Microbiology

Principal spoilage micro-organisms

• acintobacter species
• aeromonas hydrophila
• alcaligenes species
• alteromonas species
• bacillus species
• brochothrix species
• camplylobacter species
• clostridium perfringens
• coliforms
• escherichia coli
• faecal coliforms
• flavobacterium species
• micrococcus species
• moraxella species
• pseudomonas species
• salmonella species
• shewanella species
• shigella species
• staphylococcus aureus
• sulphite reducing clostridia
• vibrio species
• yeasts and moulds
• yersinia enterocolitica and related species

Acintobacter/Moraxella species

Acintobacter/Moraxella species are spoilage bacteria associated with meats, offal, poultry, gamebirds, fish, seafood, ready meals, cook-chill products, dairy products, combination products and certain beverages. The sources of Acintobacter/Moraxella species are the mucous membranes of mammals, fruits, flowers, soil and canal water. Their optimal growth temperature is 25-35°C and optimum pH for growth is 5.4-6.3. They are capable of oxidising ethanol to acetic acid and producing trimethylamine, ammonia and hydrogen sulphide from metabolising protein containing food products.

Aeromonas hydrophila

The role of Aeromonas in causing food poisoning has not been clearly established, but some strains of certain species are thought to be capable of causing diarrhoea. They are aquatic organisms but have been isolated from many foods. Aeromonas spp. grow at refrigeration temperatures.

Alcaligenes species

Alcaligenes species are spoilage bacteria which can be pathogenic under certain circumstances. The sources of Alcaligenes species are the intestinal tract of vertebrates, blood, urine, faeces, ear discharges, spinal fluids, wounds, soil and water.

Alteromonas species

Alteromonas species are aerobic spoilage bacteria which are present in coastal water and oceans and hence primarily associated with spoilage of fish and seafood products. They require a minimum concentration of salt in order to grow and their optimal growth temperature is 20°C.

Bacillus cereus and other Bacillus species

The members of the Bacillaceae produce spores as well as vegetative cells. The spores can survive adverse conditions such as drying and pasteurisation. These organisms are common in the environment and in many foods. Some strains produce enterotoxins if allowed to grow, for example Bacillus cereus in rice dishes and high moisture flour products and members of the B. subtilis - licheniformis group in meat and pastry products and meat or seafood rice dishes. The toxin of B. cereus is particularly heat resistant (126°C for 90 minutes) whereas the diarrhoeagenic toxin is inactivated by exposure to 56°C for 30 minutes. High levels (>105 per gram) are necessary to produce enough toxin to cause illness. Bacillus species can also cause spoilage.

Brochothrix species

Brochothrix species are common spoilage bacteria commonly associated with cooked, cured and processed meat, poultry and gamebird products and combination products containing these food items. Brochothrix species are capable of causing spoilage in fairly low numbers and can be the dominant microflora in MA packed and vacuum packed cooked, cured and processed meat-based products.

Camplylobacter species

Clostridium perfringens and sulphite reducing clostridia

Clostridium perfringens is a spore-forming anaerobe commonly found in mammalian faeces and soil. The spores persist in the environment and often contaminate raw food materials. Spores may survive cooking and rapid growth may occur if the food is not chilled promptly. If large numbers of the organism are eaten in a food, they can sporulate in the gut and produce an enterotoxin which causes diarrhoea.

The sulphite reducing clostridia (SRCs) include the pathogens C. perfringens and C. botulinum (the causative organism of botulism). The SRCs may be used as an indicator of plant hygiene and for the presence of other clostridia that may cause food poisoning or food spoilage.

Coloforms, faecal coliforms and Escherichia coli

Coliforms, faecal coliforms and Escherichia coli Coliforms were one of the first groups of bacterial indicators used in the water and dairy industries. They are those members of the Enterobacteriaceae that ferment lactose, and include Enterobacter, most Eshcherichia, Klebsiella and Citrobacter species. These organisms are not exclusively of faecal origin. Thermotolerant coliforms are those coliforms that can multiply at 44°C. This group includes E. coli types I and II and occasional strains of Klebsiella and Enterobacter. Most faecal coliforms are able to multiply at 44°C.

Whilst the presence of E. coli in food is generally undesirable because it indicates poor hygienic conditions, certain serotypes are pathogenic and may cause gastroenteritis.

Verocytotoxin-producing strains of E.coli (VTEC) e.g. E.coli 0157 may be particularly virulent and can cause symptoms ranging from mild diarrhoea to severe bloody diarrhoea (haemorrhagic colitis), sometimes progressing to haemolytic uraemic syndrome (HUS) and kidney failure.

Enterobacteriacea

The family Enterobacteriacea includes bacteria that naturally inhabit the mammalian gut but can also occur and multiply in other environments e.g. species of Escherichia, Citrobacter, Enterobacter, Proteus , and also some of the most important enteric pathogens such as Salmonellaspp., Shigellaspp., Yersinia enterocolitica, and pathogenic E. coli. Enterobacteriacea are useful indicators of hygiene and of post-processing contamination of heat process foods.

