Inhibition of pathogen growth by the culture supernatants of L. amylovorus
The filter-sterilized culture supernatants of the L. amylovorus strains were assayed for their abilities to inhibit the growth of various intestinal pathogens (Figure 4). All the supernatants markedly inhibited the growth of the test pathogens. For instance, the supernatants of the strains DSM 20531T and GRL 1117 reduced the growth of F4-fimbriated E.coli by more than 100 000-fold and the growth of Salmonella typhimurium almost by a factor of 10 000. The growth of F4-fimbriated E. coli was most efficiently inhibited. The pH values of the supernatants varied from 3.8 to 4.5. It is notable that the reductions in pathogen counts inversely correlated with the pH values of the supernatants (Figure 4), and culture supernatants which had been adjusted to the pH of plain MRS lowered the pathogen counts by much less than tenfold (data not shown), indicating that the inhibition was mainly due to the low pH associated with lactic acid production.
Figure 4  Reductions in pathogen counts by L. amylovous culture supernatants. Six different swine intestinal pathogens were grown in TSB medium in the presence of the filter-sterilized supernatants of the L. amylovorus strains, and the reductions in pathogen counts, expressed as log CFU values, were estimated from the area reduction percentages (ARPs) of the pathogen growth curves by linear regression. The average pH values of the supernatants are shown above the histograms. The results are the means and standard deviations of three independent experiments, each performed with fresh culture supernatants with three technical replicates.

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