Folate cross-feeding supports symbiotic homoacetogenic spirochetes

Treponema primitia, an H2-consuming CO2-reducing homoacetogenic spirochete in termite hindguts, requires an exogenous source of folate for growth. Tetrahydrofolate (THF) acts as a C1 carrier in CO2-reductive acetogenesis, a microbially mediated process important to the carbon and energy requirements of termites. To examine the hypothesis that other termite gut microbes probably supply some form of folate to T. primitia in situ, we used a bioassay to screen for and isolate folate-secreting bacteria from hindguts of Zootermopsis angusticollis, which is the host of T. primitia. Based on morphology, physiology, and 16S rRNA gene sequences, the major folate secretors were identified as strains of Lactococcus lactis and Serratia grimesii. During growth, these isolates secreted 5-formyl-THF at levels up to 146 ng/ml, and their cell-free culture fluids satisfied the folate requirement of T. primitia strains in vitro. Analysis of Z. angusticollis hindgut fluid revealed that 5-formyl-THF was the only detectable folate compound and occurred at an in situ concentration (1.3 mug/ml) which was more than sufficient to support the growth of T. primitia. These results imply that cross-feeding of 5-formyl-THF by other community members is important for growth of symbiotic hindgut spirochetes and thus termite nutrition and survival.