Enterococcus species/faecal streptococcus species

The enterococci, which include faecal streptococci, survive and grow well in factory environments. Because they are more resistant to heat and adverse conditions than the Enterobacteriacea, they are used as hygiene indicators, particularly in dried products and frozen foods.

Flavobacterium species

Flavobacterium species are aerobic spoilage bacteria which are found in soil, water, meats, poultry, fish, seafood and other foods. They have also been isolated from human clinical material and hospital environments. They have a broad optimal growth temperature range of 5°C-30°C.

Lactobacillus species and other lactic acid bacteria

This group includes bacteria capable of producing lactic acid from fermentable substrates. Some strains grown poorly in aerobic conditions. Genera commonly found in/on food include Lactobacillus, Periococcus, Leuconostoc, Lactococcus and Streptococcus. They are ubiquitous in the environment and are notable for their ability to grow at low pH levels. Starter cultures for fermented foods such as cheese and salami commonly include this group of organisms, and so their presence in high numbers should be expected. However, in some foods they may cause spoilage.

Listeria monocytogenes

Micrococcus species

Micrococci are found in water, soil and normally found on mammalian skin. They are spoilage bacteria that are not thought to be pathogenic. They can grow in the presence of salt and are capable of spoiling foods, particularly cured meat, fish and poultry products. They are aerobic bacteria but can function as facultative anaerobes.

Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen causing the disease listeriosis. The organism is ubiquitous in the environment, and scrupulous hygiene is required to minimise its presence in food production premises. It is not yet clear how many organisms are required to cause illness, but as the organism is able to grow at refrigeration temperatures, its absence or very low levels in ready to eat foods is desirable. Although the other members of the Listeria group rarely cause illness, they are often used as an indicator for the presence of Listeria monocytogenes.

Pseudomonas species

Pseudomonas species are ubiquitous in the environment and water, and some species are psychrotrophic. Pseudomonas aeruginosa is of clinical significance as an opportunistic pathogen but has rarely been implicated in gastroenteric infection. Other pseudomonads are significant in food spoilage, particularly in chilled food. Levels higher than 107 cfu/g or ml of food may result in off flavours, off odours and visual defects.

Salmonella species

Salmonella spp. are organisms of faecal origin, and the majority of species are regarded as potentially pathogenic to humans. These organisms survive and may grow well in factory environments. Salmonella spp. cause significant numbers of food poisoning cases each year.

Shewanella species

Shewanella species have been reported to be present in a wide range of different environments including soil, water, oil and spoiled food products such as fish, poultry and dairy products.

Shewanella putrefaciens from fish and seafoods can be pathogenic but is very rarely found.

Shigella species

Shigella species are members of the Enterobacteriacea. They are highly infectious enteric pathogens, easily spread by the faecal-oral route. The infective dose is known to be very low. Food may be contaminated by food-handling personnel or by contaminated irrigation water. Foodborne illness due to Shigella is relatively uncommon in the UK.

Staphylococcus aureus

Staphylococcus aureus (S. aureus) may be associated with the skin, nose and throat of healthy individuals as well as being the cause of boils and many skin and wound infections. Given suitable conditions it is capable of growth in foods, and some strains may produce a heat-stable enterotoxin. It is used as an indicator of general hygiene and adherence to good food handling procedures. Its presence, particularly in high numbers (>104 per gram), indicates the potential presence of enterotoxin and a consequent risk of food poisoning. If poor temperature control allowing growth has occurred prior to heat treatment, then the enterotoxin may remain despite the absence of viable S. aureus cells.

Vibrio parahaemolyticus and other Vibrio species

Vibrio parahaemolyticus is a marine organism capable of causing profuse diarrhoea and is most often associated with seafood and shellfish from warm waters. The infective dose is not known; high numbers e.g. >102 per gram are regarded as unacceptable. V. cholerae is the causative organism of cholera and is usually transmitted via contaminated water and the faecal-oral route. A number of other species of Vibrio are of pathogenic significance e.g. V. vulnificus, and also appear to be associated with consumption or handling of seafood. The presence of vibrios in cooked seafood indicates either inadequate heat treatment or post-processing contamination.

Yeasts and moulds

The yeasts are a common cause of food spoilage, particularly of acid foods such as fruit and fruit juices, and foods of reduced water activity (aw) such as confectionery. They have not been implicated in food poisoning. Moulds are also a significant cause of food spoilage, and some strains are able to grow at very low aw values e.g. 0.6-0.7, causing spoilage of otherwise microbiologically stable commodities such as bakery goods. Although moulds do not cause food poisoning, some strains are able to produce mycotoxins which can cause serious chronic illness if consumed.

Yersinia enterocolitica and related species

Yersinia species are also members of the Enterobacteriacea, and are capable of psychrotrophic growth. Yersinia enterocolitica and related organisms are widely distributed in animals of all types and are found in many environments. The recognised pathogenic serotypes belong to Y. enterocolitica and are mainly associated with pigs. These strains can cause gastroenteritis and many other symptoms. The infectious dose is not known.