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Aims: To use antioxidative activity and antagonistic properties of lactobacilli against selected pathogens and members of the normal microflora as a basis for screening probiotic candidates. Methods and Results: Antagonistic activity of lactobacilli against target bacteria in both microaerobic and anaerobic environments was tested. Production of antagonistic metabolites (ethanol, hydrogen peroxide (H 2 O 2 ), acetic, lactic and succinic acid) by lactobacilli as well as their total antioxidative activity were assessed. In general, the lactobacilli tested were most effective against Gram-negative bacteria and their antagonistic activity was strainspecific. However, obligately heterofermentative lactobacilli had the strongest activity when tested in a microaerobic environment. Additionally, facultatively heterofermentative lactobacilli were equally effective in either milieu and produced significant levels of acetic and lactic acid. Moreover, obligately homofermentative lactobacilli had high H 2 O 2 production and total antioxidative activity but weak antagonistic activity. Conclusions: Antioxidative and antagonistic activity of intestinal lactobacilli is strain-specific but typically can be related to their fermentation type which may be used for rapidly screening large numbers of lactobacilli for probiotic candidates. Significance and Impact of the Study: This study represents the first report on the utilization of group characteristics to screen lactobacilli intended for specific probiotic use. Such uses include the targeting of particular gut niches and pathogens as well as allowing for long-term benefits to the host.

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Indigenous intestinal microflora (including lactobacilli) has an important role in protection against Clostridium difficile infection. To assess in vitro interaction between lactobacilli and C. difficile, antagonistic activity of 50 intestinal Lactobacillus spp. strains against 23 pathogenic C. difficile strains was determined. Phenotypic properties of C. difficile strains [production of short-chain fatty acids (SCFAs) and toxin A, and antimicrobial susceptibility] and lactobacilli (production of SCFAs and H 2 O 2 ) were investigated. Five lactobacilli (Lactobacillus paracasei and Lactobacillus plantarum species) were antagonistic to all C. difficile strains, 18 were antagonistic to some C. difficile strains and 27 showed no antagonistic activity. This antagonistic activity was strain-specific and seemed to correlate with H 2 O 2 and lactic acid production. C. difficile strains that were more sensitive to lactobacilli (n ¼ 9) usually produced higher toxin levels and more SCFAs, and were more resistant to antibiotics, than strains that were resistant to lactobacilli (n ¼ 14). Compatibility of C. difficile strain properties (resistance to lactobacilli or antibiotics) with intestinal microecological conditions (e.g. presence of antagonistic lactobacilli, concentration of antibiotics) may determine expression of disease.

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