Penicillinase production by Staphylococcus aureus strains from outbreaks of food poisoning.

Abstract

In the course of the routine examination of strains of Staphylococcus aureus isolated from outbreaks of staphylococcal food poisoning in north-west England, we were surprised to find that most of them were penicillin resistant. Nine of 11 strains, all present in large numbers in foods that had caused clinically typical outbreaks of vomiting and diarrhoea, were penicillin resistant. None of the outbreaks had occurred in a hospital. This finding was particularly interesting because, when peniciflin-resistant staphylococci first became common in hospitals, the resistant strains were found to be mainly members of phage III, the to which most enterotoxic staphylococci also belong. There are several scattered references to the incrimination of penicillin-resistant staphylococci as the cause of outbreaks of food poisoning in the years before penicillin had come into general therapeutic use. Rutherford and Crowson (1945) described an outbreak of food poisoning in Canada in 1945, in which the organism responsible was resistant to 12.5 units of penicillin in a tube test. Mason (1945) examined six well-known foodpoisoning strains obtained from other workers and found that three of them were completely resistant when tested by the cylinder plate method. Segalove (1947) tested a collection of 15 wellauthenticated enterotoxic strains from the collection maintained at the University of Chicago. All were known to have had no previous contact with penicillin in the laboratory, and most had been isolated before 1941. He used a tube dilution test with an inoculum of 0.1 ml. of a 1/100 dilution of a 24-hour culture in a semi-synthetic liquid medium. Seven of the strains were inhibited by 0.1 unit per ml. of penicillin or less, and eight required 10 units or more for inhibition. Allison (1949) first noted the common association with outbreaks of food poisoning of Staph. aureus strains lysed by certain phages. He examined 47 cultures, including 26 isolated in this country and 21 from the United States, Canada, Egypt, and the Sudan, and found that 38 of them were lysed by phages of the 6/47 (including phage 42D). Others (Saint-Martin, Charest, and Desranleau, 1951 ; Williams, Rippon, and Dowsett, 1953; Parker, 1953) subsequently confirmed these findings. In 1953 Williams et al. proposed the classification of Staph. aureus into three broad phage groups, and included in III strains lysed by one or more of the phages 6, 7, 47, 53, 54, 70, 73, 75, 77, 42D, and 42E. The association of strains of III so defined with food poisoning has become even closer than that of Allison's 6/47 group since the inclusion of further III phages in the typing scheme and the introduction of the practice of testing apparently non-typable cultures with phage at 1,000 x the routine test dilution. Members of phage III form a considerable proportion of all strains isolated from human sources-between 10% and 50% according to the type of material sampled-but few of them react with phage 42D, and strains lysed only by 42D at the routine test dilution are quite rare. For example, we encountered only one pure 42D strain in a series of 1,494 human cultures. Such strains are, however, common in material of bovine origin and appear to be a frequent cause of mastitis of cattle (Macdonald, 1946; Smith, 1948; Price, Neave, Rippon, and Williams, 1954). It has, therefore, been proposed (see Rippon, 1956) that type 42D strains should be removed from phage III to a new provisional IV.