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News

Structure and Evolution of Streptomyces Interaction Networks in Soil and In Silico


 

 Streptomyces are gram-positive, filamentous bacteria that are often found in ground soils.  They are prolific produces of antibiotics.  These microbes survive by secreting toxic antibiotics in an effort to kill surrounding, competing microbes. Different strains of Streptomyces produce different secondary metabolites (antibiotics and other secreted chemicals).  The presence of secondary metabolites can either inhibit or promote the growth of a cell.  With this in mind, Kalin Vetsigian and Roy Kishony (Harvard Medical School) examined interactions between 64 different Streptomyces strains in order to catalog their positive and negative interactions.

Antibiotics, first discovered in 1928 by Sir Alexander Flemming and popularized during the Second World War, have become and integral component in treating many bacterial infections.  Interestingly, most antibiotics are actually synthesized by living microbes!  The antibiotic pills prescribed by doctors are actually produced in large fermentation tanks that contain antibiotic secreting bacteria.  The antibiotics are then isolated and distributed for medical use.

A very common type of antibiotic-producing microbe is Streptomyces.  Streptomyces are gram-positive, filamentous bacteria that are often found in ground soils.  These microbes survive by secreting toxic antibiotics in an effort to kill surrounding microbes.  The surrounding bacteria might pose a threat via increased competition for resources or they could physically harm the Streptomyces cell. 

Each strain’s growth needed to be tested in the presence of another’s chemical secretions.  This simulated an interaction between two strains in the environment.  “Sender” strains were grown on top of a fine filter that allowed the passage of small chemicals (antibiotics) into the growth medium but disallowed the passage of cells. After the cells had incubated and secreted their antibiotics, the filter was pulled off, taking the sender cells with it but leaving the antibiotics.  “Receiver” strains were grown on the “conditioned” agar that was left behind.  Movies were taken of each colony’s growth.

The tests yielded a rich array of promotions and inhibitions.  While hundreds of different Streptomyces strains might exists as a community in a single grain of soil, this research suggests that the community is not static or stable.  Populations are in constant flux.  Strains incessantly kill each other while others help encourage growth.

What’s more, Streptomyces communities do not constitute stable ecological states. Instead, they can be viewed as evolutionary hotspots where cells acquire resistances and chemically interact with others to create a dynamic population.