A alanine racemase affects spore germination and accommodates serine as a substrate.

has become one of the most common bacterial pathogens in hospital-acquired infections in the United States. Although is strictly anaerobic, it survives in aerobic environments and transmits between hosts via spores. spore germination is triggered in response to certain bile acids and glycine. Although glycine is the most effective co-germinant, other amino acids can substitute with varying efficiencies. Of these, l-alanine is an effective co-germinant and is also a germinant for most bacterial spores. Many endospore-forming bacteria embed alanine racemases into their spore coats, and these enzymes are thought to convert the l-alanine germinant into d-alanine, a spore germination inhibitor. Although the Alr2 racemase is the sixth most highly expressed gene during spore formation, a previous study reported that Alr2 has little to no role in germination of spores in rich medium. Here, we hypothesized that Alr2 could affect l-alanine-induced spore germination in a defined medium. We found that mutant spores more readily germinate in response to l-alanine as a co-germinant. Surprisingly, d-alanine also functioned as a co-germinant. Moreover, we found that Alr2 could interconvert l- and d-serine and that Alr2 bound to l- and d-serine with ∼2-fold weaker affinity to that of l- and d-alanine. Finally, we demonstrate that l- and d-serine are also co-germinants for spores. These results suggest that spores can respond to a diverse set of amino acid co-germinants and reveal that Alr2 can accommodate serine as a substrate.