Lect 2 : Microorganisms in foods pt 2

Question 1

What are the 3 broad categories of factors affecting microbial growth in foods?

Answer 1

1. Intrinsic factors
2. Extrinsic factors
3. Implicit factors

Question 2

What is the definition of intrinsic factors? List 5 intrinsic factors.

Answer 2

Intrinsic factors are factors inherent to the food
1. nutrient availability
2. pH
3. Water activity
4. Oxidation reduction potential
5. Antimicrobial constituents

Question 3

What is the definition of extrinsic factors? List 3 extrinsic factors.

Answer 3

Extrinsic factors are properties related to the storage / processing environment

1. Temperature
2. Relative humidity
3. Presence of gasses

Question 4

What is the definition of implicit factors?

Answer 4

Implicit factors are the interactions between microorganisms.

Question 5

What nutrients do MOs require to grow, and what are the functions of these nutrients ?

Answer 5

1. Source of energy : Carbs, proteins, lipids, vitamins, minerals
2. source of nitrogen for growth : Amino acids and nucleotides
3. For growth : Vitamins, minerals and growth factors

Question 6

Which nutrient does MO mainly rely on as a source of energy?

Answer 6

Glucose (carbs)

If insufficient, proceeds with amino acids (protein) –> then lipids

Question 7

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

Answer 7

1. Gram negative bacteria (Aw : 0.95)
2. Gram positive bacteria (0.91)
3. Yeast (0.88)
4. Mould (0.80)

Question 8

Why do gram negative bacteria need a higher water activity to survive than gram positive bacteria? List 2 reasons.

Answer 8

1. 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
2. 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

Question 9

What is the pH most MOs grow at?

Answer 9

Neutral pH (6.6-7.5)

Question 10

Rank the MOs, from being able to survive in lowest pH to highest pH (ascending order)

Answer 10

1. Moulds – pH 1.5-9.0 (most pH resistant)
2. Yeasts – pH 2.0 - 8.5

Gram positive – pH 4.0 - 8.5

Gram negative – pH 4.5 - 9.0

Question 11

What is meant by Oxidation-Reduction potential?

Answer 11

Ability of substrate to gain or lose electrons

Question 12

What are differentnames for sub categories of bacteria (require free oxygen to carry out respiration vs dont require free oxygen)?

Answer 12

1. Aerobic bacteria (obligate aerobes)

Anaerobic bacteria
2. Obligate anaerobes
3. Facultative anaerobes
- can livee in presence / absence of O2

Question 13

What are antimicrobial constituents? Give some examples of foods with natural antimicrobial constituents

Answer 13

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)

Question 14

At what temperatures must food be at to inhibit / inactivate MOs in food?

Answer 14

below 4℃ or above 60℃

more detailed
- Refrigerated foods : <= 4℃
- Frozen foods : <= -18℃

• Hot foods : above 60℃
• Cooked foods : 72-82℃

Question 15

What is the range of possible growth temperatures of MOs?

Answer 15

-18℃ to 70℃

Question 16

What are the optimal temperature growth of psychropiles. mesophiles, thermophiles?

Answer 16

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℃

Question 17

Psychrotrophs are able to grow at < __℃, but have optimum T℃ similar to mesophiles, which is __ - __ ℃

Answer 17

7
30
37

Question 18

PSYCHOTROPHS
1. What are the names of the 2 most common psychrotophic bacteria?

2. What are names of the 2 pathogens that are psychrotrophic?

Answer 18

1. Pseudomonas
   Enterococcus
   (PE)
2. Listeria monocytogenes
   Clostridium Botulinium (types B,E,F)

Question 19

Mesophiles generally dont grow well. True or False?

Answer 19

True. They are often out-competed by psychrotrophs

Question 20

Name 2 species of thermophiles

Answer 20

CB
Clostridium
Bacillus

Question 21

Under what conditions do mesophiles grow more rapidly?

Answer 21

When temperature is abused

Question 22

___ (of bacteria/mould) can survive heat processing conditions well

Answer 22

Spores

Question 23

Give 2 examples where implicit factors can affect microbial growthj

Answer 23

1. 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)

2. Antagonistic effect – hostility that results in active resistance, opposition or contentiousness

Question 24

What are the 2 kinds of antagonsim and how are they carried out?

