The routine methods for propagation of staphylococcal typing bacteriophages, and for their use in identifying strains of staphylococci, are described.Most of the phages can be propagated in fluid media as well as on agar, and for some glucose-peptone-water is a better medium than nutrient or Todd-Hewitt broth.Many phage filtrates derived from broth or agar propagation contain, in addition to the phage, an agent that inhibits the growth of staphylococci.Investigations of variations in the routine typing technique showed that considerable latitude is permissible in the age of cultures used for typing and in the inoculation procedures. It is, however, important to test all phage filtrates after propagation for purity and freedom from the inhibitory agent; and to repeat the tests frequently during use to detect alterations in titre.About 40% of staphylococci are not lysed by the phages used at their test dilution; about half of these untypable strains are lysed by undiluted phage filtrates.An analysis was made of the results of typing 567 independent strains of staphylococci; 229 of these showed strong lysis by one or more phages, but there were no fewer than 82 distinct phage patterns represented, and only 118 strains were lysed strongly by a single phage. Certain phages tend to appear together in patterns and on the basis of such associations three main phage groups can be distinguished; they are known respectively as the ‘3A’, ‘6/47’ and ‘52’ groups.No method was discovered for segregating patterns into ‘types’, but conventions have been devised on the basis of the variation observed in sets of strains from various sources, for distinguishing between different patterns.We are deeply indebted to Dr V. D. Allison for teaching us the bacteriophage typing methods and for giving us the benefit of his wide experience.We are grateful to the following for sending us phages, and in some cases discussing their results with us: Dr Phyllis Rountree, Sydney; Dr R. Wahl, Paris; Dr G. Wallmark, Stockholm; Dr H. Williams Smith, Poultry Research Station, and Mr A. M. Hood, Birmingham.Our thanks are also due to Miss S. Mayo, Mrs E. Lyons and Mr D. Woodroof for technical assistance. The photographs for PI. 14 were taken by Mr W. Clifford.

Staphylococcal phages may be divided into three broad divisions:(1) Phages lysing coagulase-positive staphylococci with a restricted host range. Phages in this division may be further divided into five lytic groups I, II, III, IV and Miscellaneous.(2) ‘Polyvalent’ phages; phages with a wide host range among coagulase-positive cocci, and sometimes also active on coagulase-negative strains.(3) Phages lysing only coagulase-negative staphylococci.Within each division the phages can be further grouped by their serological reactions. Nine serological groups have been defined. Phages of any single serological group also share other characteristics, e.g. stability, and the ability to form lysogenic systems.

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SUMMARY : Three temperate phages isolated from lysogenic staphylococci of phage type 52/52A/80 were used to lysogenize strains of phage-type 80. Two of these phages belonged to serological group A and one to serological group F. Lysogenization of a strain of phage-type 80 with one of the phages (287') resulted in a change in the typing pattern from 80/81 to 52/52A/80. There was, therefore, a gain in sensitivity to phage 52 and 52A and a loss in sensitivity to phage 81. Lysogenization of type 80 strains with the second phage (581') caused a loss in sensitivity to phage 81, while lysogenization with phage 7287' caused a gain in sensitivity to phage 52 and 52A.Loss in sensitivity to phage 81 after lysogenization with phage 287' seems most probably due to specific prophage immunity since phages 287' and 81 belong to the same serological group (A) and may be closely related. Loss in sensitivity to phage 81 after lysogenization with phage 581' does not appear to be a case of specific prophage immunity since the two phages are serologically distinct and the phenomenon resembles examples of prophage interference observed in other organisms.Rountree & Freeman (1955) described a number of hospital epidemics in Australia caused by a strain of staphylococcus which was almost completely resistant to the routine typing phages then in use. However, the two group-I phages 52 and 52A produced a few plaques when applied undiluted to a lawn of the organisms, and phage 52A was 'adapted' to this new type of staphylococcus. The adapted phage is now known as phage 80 and strains lysed specifically by this phage as phage-type 80. A second phage known as 81 was isolated independently in Canada (Bynoe, Elder & Comtois, 1956); it is capable of lysing the majority of phage-type 80 strains. Such strains are therefore more accurately described as phage-type 80/81. Since, however, phage 81 is not included in the set of 21 basic phages used in Great Britain, the strains are usually reported simply as phage-type 80. The 81 reaction will be referred to in this paper only when relevant.Since the original description by Rountree & Freeman many epidemics caused by staphylococci of phage-type 80 have been described in different countries. During the past two or three years it has been noticed in this laboratory that, in some prolonged epidemics caused by phage-type 80, strains giving the typing pattern 52/52 A/80 were also isolated, particularly during the later part of the epidemic. The 'classical' phage-type 80 coccus is lysed only by phage 80 a t the routine test dilution (RTD), using the basic set of

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