The responsibility for preventing food-poisoning is shared by many people, including public health authorities, wholesale and retail food distributors, caterers and their staffs, and housewives. The following are among the more obvious precautions:
The nature of the organism responsible for a food-poisoning outbreak is often suggested by the pattern of the outbreak. Whether it is confined to a household or involves large numbers of people, the time relationships, the nature and severity and duration of the symptoms, and so on.
Except in salmonellosis the investigation of an outbreak has little bearing upon treatment and is principally concerned with finding out what went wrong and preventing further outbreaks. The laboratory's contribution is to try to isolate the responsible organism, by culture of faeces and vomit (if available) from a manageable proportion of the patients and by microscopy and culture of the offending food if it can be identified and if some of it is still available.
Sometimes the history of the outbreak clearly incriminates a particular item or at least a meal, but in other cases detailed detective work is necessary. If the food is available and either salmonella, a heat-resistant Clostridium welchii, or a Bacillus cereus is involved, there is not usually much difficulty in growing the organisms from the food and from at least some of the patients, and in. showing that they are identical.
Clostridium botulinum is also likely to be recoverable from the food, and although it is unlikely to be recovered from the patients it produces a highly characteristic clinical picture. The case against a staphylococcus, however, may be difficult or impossible to prove. These organisms, being less heat-resistant than their enterotoxin, may have been killed during cooking, and there is no simple and satisfactory procedure for demonstrating the presence of the enterotoxin, though it can be detect tests used in specialized food-hygiene laboratories.
Human milk taken by the baby straight from the mother is seldom a vector of pathogens. Only minor bacteriological problems are associated with the collection and storage of expressed human milk. Cow's milk, on the other hand, presents many important problems. It may contain pathogens derived from the cow; the circumstances of its collection, unless carefully controlled, permit it to become heavily contaminated with a wide variety of micro-organisms; it is a good culture medium for many of these; it may spend hours or days at temperatures suitable for bacterial multiplication before it is consumed; and, as the result of pooling, milk from a single cow may be distributed to a large number of human beings.
Organisms which may be present in cow's milk include the following:-
Multiplication of organisms can be kept to a minimum by cooling the milk as soon as it is collected, keeping it cool during transmission, and delivering it to the consumer early in the day while the atmospheric temperature is still low. But even a combination of all these measures does not guarantee that the milk will be safe to drink. This can be achieved only by heating it.
BACTERIOLOGICAL EXAMINATION OF MILK
In this test, haematoxylin-stained dead brucellae are added to a sample of the milk; if they are agglutinated, they rise up in the fat globules and form a blue ring in the cream layer. Milk products, such as butter and cheese, and milk-containing foods, such as ice-cream and custards, are of course liable to contain pathogens similar to those found in milk, and are exposed to greater risks of contamination by handlers. It is, however, very much more difficult to devise bacteriological standards and tests for these products, apart from cultures to exclude the presence of named pathogens.
Factors which determine the total bacterial content of a water supply include the following:
It is not the total bacterial content, however, which is important in assessing the suitability of a water supply for human consumption. What matters is the possibility that it contains organisms capable of causing disease in those who drink the water. Important among these organisms are the bacilli of typhoid, paratyphoid, dysentery and cholera; some pathogenic viruses can also be water-borne—notably those of poliomyelitis and hepatitis A.
Presence of such organisms results from contamination with human excreta, or in some cases with animal or bird droppings. There are a number of obvious precautions which can be taken to reduce the risk of such contamination—e.g. drawing supplies only from relatively uninhabited catchment areas, seeing that one community is not discharging its untreated sewage into a river upstream from the point at which another is drawing off its supply, or, in places where there is no main water supply or main drainage, seeing that wells are so situated and protected that there cannot be any leakage into them from pit-latrines and the like.
There are few occasions, however, when it is safe to assume that a water supply has not been contaminated—though a dangerous level of contamination is highly improbable in the case of water taken from a fast-running stream in a hill or mountain area above the level of human habitation. With this exception it is virtually always unwise to drink unboiled water derived from a source that has not been subjected to thorough and repeated testing, as described below, or to filtration and chlorination with adequate bacteriological control. Although most non-chlorinated water supplies contain Gram-negative bacilli, including enterobacteria such as Klebsiella aerogenes, the presence of 'faecal coliforms' such as Escherichia coli indicates contamination with human or animal faeces, and their presence in more than very small numbers indicates that the water is not safe for human consumption. Streptococcus faecalis and Clostridium welchii have a similar significance, though the latter, being a sporing organism, survives longer than other faecal organisms, and its presence without them suggests that contamination was not recent.
There is seldom any point in examining a water supply directly for the presence of pathogens, for if contamination has occurred other faecal organisms are likely to be far more numerous and easier to detect, and the presence of these organisms in a supply that does not at present contain detectable pathogens implies that it may well do so on other occasions.
Since contamination may be intermittent, a single satisfactory bacteriological examination does not guarantee the safety of a water supply. Regular testing of any supply to be used for drinking is essential, and in the case of a non-chlorinated supply any deviation from the pattern of results obtained over the course of previous years must be taken as indicating that there has been a change in the source of the supply and that it must be regarded with suspicion. Almost all piped drinking water in Britain is chlorinated. The presence of any coliform bacilli at all in a too ml sample of chlorinated water collected at its point of entry into a public supply indicates that the chlorination procedure was defective.
BACTERIOLOGICAL EXAMINATION OF WATER
As we have indicated, the most important part of the bacteriological examination of water is the detection and enumeration of coliform bacilli, and in particular of those that are of faecal origin. This can be done by adding portions of the water sample—e.g. one of 5o ml, 5 of to ml and 5 of I ml—to bottles or tubes containing a suitable liquid lactose containing medium and incubating overnight at 37 C.
Production of acid and gas in a bottle or tube indicates the presence of 'presumptive coliforms'. Because organisms may be irregularly distributed in the water, it may well happen that, for example, growth of `coliforms' occurs from some of the 1 ml samples but fails to occur from some of the 10 ml samples.
However, by comparing the results obtained with McCrady's probability table worked out for this purpose, it is possible to arrive at a figure for the most likely number of `coliforms' per 100 ml of the water. Not all bacteria that produce acid and gas under the conditions of the presumptive coliform count are in fact faecal organisms. If any such bacteria are detected, they are further tested for ability to grow and to produce gas in the same or a modified fluid medium incubated at 44 C.
With rare exceptions, the ability to do this is confined to true 'faecal coliforms'. The finding of any such indicates that the water is not fit for human consumption. An alternative procedure for the bacteriological sampling of water is to pass a known volume of water under pressure through a special porous cellulose acetate membrane which holds back all bacteria. The membrane can then be placed in a Petri dish on top of a pad of filter paper or other suitable absorbent material which is saturated with a fluid culture medium.
On incubation, colonies form on the surface of the membrane. By using appropriate media and incubation temperatures it is possible to carry out total counts, counts of lactose-fermenting organisms that will grow on bile-salt lactose medium and counts of those which will do so at 44 C. However, gas production cannot be detected and the results are therefore not strictly parallel with those of the tests in liquid media.
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