D3332: Antimicrobial peptide expression in the cockroach gut during enterobacterial infection is specific and influenced by type III secretion

The German cockroach (Blattella germanica) is a reservoir and vector of enteric bacterial pathogens. A lifestyle conducive to frequent encounters with high loads of diverse bacteria may have led to the evolution of unique innate immune systems in this insect. Investigating how cockroaches respond to enterobacteria provides the opportunity to expand fundamental knowledge of insect innate immunity in a context that is biologically and medically relevant. German cockroaches can harbor both Salmonella Typhimurium and Escherichia coli in their gut. The former colonizes the gut and replicates while the latter persists only transiently. We hypothesized that differences in the innate immune response may contribute to or result from the difference in infection dynamics between the two enterobacteria. To test this, we used qRT-PCR to analyze expression of five genes encoding representative AMPs in the gut of German cockroaches after ingestion of live or heat-killed enterobacteria. We found AMP expression was induced in response to ingestion of live wild-type S. Typhimurium, but not in response to live E. coli, heat-killed S. Typhimurium, or a live mutant strain of S. Typhimurium lacking type III secretion systems. These results indicate the cockroach immune system does not respond to stimulation with high levels of ingested bacterial PAMPs such as peptidoglycan. Rather, AMP expression in the gut appears to be induced by active bacterial colonization involving type III secretion. We speculate that this form of regulation may have evolved to prevent over activation of the immune system from frequent ingestion of innocuous, non-colonizing, or non-viable bacteria.