The differentiation of Streptococcus cremoris and Streptococcus lactis by means of bacteriophage action.

Many pure cultures of lactic streptococci have been isolated at this Institute during investigations of the various problems associated with the behaviour of cheese starters in commercial factories. These cultures have been derived mainly from commercial starters consisting of mixtures of several types of streptococci. The strains most suitable for use as 'single-strain starters' have proved to be those which do not ferment maltose and dextrin, which form long chains when grown in milk for 6—7 hr. at 37° C, and show little involution at this temperature. Consideration of these characteristics, along with other general physiological properties, has led us to classify these streptococci as strains of Str. cremoris (Orla Jensen). Recent isolations of streptococci from samples of sour milk have given an entirely different picture, characterized by the almost constant occurrence of predominantly diplococcal and short-chained types which ferment maltose and dextrin, and conform in a general way to the standard descriptions of the species Str. lactis. Thus at this Institute we have come to regard these two races as separate and distinct species. This division of species, however, has not been generally accepted by many investigators in the same field, and a review of the literature shows that some confusion exists as to the true identity of the lactic streptococci and their relationships to other groups or species of the genus Streptococcus. Hammer & Baker (1926), for example, do not consider Str. cremoris as a separate species and include in the ' Str. lactis group', ' any organism coagulating litmus milk rapidly or fairly rapidly with reduction of the litmus but without digestion or formation of gas, and which appears in' strains from milk as a Grampositive coccus arranged in chains or in pairs, the pairs rarely being grouped in clumps'. Hammer (1936) is further inclined to the view that divisions within the group should be made from a practical standpoint, e.g. the production of a malty flavour, ropiness, etc., rather than on morphological differences or fermentation reactions. Again, many of the reactions given by the 'milk-souring' streptococci are similar to those given by such closely related organisms as Str. faecalis and the Lancefield group D streptococci, sometimes termed enterococci, and consequently some difficulty arises in segregating the organisms into clearly defined species or groups. Sherman (1937) has drawn attention to these difficulties in an excellent review, and has suggested an approach to the classification of the streptococci as a whole. Recently, Shattock & Mattick (1943) have brought forward evidence on serological and biochemical grounds to show that Str. lactis can be clearly differentiated from Str. faecalis and other members of the group D streptococci. They have shown that Str. faecalis falls into Lancefield group D, and have assigned Str. lactis, along with Str. cremoris, to a new serological group 'N ' . Without presenting a historical survey of the literature, it may be fairly stated that, although there appears to be little doubt that Str. lactis is a distinct species, the claim for Str. cremoris remains unsettled. Having the opportunity to collect different phage races in connexion with cheese-starter problems in New Zealand, it seemed of value, in view of the general uncertainty of the relationship of the various species of streptococci, to ascertain whether phage reactions could help in the definition of species or groups.