A differential medium for lactic acid-producing bacteria in a mixed culture

Culture media for differential isolation of Lactobacillus casei Shirota from oral samples

This study aimed to develop a solid culture medium for differential isolation of the probiotic strain Lactobacillus casei Shirota (LcS) and for selective cultivation of lactobacilli present in oral samples. Type strains of lactobacilli and isolates from commercial probiotic products were inoculated onto modified de Man Rogosa Sharpe agar (termed 'LcS Select'), containing bromophenol blue pH indicator, vancomycin and reducing agent L-cysteine hydrochloride for differential colony morphology development. L. casei Shirota cultured on the novel medium produced distinctive colony morphologies, different from other lactobacilli tested. LcS-characteristic colonies were recovered on LcS Select medium from samples of saliva and tongue plaque following a four-week probiotic intervention study. The viable count of presumptive LcS colonies correlated with those isolated on a non-commercial lactitol-LBS-vancomycin agar (LLV) developed for a selective isolation of LcS from faeces. The novel LcS Select medium proved suitable for differential isolation of the probiotic strain L. casei Shirota from oral samples containing mixed microbial populations. It can also be used for selective growth of vancomycin-resistant lactobacilli. There are few available culture media that are sufficiently selective to enable isolation of probiotic strains from mixed populations. LcS Select medium provides a cheaper, yet effective tool in this context.

Improved viability of bifidobacteria in fermented milk by cocultivation with Lactococcus lactis subspecies lactis

The poor survival of probiotic bacteria in commercial yogurts may limit their potential to exert health benefits in humans. The objective was to improve the survival of bifidobacteria in fermented milk. Cocultivation with some strains of Lactococcus lactis ssp. lactis improved the survival of bifidobacteria in fermented milk during refrigerated storage. Studies on one strain, Lc. lactis ssp. lactis MCC866, showed that the concentrations of dissolved oxygen were kept lower in the cocultivated fermented milk during storage compared with monocultured Bifidobacterium longum BB536 or samples cocultured with another noneffective Lc. lactis ssp. lactis strain. Degradation of genomic DNA was suppressed in the cocultivating system with Lc. lactis ssp. lactis MCC866. Several genes that participated in protection from active oxygen species (e.g., genes coding for alkyl hydroperoxide reductase and Fe(2+) transport system) were expressed at higher levels during refrigerated storage in Lc. lactis ssp. lactis MCC 866 compared with another noneffective Lc. lactis ssp. lactis strain. Concentration of free iron ion was also lower in supernatants of fermented milk cocultivated with B. longum BB536 and Lc. lactis ssp. lactis MCC866. These results suggest that Lc. lactis ssp. lactis MCC 866 is potentially superior in reducing oxygen damage and consequently improves the survival of bifidobacteria in the cocultivating system. This cocultivation system is of industrial interest for producing fermented milk containing viable bifidobacteria with long shelf life.