Food Safety. Intoxications. Infections

Food poisoning can be classified by two distinct descriptors, intoxications and infections. The former is the result of the ingestion of toxins that may be the product of foodborne microorganisms. As such, the microorganisms need not be consumed nor vestiges of their presence in the food even detected. In a broader sense, the ingestion of any chemical toxicant that results in disease is an intoxication, with the exception of those reactions that are due to an allergic response or are mediated by an inherited metabolic disorder.

Infections, on the other hand, are the result of the consumption of the pathogenic organisms. The nature of the poisoning is expressly a function of the etiologic agent. Certain microorganisms cause infections, including Salmonella, Escherichia coli, Clostridium perfringens, Campylobacter, Yersinia, and Listeria. Other, such as Staphylococcus, Clostridium botulinum, and Bacillus cereus, cause intoxications. The severity of the disease varies with both infections and intoxications, with some of each class being fatal. Other food poisoning agents include viruses and parasites, whereas mycotoxins can cause illness but are more of concern due to their long-term health consequences.

A. Intoxications. 1. Staphylococcus gastroenteritis. Staphylococcus aureus is a gram-positive, non-sporeforming coccus that grows in chains. It is a normal inhabitant of a number of environments and is a common resident of human skin. It grows under a wide variety of conditions and can grow at lower water activities than most other bacteria (Aw = 0.85). As a consequence, S. aureus food intoxications are frequently associated with foods that have low water activity.

Although S. aureus is able to cause infection, most foodborne illness is due to intoxication. This microorganism produces a powerful enterotoxin that causes gastrointestinal distress with symptoms including nausea, retching, abdominal cramps, and diarrhea. Staphylococcus aureus enterotoxin is notable because it is heat stable, able to withstand heating at, for example, 110°C for >60 min. Therefore, in certain processed foods, the enterotoxin can be present in the absence of any viable S. aureus. The term preformed enterotoxin is used to define illness from enterotoxin where no evidence for S. aureus cells is found.

2. Gram-Positive Spore Formers. The gram-positive spore-forming bacteria include the genera Bacillus and Clostridium. These organisms are different in that the former is an aerobe whereas the latter is an anaerobe. They are notable not only because they include one of the more deadly foodborne microorganisms, Colstridium botulinum, but also because of their extreme heat resistance. The current regulations regarding the proper thermal processing of canned foods is largely based on the heat resistance of these spore formers (see Section IV).

Bacillus cereus is a frequent contaminant of dried dairy ingredients largely due to its ability to survive spray drying. It produces two types of toxins, an enterotoxin and an emetic toxin. The former causes gastroenteritis and the latter causes vomiting. It is not considered to be a life-threatening foodborne pathogen, although because of its potential contamination of infant formula (which may include milk powder as a major ingredient), specific regulations on maximum levels of B. cereus in infant formula do exist.

Clostridium perfringens causes gastroenteritis due to the production of an enterotoxin. Because the enterotoxin can be produced in the intestinal tract, this organism is typically an infective disease. The enterotoxin is spore specific, being produced during sporulation of the vegetative cell. As an anaerobe, C. perfringens proliferates under low-oxygen conditions and requires a relatively rich medium for growth. An increased incidence of illness due to C. perfringens has been reported in slow-cooked meat products including roast beef. Reaching proper cooking temperatures in excess of 165°C and then rapidly cooling the food product after cooking helps to reduce the likelihood of C. pefringens contamination.

B. Infections. I. Listeria monocytogenes. The gram-positive bacterium Listeria monocytogenes causes a variety of diseases in humans as well as in animals. Clinically, infection with L. monocytogenes can lead to septicemia, encephalitis, or abortion depending on the infected host and perhaps the strain present. The organism, similar to other bacteria such as Salmonella and Yersinia, is capable of intracellular growth. It has the unique ability to evade the normal host defense mechanisms that include the ability of macrophages to kill bacteria. Listeria monocytogenes possesses a number of virulence factors that encode the ability to lyse blood cells (hemolysins) and to transit both within and between host cells.

Listeria monocytogenes was recognized as a pathogen almost 50 years ago, but only in the last 15 years did it come to prominence as a food pathogen. It has the ability to grow at refrigerator temperatures (4°C), making it problematic in foods normally assumed to be safe under those storage conditions. Because it can grow at refrigeration temperatures, the normal warning systems that protect the consumer are not effective. A typical consumer would not ordinarily consume spoiled food, which is characterized by an off-odor or appearance. Spoilage organisms are usually not pathogenic and they are useful indicators that a food has exceeded its effective shelf-life. Under refrigeration, these spoilage organisms do not grow and therefore do not spoil the food, while L. monocytogenes is still capable of proliferating.

2. Salmonella. Salmonella is a gram-negative bacterium that is closely related to Escherichia coli. Speciation of Salmonella is based on serology and there are greater than 2200 different serotypes. Salmonella is responsible for the most reported foodborne disease outbreaks involving approximately 2000-3000 cases per year in the United States. This is probably a vast underestimate, as many cases of foodborne poisoning go unreported due to their relatively mild consequences or the uncertain nature of their origin. Some reported single outbreaks of salmonellosis have been estimated to involve more than 200,000 people on the basis of the consumption of contaminated product. Most of these outbreaks involve either water or foods of animal origin.

In general this is due to the normal habitat of Salmonella, which is the intestinal tract of animals. The ability of certain animals to carry this organism yet remain virtually asymptomatic is a function of the particular host range of the Salmonella strain. Among the wide variety of Salmonella species, some are particularly linked to a given host whereas others are not “host adapted.”

The prevalence of Salmonella contamination of poultry is of primary concern as it affects human health. Estimates ranging as high as 100% have been reported for its contamination of poultry. As a consequence, significant efforts have been put forward to control contamination in flocks. One promising avenue is the administration of competitive microflora that exclude the colonization of the bird’s intestinal tract by Salmonella. However, the most effective measure for reducing salmonellosis in humans is educational, that is, making the consumer aware of proper handling techniques and the need to thoroughly cook poultry products.

3. Escherichia coli. Escherichia coli, similar to Salmonella, is a gramnegative bacterium. It can cause a number of different diseases depending on its complement of virulence factors and the immune status of the host that it infects. The most common types of disease caused by E. coli include enteropathogenic, enteroinvasive, enterotoxigenic, and enterohemorrhagic. In general the most common symptom is diarrhea, which when accompanied by blood is a vivid indication of E. coli. Similar to Salmonella, E. coli is also a normal inhabitant of the intestinal tract of animals making it ubiquitous in situations where fecal material might contaminate the environment. Few strains of E. coli are pathogenic and hence its mere detection in a food product is not necessarily unusual. However, it is a good indication of fecal contamination and E. coli along with coliforms and, more narrowly, fecal coliforms are common indicators of food safety.

Of recent note is the attention given to one serotype of E. coli, 0157:H7. Like Salmonella, E. coli can be classified by serotyping, which is dependent on the O and H antigens carried on the cell surface. This particular serotype has been associated with a number of recent food-poisoning outbreaks involving ground beef and apple cider. First recognized in 1981 when it was the causative agent in a foodborne outbreak involving ground beef, E. coli 0157:H7 has been the object of considerable attention in the United States.

One unexpected source of E. coli 0157:H7 has been unpasteurized apple cider and products formulated with unpasteurized apple juice. This food product was not assumed to be a likely source of enteric contamination due to its relatively low pH. Recent evidence has suggested that E. coli 0157:H7 may be more tolerant to low pH relative to other E. coli strains.