Effect of different initial pH on the growth of Streptococcus macedonicus and Streptococcus thermophilus strains

Streptococcus thermophilus and Streptococcus macedonicus are two known species of the genus related to foods, but little information is available for the latter species, introduced quite recently. Streptococci are widely used as starter cultures to rapidly lower food pH, thus inhibiting the development of deleterious and pathogenic microbes. We compared the growth kinetics over 24 h of nine S. thermophilus and eight S. macedonicus strains, isolated from various dairy environments, at three different pH values, namely 7.0, 6.0 and 5.5. The Gompertz model was used to estimate the three main growth parameters, namely lag phase duration (λ), maximum growth rate (μmax) and maximum cell number at the stationary phase (Nmax). Our results report significantly shorter lag phases and faster growth rates at all the pH conditions tested for S. macedonicus strains, thus indicating this species as a possible interesting substitute for S. thermophilus in selected food productions.