Food Microbiology 1 Unit 3 Food borne Pathogens
Food Microbiology 1 Unit 3 Food borne Pathogens III
Infection
Listeria monocytogenes • Facultative anaerobe, Gram positive rods • Non-spore forming • Hemolytic: which means that the bacteria have the ability to lyse or break red blood cells.
• Psychrotrophic: can grow at low temperatures • Capable of growing at a very broad range of temperature (1 -44 o. C) • Growth is optimum at 30 -37 o. C • p. H 4 -9. 6 (prefers a neutral p. H) • Salt tolerant, can survive in 25 -30% Na. Cl solutions
Listeria monocytogenes • It is the only one of seven listerial species which is of concern to humans • It is extremely prevalent in the animal population • Listeriosis : is the infection which results from ingestion of food containing Listeria monocytogenes • Most serious cases in humans are sporadic and the source and route of infection is often unknown
Illness • In healthy individuals the symptoms are “flu-like” • Susceptible individuals (pregnant women, elderly, newborns and infants, immunosuppressed individuals: Ø Stillbirth or abortion Ø Meningitis Ø Pneumonia Ø Septicemia (infection of the blood) Ø Endocarditis (inflammation of the heart)
• Infective dose for susceptible individuals is 103 CFU/ml • Incubation period required for disease to occur is a few days to 35 days • High mortality rate (30 -40%)
Double Life At < 30 o. C (i. e in the environment) Virulent genes switched off Motility genes switched on At > 30 o. C (in the body) Virulent genes switched on Motility genes switched off • Listeria monocytogenes adapts to life both in the environment and within the body
Sources • Found widely distributed in the environment (water, soil, decaying plant matter) • Present in many processing facilities especially dairy and vegetable processing • 5 -10% carriage by humans in GI tract
Foods Implicated • Raw and pasteurized Milk • Soft cheeses • Ice cream • Salads and raw vegetables • Fish and fish products • Meat and meat products
Significance of Listeria • Listeria monocytogenes can grow and multiply at low temperatures where background microflora are restricted No Competition • Decontaminated food where natural microflora are reduced provide a good environment for Listeria monocytogenes (e. g. washed salad vegetables
Prevention • High risk groups to avoid high risk foods • Sanitation within processing facilities • Avoid post-processing contamination • Cook foods thoroughly
Toxico-Infection • Pathogenic bacteria is ingested via foods • Toxins then are produced inside the intestinal tract during growth and released into the bloodstream
Main Bacteria that cause toxico-infection Clostridium perfringens E. coli O 157: H 7 Bacillus cereus
Clostridium perfringens • Gram positive, anaerobic rods • Spore-forming • Temperature growth between 6. 5 and 55 o. C but growth is very slow at temperatures of less than 20 o. C • In rich medium (cooked meats) at 45 o. C the doubling time can be very short (7 to 10 minutes)
Illness • Vegetative cells ingested, grow and multiply in the GI tract • In the course of sporulation inside the intestine, enterotoxin is produced that changes permeability of the intestinal tract
Symptoms • Nausea and diarrhea • Severe abdominal pain • Fever and vomiting less common Source: FDA
• Food poisoning occurs upon consumption of large numbers of the organism (often > 108 cells) • Incubation period is 8 to 22 hours • Duration of the illness is usually short (12 -24 hours), that’s why few single cases are reported • Usually non-lethal
Sources • Present in soil • GI tract of animals and human
Foods Implicated • Soups or gravies are often involved • Foods high in protein, typically beef and other meat products • Cooking does not destroy spores but reduces the competitive microbiota and activates the spores • If food was held at room temperatures (20 -50 o. C) spores can germinate and grow in the food
• Large volumes of food and heating drives off oxygen to give anaerobic environment; ideal for clostridial growth • Adequate re-heating can destroy vegetative cells and toxin
Prevention • Cook foods thoroughly and cool rapidly • Avoid large volumes and preparing foods too far in advance • Store foods at > 63 o. C • Avoid leftovers or re-heat thoroughly
- Slides: 22