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A new collection of phages for typing methicillin-resistant S. aureus (MRSA) is proposed. The collection includes phage 85 (modified by the MRSA strain), capable of selecting strains with the similar specificity of the restriction-modification system, and 9 MRSA-induced phages. The latter differentiate MRSA strains according to the specificity of prophages present in bacterial cells. The use of this phage collection has permitted the typing of MRSA strains insensitive to the phages of the international collection. Among these cultures an epidemic strain has been detected and the source of its spread in the burn center has been established.

A new approach to using experimental phages for typing methicillin-resistant S. aureus (MRSA) non-sensitive to the phages of International Basic Set (IBS) is described. The collection Includes phage 85, modified on a culture of MRSA, and 5 phages induced from MRSA strains isolated in clinics of Moscow in 1975-76. Firstly, the modified phage selects cultures according to the specific character of its restriction-modification system, then the induced phages differentiate the selected strains into 5 groups (1, 2, 3, 4, 5) based on the specificity of the prophages they contain. Group 1 strains can further be differentiated into 5 subgroups (A, B, C, D, E) by additional phages. Forty-one MRSA strains isolated in 1987-90 in various hospitals of Moscow showing no sensitivity to IBS phages, were lysed by the modified phage, 15 of them belonging to Group 2 and isolated in the traumatological hospital, 26 belonging to Group 1 and were circulating in the burn center. Twenty-three strains of Group 1 appertain to subgroup 1B and were isolated over a 4-year period from the burned surface of patients and from the throat of a medical staff carrier.

The possibility of using phages, isolated from the lysogenic cultures of methicillin-resistant staphylococci and modified in methicillin-resistant cultures of phage 85, for the differentiation of nontypable staphylococcal strains has been studied. The variants of phage 85 cannot be used for the determination of differences between the strains of methicillin-resistant cultures; they are not suited for typing. For this purpose the collection of phages isolated from lysogenic methicillin-resistant cultures should be used.