This group of bacteria of is comprised of a large number of different serotypes of the species Salmonella enterica. However, in general, a few serotypes tend to dominate those identified as causing disease. These serotypes are normally denoted as a non-italicised specific name, e.g. Salmonella Enteritidis. Which drives microbial taxonomists nuts. You never capitalise the specific name, FFS.
So, there’s lots of Salmonella. These are bad enough, this one is really bad.
Serotypes are often associated with particular geographical areas.
Growth and Control
Temperature: Minimum 7°C, growth greatly reduced at <15°C. Maximum 49.5°C. Optimum 35-37°C. Some evidence for growth at 5.2°C exists, but this is serotype specific and the data are still not universally accepted.
Water Activity: Minimum 0.94, optimum 0.99, maximum >0.99.
pH: Minimum 3.8, optimum, 7-7.5, maximum 9.5. The minimum pH is influenced by other factors such as temperature, the acid present, and the presence of nitrite etc. For example at 10°C the minimum pH allowing growth was 4.4-4.8 (13 isolates tested), while at 30°C it was 3.8-4.0.
Atmosphere: Can grow in the presence or absence of air. Growth under nitrogen is only slightly less than that under air. Grows at 8-11°C in the presence of 20-50% CO2. Growth at low temperatures is retarded in the presence of 80% CO2 compared to air.
Survival: Salmonella is known to survive well in foods and on surfaces.
Temperature: Survival for >10 weeks in butter held at –23 and 25°C has been noted. Salmonellae can survive for 28 days on the surfaces of vegetables under refrigeration. In essence Salmonella can survive for long periods under refrigeration.
Water Activity: Survival in dry environments is a characteristic of these organisms. For example can survive in chocolate (aw 0.3-0.5) for months. Exposure to low aw environments can greatly increase the subsequent heat resistance of these organisms.
pH: Salmonella are less acid resistant at low pH than – for example – E. coli.
Inactivation (CCPs and Hurdles)
Death can occur during the freezing process, but those that survive remain viable during frozen storage. Freezing does not ensure the inactivation of salmonellae in foods.
60°C usually 2-6 min. 70°C usually 1 min or less.
N.B. Extremely high D times have been reported for experiments with milk chocolate. Values reported were up to 1,050 min at 70°C, 222 min at 80°C and 78 min at 90°C. This also applies to other low water content foods. Some rare serotypes (e.g. S. senftenberg) are significantly more heat resistant than the others which are not particularly resistant to heat.
pH: Inactivation at sub-optimal pH depends on many factors including the type of acid present and the temperature. For example inactivation is more rapid in commercial mayonnaise at 20°C than it is at 4°C.
Water Activity: Decline in numbers is reduced at lower aw values.
Preservatives: Growth was inhibited in the presence of 0.1% acetic acid (pH 5.1).
Sanitisers/Disinfectants: not resistant to disinfectants used in the food industry.
Radiation: D value around 0.5 kGy, up to 0.8. D times are higher in drier foods such as desiccated coconut.
Incubation: 6-48 hours (usually 12-36 hours).
Symptoms: Diarrhoea, abdominal pain, vomiting, nausea and fever lasting 1-7 days. Hospitalisation rate estimated at 22.1% cases fatality rate 0.8%.
Toxins: Toxins are not produced in foods.
At Risk Groups: The young, old, and immunocompromised are particularly at risk. In addition people of less privileged socioeconomic groups and those living in higher population densities are more at risk.
Long Term Effects: Septicaemia and subsequent non-intestinal infections can occur. Reactive arthritis may occur 3-4 weeks after gastrointestinal symptoms.
Dose: The dose required to cause disease varies with many factors. Low attack rates have been observed in one outbreak where 4-45 cells were consumed, and another where the dose was 6 cells. Different serotypes will have different dose responses, and generally recognised doses to cause disease at high attack rates are in the range of 105 to 106 cells. Foods with high fat content, like chocolate or peanut butter may protect cells from gastric juices so permitting a lower dose than usual to cause infection.
Treatment: The infection is usually self-limiting although fluid replacement may be required. Antibiotic treatment seems to be either ineffective or results in relapse or prolonged faecal shedding. Certain groups, e.g. neonates, may benefit from antibiotic treatment.
Reservoirs / Sources
Human: Some serotypes are confined to humans (e.g. S. typhi and S. paratyphi B). Faeces of infected people contain large numbers of the organism and shedding may continue for up to 3 months. The median period for shedding is 5 weeks, <1% become chronic carriers.
Animal: Some serotypes are confined to particular animal reservoirs, but many are capable of crossing between species to cause disease in man, often via food. Most Salmonella infections in animals are asymptomatic. Poultry and pigs are regarded as major reservoirs of the organism. Animal feeds made from animal products may be contaminated by Salmonella. Salmonella can also be found in fish, terrapins, frogs and birds.
Food: Meat or other products derived from infected animals are important vehicles of salmonellosis. Other animal products, e.g. re-contaminated pasteurised or raw (unpasteurised) milk can act as vehicles. Which isn’t surprising as raw milk is – microbiologically speaking – fucking sewage.
Environment: Salmonella shed in faeces can contaminate pasture, soil and water. It can survive for months in the soil. Contamination in the environment can serve to act as a source of infection of other animals.
Transmission Routes: May be transmitted to humans via contaminated food or water, animal contact, or from a contaminated environment.
Plague and Pestilence
Foods identified by epidemiological studies as acting as outbreak vehicles include poultry meat and eggs (in the USA and UK), mung bean sprouts, paprika flavoured potato chips and salami sticks.