Differences and Similarities Among Proteolytic and Nonproteolytic Strains of Clostridium botulinum Types A, B, E and F: A Review

Abstract

Cultures of Clostridium botulinum types A, B, E and F, which are responsible for human botulism, fall into two groups with different characteristics unrelated to toxin type. These groups differ primarily with respect to proteolysis, but also have different somatic and spore antigens and DNA; the heat resistance of their spores, their growth at low temperatures and their salt tolerance also differ. All known type A strains are proteolytic and all type E strains are non proteolytic, but types B and F have some proteolytic and some nonproteolytic strains. Although proteolytic strains can activate their own toxins, nonproteolytic strains cannot do so and therefore require trypsinization for maximum toxicity. Proteolytic strains are unable to grow at temperatures below 10 C, but have relatively high salt tolerance and spores of high heat resistance. Nonproteolytic strains can grow at 3.3 C and have a lower salt tolerance; their spores have a much lower heat resistance than those of proteolytic strains. Clostridium botulinum types A, B, E and F, which are responsible for human botulism, contain both proteolytic and nonproteolytic strains. Proteolysis and other phenotypic differences separate the strains of these four types into two distinct groups, unrelated to toxin type, i.e., immunologically identical toxins may be produced by both proteolytic and nonproteolytic strains. Smith (95) refers to these groups as group I and group II; the latest edition of Bergey's Manual ofDeterminativeBacteriology (14) also recognizes them, but without numerals. The two groups differ with respect to the need for potentiation of their toxins by proteolytic enzymes, their common somatic and spore antigens, their common DNA, the heat resistance of their spores, their ability to grow under refrigeration and their tolerance for salt. Van Ermengem (36,37), who first isolated the organism, described Bacillus botulinus as a rod with spores readily destroyed at 80 C. The second strain of "B. botulinus" to be isolated differed somewhat from van Ermengem's both culturally (41.59) and with respect to the immunological specificity of its toxin (61). Both of these strains were isolated in Europe. By the time antigenic differences in the toxins were assigned type designations, however, the two original strains had been lost and two other strains known to differ from one another, both isolated in the United States, were designated types A and B. All other strains tested at that time produced toxins corresponding to one or the other of these two types; they were also proteolytic strains isolated in the United States (16). Differences in the description of strains caused doubts about the validity of proteolytic activity as an attribute of C. botulinum by some workers and the toxigenicity of truly nonproteolytic strains by others. Standardization of media and methods and thorough study of a great number of pure cultures, including some obtained by single-cell isolation, showed that all type A strains were proteolytic, whereas type B strains were either proteolytic or nonproteolytic (8,46, 72). To date there are no known nonproteolytic strains of type A or proteolytic strains of type E; however, types B and F each have some proteolytic and some nonproteolytic strains. Since its low heat resistance indicated that the van Ermengem strain was probably nonproteolytic type B, the species name, C.