Food Microbiology &
Helen Billman-Jacobe
Microbial responses to stress_Sporulation
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Ray and Bhunina Ch 8
Intended learning outcomes
Distinguish between spores of molds, yeasts and bacteria
Describe the process of bacteria sporulation, dormancy, activation and germination Discuss the importance of spores in food
For bacteria, sporulation is a response to an unfavourable environment
For moulds and yeast, sporulation can occur through sexual and asexual reproduction
Spores are more resistant to environmental conditions than vegetative cells Bacterial spores are much more resistant than mould or yeast spores
For convenience, we refer to bacterial spores as endospores. Refer back to earlier lectures on different sorts of mould spores; sporangiospores (with sac) and conidiospores (without sac)
Mould spores
Mould form large numbers of spores • Condiospores (condia)
– Aspergillus and Penicillium species form conidia
Mould spores
• Sporangiospores
– Mucor and Rhizopus form sporangiospores
Mould spores
• Arthrospores
– Geotrichium and Pestalotiopsis for arthrospores
– Arthrospores are a primitive version of spores formed by breaking up of fungal mycelium
Geotrichium
Mixed culture. Rhodotorula, pink
Yeast spores
Yeasts that are important in food can be divided into two groups
Those that do not form spores (false yeasts) Examples: Candida, Rhodotorula
Yeasts that produce sexual ascospores (true yeasts) Examples: Saccharomyces, Pichia, Hansenula
Sexual reproduction is rarely observed in yeast which are important in food
S. cerevisiae on SAB agar
Bacterial spores
• The ability to form spores is confined to few gram-positive bacterial genera including Bacillus and Clostridium and a few gram-negative species
• Bacteria form endospores ( inside cell, one spore per cell)
Activation and germination
Bacterial spores remain dominant when environmental conditions are unfavourable for growth
Bacterial spores are very resistant to extreme environmental conditions and they are essentially metabolically inactive (dormant)
Bacterial spores can be activated by – Sublethal heat treatment
– Radiation
– Exposure to extreme pH
– Treatment with high-pressure – Sonication (soundwaves)
Bacterial spores which are activated undergo structural changes as they emerge from the dormant state and entered germination. This process is irreversible once it has started
Important of spores in food
Spores are able to survive for a long time and I spread easily by dust and in air
This enables spores to contaminate food and if the food environment is suitable spores germinate, grow and produce undesirable effects such as spoilage
Moulds and yeasts
• spores can be readily destroyed by heat treatments.
• Germination of spores can only happen in air. The growth can be prevented by storing foods in the
absence of air
• Species of Bacillus and Clostridium are associated with food spoilage and foodborne disease
• Bacterial spores have high heat resistance
• Special effort is made to prevent germination and outgrowth
Prevention of spore germination
Spores are a major concern in the canning of low acid foods
Very high heat treatment is employed to achieve commercial sterility that kills spores of pathogenic bacteria
Spore germination can be inhibited with
• Nitrite (processed meats such as salami)
• Low pH ( acid products)
• Low-water activity
• High salt
If a combination of different measures is used to prevent a food safety problem we called this Hurdle technology.
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