Answer 24

1. 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
2. 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)

Question 25

What are microbiological indicators?

Answer 25

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

Question 26

Why do we use microbiological indicators instead of detecting the specific MO of interest? [4]

Answer 26

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

Question 27

What are the 4 different spoilage (quality) indicators of food products ?

Answer 27

1. Total aerobic / anaerobic colony count 2. Aerobic or anaerobic SPORE FORMING BACTERIA and/or spores 3. Lactic acid bacteria 4. Yeasts and moulds

Question 28

When incubating a sample in an agar medium, the colony count that is determined is biased by ? [3]

Answer 28

1. Time 2. Temperature 3. The agar medium used (different agar mediums have different types of nutrients, may support growth of some MO better)

Question 29

For aerobic count, what kinds of conditions is the sample incubated in? What kinds of bacteria will grow?

Answer 29

- incubated in air - Strict aerobic and facultative anaerobic

Question 30

For anaerobic count, what kinds of conditions is the sample incubated in? What kinds of bacteria will grow?

Answer 30

- 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

Question 31

What are the 3 kinds of dedicated medium and what are they used for?

Answer 31

1. Plate Count Agar (PCA) : most commonly used agar 2. Reinforced CLOSTRIDIAL agar (RCA) : anaerobic / facultative anaerobes 3. Marine Agar (MA) : samples from marine origin [PCM]

Question 32

What are the steps involved in determining spore count in a food sample?

Answer 32

- 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)

Question 33

Lactic Acid Bacteria (LAB) are a large group of gram ____ bacteria and occur as ____ and ____ (shape)

Answer 33

positive ;; rod ;; cocci

Question 34

LAB is a very competitive bacteria. Under what conditions are they very competitive?

Answer 34

- When sugars are present - Low pH (minimum of pH 3.3 for some types of LAB)

Question 35

LAB is a strictly aerobic bacteria. True or False?

Answer 35

False, they are facultative anaerobes, and are not sensitive for CO2

Question 36

** LAB are able to grow at a _____ range of temperatures, and at ____ temperatures

Answer 36

wide ;; low

Question 37

What is the name of the specific agar medium for LAB?

Answer 37

De Man, Rogosa Sharpe (MRS) - However, do note that not all bacteria will grow

Question 38

At what kinds of conidtions do yeast and moulds dominate?

Answer 38

Low pH and low water activity [conditions that are less favourable for bacteria growth]

Question 39

What kind of MO is mould? 1. Strict anaerobe 2. Strict aerobe 3. Facultative anaerobe

Answer 39

2. Strict aerobe

Question 40

Why are yeast less taken into account than mould?

Answer 40

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)

Question 41

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.

Answer 41

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)

Question 42

Name 2 groups of MO that are used as hygiene indicators

Answer 42

1. coli-aerogenes group (indicator in ice cream & pasteurised milk) 2. Coliform

Question 43

What kind of information do safety indicators provide us with?

Answer 43

The likelihood for product contamination by pathogens originating from the PROCESSING ENVT

Question 44

A safety indicator was used to test food but the result came back negative. Does this mean that pathogens are absent?

Answer 44

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

Question 45

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)

Answer 45

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

Question 46

At which stage of the food supply chain is Enterobacteriaceae used as a hygiene indicator?

Answer 46

- 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

Question 47

Embacteriaceae family is a group of effective indicators for hygiene. What does it mean by they are sensitive to control measures?

Answer 47

It means that the indicator is responsive to the effectiveness of the hygiene and sanitation measures implemented in a food processing or handling environment.

Question 48

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?

Answer 48

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

Question 49

What is the definition of coliform, according to laboratory methods used to detect them??

Answer 49

[Gram-negative non-spore forming Bacill] Rapidly ferment lactose within 24-48h (at 37℃), producing acid and gas

Question 50

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?

Answer 50

E.Coli is a marker organism for... 1. Salmonella, on beef and pork meat 2. 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.

Question 51

What are the 3 features of an ideal indicator ?

Answer 51

1. Non-pathogenic 2. Rapid, simple, inexpensive detection of target MO. MO should have readily identifiable characteristics 3. 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

Question 52

Why should the indicator microorganism be more persistent ad be in higher numbers than target pathogen?

Answer 52

- 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