Microbial ‘bodyguard’ oozes antibiotics

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A group of cotton leafworms, Spodoptera littoralis, chomping through a leafy meal. The older specimens have more of a specific bacterium in their gut that helps keep them healthy.
Credit: Yongqi Shao

A common caterpillar has been found harbouring its own private pharmacy: an anti-microbial bacterium living in its gut.

Researchers in China and Germany report the cotton leafworm’s disease defence system in Cell Chemical Biology, saying the work could provide new avenues for antibiotic discovery.

A herbivorous insect’s diet also includes the tiny microorganisms that hitch a ride on plant matter. These microbial clingers-on should be dangerous, but insects have some of the strongest antimicrobial strategies in the animal world. 

Exactly how they combat nasties has puzzled scientists, but research suggests that an insect’s main line of defence against microbial trespassers is its gut bacteria.

Indeed, Yongqi Shao at Zhejiang University in China and colleagues studying the cotton leafworm (Spodoptera littoralis) saw its gut bacteria populations shift as the larvae matured.

While younger larvae played host to a range of different gut bacteria from the Enterococci genus, older larvae guts were dominated by one particular species, Enterococcus mundtii

It turns out E. mundtii secretes an antimicrobial peptide – or bacteriocin – called mundticin KS, which targets invading or competitive bacteria, helping keep the leafworm healthy.

This relationship also benefits E. mundtii, giving the bacterium a safe living space and first dibs on nutrients, ensuring its dominance in a complex environment.

A similar symbiosis between host and gut bacteria has previously been observed among ants, locusts and beetles, suggesting this research may have applications throughout the animal world – even, potentially, in human medicine.

“Many conventional antibiotics are facing increasing problems of resistance,” Shao explains.

“As evolutionarily conserved weapons, bacteriocins have great potential as alternatives to conventional antibiotics.”

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