FOODBORNE BACTERIAL PATHOGENS

Food Poisoning – a Brief Overview

Foodborne illness is a major public health concern worldwide. The costs in terms of human illness and economic losses to individual companies and to the public health sector can be immense. Recalls, closure of factories, legal proceedings and adverse publicity to food companies involved in 'food scares' can result in both acute and long-term losses, with repercussions that may continue for many years. For example, sales of corned beef after the 1964 typhoid outbreak in Aberdeen, Scotland, were not restored to pre-outbreak levels for about twenty years. More recently, an outbreak of E. coli O157 infection in the UK caused the closure of a meat processor and was the subject of major headlines in the media, causing as it did the death of a five year old boy as a result of eating a contaminated product.

We can see by observing the scientific literature and reviewing the media that foodborne illness is a worldwide problem, not restricted to any particular country. Solutions to problems will often have to be considered from outside the European community if they are to be effective. Nevertheless, there was a real increase in food poisoning in the UK in the 1980s and 1990s, reaching a peak of approximately 100,000 reported cases in 1997/1998.

There were many different causes for this increase, and the 'Richmond' Committee on the Microbiological Safety of Food concluded in 1990 that poultry and their products were the most important source of human gastrointestinal infections arising from food, both from carcass meat contaminated with Salmonella and Campylobacter, and shell eggs that could also be a source of Salmonella. They considered that many factors were involved in this increase, with no factor more important than the next.

More recently, the number of recorded cases of food poisoning in the UK has fallen significantly, and this is thought to be due principally to action taken by the poultry industry to eliminate Salmonella from laying flocks and broilers by vaccination programmes and improved hygiene. In contrast, however, reports of Campylobacter infection have risen over the same period, and the organism is now the most important cause of bacterial foodborne disease in the UK. The same trends are noted in other European countries and the US, and much attention is now being directed to the control of Campylobacter in poultry. Some countries remain free of Salmonella and Campylobacter, by the effective application of biosecurity measures designed to contain the organism.

The global nature of the food chain itself can cause problems, with a lack of control over the ingredient supply chain resulting in large-scale chemical contamination issues. Several recent large food poisoning outbreaks in the UK, caused by Salmonella, have had, at their root, the purchase of (presumably) cheaper eggs from outside the UK, where there is no vaccination policy.

'Emerging' pathogens are the subject of much research and discussion. They continue to present new challenges for identification and control. Changing consumer demands mean that some unlikely organisms may emerge over the next 10 years, and it is the job of microbiologists to anticipate and control these new organisms. It is important to remember that less than 20 years ago, L. monocytogenes was an obscure species, almost unknown outside the veterinary field. Campylobacter has emerged more recently as a significant cause of food poisoning from the consumption of poultry, and E. coli O157 as a cause of food poisoning in ground beef.

An explanation of the term food poisoning'

For the purposes of Micro-Facts, the rather loosely applied term 'food poisoning' encompasses all those types of illness that are caused by the consumption of food contaminated with pathogenic micro-organisms and/or their toxins. In this context, therefore, Campylobacter has been included as a 'food-poisoning' bacterium, despite the fact that it does not grow in food - food acts only as a vehicle for the organism.

In most cases food poisoning involves gastroenteritis - vomiting and/or diarrhoea - but in the cases of botulism and listeriosis, the main symptoms are caused by effects on other ('extra-intestinal') parts of the body.

Intoxication vs infection

Food poisoning can be split further into three types: 'intoxication', 'infection', or 'intermediate'.

Infection is caused by the in vivo multiplication of bacteria that are taken in with food; subsequently, live food-poisoning organisms (usually many) need to be ingested and there is normally a delayed response (typically involving diarrhoea) reflecting the time needed for an infection to develop. Examples of food-poisoning bacteria that cause infection are Salmonella, Campylobacter and Vibrio parahaemolyticus.

Intoxication is caused by the ingestion of toxin(s) that has (have) been pre-formed in the food. Hence, there is no requirement for live organisms to be present and the onset of symptoms (usually starting with vomiting) is soon after the toxic food is ingested. Examples are Bacillus cereus and Staphylococcus aureus intoxication.

The 'intermediate' type of food poisoning is caused by the formation of toxin(s) in the bowel as a consequence of consumption of contaminated food, as in the case of Clostridium perfringens food poisoning. However, for the purposes of Micro-Facts, the 'intermediate type' is included under 'Infection'.

In each individual section, descriptions are provided of all the main food-poisoning bacteria, including those that cause intoxication, infection, or the intermediate type of food poisoning. Micro-organisms that are known to be occasionally associated with foods, but not to cause 'food-poisoning' symptoms (viz. the causative agents of tuberculosis, typhoid, brucellosis, Q-fever, etc.), are not included. Other foodborne - or potentially foodborne - pathogens are also mentioned, as an aid to awareness.

Summaries of the key characteristics of the main organisms considered are to be found in the convenient tables to follow.

Methods for Detecting/Counting Foodborne Pathogens in Foods

The sections in Micro-Facts that describe different foodborne pathogens include certain references to methods for their isolation. These references have been selected, in the main, on the basis that they either describe well - verified and established methods, or they review a number of methods. The decision to exclude most references to 'rapid methods' - other than a few selected articles and reviews/books - was a conscious one, and made after some deliberation. As this field is still developing at such a fast rate, and because the choice of a suitable rapid method is one that needs to be taken with regard to costs, facilities available, etc., the vast majority...