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 of references have been omitted here. However, the brief list of references on the subject of rapid methods has been extended from the previous edition and is given at the end of this section.

The selection and use of detection methods - whether traditional or 'rapid' - should be carefully considered. Microbiological testing alone - especially end-product testing - cannot ensure food safety (you cannot 'test in' safety), and should be done as part of a HACCP scheme. Unfortunately, many people still seem to feel that end product testing can be a useful means of determining overall safety of food products, coupled with 'positive release'.

Before embarking on any microbiological examination of foods, the questions should be asked: "What are we testing for and why? What will the results mean? What action will need to be taken on finding a particular result?"

Account also needs to be taken of the uneven distribution of micro - organisms in non-liquid foods, and appropriate sampling plans should be used (see, for example, the sampling plans described by the International Commission on Microbiological Specifications for Foods (ICMSF) 1986).

Furthermore, the detection of low numbers of foodborne pathogens may require the careful selection of appropriate resuscitation, enrichment, isolation and confirmatory tests. Pathogens may be damaged, and may be greatly outnumbered by other, competitor, organisms. These facts need to be taken into account when considering methods to be used.

Many standard methods are described by the British Standards Institution (BSI) and the International Standards Organization (ISO) (available through BSI). Other standard methods, relating to dairy products or chocolate and confectionery, are available from the International Dairy Federation (IDF) (through the United Kingdom Dairy Association), or the International Office of Cocoa, Chocolate and Sugar Confectionery (IOCCC), respectively (see chapter on Addresses of Authorities/Sources of Further Information, Etc.). The ICMSF and the Association of Official Analytical Chemists (AOAC) also describe recommended/standard isolation procedures. Certain books, such as 'The Microbiological Safety and Quality of Food: Volume 2' (by Lund, Baird-Parker & Gould), and 'Foodborne Pathogens: An Illustrated Text' (by Vamam & Evans), review or describe many methods for various pathogens, and major media suppliers provide useful guidance on methods, through their catalogues and their technical support.

American standard methods are described in the FDA Bacteriology Analytical Manual (BAM) (see * below) and/or in the APHA Compendium of Methods for the Microbiological Examination of Foods (see + below).

Advice concerning the use of different isolation procedures, whether 'traditional' or rapid, can be obtained through Leatherhead Food International and other research associations.

BACILLUS CEREUS

Bacillus cereus is now recognised as a significant cause of foodborne illness in humans. It can cause two distinct forms of food poisoning. These are the so-called "diarrhoeal" type and the "emetic" (vomiting) type, caused by the formation of toxins. The emetic type is considered to be most common in the UK.

The Organism Bacillus cereus

Bacillus cereus is a large Gram-positive, endospore-forming, motile rod-shaped bacterium with peritrichous flagellae belonging to the family Bacillaceae. It grows best in the presence of oxygen, but also grows well anaerobically. Sporulation readily occurs only in the presence of oxygen. It tends to grow in long chains. The cell width is ≥ 0.9 µm and it produces central to terminal ellipsoid or cylindrical spores that do not distend the sporangia. Under anaerobic conditions both growth and toxin production are reduced.

The genus Bacillus encompasses a wide variety of species and strains. Although other Bacillus species have been associated with food poisoning, Bacillus cereus is the species of most importance to food microbiologists. These other species are covered in more detail at the end of this section.

B. cereus Food Poisoning

Bacillus cereus has been known to be a cause of food poisoning since the early 1950s. B. cereus food poisoning occurs after the ingestion of foods in which the organism has grown and formed its toxin(s). Emetic food poisoning arises from the ingestion of emetic toxin that has been pre-formed in the food (i.e. it is intoxication). The emetic toxin has been characterised as a ring form peptide (cereulide) and is thought to be associated with sporulation (1). The less common diarrhoeal type of B. cereus poisoning arises from the formation and release of enterotoxin in the small intestine, although the enterotoxin can also be pre-formed in food (i.e. it is an intermediate type of food poisoning) (2). At least two different enterotoxins have been found, but their role in food poisoning is unclear.

