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| The Bug Bible > Salmonella spp
Salmonella spp Description
Salmonella are Gram negative rods that are usually motile by peritrichous flagella. They are facultative anaerobes, catalase positive and oxidase negative. There are over 2,500 different serotypes known. Most but not all are human pathogens. They are widely distributed in nature and the primary reservoir is the intestinal tract of vertebrates.
They are found in a wide variety of animals –including domesticated, pets or wild, mammalian or avian, warm blooded or cold blooded. Most animals colonised with salmonellae show no sign of illness.
Environmental sources can include water, soil, insects, factory surfaces, kitchen surfaces and animal faeces.
The organism can grow at temperatures ranging from 7 to 47°, and at pH values of between 4.0 and 9.5. The minimum water activity value for growth is 0.96, and the organism can survive for long periods of time in foods containing fats and at low water activities. They are easily destroyed by heating.
Characteristics of Illness
The ingestion of live cells of Salmonella can cause gastroenteritis. The micro-organism will adhere to the walls of the small intestine and grow and release enterotoxin. Invasion of salmonellae results in varying degrees of damage to the mucous membrane of the small intestine and colon. In addition to enterotoxin, cytotoxin is also produced by most strains of Salmonella. The disease is often referred to as salmonellosis.
Infective dose: The number of Salmonella required to cause an illness varies according to the individual and food material. Generally around 105 Salmonella per gram of food is enough to cause illness, although outbreaks have occurred where between 1 to 10 cells per gram of food were detected.
Time of Onset of Symptoms: The illness usually occurs 8 to 72 hours after the ingestion of the contaminated food.
Symptoms: Symptoms include abdominal pains, nausea and watery diarrhoea, sometimes with mild fever, vomiting and headaches. Dehydration can lead to prostration.
Duration of symptoms: Gastroenteritis symptoms usually last 2 to 5 days.
Diagnosis of human illness
Serological identification of culture isolated from stool.
At the onset of illness large numbers of salmonellae occur in the faeces. The numbers shed in the faeces decreases with time however, the median duration of excretion has been estimated at 5 weeks with less than 1% of patients becoming chronic carriers.
Complications
Serious complication may include septicaemia, with patients with an underlying disease more susceptible. Reactive arthritis and Reiter's syndrome may occur in a small percentage of patients 3 to 4 weeks after the enteritis.
Susceptible individuals
All age groups are susceptible but symptoms are most severe in the elderly, infants, and the infirm. AIDS patients suffer salmonellosis more frequently (estimated 20-fold more than general population) and suffer from recurrent episodes.
Associated foods
Salmonella is typically associated with animal foods such as meat, eggs and milk. It can also be present in many other foods through contamination from the environment.
Many foods have been implicated in foodborne disease caused by Salmonella, including poultry, eggs, milk and dairy products, fish, shrimp, frog legs, yeast, coconut, sauces, salad dressing, cake mixes, cream-filled desserts and toppings, dried gelatine, peanut butter, cocoa, and chocolate.
Pepper has caused at least two outbreaks one in Canada with S. Weltwvreden and another in Norway with S. Oranienburg. Contaminated paprika was responsible for an outbreak in Germany involving potato chips with an estimated 1000 cases. The number of cells of salmonellae in the food was low (2.5 cells/g of paprika: 0.04-0.45 Salmonella cells per gram of chips. Recent outbreaks in Australia include 36 cases from Iceberg lettuce (S. Bovismorbificans) and 500 cases from unpasteurized orange juice (S.typhimurium).
Food analysis
Conventional culture methods require 5 days for presumptive results, although several rapid methods are available which require only 2 days. Confirmation requires serological testing.
Epidemiology
Notifications of Salmonellosis indicate that over 5,000 cases are reported in Australia each year. The incidence of salmonellosis appears to be rising in Australia, as in other industrialised nations. Most outbreaks occur through cross-contamination from raw to cooked food and subsequently the food remaining in the Temperature Danger Zone (5°C to 60°C) for too long. Inadequate cooking or reheating is an other contributing factor.
The following is a list of the most recent outbreaks.
In March 1999, an outbreak of Salmonella typhimurium 135A occurred in South Australia. It was linked to consumption of fresh fruit juice and 74 cases were reported.
In 1996, an outbreak of Salmonellosis caused by the serovar Salmonella mbandaka occurred in Australia. 54 cases occurred linked to the consumption of peanut butter. The source of the serovar was thought to be the peanuts.
In 1996-1997 an outbreak of Salmonellosis caused by a strain of Salmonella typhimurium occurred in South Australia. At least 71 people became ill after consuming meat-filled bread rolls.
In early 1997, five separate outbreaks of Salmonellosis occurred in Victoria:
- 24 cases of Salmonella muenchen linked to a local meat product manufacturer.
- 808 cases of Salmonella typhimurium 1, linked to the consumption of Vietnamese pork rolls.
- 25 cases of Salmonella anatum, linked to a smallgoods manufacturer.
- 5 cases of Salmonella chester linked to a local delicatessen.
- 7 cases of Salmonella typhimurium 43 linked to a Vietnamese/Chinese restaurant.
Prevention
- In the home the following steps will minimize the possibility of salmonellosis:
- Obtain food from safe sources.
- Store perishable foods chilled or frozen.
- Wash vegetables and fruit well if these are to be eaten raw.
- Cook food to at least an internal temperature of 75ºC and if not eaten immediately cool rapidly and refrigerate at 50C or less.
- Do not hold reconstituted foods such as dried milk powder and infant formula at ambient temperature.
- Prevent cross-contamination by not using the same utensils and equipment for raw and cooked foods.
- Practise good personal hygiene making sure that hands are washed regularly with soap, particularly after going to the toilet and after handling raw foods.
- Thaw, store and cool perishable food in a refrigerator at less that 5°C.
- Reheat food to 75°C. Hold hot food at above 60°C.
- Wipe up meat and vegetable residue with disposable paper towels.
- Ensure that kitchen sponges and clothes are disinfected and replaced regularly.
- Ensure that people recovering from gastric illness particularly salmonellosis do not directly handle ready to eat foods.
Further Information
FDA Bad Bug Book
Literature:
Hocking, A.D. et al. (2003). Foodborne Microorganisms of Public Health Significance. 6th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.
Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York. |
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