Serological properties of milk and their role in regard to the inhibition of certain strains of Streptococcus cremoris

Abstract

Serological aspects involved in the inhibition of certain strains of Streptococcus cremoris have been examined. The study has included an investigation into the presence of antibodies occurring naturally in milk as well as an examination of the effects of adding homologous antisera to milk containing either susceptible or resistant strains.
Whey agglutination tests showed that there were no antibodies present for the resistant strains, but tests carried out on susceptible strains were unsatisfactory because of the instability of the antigen.
A modified ring test gave positive results for the susceptible strains but was negative for the resistant strains. As the susceptible strains were found to be rough variants and the resistant ones were smooth, it is considered that the positive tests were due to non-specific
agglutination.
Microscopic examination revealed that all susceptible strains were agglutinated in both pasteurized wholemilk and skim-milk. Only the long-chain resistant strains, however, showed evidence of agglutination. Long-chain susceptible strains exhibited a tendency to agglutinate in sterilized milk, but short-chain susceptible strains did not. Such agglutination in itself had little effect on acidity production.
Addition of homologous antiserum to pasteurized milk inoculated with a resistant strain resulted in it behaving in all respects like a susceptible strain in pasteurized milk.
By adding homologous antibody to recombined milk (sterile skim-milk and heated cream) inoculated with a susceptible strain, it was shown that the addition of specific antibody caused the susceptible strain to react in the same way as in whole separated milk. However, in this case the resulting inhibition was not quite so marked.
Inoculation of susceptible strains into sterile skim-milk containing cream washings gave results which showed that the inhibitory property was transferable. It was also found that when a short-chain susceptible strain was used there was no evidence of any agglutination of the starter cells.
Conclusions have been drawn from the data obtained and an explanation for the inhibition of the susceptible strains has been advanced. It appears that both skim-milk and cream fractions donate factors which separately cause inhibition of acidity production by the susceptible strains but both these factors are required for maximum inhibition.