Incubation time

The emetic type of illness, characterised by nausea and vomiting has a short onset period of between 1 and 5 h. The diarrhoeal illness, characterised by watery diarrhoea, typically has a longer incubation time of approximately 8-16 h (more usually 10-12 h) (3,4).

Symptoms

Emetic food poisoning symptoms, which usually last for between 6 and 24 h (3,4), include nausea, vomiting and general malaise, and occasionally diarrhoea. This type of food poisoning closely resembles that of S. aureus.

Predominant symptoms of diarrhoeal food poisoning are diarrhoea and abdominal cramps, with only occasional nausea and vomiting. Symptoms usually persist for between 12 and 24 h (3,4). However, body aches, fever, chills, and longer incubation and duration times were reported in a large outbreak in the USA (5). This type of food poisoning resembles that of Clostridium perfringens.

Mortality

Although recovery from B. cereus food poisoning is normally within 24 hr with no complications, there have been reported fatalities due to ingestion of emetic toxin (6).

Infective dose

Large numbers of B. cereus are required to cause food poisoning. Numbers found in foods implicated in B. cereus poisoning are typically within the range of 105-109/g, although smaller numbers have occasionally been reported. B. cereus food poisoning may also occur after ingestion of food containing the pre-formed heat-stable emetic toxin, but which, as a result of further processing or re-heating, contains few viable organisms (1,5). Any food containing more than 103B. cereus /g cannot be considered completely safe for consumption.

Foods involved

Emetic food poisoning is almost exclusively associated with farinaceous foods, especially boiled or fried rice, and cooked pasta and noodle dishes. Most of the outbreaks in the UK have been associated with cooked rice from Chinese restaurants and take-away outlets. In Chinese restaurants, cooked rice (prepared in bulk) is allowed to cool at room temperature overnight, after which it may be reheated/fried for serving. These practices result in the survival and growth of strains of B. cereus, producing the pre-formed toxin in the food (2,7).

The diarrhoeal type of B. cereus food poisoning has been associated with a wide range of proteinaceous foods, most commonly meat or vegetable dishes, soups, sauces and puddings. In general, this type of food poisoning occurs when spores survive the cooking process, and unsatisfactory post-cooking storage conditions permit the spores to germinate and growth to occur (2).

Incidence of B. cereus Food Poisoning

The worldwide reported incidence of B. cereus food poisoning ranges from <1-22% of total food-poisoning outbreaks (<1-18% of cases) of known bacterial origin, according to country (2).

In the US during 1988-1992, 21% of food poisoning outbreaks were due to B. cereus. On average, about 14% of food poisoning outbreaks in Canada are associated with B. cereus per year. In the UK, during 1989-1993, 68% of food poisoning outbreaks were due to B. cereus food poisoning (9). Because of the relatively mild and short-lived nature of the illness, a large proportion of food-poisoning incidents caused by this organism may go unreported.

It has been estimated that there are 84,000 cases of B. cereus food poisoning in the US each year, with an annual cost of USD36m. Canada has an estimated 23,000 cases at a cost of USD 10m (9).

Sources

Humans

Bacillus species are often present in low numbers in normal stools as transient flora, which reflects dietary intake (low numbers of B. cereus can be ingested with no ill effect). In cases of food poisoning, the organism can be excreted in fairly high numbers for up to 48 h after onset (7,10).

Animals and environment

Bacillus cereus is widely distributed in nature. It is common in a wide range of environments, including soils (especially rice paddy soil), sediments, dust, natural water, animal hair and vegetation.

Foods

Because the Bacillus cereus group is ubiquitously distributed in the environment, it can easily contaminate various types of foods, especially products of plant origin. It is common in many foods, notably cereals and cereal derivatives, as well as spices, meat and meat products, pasteurised liquid egg, rice, ready-to-eat vegetables, milk and dairy products, and dried foods. Its association with milk, where it can be a significant cause of spoilage in raw and heat-treated products (especially 'bitty cream'), as well as with rice, may be a particular cause for concern. However, proper cold storage appears to be effective in preventing spore outgrowth and toxin production.