Public health microbiology

Organisms which cause infection in man

The field of microbes and their activities is a vast one including everything from the useful to the downright dangerous. These notes are intended to provide basic information on those organisms which cause intoxication and infection in man and are either water or food borne.

Staphylococcus aureus

Staphylococcus aureus is a spherical shaped cell which tends to occur in clusters resembling bunches of grapes. It can cause infections in man and is often found in the nose, throat, skin, and in septic lesions. Symptomless carriers are often found. The most important factor of this organism is its ability to produce a heat-stable enterotoxin which, when ingested, gives rise to nausea, cramps and diarrhoea. The incubation period between ingestion and illness is usually 2 - 4 hours. The organism is normally transmitted to the food from the hands of food handlers and, if the food is kept warm for some time, growth occurs and toxin is produced. The addition of preservatives or low temperature storage will prevent growth, as will adverse pH or lowered water content. Once the toxin has been formed, it is very difficult to destroy and often survives treatments which kill the causative organism.

Salmonella spp.

Salmonella spp. form a large group of cylindrical shaped organisms which is commonly associated with the intestinal tracts of warm-blooded animals. Although this is their principal habitat, they are capable of prolific growth on a wide variety of foodstuffs. As with Staphylococcus aureus, symptomless carriers do occur but at much lower rates. The organisms are passed in the faeces and are transmitted thence to the food. Salmonella produces infections in man and there is no toxin production in food; this means that sufficient cells (the minimum infective dose) must be ingested in a living state to cause disease. Following contamination of a food there must, therefore, be a period during which growth can occur followed by ingestion; if there is any form of cooking, then the Salmonella will probably be killed and the food will be rendered safe. The heat-resistance of Salmonella is, except for a few strains, low and pasteurisation is normally adequate to eliminate them. The symptoms of salmonellosis are fever, abdominal pain, diarrhoea, prostration and frequent vomiting. The symptoms usually occur between 12 and 24 hours after ingestion but can occur between 6 and 48 hours. The illness may last for & days or even longer. During the period of the illness, and for some time afterwards, the patient will be passing Salmonella in the faeces and for this reason should not be allowed to handle food until he is fully recovered. Control of this organism in the factory is effected by the exclusion of carriers and an insistence that all staff wash their hands before entering a food handling area.

Clostridium perfringens (welchii)

This organism is widely distributed in nature, it is a commonly found inhabitant of the bowels of man and many other animals, and its spores can persist in soil, dust and water for considerable periods. The cells are cylindrical and may contain a resistant spore. This organism is anaerobic and grows most rapidly when oxygen is absent or in low concentrations. When food is cooked, oxygen is driven off and, thus, if cooked food is kept warm for any length of time, the spores which may well have survived the cooking process will germinate and growth will occur. if food in which C. perfringens has proliferated is eaten, the illness which usually develops within 10 - 12 hours is thought to be due to the release of a toxin within the intestine. The symptoms are abdominal pain, diarrhoea and prostration and they may last for 12 - 24 hours. Due to widespread distribution of this organism, it is difficult to ensure its absence from foods although normal procedures for hand washing and plant sanitation will help. The principle method of control must be to ensure that all cooked foods are stored at temperatures outside the normal growth range for the organism. For most practical purposes, the safe temperatures are below 10 or above 60°C, and food should only be at temperatures in between whilst being actively heated or cooled. Foods in which a preservative is incorporated or some other physical condition is growth limiting may be exempt from this requirement.

Clostridium botulinum

This is an organism which is in many respects similar to C. perfringens but differs in that, during growth on suitable substrates, an extremely potent toxin is released. The toxin, when ingested, attacks the nervous system causing paralysis and frequently causes death. Fortunately, the toxin is sensitive to heat and foods can be rendered safe by boiling for a few minutes.

Spores of C. botulinum are widely distributed in nature and it is not uncommon to find them on raw materials. If it is accepted that there is a high probability that raw materials will be contaminated, then control measures must centre on the prevention of growth and subsequent elaboration of toxin. Except in the case of canned seafoods, there is little hope of destroying the spores by heat treatment and it is necessary to control growth by physical or chemical means. Commonly used methods are: refrigeration/freezing, drying, pickling, salting, and curing with nitrite. If, despite such precautions, toxin is produced and the food is consumed without further cooking, then the symptoms mentioned above will occur after approximately 6 hours but may be delayed for up to 24 hours and sometimes even longer. Death, if it occurs, will follow at the earliest at 24 hours but sometimes as late as one week after ingestion of the toxin.

Vibrio parahaemolyticus

This is an organism which is frequently found in seafoods and in coastal waters. The organism grows on the food, and ingestion of the living cells gives rise to the symptoms which are as follows: abdominal pain, nausea, vomiting and diarrhoea. Symptoms may start within 2 hours but may be delayed for up to 48 hours. The illness usually lasts for 1 - 2 days. This organism does not produce resistant spores and can be easily destroyed by heat. For this reason, it is only a problem where seafood is consumed raw, e.g., in Japan. There is still a great deal to be discovered about the pathogenicity of this organism and its significance in seafoods which will be cooked before consumption. After initially over-reacting to the presence of V. parahaemolyticus, many importing countries now accept shipments containing the organism provided that they are raw products.

Vibrio cholerae

This organism is traditionally regarded as waterborne but the use of polluted water in a seafood processing plant can lead to contamination of the product. Mild forms of the disease do occur and there is a danger that personnel with mild diarrhoea may in fact be excreting cholera vibrios, which might be transferred to the food. This in itself is sufficient reason to exclude anyone suffering from a gastro-intestinal disturbance from the processing plant. The symptoms are similar to many of the other types of food poisoning but the passing of rice water stools is peculiar to cholera. The illness starts 1 - 5 days after ingesting the contaminated food and may cause death due to loss of fluid and electrolyte depletion. V. cholerae is not heat resistant so that, like V. parahaemolyticus, it is destroyed by normal cooking procedures.

Bacillus cereus

This is a rod shaped organism which can form heat-resistant spores. It is common in many foods and in soil and produces two fairly distinct types of illness. The first, or classical, form resembles C. welchii poisoning, with an incubation period of 10 - 13 hours. The second form is more acute and resembles staphylococcal intoxication and has an incubation period of 1 - 5 hours. This suggests that, in the first form, there is some type of infective process taking place, whilst in the second form the symptoms are indicative of a straightforward intoxication. In either case, illness only develops when the food contains extremely large numbers of the organism (usually in excess of 10 million per gram). This fact means that effective control of the growth of B. cereus by means of temperature control, drying, salting etc. will greatly reduce the possibility of illness. Foods are most susceptible when they are cooked and then held in a warm state for protracted periods. Under these conditions, the spores which survive cooking grow very rapidly and large numbers will be produced in a matter of hours.

Indicator organisms

The tests needed to ascertain the presence of the principal intestinal pathogens are very elaborate and, particularly in the case of water samples, it has been found to be more expedient to look for organisms which indicate that contamination with faecal material has taken place. Organisms used in this way include C. perfringens, Escherichia cold type I and Streptococcus faecalis, al I of which can be isolated from human faeces. From time to time, these organisms can cause diseases in their own right but their principal importance is in assessing plant hygiene. This is true of the coliform group in general and particularly of E. cold I with its association with faecal matter from humans and other warm blooded animals. It should not be assumed that the presence of the indicators is always associated with the presence of pathogens and, indeed, on some occasions, organisms such as Salmonella can be found when there are no indicator bacteria. Nevertheless, tests for these organisms do provide a fair indication of the hygienic state of the production facilities and the personal hygiene of the staff working in them.