Food Microbiology &
Helen Billman-Jacobe
Microbial responses to stress
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RayandBhunia,5th edCh10
Intended learning outcomes
Describe how microorganisms adapt to stress in the food environment
Explain how sublethal stress and injury affects detection of microorganisms in food samples
Microbial stress response in the food environment
Food borne microorganisms become stressed when they are exposed to some of the physical and chemical environments during
• Production
• Processing
• Preservation
• Transportation
• Consumption of food
Some cells develop resistance to the stress, or suffer reversible injury or lose the ability to be cultured or to multiply.
Responses to stress
Microorganisms have specific conditions where the growth is optimal however they can multiply slowly under suboptimal conditions
Optimal + sub optimal = growth range
Cells suffer stress when they are outside their optimal conditions In the sublethal ranges they will be damaged
In the lethal ranges they will be killed
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Stress adaptation
Exposure to sub optimal physical or chemical environments triggers stress adaptation or stress response in bacterial cells
Cells that have a brief exposure to suboptimal conditions and adapt are able to withstand harsh conditions Examples of stress
• Temperature (cold or warm)
• Low hydrostatic pressure
• UV light
• High salt concentrations
• Antibacterial chemicals (preservatives, disinfectants, antibiotics)
Stress adaptation is not “resistance”
Stress adaptation is a temporary change in the microorganisms in response to brief exposure to suboptimal environments
When cells are returned to optimum conditions and are allowed to multiply then they revert to their original state
This is different than when genetically different variants are selected out of a population
For example, antibiotic resistant organisms may have acquired new genes that make them permanently resistant
Mechanisms for stress adaptation
Adaptation to stress involves a change in the expression of specific genes which encode heat shock proteins or stress proteins
When cells detect the stress they synthesise a special component of RNA polymerase
The RNA polymerase then transcribes the genes which encode stress proteins
The stress proteins protect cells from the stress
When the stimulus (stress) goes away then the gene expression ceases
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Low pH foods
Many foodborne pathogens and spoilage bacteria are sensitive to low pH and die in high acid foods (pH≤4.5)
Cells most enteric pathogens die in the low pH of the stomach (pH2)
A few cells survive the stomach, into the GI tract and establish infections
If a pathogen in a food is stress adapted then it is better able to survive in the stomach and cause infection
Food safety = eliminate or reduce the presence of stress adapted pathogens in ready to eat food
Three populations of stressed cells
1. Uninjured normal cells
2. Reversibly injured cells (injured)
3. Irreversibly injured cells (dead)
The relative percentages depend on the
implicit nature of the organisms
the nature and duration of the stress and the methods of detection
Injured Dead
Injured cells differ from normal cells
Increased sensitivity to many compounds
– Surface active compounds (bile salts, deoxy cholate or SDS)
– Some chemicals
– Enzymes (lysozyme or RNase)
– Antibiotics
– Dyes (crystal violet or brilliant green)
– undissociated acids
Brilliant green agar MacConkey agar
Injured cells do not multiply unless the injury has been repaired
Nutrient agar (non- selective)
MacConkey agar (selective )
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Repair of reversible injury
Injured cells are able to repair the injury and become similar to normal cells if they are given the right conditions
Suspending a subject easily stressed population repair medium and giving them time to recover will increase the number of colony-forming units
Injured survivors a be able to form colonies on nonselective media but are unable to tolerate selective agents
If they repair then they regain resistance and conform colonies on selective media
Recovery from stress differs according to the level of stress
Ray and Bhunina
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