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A Fine-Tuned Management between Physiology and Immunity Maintains the Gut Microbiota in Insects

Mithilesh Kajla, Kuldeep Gupta, Lalita Gupta and Sanjeev Kumar

The association of microbial community with the digestive system is a distinct phenomenon and the insect gut is an excellent model to understand these interactions. Insects are omnivorous, feed on all kinds of food and encounter a variety of microbes. The diversity of these large and varied microbial communities inhabiting the gut depends on the feeding behavior of insects. Insect gut is also the foremost immune organ that encounters foreign food particles and exogenous pathogenic/non pathogenic microbes. Thus, it should be equipped by some mechanism that can distinguish between the food and pathogens. In most of the insects, the synthesis of an acellular chitinous peritrophic matrix (PM) around the ingested food compartmentalizes the gut to keep exogenous/endogenous microbes containing food bolus detached from the immunoreactive gut epithelium. This barrier-like functioning of the PM blocks the induction of insect immunity against the microbes present in the gut bolus. In addition to the PM, an extensively cross-linked mucin barrier also suppresses gut immunity against soluble microbial elicitors in the mosquito. Eventually, these acellular barriers maintain ‘low immunity zone’ in the gut to support the survival and proliferation of endosymbiotic microbes. In this review, we discuss that the ‘fine-tuned’ regulation of physiological state of digestion and immunity maintains the fitness-relevant traits such as growth and fecundity in insects.