THE GUT MICROFLORA OF RETICULITERMES FLAVIPES, ITS RELATION TO OXYGEN, AND EVIDENCE FOR OXYGEN-DEPENDENT ACETOGENESIS BY THE MOST ABUNDANT ENTEROCOCCUS SP

We previously demonstrated a potentially deep penetration of oxygen in to the hindgut lumen of wood-feeding termites, high oxygen consumption rates of the symbiotic gut microbiota, and an indispensable role of o xygen in the mineralization of aromatic compounds. Here we characteriz e the gut microflora of the wood-feeding termite Reticulitermes flavip es, focusing on metabolic capacities and relation to oxygen of the mos t prevalent physiotypes. The carbohydrate-utilizing isolates, enumerat ed on liquid and solid growth media (3.1 X 10(5) cells per gut; gut li quid volume similar to 0.3 mu l), consisted of aerotolerant lactic aci d bacteria (58%) and surprisingly large numbers of facultatively aerob ic (20%) and even strictly aerobic bacteria (22%). The second largest group of cultivable gut bacteria were hydrogen-oxidizing methanogens ( 3.0 X 10(5) cells per gut), which were cultivated only on liquid media . Together, these two groups represented 10% of the total microscopic counts obtained by DAPI staining. The large number of lactic acid bact eria agreed with the metabolic product profiles in anoxic serial dilut ions of gut homogenates in which lactate was the main product from glu cose in the highest dilutions. Other physiological groups, e.g. propio nate and butyrate producers, were much less abundant. Lactate was not, however, a major metabolite in the hindgut fluid, a phenomenon hither to explained by a rapid turnover of lactate by other gut bacteria. In view of the relatively small number of lactate-oxidizing and lactate-f ermenting bacteria, this explanation is not sufficient. We show that t he isolate Enterococcus strain RfL6, representing the most abundant ph ysiotype among the carbohydrate-utilizing gut bacteria, was not purely fermentative, but consumed oxygen during growth on glucose, accompani ed by a complete shift in the product spectrum from lactate to acetate , and was able to oxidize lactate to acetate when oxygen was present. This agrees with the observation that acetate is the major product of the hindgut metabolism of R flavipes (similar to 70 mM in the hindgut fluid). Together with previous findings, these results underline the o bvious but so far neglected importance of oxygen and contribute to the emerging concept that the termite hindguts are far from simple, anoxi c fermenters, but axially and radially structured, heterogeneous syste ms characterized by steep gradients of metabolites.