4 - investigating foodborne diseases and controlling food spoilage

List 3 levels that microbes can be observed as

• single cells

• colonies

• populations

Define selective culture media

media that selects for a particular group of bacteria, only allowing them to grow (gram positive or negative)

Give an example of selective culture media

MacConkey agar selects for gram negative bacteria

Define differential culture media

media that differentiates between bacteria based on their ability to perform certain biochemical functions. Indicated by changes in media colour

Give an example of differential culture media

mannitol salt agar

How do microbial populations grow?

exponentially

How is bacterial growth calculated?

2^n where n = number of generations

Define generation doubling time

the time taken for the population to double

List 3 methods of detecting foodborne pathogens

• direct counts using chambers

• plate count

• selective media

Give one advantage of direct cell counts

quick, cheap and easy

Give 2 disadvantages of direct cell counts

• its sometimes hard to distinguish between alive and dead cultures

• used to be thought that cells were alive if culturable

How do you perform a plate count?

perform several dilutions of innoculum until 30-300 discrete colonies are countable

Give two methods of testing water sampling

• total plate counting

• using indicators

Discuss total plate counting with water samples

estimate the total number of colony forming units per weight of solid or volume of liquid

Define microbial indicators

microorganisms that indicate the potential presence of pathogenic bacteria

Microbial indicator criteria: sources

should be suitable for analysis of many water sources

Microbial indicator criteria: presence

should be present wherever enteric pathogens are present

Microbial indicator criteria: survival

should survive longer than the hardest enteric pathogen

Microbial indicator criteria: reproduction

shouldn’t reproduce in contaminated water

Microbial indicator criteria: harm

should be harmless to humans

Microbial indicator criteria: relationship

its level in the contaminated water should have a direct relationship to the degree of faecal pollution

Microbial indicator criteria: assay

the assay for identifying indicator should be specific

Microbial indicator criteria: testing method

the testing method should be easy to perform

What’s the most probable number method (MPN)?

a statistical technique used to estimate the concentration of microorganisms in a sample when direct counting is not feasible

What are coliforms?

bacteria that possess β-galactosidase to produce acids and gases

What are the 3 steps of the MPN test

1. presumptive test

2. confirmatory test

3. completed test

What does the presumptive test do?

samples water for the presence of coliform organisms

List 3 mediums that can be used for presumptive testing

• lactose broth

• MacConkey broth

• Lauryl tryptose broth

What is the main principle of presumtive testing?

water sample is added to a lactose broth

if coliforms are present they will metabolise the lactose present in the broth with β-galactosidase and gas bubbles will appear

Presumptive test on untreated water: 3 steps

• take 5 tubes of double strength and 10 tubes of single strength medium

  • add 10ml of water to 5 tubes

  • add 1ml to 5 tubes

  • add 0.1ml to 5 tubes

• incubate at 37°C for 24 hours

• compare the number of tubes giving  a positive reaction to a standard chart and record the MPN of bacteria present

Presumptive test on treated water: 3 steps

• take 5 tubes of double strength (50ml) and 1 tube of single strength medium

  • add 50ml of water to 1 tube

  • add 10ml to 5 tubes

• incubate at 37°C for 24 hours

• compare the number of tubes giving  a positive reaction to a standard chart and record the MPN of bacteria present

What happens if the presumptive test is negative?

no further testing is performed and the water source is considered microbiologically safe

What happens if the presumptive test is posititive?

a confirmatory test is performed

Give 3 advantages of MPN

• ease of interpretation

• sample toxins are diluted

• effective method of analysing highly turbid samples like mud that can’t easily done by membrane filtration

Give 3 disadvantages of MPN

• takes a long time to get results

• requires more equipment

• greater probability of false positives

How is membrane filtration performed? (3 steps)

• water sample is filtered through a membrane filter (0.45μm)

• membrane filter is removed and placed in a plate containing appropriate medium

• plate is incubated for 24 hours

Give 3 advantages of membrane filter technique

• lower total cost than MPN

• easily reproducible

• single step results are often possible

Give 2 disadvantages of membrane filter technique

• high populations of background bacteria cause overgrowth

• metals and phenols can adhere to filter

Louis Pasteur (2 points)

• established modern era of food microbiology in 1857

• his work in the 1860s proved that heat could be used to control spoilage organisms in wines and beers

List 3 food prevention methods

• low/high temp

• radiation

• chemical based preservation

Climate change link to water borne diseases: extreme weather events

increased rainfall, floods, and storms can overwhelm treatment systems leading to contamination

Climate change link to water borne diseases: temperature rise

warmer temperatures accelerate the replication of water borne pathogens

Climate change link to water borne diseases: flooding impact

heavy rain can flush pathogens from land into water sources, leading to outbreaks (leptospirosis)

Give an example of water borne diseases being affected by climate change

vibrio infections in the Baltic region are linked to rising sea surface temperatures

Climate change link to food borne diseases: heat

higher ambient temperatures increase the survival and replication rates of pathogens like Shigella and Campylobacter

Climate change link to food borne diseases: contamination pathways

extreme weather and warmer temperatures can disrupt food storage, handling, and distribution, increasing risk of contamination

Climate change link to food borne diseases: seasonal variations

pathogens like Campylobacter show increased activity during warmer months, linked to behavioral changes in food consumption and preparation

List 4 adaptation strategies to climate sensitive diseases

• better surveillance

• improved food safety regulations

• waste management

• community and education