Spermidine promotes lysozyme tolerance and acid stress resistance in M3.

are Gram-positive opportunistic pathogens residing in the human nasopharynx and skin. Changes in environmental conditions, such as pH, temperature and availability of essential ions, can stimulate the expression of virulence factors. One such factor could be the availability of an extracellular pool of polyamines. Polyamines are synthesized from amino acids, and are universally present in the environment. Polyamines have been implicated in the ecology of pathogenesis by modulating quorum sensing, host adaptation and virulence. Polyamines mediate pathogenesis and help the pathogen resist environmental stress. In this study, we investigated the ability of the polyamine, spermidine, to promote acid stress survival of does not synthesize spermidine, but the extracellular pool of spermidine constituted by the host and microbiome could be utilized as a signalling molecule. We report that spermidine promotes acid stress resistance in . Moreover, spermidine affects the morphology of by decreasing the cell size and increasing the gene expression. Along with , spermidine upregulated the gene expression of cell wall-modifying genes such as , , and , which might help the bacteria to resist acidic stress.