Folate Cross-Feeding Supports Symbiotic Homoacetogenic Spirochetes

Abstract

Treponema primitia, an H 2 -consuming CO 2 -reducing homoacetogenic spirochete in termite hindguts, requires an exogenous source of folate for growth. Tetrahydrofolate (THF) acts as a C 1 carrier in CO 2 -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 g/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.Spirochetes are a major component of the hindgut microbiota of termites, accounting for as many as 50% of all prokaryotic cells (37), but for many years their role in the gut remained obscure as none had ever been isolated and studied in vitro. Recently, the first pure cultures of the following termite gut spirochetes were obtained: Treponema primitia strains ZAS-1 and ZAS-2 and Treponema azotonutricium strain 29,30). These organisms are believed to be important for termite nutrition because of their ability to fix dinitrogen and produce acetate, a major carbon and energy source for termites. In fact, T. primitia is a homoacetogen and possesses the Wood-Ljungdahl (acetyl coenzyme A) pathway for CO 2 -reductive acetogenesis (14). This enables T. primitia to conserve energy by homoacetogenesis from H 2 plus CO 2 , as well as from carbohydrates and methoxylated benzenoids, and to use CO 2 as a carbon source for growth. The Wood-Ljungdahl pathway is undoubtedly important in situ, because a significant portion of acetate formed in the hindgut arises from CO 2 reduction (5, 56). Given the abundance and species diversity of treponemes in termite hindguts (31), it is likely that many of the other not-yet-cultivated spirochetes are also CO 2 -reducing acetogens. This notion is supported by the large number of formyltetrahydrofolate synthetase (a key enzyme of the WoodLjungdahl pathway) genes similar to those of T. primitia detected in hindguts of Zootermopsis angusticollis, the termite that is the host of T. primitia (45).During recent studies of the physiology and nutrition of T. primitia, cells were found to require folate for growth (14). Requirements for exogenous folate compounds are not uncommon among...

Disruption of the Termite Gut Microbiota and Its Prolonged Consequences for Fitness

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