Gut Check

Fluorescent imaging reveals that a high density of Enterococcus mundtii bacterial cells (yellow) adhere to the mucus layer lining the gut epithelium of the African cotton leafworm.

Yongqi Shao, Zhejiang University

Insects rarely get sick, although they eat plants and consume a multitude of potentially harmful microorganisms in the process. Credit for their robust health, ironically, goes to some of the microbes they ingest, which take up residence in their guts. One such bacterium, Enterococcus mundtii, is highly prevalent in moths and butterflies. New research involving the caterpillar larvae of the African cotton leafworm (Spodoptera littoralis) has revealed the benefits of this relationship.

Entomologist Yongqi Shao of the College of Animal Sciences at Zhejiang University in China and colleagues had previously observed that the resident bacteria populations in the cotton leafworm larvae radically change as the larvae mature. In a larva’s first few days, it is populated by potentially pathogenic bacteria. Shortly thereafter, though, E. mundtii begins to dominate, and does so for the rest of the caterpillar’s life.

These findings led the researchers to ask whether E. mundtii directly inhibits harmful bacteria. By growing several bacterial species together in a dish, they observed that other bacteria did not grow in a region surrounding cultures of E. mundtii. Furthermore, they discovered that E. mundtii produces a potent antibiotic, mundticin KS. The microbe-fighting action of mundticin KS was verified by genetically creating a version of E. mundtii that did not secrete it; the modified bacterium could not hold competitor bacteria at bay. Finally, exposing infected caterpillars to the antibiotic cured them of their disease.

Shao and colleagues conclude that by producing mundticin KS, E. mundtii effectively kills off invading bacteria that could make its caterpillar host ill. This protection from pathogens has helped the cotton leafworm excel; the crop-devouring pest species is among the most widespread herbivorous insects in temperate regions worldwide.

The researchers suspect that symbioses of this sort underlie a great deal of the survival ability of insects in their microbe-laden environments. Identifying more examples could point the way to new food preservatives, or even human antibiotic drugs. Additionally, understanding the interactions between insect gut microbes and the environment could lead to new forms of agricultural pest control. (Cell Chemical Biology)