Objective: This study assessed the anti-pathogenic potential of culture supernatants
from three probiotic strains viz. Lactobacillus plantarum, Bifidobacterium bifidum, and Enterococcus
faecium against five different gram-positive and gram-negative pathogenic bacteria.
Methods: The effect of probiotic culture supernatants on growth and production of quorum sensing
(QS)-regulated pigments was assessed through broth dilution assay. In vivo anti-infective efficacy
of these supernatants was evaluated by challenging the model nematode host Caenorhabditis
elegans with pathogenic bacteria pre-treated with probiotic culture supernatants, and worm survival
was monitored over a period of 8 days.
Results: All the probiotic culture supernatants (at 5-10 %v/v) exhibited bactericidal activity
against four of the test pathogens (including three multi-drug resistant gram-negative bacteria) and
they (at ≥ 1%v/v) could also modulate QS in pathogenic bacteria. Pre-treatment with culture supernatants
from all the three probiotic strains were able to attenuate virulence of four of the test
pathogens towards C. elegans. Worms challenged with supernatant-pre-treated pathogens recorded
75-100% better survival than those challenged with pathogens receiving no such pretreatment.
Virulence-attenuating effect of the culture supernatants was also carried forward to
daughter populations of the supernatant-treated parent pathogens. Lactic acid seemed to contribute
notably towards anti-virulence activity of the culture supernatants, but acidic pH did not seem to
be important for such activity.
Conclusion: Culture supernatants of probiotic strains used in this study can be said to possess appreciable
virulence-attenuating potential. Further investigation for characterization of these supernatants
and identification of the bioactive constituents is warranted. Elucidation of the molecular
basis associated with the anti-pathogenic potential of these supernatants may lead to the identification
of novel targets in pathogenic bacterial populations.