ARS | Publication request: Bacterial communities associated with the digestive tract of the predatory ground beetle, Poecilus chalcites, and their response to laboratory rearing and antibiotic treatment

Submitted to: Microbial Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 12, 2008
Publication Date: N/A

Interpretive Summary: Ground beetles such as Poecilus chalcites (Coleoptera:Carabidae) are beneficial insects in agricultural systems where they contribute to the control of insect and weed pests. In an effort to understand beneficial ground beetle ecology and to understand the factors that may influence their abundance and fitness, we are investigating the bacterial communities inhabiting their digestive tracts. Previous research on non-pathogenic intestinal bacteria in insects has been reviewed and there is a little information for Carabidae, particularly using culture-independent (i.e., molecular biology) approaches. We determined the number and identity of bacterial populations occurring in the digestive tracts of field-collected P. chalcites using a combination of DNA-based molecular methods. The digestive tract of field-collected P. chalcites was colonized by an average of about five bacterial populations per beetle. Lactobacilli were most abundant, followed by Enterobacteriaceae, Clostridia, and Bacteriodetes. The majority of the sequences recovered were closely related to sequences reported from other insect guts. Lab rearing of the beetles resulted in a reduced number of bacterial populations per beetle (ca. three) and selected for a different set of populations dominated by Enterobacteriaceae compared to the field-collected beetles. Antibiotic treatment significantly (p<0.05) reduced the number of bacterial populations per beetle and selected for a less diverse set of populations. We conclude that the digestive tract of P. chalcites is colonized by a simple community of bacterial populations that possess autochthonous characteristics and that lab rearing conditions may manipulate these populations. Based on taxonomic affiliation of the clone sequences, at least one possible function of the gut bacteria is nutritional ¿ the production of organic acids from more complex substrates.

Technical Abstract: Ground beetles such as Poecilus chalcites (Coleoptera:Carabidae) are beneficial insects in agricultural systems where they contribute to the control of insect and weed pests. We determined the number and identity of bacterial populations occurring in the digestive tracts of field-collected P. chalcites using a combination of culturing and direct DNA extraction followed by terminal restriction fragment length polymorphism (tRFLP) and sequence analysis of 16S rRNA genes. The bacterial populations in field-collected beetles were then compared to those from groups of beetles that were reared in the lab on an artificial diet with and without antibiotics. Direct cell counts estimated 1.5 x 108 bacteria per cc of gut. The digestive tract of field-collected P. chalcites was colonized by an average of about five bacterial populations per beetle. Lactobacilli were most abundant, followed by Enterobacteriaceae, Clostridia, and Bacteriodetes. The majority of the sequences recovered were closely related to sequences reported from other insect guts. Lab rearing of the beetles resulted in a reduced number of bacterial populations per beetle (ca. three) and selected for a different set of populations dominated by Enterobacteriaceae compared to the field-collected beetles. Antibiotic treatment significantly (p<0.05) reduced the number of bacterial populations per beetle and selected for a less diverse set of populations. We conclude that the digestive tract of P. chalcites is colonized by a simple community of bacterial populations that possess autochthonous characteristics and that lab rearing conditions may manipulate these populations. Based on taxonomic affiliation of the clone sequences, at least one possible function of the gut bacteria is nutritional ¿ the production of organic acids from more complex substrates.