Polyphosphate in Lactobacillus and Its Link to Stress Tolerance and Probiotic Properties

Abstract

The synthesis of the inorganic polymer polyphosphate (poly-P) in bacteria has been linked to stress survival and to the capacity of some strains to sequester heavy metals. In addition, synthesis of poly-P by certain strains of probiotic lactobacilli has been evidenced as a probiotic mechanism due to the homeostatic properties of this compound at the intestinal epithelium. We analyzed the link between poly-P synthesis, stress response, and mercury toxicity/accumulation by comparing wild-type strains of <i>Lactobacillus</i> and their corresponding mutants devoid of poly-P synthesis capacity (defective in the poly-P kinase, <i>ppk</i>, gene). Results showed that resistance to salt (NaCl) and acidic (pH 4) stresses upon <i>ppk</i> mutation was affected in <i>Lactobacillus casei</i>, while no effect was observed in two different <i>Lactobacillus plantarum</i> strains. Inorganic [Hg(II)] and organic (CH₃Hg) mercury toxicity was generally increased upon <i>ppk</i> mutation, but no influence was seen on the capacity to retain both mercurial forms by the bacteria. Notwithstanding, the culture supernatants of <i>ppk</i>-defective <i>L. plantarum</i> strains possessed a diminished capacity to induce HSP27 expression, a marker for cell protection, in cultured Caco-2 cells compared to wild-type strains. In summary, our results illustrate that the role of poly-P in stress tolerance can vary between strains and they reinforce the idea of probiotic-derived poly-P as a molecule that modulates host-signaling pathways. They also question the relevance of this polymer to the capacity to retain mercury of probiotics.