Agnes A. Nichols

387. Bacteriophage in typing lactic streptococci

1. The present position in the problem of ‘slow’ starter due to phage lysis has been reviewed. 2. The sources of some 450 strains of lactic streptococci, collected from a wide field, are given. The methods used in their selection for use as starter together with storage conditions, suitable to maintain their activity, have been discussed. 3. The sources from which phages for the lactic streptococci have been obtained and the methods of ‘building up’ a phage of high titre from dairy products, have been discussed. Attempts have been made to isolate phages from pig-faeces, after feeding starter strains to pigs, to adapt phages to strains previously unattacked and to establish interrelationship of the phages and strains of lactic streptococci with phages and strains of enterococci. 4. The strains of lactic streptococci have been classified in eleven phage types by modifying a method described for the phage typing of staphylococci. Identification of a type has been based on the reactions of a ‘phage pattern’ and not on the reaction of a single phage. It has not been possible to adapt one phage to lyse all the strains of one type and to replace the several phages on which type identification originally depended but, adapted phages have indicated the subdivision of some types. 5. The application of the ‘phage-resistant carrier’ strain phenomenon to phage typing has been investigated. The results have helped in the typing of some strains. 6. Phages used in typing the strains have been classified by means of antiphage sera, prepared from a selection of the test phages. On the data presented the majority of the phages were divided into three groups and the results have been discussed with reference to the phage types suggested. 7. The significance of phage typing, adaptation of phage and phage carrying in the selection of starter strains has been considered and the best method of applying this information to the control of ‘slowness’ in cheese factories has been discussed.

373. Inhibitory strains of lactic streptococci and their significance in the selection of cultures for starter

1. Inhibitory strains of lactic streptococci have been encountered amongst strains of Str. cremoris isolated from starter and amongst strains of Str. lactis, isolated from sour milk, which had otherwise been regarded as suitable for cheesemaking. 2. Methods are described for determining the concentration of inhibitory substance produced and the effect of these inhibitory strains on a wide selection of lactic streptococci. 3. The significance of the presence of strains of lactic streptococci having inhibitory properties, if incorporated in starters, particularly when the strains are prepared separately and mixed in the vat, is discussed. Recommendations for the exclusion of inhibitory strains from such starter mixtures are made.

374. The identity of streptococci from starter and of streptococci, suitable for use as starter, isolated from sour milk

1. 277 strains of lactic streptococci isolated from commercial starters and 72 ‘wild’ strains from souring milk, suitable for cheesemaking, have been identified. All the starter strains examined were Str. cremoris and those (with two possible exceptions) from sour milk were Str. lactis. 2. Since the strains from each source were so similar, the information obtained did not help in the selection of the strains, within each group, most suitable for starter. 3. It is suggested that three differential tests are sufficient to distinguish Str. lactis and Str. cremoris and to identify them when isolating strains suitable for starter.

324. The heat resistance of the bacteriophages of cheese starter with observations on the estimation of phage concentration

The heat resistance, in milk, of bacteriophages attacking strains of lactic streptococci isolated from cheese starters has been determined. Active phages did not usually survive a temperature of 75°C. for 7½ min. but many phages were not destroyed by heating for 50–60 min. at 65–67°C. and most survived at 70°C. for 10–15 min. The practical implications of these findings in the ‘sterilization’ of milk for starter making have been discussed. Growth in yeast dextrose broth was found to be superior to methylene blue reduction for the demonstration of residual phages after heating.

323. Observations on cheese starters with reference to bacteriophage and the phageorganism relationships of strains isolated

1. Frequent examination of the mother and bulk starters from several cheese-making creameries during the 1942 season has shown bacteriophage to be prevalent in some factories and apparently absent from others. Outbreaks of phage attack could not be correlated with heat treatment of the cheese milk, and were geographically widespread over the country. 2. The phage-organism relationships of these starter strains and their phage races were studied, and on this basis it was possible to arrive at the relationship and in some cases the identity of some of the strains circulating in commerce. It was found that for the isolation of some phaging strains, milk was a more sensitive medium than broth. Commercial starters appear to be composed of more than one distinct strain, and no starter can be judged by the reactions of one of its component strains. The nascent phage phenomenon was found not to be common in the selected starter strains. The reaction itself may, however, possibly be used to demonstrate fundamental relationships between strains.