Everybody wants good neighbours
Streptomyces are weird and wonderful, even among the Bacteria (and this is a kingdom not short on oddities). They look and grow like fungi but are 1000 times smaller. That characteristic earthy smell you get walking in the countryside? That’s made by Streptomyces bacteria. They also make about 60 per cent of all the antibiotics and anticancer drugs that we use clinically, in addition to numerous immunosuppressants and antiparasitic drugs that helped to revolutionise medicine in the last century. In other words, Streptomyces are very friendly, very useful bacteria.
Streptomyces bacteria probably make antibiotics as chemical weapons to kill off competitors for the scarce nutrients in the soil and to use as signalling molecules to communicate with their neighbours. We know they are very good at sharing genes among themselves and other bacterial species through horizontal gene transfer – many antibiotic resistance genes originate in Streptomyces bacteria because they have to be resistant to their own antibiotics to avoid suicide – but, until now, very little was known about the way Streptomyces bacteria coexist and interact within the soil.
Kishony and colleagues, from Harvard Medical School, examined the interactions between all the Streptomyces species they isolated from three single grains of soil. Specifically, they looked at whether antibiotics secreted by individual ‘sender’ strains promoted or inhibited the growth of ‘receiver’ strains isolated from the same soil grains.
The results showed that closely related species were just as likely to inhibit each other’s growth as distant relatives. Remarkably, if a sender strain promoted (or inhibited) the growth of a receiver, the relationship was likely to be reciprocated. What’s more, two strains isolated from the same grain of soil were more likely to show this reciprocal relationship than two strains isolated from different grains of soil, regardless of how closely they are related.
This study is important because it overturns previously held assumptions that Streptomyces species happily share their antibiotic resistance genes with each other while competing with non-streptomycetes. In fact, they’ll compete with anyone, including their closest relatives. The reciprocal interactions suggest they can form mutually beneficial relationships or go to all-out war.
This study is impressive in its technical achievements, but it also provides a vital first step towards addressing the role of antibiotic production and resistance in the soil. Longer term, it could enable scientists to discover new antibiotics that are only made when Streptomyces species (positively or negatively) interact with their neighbours.
Vetsigian K, Jajoo R, & Kishony R (2011). Structure and evolution of streptomyces interaction networks in soil and in silico. PLoS biology, 9 (10) PMID: 22039352