EFFECT OF HOST DIET ON PRODUCTION OF ORGANIC-ACIDS AND METHANE BY COCKROACH GUT BACTERIA

The effect of high-fiber diets on microbial populations and processes in cockroach guts was investigated by feeding American cockroaches (Periplaneta americana) milled cereal leaves, milled corn cob, or commercial bran-type breakfast cereal in place of the commonly used laboratory diet of dog chow. The activities and numbers of specific gut bacteria varied significantly with the insect's diet and developmental stage. Acetate and lactate were the principal organic acids present in the gut fluid of adult cockroaches and occurred at concentrations of up to 17 and 8 mM, respectively. These acids were most abundant in the gut fluid of dog chow-fed insects, and the greatest amounts were generally found in the foregut and midgut regions. Foreguts of dog chow-fed cockroaches contained an abundant population of lactic acid bacteria that formed acetate and lactate from endogenous hexoses present in the foregut. When adult cockroaches were fed dog chow amended with antibacterial drugs, (i) the concentrations of acetate, lactate, and total hexoses in gut fluid decreased significantly, (ii) the numbers of lactic acid bacteria in the foregut also decreased significantly, and (iii) the production of acetate and lactate by foregut homogenates was suppressed. It was estimated that acetate and lactate produced by bacteria in the foregut of dog chow-fed adult P. americana could support up to 14% of the insect's respiratory requirement if taken up and used by the animal. When insects were fed high-fiber diets of bran cereal, cereal leaves, or corn cob, bacterial production of acetate and lactate in the foregut diminished. The main terminal electron sink processes accompanying gut fermentation in P. americana were microbial reduction of CO2 to CH4, to formate, and to acetate. Methanogenesis occurred exclusively in the hindgut region and was favored when insects were fed high-fiber diets. This was particularly true for larvae, which generally emitted more CH4 per gram (fresh weight) than adults. In contrast, formico- and acetogenesis from CO2 were favored when cockroaches were fed the low-fiber, dog chow diet. These results indicate that host diet has a significant effect on the intestinal microbiota of P. americana and its activities.