Lysis of lysogenic strains of Lactobacillus helveticus in Swiss cheeses and first evidence of concomitant Streptococcus thermophilus lysis
Résumé
Lactobacillus helveticus is used as starter in Swiss cheese manufacture. To characterize the technological behavior of lysogenic L.helveticus , 21 strains were screened for prophage inducibility using mitomycin C. Eight out of the 11 inducible strains, i.e. CNRZ 32, CNRZ 243, CNRZ 303, IL1, T649, T650, T651, T653, were assayed in experimental Swiss cheeses, using the same Streptococcus thermophilus and Propionibacterium freudenreichii industrial starters. Six strains, CNRZ 32, CNRZ 243, CNRZ 303, T649, T650,
and T651 grew well during pressing, exhausting galactose as expected. They lysed then extensively and early during the ripening as shown by the detection of d-Lactate dehydrogenase in the cheese aqueous extracts. Phages were detected at day 1 in four cases out of six, indicating a spontaneous release in situ.
Except CNRZ 32, renaturing SDS-PAGE of the cheese extracts revealed the presence of a lytic activity (30kDa). For strains IL1 and T653, growth was not sufficient during pressing leading to a higher pH (5.4 versus
5.2) and to residual galactose (about 600mg/100g) that had a strong impact on the subsequent growth of propionibacteria and non-starter lactic acid bacteria. No free d-Lactate dehydrogenase, no phages and no lytic activity were detected at day 1. Both strains lysed later between days 20 and 36 concomitantly with galactose exhaustion. Interestingly, decrease of viability and lysis of S. thermophilus during the ripening were observed and were correlated to those of the L. helveticus strain, except for CNRZ 32. Thus, lysis of L. helveticus
can seemingly induce S. thermophilus death and lysis during ripening. Such an in situ interaction, whatever the mechanism involved, was further supported by scanning electron microscopy showing large mixed colonies of
L. helveticus and S. thermophilus.