Phage type/biotype groups of Salmonella typhimurium in Scotland 1974-6: variation during spread of epidemic clones.

Biotyping by the scheme of Duguid et al. (1975) of 2010 cultures of Salmonella typhimurium received by the Scottish Salmonella Reference Laboratory in 1974--6, the definitive phage types of which were known, revealed 137 different phage type/biotype groups. Four major epidemic clones, comprising 52% of the cultures, were recognized: 1/2a, 49/26a, 56/17g and 141/9f. The sources of each of these four groups of strains were primarily bovine (587 cultures) and human (361), suggesting a close association between infections in the two hosts. Epidemiological evidence showed that most of the outbreaks were caused by cultures of a single phage type/biotype, suggesting that both phage typing and biotyping characters were usually stable in the course of spread of epidemic strains. Thirty-two of the 63 phage types contained strains of more than one biotype. Cultures from 11 of the phage types were of two or more closely related biotypes and those from 21 others were of unrelated or distantly related biotypes. The combined use of phage typing and biotyping made it possible to detect occasional variations in the phage type or biotype in epidemic clones during their spread, e.g. phage type 49 to 204, 56 to 193, 141 to 193 and biotype 2a to 10a, 9f to 9bf, or 9cf, 26a to 26f.