Extracellular microbial products

Microbes, including SBW25, produce a range of soluble products that are commonly referred to as “public goods”.  One that has received much attention is the fluorescent iron-chelating agent pyoverdin.  

Over the last decade the idea that the microbial world is inherently social has moved from interesting hypothesis, to seemingly established fact. Claims are based largely on the discovery — and for the most part in vitro generation — of mutants that do not produce extracellular products.  Those advocating a social perspective label non-producers “social cheats” (types that gain advantage from producing types) and thus it follows (according to the social evolution perspective) that producing types are “cooperators”; any excreted product is termed a “public good”. While this maybe true, there exist alternate non-social explanations for producers and for non-producers.

In 2013 Zhang and Rainey published an in-depth investigation into the origin of pyoverdin-non-producing mutants that arose during the course of a selection experiment.  Among various things they showed the non-producers were not cheats, but in fact adaptive mutants that enjoyed a selective advantage because they avoided the cost of producing pyoverdin when it was not required.  This led the authors to question the generality of the conclusion that pyoverdin is a social trait. The paper was met with hostility but countered with a firm rebuttal.  Sanity seems now to prevail with others also calling into question the naive assumption that all extracellular products are “public goods”.  See for example Corina Tarnita’s paper.  

One the findings that that particularly intrigued was ability of pyoverdin-producing cells to — in some media — invade a population of numerically dominant non-producing cells.  This pointed to personalisation and has fuelled Clara Moreno Fenoll‘s extensive and elegant time-resolved, single-cell analysis of pyoverdin production and conditions promoting (polar) personalisation including its reversibility.

Polarisation of pyoverdin (image from Clara Moreno Fenoll)
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