Polyphosphate is an extracellular signal that can facilitate bacterial survival in eukaryotic cells.

Abstract

Polyphosphate is a linear chain of phosphate residues and is present in organisms ranging from bacteria to humans. Pathogens such as <i>Mycobacterium tuberculosis</i> accumulate polyphosphate, and reduced expression of the polyphosphate kinase that synthesizes polyphosphate decreases their survival. How polyphosphate potentiates pathogenicity is poorly understood. <i>Escherichia coli</i> K-12 do not accumulate detectable levels of extracellular polyphosphate and have poor survival after phagocytosis by <i>Dictyostelium discoideum</i> or human macrophages. In contrast, <i>Mycobacterium smegmatis</i> and <i>Mycobacterium tuberculosis</i> accumulate detectable levels of extracellular polyphosphate, and have relatively better survival after phagocytosis by <i>D. discoideum</i> or macrophages. Adding extracellular polyphosphate increased <i>E. coli</i> survival after phagocytosis by <i>D. discoideum</i> and macrophages. Reducing expression of polyphosphate kinase 1 in <i>M. smegmatis</i> reduced extracellular polyphosphate and reduced survival in <i>D. discoideum</i> and macrophages, and this was reversed by the addition of extracellular polyphosphate. Conversely, treatment of <i>D. discoideum</i> and macrophages with recombinant yeast exopolyphosphatase reduced the survival of phagocytosed <i>M. smegmatis</i> or <i>M. tuberculosis</i><i>D. discoideum</i> cells lacking the putative polyphosphate receptor GrlD had reduced sensitivity to polyphosphate and, compared to wild-type cells, showed increased killing of phagocytosed <i>E. coli</i> and <i>M. smegmatis</i> Polyphosphate inhibited phagosome acidification and lysosome activity in <i>D. discoideum</i> and macrophages and reduced early endosomal markers in macrophages. Together, these results suggest that bacterial polyphosphate potentiates pathogenicity by acting as an extracellular signal that inhibits phagosome maturation.