Chapter 12: Clostridium botulinum

The following information on Clostridium botulinum is from Solomon and Lilly (1998).

C. botulinum is an anaerobic, rod-shaped sporeformer that produces a protein with characteristic neurotoxicity. Botulism, a severe food poisoning, results from ingestion of food containing botulinal toxin produced during the growth of these organisms in food. Although this food poisoning is rare, the mortality rate is high; the 962 recorded botulism outbreaks in the United States from 1899 to 1990 (CDC, 1979) involved 2320 cases and 1036 deaths. In outbreaks in which the toxin type was determined, 384 were caused by type A, 106 by type B, 105 by type E, and 3 by type F.

In two outbreaks, the foods implicated contained both types A and B toxins. The limited number of reports of C and D toxins as the causative agent of human botulism have not been generally accepted. However, all types except F and G, which have not been as thoroughly studied, are important causes of animal botulism.

Antigenic types of C. botulinum are identified by complete neutralization of their toxins by the homologous antitoxin; cross-neutralization by heterologous antitoxins does not occur or is minimal. There are seven recognized antigenic types: A, B, C, D, E, F, and G. Cultures of five of these types apparently produce only one type of toxin but all are given type designations corresponding to their toxin production. Types C and D cross-react with antitoxins to each other because they each produce more than one toxin and have at least one common toxin component. Type C produces predominantly C₁ toxin with lesser amounts of D and C₂, or only C₂, and type D produces predominantly type D toxin along with smaller amounts of C₁ and C₂. Mixed toxin production by a single strain of C. botulinum may be more common than previously realized. There is a slight reciprocal cross-neutralization with types E and F, and recently a strain of C. botulinum was shown to produce a mixture of predominantly type A toxin, with a small amount of type F.

Aside from toxin type, C. botulinum can be differentiated into general groups on the basis of cultural, biochemical, and physiological characteristics. Cultures producing types C and D toxins are not proteolytic on coagulated egg white or meat and have a common metabolic pattern which sets them apart from the others. All cultures that produce type A toxin and some that produce B and F toxins are proteolytic. All type E strains and the remaining B and F strains are nonproteolytic, with carbohydrate metabolic patterns differing from the C and D nonproteolytic groups. Strains that produce type G toxin have not been studied in sufficient detail for effective and satisfactory characterization.

C. botulinum is widely distributed in soils and in sediments of oceans and lakes. The finding of type E in aquatic environments by many investigators correlates with cases of type E botulism that were traced to contaminated fish or other seafoods. Types A and B are most commonly encountered in foods subjected to soil contamination. In the United States, home-canned vegetables are most commonly contaminated with types A and B, but in Europe, meat products have also been important vehicles of food-borne illness caused by these types.

Optimum temperature for growth and toxin production of proteolytic strains is close to 35ºC; for nonproteolytic strains, it is 26-28ºC. Nonproteolytic types B, E, and F can produce toxin at refrigeration temperatures (3-4ºC). Toxins of the nonproteolytics do not manifest maximum potential toxicity until they are activated with trypsin; toxins of the proteolytics generally occur in fully (or close to fully) activated form. These and other differences can be important in epidemiological and laboratory considerations of botulism outbreaks. Clinical diagnosis of botulism is most effectively confirmed by identifying botulinal toxin in the blood, feces, or vomitus of the patient. Specimens must be collected before botulinal antitoxin is administered to the patient. Identifying the causative food is most important in preventing additional cases of botulism. See Examination of Canned Foods, Chapter 5.

Botulism in infants 6 weeks to 1 year of age was first recognized as a distinct clinical entity in 1976. This form of botulism results from growth and toxin production by C. botulinum within the intestinal tract of infants rather than from ingestion of preformed toxin. It is usually caused by C. botulinum types A or B, but a few cases have been caused by other types. Infant botulism has been diagnosed in most U.S. states and in every populated continent except Africa (Arnon, 1987).

Constipation almost always occurs in infant botulism and usually precedes characteristic signs of neuromuscular paralysis by a few days or weeks. There is a broad range of severity of illness. Some infants show only mild weakness, lethargy, and reduced feeding and do not require hospitalization. Many have shown more severe symptoms such as weakened suck, swallowing, and cry; generalized muscle weakness; and diminished gag reflex with a pooling of oral secretions. Generalized muscle weakness and loss of head control in some infants reaches such a degree of severity that the patient appears "floppy." In some hospitalized cases, respiratory arrest has occurred, but most were successfully resuscitated, and with intense supportive care have ultimately recovered. As a result, the case-fatality rate (2%) for this form of botulism is low. Recovery usually requires at least several weeks of hospitalization (Arnon, 1987).

Honey, a known source of C. botulinum spores, has been implicated in some cases of infant botulism. In studies of honey, up to 13% of the test samples contained low numbers of C. botulinum spores (Hauschild et al., 1988). For this reason, the FDA, the Centers for Disease Control and Prevention (CDC), and the American Academy of Pediatrics recommend not feeding honey to infants under the age of 1 year.

Measures to prevent botulism include reduction of the microbial contamination level, acidification, reduction of moisture level, and whenever possible, destruction of all botulinal spores in the food. Heat processing is the most common method of destruction. Properly processed canned foods will not contain viable C. botulinum. Home-canned foods are more often a source of botulism than are commercially canned foods, which probably reflects the commercial canners' great awareness and better control of the required heat treatment.

A food may contain viable C. botulinum and still not be capable of causing botulism. If the organisms do not grow, no toxin is produced. Although many foods satisfy the nutritional requirements for the growth of C. botulinum, not all of them provide the necessary anaerobic conditions. Both nutritional and anaerobic requirements are supplied by many canned foods and by various meat and fish products. Growth in otherwise suitable foods can be prevented if the product, naturally or by design, is acidic (of low pH), has low water activity, a high concentration of NaCl, an inhibitory concentration of NaNO₂ or other preservative, or two or more of these conditions in combination. Refrigeration will not prevent growth and toxin formation by nonproteolytic strains unless the temperature is precisely controlled and kept below 3ºC. Foods processed to prevent spoilage but not usually refrigerated are the most common vehicles of botulism (Solomon and Lilly, 1998).