Abstract
Legionella pneumophila is an organism of public health interest for its presence in water supply systems and other humid thermal habitats. In this study, ten cell-free supernatants produced by Lactobacillus strains were evaluated for their ability to inhibit L. pneumophila strains isolated from hot tap water. Production of antimicrobial substances by Lactobacillus strains were assessed by agar well diffusion test on BCYE agar plates pre-inoculated with L. pneumophila. Cell-free supernatants (CFS) showed antimicrobial activity against all Legionella strains tested: L. rhamnosus and L. salivarius showed the highest activity. By means of a proton-based nuclear magnetic resonance (1H-NMR) spectroscopy, we detected and quantified the Lactobacillus metabolites of these CFSs, so to gain information about which metabolic pathway was likely to be connected to the observed inhibition activity. A panel of metabolites with variations in concentration were revealed, but considerable differences among inter-species were not showed as reported in a similar work by Foschi et al. (2018). More than fifty molecules belonging mainly to the groups of amino acids, organic acids, monosaccharides, ketones, and alcohols were identified in the metabolome. Significant differences were recorded comparing the metabolites found in the supernatants of strains grown in MRS with glycerol and the same strains grown in MRS without supplements. Indeed, pathway analysis revealed that glycine, serine and threonine, pyruvate, and sulfur metabolic pathways had a higher impact when strains were grown in MRS medium with a supplement such as glycerol. Among the metabolites identified, many were amino acids, suggesting the possible presence of bacteriocins which could be linked to the anti-Legionella activity shown by cell-free supernatants.
Highlights
Lactic acid bacteria (LAB) are commensal bacteria widely studied for their postbiotic products, a term which indicates the majority of the metabolites referring to soluble factors secreted by bacteria during their cycle-life and/or released after membrane lysis (Aguilar-Toala et al, 2018)
For the evaluation of antimicrobial activity, agar well-diffusion test bioassay was performed with Cell-free supernatants (CFS) obtained from the growth in De Man (MRS) with and without supplement (Fuochi, 2016; Clinical and Laboratory Standards Institute [CLSI], 2018). 100 μL of each CFS were inoculated into wells made on pre-inoculated BCYE agar plates (Buffered Charcoal Yeast Extract Agar, Merck KGaA, Darmstadt, Germany) and inhibitory activity was measured against Legionella pneumophila strains (Clinical and Laboratory Standards Institute [CLSI], 2018)
Ten CFSs were produced by different species of Lactobacillus: L. crispatus, L. gasseri, L. jensenii, L. fermentum, L. rhamnosus, and L. salivarius grown up with two different culture media
Summary
Lactic acid bacteria (LAB) are commensal bacteria widely studied for their postbiotic products, a term which indicates the majority of the metabolites referring to soluble factors secreted by bacteria during their cycle-life and/or released after membrane lysis (Aguilar-Toala et al, 2018) Among these by-products, there are both biosurfactants and bacteriocins (Dykes, 1995; Van Hamme et al, 2006). The former are surface-active compounds including glycolipids, lipopeptides, lipoproteins, phospholipids, and fatty acids that reduce surface and interfacial tension in both aqueous solutions and hydrocarbon mixtures (Desai and Banat, 1997) As for bacteriocins, they are cationic and amphipathic small molecules with antagonistic activity usually against microorganisms closely related to the producing strain (Banerjee et al, 2013). The mechanism of action of bacteriocins produced by Gram positive bacteria is mostly related to the proton motive force (PMF) dissipation and can be further divided into subclasses on the basis of the energy required (Bruno and Montville, 1993)
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call