AMONG the bacteria there are many instances where growth of one organism renders the medium unfavourable for growth of some other type of organism; but cases where an organism militates against growth of other strains of its own species, except by accumulation of the normal end-products of growth, are rare. The following account deals with an investigation into the biochemistry of two strains of lactic streptococci which produce during their growth some substance which very strongly inhibits the growth of other strains of lactic streptococci. The organisms were isolated from milk in the course of an investigation of trouble occurring during the manufacture of Cheddar cheese in a dairy factory [Whitehead and Riddet, 1933]. As is well known, development of lactic acid in the milk and curd is an essential part of the process of cheese manufacture, and the formation of acid can occur only if the lactic streptococci which are added as a " starter" culture grow readily in the milk and ferment the lactose. In the particular case under investigation most of the milk received at the dairy factory readily supported the growth of lactic streptococci and hence gave no trouble to the cheese-maker; but the milk from one particular source of supply proved entirely abnormal in that very little acid could be developed in it. The inhibitory action of the milk was so marked as to suggest the presence of some strong disinfectant, but none could be detected. Moreover, the bacteria present in the milk as contaminants were quite active, since added methylene blue was rapidly decolorised. The various types of bacteria present in the milk were isolated in pure culture, and it was found that there were present in large numbers some Gram positive cocci which, when allowed to grow in any sample of normal milk, rendered it unsuitable for the subsequent growth of lactic streptococci and hence prevented the development of acidity. Even when the cocci were destroyed by heat after they had grown for twelve or twenty-four hours the milk still retained its inhibitory power. Thus it became evident that the phenomenon was not due to an antibiosis between living organisms but to the production of some heat-stable substance which was inhibitory towards lactic streptococci.Later, another Gram positive coccus which produced a similar effect was isolated from a sample of milk obtained from quite a different locality. There was a slight difference in this case in that the coccus itself produced appreciable quantities of acid. Thus milk in which this organism was growing appeared to allow acid production by lactic streptococci to proceed quite normally. When however the milk was pasteurised or sterilised, lactic streptococci were strongly inhibited just as was the case in the first example. Biochem. 1933 xxvii 113

The occurrence and isolation of bacteriophages from cultures of lactic streptococci (“starter cultures”) is described. Aeration of the milk in which the phage-infected cultures are grown is shown to have a stimulating action on the development of the phage. The characteristics of phages for lactic streptococci are described.

No abstract

Cheesemaking experiments were carried out to investigate the moisture-retaining characteristics of curd made from milks of varying fat and casein content. The results indicate:(a) That milk fat helps to retain moisture in a cheese curd. The higher the proportion of fat present the more drastic the treatment required in the cheese vat to reduce the moisture content of the finished cheese to the desired level. A higher cooking temperature, more dry-stirring of the curd and a larger proportion of salt are all necessary with rise in fat content of the curd.(b) That curd formed from milk of a low-casein content (from Friesian cows) retains moisture in the cheesemaking process more tenaciously than curd from milk of a highcasein content (from Jersey cows).

It is now generally recognized that' slowness' in cheese manufacture is commonly due to destruction of the acid-producing starter streptococci in the milk and curd by bacteriophage. Methods for preventing the phage infection in commercial practice are still being worked out, and general agreement has not yet been reacted among workers in this field although basic principles are well established. Apparently the tendency in England (l, 2) is to use starters containing a mixture of several strains of streptococci (mixed starters) and, guided by a 'phage-development test' on the cheese whey, to change the starter in use as soon as a significant concentration of phage for the starter appears in the cheese vats. In New Zealand, on the other hand, the tendency is towards the use of 'singlestrain ' starters because of their greater and more regular activity and their capacity to produce a cheese of closer texture. Wherever one of the few stock single-strain cultures is in use, phage for the starter is constantly present in the cheese whey, but experience has shown that this has no untoward effect provided that the concentration is kept below a certain level, by maintaining the starter itself free from phage and by sterilization of the factory and farm utensils. Even where one of the stock cultures has not been used in a given factory for several months the specific phage is present (presumably in dormant form) in the surroundings, and it rarely fails to appear in the whey if not on the first then on the second day after the starter is used again. It seems to us, therefore, that a large number of unrelated mixed starters would be required under the English system if the whey is to be kept entirely free from phage. It will be interesting to see whether the system can be maintained on its present basis in England over several cheese-making seasons.Where a starter has been in use in a cheese factory for a lengthy period there is invariably a low and harmless concentration of phage daily in the whey, which is extremely difficult to eliminate. It is probably due to air-borne contamination of the milk while the vats are being filled. Apart from this there are three possible sources for the dangerously high concentrations which sometimes occur:(i) The starter culture itself.(ii) The surfaces of factory utensils and equipment.(iii) The milk delivered to the factory. The first two can be dealt with by isolation and protection of the starter (3) and by sterilization of equipment by heat or disinfectants. The third source exists in New Zealand dairy practice because farmers collect cheese whey from the factories in the cans in which they deliver the milk and sometimes fail to sterilize the cans completely before they are used again for milk. If special arrangements had to be made for the transport of whey back to farms, the pig-rearing industry might be rendered uneconomic. Farmers who deliver only a small amount of milk are able to collect whey in special