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Shigella

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Bacillary dysentery – more likely to be sexually transmitted in the UK than foodborne

Shigella is responsible for 90 million infections and 100,000 deaths annually, principally in children in the developing world. It is closely related to Salmonella and the genus includes four species; S. dysenteriae, S. flexneri, S. boydii and S. sonnei, which are all able to cause bacillary dysentery. These are also very similar to Escherichia coli and are serologically cross reactive but have remained separate species for clinical reasons. Shigella sonnei is the most common species found in the UK – which tends to cause a mild illness. Traditionally  Shigella infections in the UK  were usually (but not always) the result of travel to a country where sanitation or hygiene is poor, but in the last few years UK-acquired cases of Shigella flexneri have been on the rise in men who have sex with men. The tabs below deal with foodborne transmission only.

Growth and Control Survival Inactivation (CCPs and Hurdles) Clinical Notes Reservoirs / Sources Plague and Pestilence
Not much is known about the growth and survival of the organism in food. S. sonnei is more robust than S. flexneri .

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, cheese and 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.

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.

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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.

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.

Very common global disease – 90 million cases per year with 100,000 deaths primarily in children in the developing world. In the UK it is associated with men who have sex with men and also with dips, lettuces, parsley and salads generally. Control measure failures are likely faecal contamination of ready-to-eat food / possible contamination by sick food handler / contaminated water and ice.

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