Pr. PIERRE BOURLIOUX

Food-borne diseases are most often due to bacteria per se or to toxins they release. Onset of food-borne diseases is mostly due to food harvesting, reparation and preservation conditions than to the nature of the food itself. It is therefore important to follow basic hygienic rules both in the agri-food industry and in the family kitchen.

Food-borne disease is a condition caused by bacteria, viruses or parasites and is induced by consumption of a contaminated food. A microbial attack can be due to the invasive properties of a micro-organism itself (food-borne infection) and/or to the toxins it may produce during its growth (food intoxication). Food-borne diseases comprise classical infections such as from Staphylococcus aureus, Salmonella sp or Clostridium perfringens, as well as less common ones due to consumption of food contaminated by viruses, parasites or prions. Bacterial food infections, however, represent the majority of cases.

Foods are consumed by man to fulfil his nutritional needs. However, foods also provide many micro-organisms (especially bacteria) an environment where they find everything needed to multiply very quickly and produce eventual poisonous metabolites. Some foods such as milk and meat are particularly at risk. Virtually all foods can induce a food-borne disease if proper care is not taken during their harvesting, preparation and conservation (Table 1).

Principal infectious agents.

Germs causing food-borne diseases can be classified into two main categories:
- Those acting directly on the intestinal mucosa, adhering and penetrating the intestinal cells. Most of these bacteria are of the Salmonella, Shighella, Escherichia (entero invasive E. coli), Yersinia (Y. enterocolitica), Vibrio (V. parahemolyticus), Campylobacter, and Lister (L. monocytogenes) strains.
They induce food-borne infections.
- Those acting through toxins and mostly belonging to the Staphylococcus (S. aureus), Clostridium (C. perfringens, C., botulinum) and Bacillus (B. cereus) genus. They lead to food intoxication.

Mechanism
Bacteria with direct action on intestinal mucosa adhere and penetrate the intestinal cells. Their virulence factors are coded by chromosomal and/or plasmidic genes that determine their invasive properties. Two processes may unfold. Either bacteria destroy the cell and induce a food-borne infection with local inflammatory reaction symptoms, febrile infectious diarrhoea and presence of blood and leukocytes in stools. Or either it remains engulfed in the cell and multiply in vacuoles without cell lysis, initiating a cascade of reactions responsible for febrile diarrhoea and vomiting (gastro-enteritis).

Onset of food-borne infections
Conditions for the onset of food-borne infections are determined by bacteria and host characteristics. Bacterial aggression first depends on the full or partial pathogenic potential of bacteria (surface antigen, adhesins, invasin, lipopolysaccharides, siderophores), and secondly, on the number of bacteria comprised in the infectious inoculum. Indeed, the minimal infectious dose varies according to bacteria species and depends extensively on their virulence factors.
In response to this aggression, the host will trigger localised or general (immune response) defences. If these defences are overwhelmed, symptoms of a food-borne infection appear.

Symptoms Symptoms only appear insofar as bacteria have grown in the food, that they survived through the digestive tract and reached their site of action in large enough number. Symptoms (vomiting, fever, and diarrhoea) occur as rapidly as the inoculum is considerable.

Mechanism
Bacterial toxins causing food intoxication are holoproteinacious molecules and have a direct impact on cellular metabolism. Staphylococcus toxins act on water and electrolytes metabolism, whereas botulinum toxin attacks the central nervous system. Toxins induce liquid diarrhoeas of varying gravity without any fever. Because toxins act so fast, the organism is usually unable to produce antibodies: although staphyloccocci antibodies have been detected in man, their protective capacity has not been yet demonstrated.

Onset of food intoxication
Toxins are heat-resistant, which explains that even cooked foods can lead to food poisoning.

Symptoms Symptoms occur in general very quickly after ingestion of a contaminated food: two to four hours following consumption, an important and very liquid diarrhoea appears without any fever and without any sign of invasiveness (no blood, nor leukocytes in stools). Usually, diarrhoea doesn't last more than twenty-four hours.

Epidemiology of food-borne diseases has been studied extensively in community where salmonellas are predominant: in 1997, in France, salmonellas were at the origin of 76% of community food-borne infection where the agent has been identified. Among other bacteria involved in community food-borne infections are namely S. aureus (12%) and C. perfringens (5%).
Beside communities, salmonellas are also responsible for the majority of cases of food-borne infection in France and in Europe. Contamination is often linked to consumption of raw or insufficiently cooked products of animal origin (milk, eggs or raw egg-based preparations, pork meat, poultry, cold cuts…).
Food-borne infection from Listeria monocytogenes recently got a lot of media attention. Listeria monocytogenes is a saprophytic bacterium, very common in nature and present in many foods. Because of its saprophytic characteristic, it is likely to cause infection in animals, but most often without clinical consequences. It is presumably the same thing in man. This bacterium is especially found in patients with decreased immunity but also, for the past twenty years, in subjects apparently in good health. Progress made in the identification of this bacterium in the environment and in foods led to isolating it more frequently. It enters the body through respiratory and digestive mucosas and the onset of disease is presumably due to the virulence of its strain and the genetics the host.

Food intoxication from Clostridum botulinum is quite different: its toxins are neurotoxins causing flaccid paralysis. This food intoxication is often due to consumption of insufficiently sterilised home-made preserves.

Bacteriological safety is the basic criteria for food safety.
Presence of potentially pathogenic bacteria in a food doesn't necessarily lead to symptomatic food-borne disease. Indeed, germs comprised in foods are in a variable physico-chemical environment and these conditions may not be favourable to their development (temperature, pH, inhibitory compounds…). On the other hand, a micro-organism is not alone, but within a competitive bacterial population of different species. Heating foods at 70° C for ten minutes, when possible, eliminates all bacteria in a vegetative state; however, spores and toxins are still active at this temperature.
Therefore, if bacteria are already present in a food or if they are added involuntarily during their preparation, it is necessary to avoid their multiplication before their consumption. Proper refrigeration from preparation to consumption must be followed, and fragile dishes (fish, dairies, meat…) should not stand at an ambient temperature that promotes the multiplication of bacteria.

In practice, at the industrial or community level, it is primordial to handle controlled raw materials like being microbiologically safe. In addition, fragile products must always be stored in a maintained refrigerated environment. The conditions of preparation will be even more rigorous when foods are not cooked and require manipulation.
At the individual and domestic levels, rules are similar to those recommended for communities. It is necessary to maintain foods in a constant refrigerated environment, to keep products in the refrigerator, to wash hands before handling and to verify the cleanliness of instruments used in the kitchen.

TEMPERATURE AND SHELF-LIFE OF DIFFERENT FOODS
Nature of the food	Temperature	Shelf-life
Quarter of meat	[0-7°C]	2 weeks
Meat cut	[0-3°C]	1 week
Ground meat	[0-3°C]	1-2 days
Fresh fish	[0-2°C]	3-7 days
Living shellfish	5-10°C	1-2 weeks
Eggs	[0-8°C]	2 weeks
Semi-preserved foods	5-10°C	6 months

From : Association Française du Froid

Food-borne diseases are still frequent in France, mainly as cases of community food-borne infections that increase rapidly with the development of the restaurant industry. Surveillance and strict control of the food in restaurants must therefore be applied. All community food-borne diseases must be declared to the Departmental Sanitary Authorities (DASS). This obligation concerns all actors of the health system, namely general practitioners and work physicians.

Pr. Pierre BOURLIOUX
University of Paris Sud-Châtenay-Malabry