Intracellular granule formation in response to oxidative stress in Bifidobacterium

Bacteria in the genus Bifidobacterium are commonly known as beneficial colonizers in the human gastrointestinal tract. We found that, when these anaerobic organisms were grown in culture media without the reducing agent, cysteine, they produced intensely stained intracellular granules reminiscent of polyphosphate granules (poly P) produced by other bacteria in response to certain environmental signals, such as starvation and oxidative stress. The addition of cysteine led to a significant reduction in granule formation in bifidobacteria. Specific microscopic staining showed that the intracellular granules in Bifidobacterium scardovii were consistent with the poly P granules. In addition, the expression of the putative polyphosphate kinase gene responsible for poly P synthesis showed a 16-fold increase in the granule-forming cultures of B. scardovii compared with the nongranule-forming cultures, suggesting a role of poly P production in the oxidative stress response. Furthermore, the granule-forming cells exhibited a higher acid tolerance and a higher degree of cell surface hydrophobicity than the nongranule-forming cells. Therefore, we propose that Bifidobacterium cells produce poly P as a part of the oxidative stress response, which in turn allows the cells to better tolerate other environmental stresses such as acidic pH and perhaps allows better host colonization in vivo.