The initial rate of growth of Str. lactis and Str. cremoris in skim-milk depends upon the proportion of CO2 present in solution. When CO2 is constantly swept from the milk medium by a stream of CO2-free gas all strains show a very prolonged lag period. They eventually emerge from this lag period after more than 8 hr. and grow well enough to coagulate the milk within 24 hr. For optimal initial growth the lactic streptococci require the presence in solution in the milk of proportions of CO2 within the range 0·2 and 2·3% by volume. Some strains have a higher CO2 requirement within this range than others. Skim-milk after sterilization contains less than the optimum proportion of CO2 for many strains of lactic streptococci.

Yeast extract in a proportion of about 0·5% can substitute for CO2 in skim-milk.

The periodic inversion of cultures in plain skim-milk reduces the initial rate of growth of some of the streptococci, particularly those which form long chains. The reason for this is unknown. It does not occur when yeast extract is present.