similar conditions as Salmonella, so by incubating E. Coli, its presence can be asssociated with the presence of Salmonella.
However, I don’t want to isolate Salmonella since it is pathogenic. (tho salmonella is not commonly transmitted via air, briefly inhaling it may cause infections, though it is of a low risk if hygiene is maintained)
Name 2 groups of MO that are used as hygiene indicators
- coli-aerogenes group (indicator in ice cream & pasteurised milk)
- Coliform
What kind of information do safety indicators provide us with?
The likelihood for product contamination by pathogens originating from the PROCESSING ENVT
A safety indicator was used to test food but the result came back negative. Does this mean that pathogens are absent?
No, maybe the safety indicators are at a level too low (below detection limit) / pathogens can be unevenly distributed across samples –> the sample happened to have v little pathogens and came back w neg results etc
Name the family of bacteria that is most widely used as a hygiene indicator.
Name the characteristics of these bacteria (thermophilic/mesophilic/psychrophilic, gram___, shape)
Enterobacteriaceae
They are mesophilic, gram negative, rod-shaped bacteria
- mesophilic : rmb in expt, coliform incubated at 37℃
- gram negative : since indicators should be non-pathogenic, they are probably non-virulent –> thus gram negative
At which stage of the food supply chain is Enterobacteriaceae used as a hygiene indicator?
- Used as indicators at the end of manufacturing process, but NOT during their shelf life
- Cuz you want to check the microbial load of food after processing. If high load, means it wasnt hygienic since u tested it RIGHT aft processing
- If you test during shelf life it is useless. A food can have very low microbial load but during shelf life, bacteria can multiply due to intrinsic factors / envt in packaging. So you cant really determine if the microbial load is so high due to lack of hygiene during processing
Embacteriaceae family is a group of effective indicators for hygiene. What does it mean by they are sensitive to control measures?
It means that the indicator is responsive to the effectiveness of the hygiene and sanitation measures implemented in a food processing or handling environment.
Pyramaid for enterobacteriaceae family : Entereobacteriaceae > Coliforms > Fecal coliforms > E.coli.
Which 2 categories in the taxonomic pyramaid can be used as safety indicators, on top of hygiene indicator?
Fecal coliforms and E.coli
Do note : fecal coliforms and E. coli exist in intestines of warm-bloodd animals, are MOSTLY non-pathogenic.
But they could possibly indicate the presence of pathogenic organisms
What is the definition of coliform, according to laboratory methods used to detect them??
[Gram-negative non-spore forming Bacill]
Rapidly ferment lactose within 24-48h (at 37℃), producing acid and gas
E. Coli is used as both a hygiene and safety indicator. E. Coli is a marker organism for which 2 microbes, on what kinds of food products?
E.Coli is a marker organism for…
- Salmonella, on beef and pork meat
- Campylobacter, indicating contamination of broiler meat during processing steps up to the point of chilling
- broiler meat : chicken that is bred and raised specifically for meat production.
What are the 3 features of an ideal indicator ?
- Non-pathogenic
- Rapid, simple, inexpensive detection of target MO. MO should have readily identifiable characteristics
- Characteristics of indicator should correlate with characteristics of target organism.
- Correlation between level of target pathogen and level of indicator
- Correlaation between growth
conditions for indicator and target MO - Indicator organism persists slightly longer than target pathogen
Why should the indicator microorganism be more persistent ad be in higher numbers than target pathogen?
- Even if pathogen dies, but indicator still persists, it is better to have a false positive to play safe (tho food dh pathogen, test say have, so btr not to consume)
- However, if it is the opposite, where indicator is less persistent than pathogen, it can result in false negative
(i.e. food have pathogen but test say dont have)
And thus false negatives are dangerous
