This genus includes four species; S. dysenteriae, S. flexneri, S. boydii and S. sonnei, which are able to cause bacillary dysentery. They are very similar to Escherichia coli and are serologically cross reactive but have remained separate species for clinical reasons.
Growth and Control
Very little is known about the growth and survival of the organism in food. S. sonnei is more robust than S. flexneri (data for S. sonnei given). It has been shown to be able to grow on foods (e.g. parsley).
Temperature: Minimum 6-7°C, maximum 45-47°C.
Will grow on sliced fruits at room temperature.
Water Activity: Maximum 5.2% NaCl.
pH: Minimum 4.8-5.0 in 3.8-5.2% NaCl, 5.5 in the presence of 300-700 mg/litre NaNO2. Maximum 9.3 in the presence of 5.2% NaCl.
Atmosphere: Able to grow in the absence of oxygen.
Temperature: In general they survive best at low temperatures (subzero and refrigeration). Can survive storage in butter for more than 100 days at –20°C and 4°C. Can survive storage on soil, cheesevand herbs for 50 days, in orange juice for 1-6 days and in white cheese, cheese curd and salad with mayonnaise from 19-32 days. Persisted for 11-20 days on salads, depending on salad type.
Survives heating to 63°C for 2-3 min.
Water Activity: In general they survive better in low moisture foods. Some strains can survive 15% NaCl for 1 day.
pH: Despite its relatively high minimum pH for growth, Shigella is among the most acid resistant of foodborne pathogens. Some strains can survive exposure to pH 2.5 or 3.0 for 2 hours, and for a few hours to a day in fruit juices of various pH values. Organic acids are more inhibitory than mineral acids.
Inactivation (CCPs and Hurdles)
Temperature: Rapidly inactivated at temperatures above 65°C.
pH: Are inactivated at pH values <4.0 (but can persist for some time, see above).
Water Activity: Numbers decline slowly (over days/weeks) at 6% NaCl. Two of 21 isolates survived for 4 days in 10% NaCl.
Preservatives: S. flexneri is inhibited by plastic containing 1,500 ppm of triclosan. At pH 5.5 450ppm nitrite was required to inhibit S. flexneri but 700ppm was required to inhibit S. sonnei.
Sanitisers/Disinfectants: QACs or chlorine. 90% inactivation was produced by sodium hypochlorite at a concentration 0.5-1.5 mg/litre free chlorine and at 4°C. A 6D reduction was achieved on inoculated parsley with 5.2% acetic acid or 200 ppm free chlorine after 5 minutes exposure at 21°C. Treatment with 7.6% acetic acid or 250 ppm chlorine increased the kill to >7D.
Radiation: Sensitive to γ radiation, a dose of 3 kGy results in a 7D kill. D values are of the order of 0.2-0.4 kGy.
Incubation: 12 hours to 4 days. In outbreaks incubation times of up to 36 hours are observed.
Symptoms: Abdominal pain, diarrhoea, fatigue, malaise and fever. Mucus and occasionally blood appear in the faeces. The illness may progress to the colonic phase within 1-3 days where the symptoms are intense cramps as well as frequent and painful bowel movements. Lasts for 3 to 14 days. Estimated 13.9% hospitalisation rate, 0.16% case fatality rate.
Condition: Bacillary dysentery or shigellosis.
Toxins: Toxins are not produced in foods.
At Risk Groups: Some groups are more predisposed to infection: children under 6 in day care centres, people in nursing homes or prisons, men who have sex with men.
Long Term Effects: Septicaemia sometimes occurs in the immunocompromised host with an associated high fatality rate. May rarely cause haemolytic uraemic syndrome.
Dose: The dose required to cause disease is small at 10-100 cells.
Treatment: Antibiotic treatment is possible, but is not required in milder cases. Oral replacement of fluids may be required. Antibiotic resistance is common. No effective vaccine exists.
Reservoirs / Sources
Human: Humans and the higher primates are the reservoir for this organism. The organism can be found in the faeces for weeks after symptoms have ceased. It can survive in human faeces for days if the samples remain moist.
Animal: Not carried by animals other than primates.
Food: Foods can become contaminated by water or soiled hands.
Environment: Water that is contaminated by sewage may act as a vehicle for this organism.
Transmission Routes: Person-to-person spread (during convalescence) is important, but in many countries food and waterborne transmission are more significant. Most meals implicated in causing shigellosis comprise cooked food that is served cold and that has been contaminated by a food handler. Food can become contaminated by flies carrying sewage or faeces.
Plague and Pestilence
Associated with dips, lettuces, parsley, salads generally. Control measure failures are likely faecal contamination of ready-to-eat food / possible contamination by sick food handler / contaminated water and ice.