Monday, May 22, 2006

Synergistic Substances and Several Siderophores Synthesised by Sessile Soil Saphrophytes.

Here is an link to a published essay about synergy of antibiotics, as promised by Dr Pundit.

Students may wish to stop at the abstract, as the full essay was written for professional scientists who are investigating streptomycetes in research. The two key words though, to remember, are synergy and contingency. The discussion in the essay about how ability to make several siderophores deals with contingencies faced by streptomyces in the soil is really relevant to classroom discussions.

But before moving on to that, when Pundit read through this fine essay, it reminded him of important points about streptomyces life-cycle that were not explicitly emphasised in the classes, but should be added to your notes.

Note Bene

  • Aerial hyphae derive their nutrients from dead and lysing vegetative hyphae.
  • In other words, vegetative hyphae sacrifice themselves for their spores.
  • This involves substantial reorganisation of cell activities during the transition from vegetative to aerial hyphal growth.

Before reading on, it would be sensible to be clear in your mind what "contingency" means. Helpfully, Challis and Hopwood provide a definition:

Note that our use of "contingency" in this article relates to multiple metabolites acting on the same biological target to provide an organism with a contingency plan to combat unforeseeable biological competition. Moxon and coworkers have used contingency to describe hypermutable loci coding for variable surface proteins in Haemophilus influenzae and Nesseria meningitidis. The two uses of the word should not be confused
Roughly speaking then, contingency means "just in case".

If you read further, as a bonus you also discover the Fatal Attraction hypothesis of soil ecology. But to see the nitty-gritty of Fatal Attraction you have to read the whole thing.

Synergy and contingency as driving forces for the evolution of multiple secondary metabolite production by Streptomyces species.

Challis GL, Hopwood DA.

In this article we briefly review theories about the ecological roles of microbial secondary metabolites and discuss the prevalence of multiple secondary metabolite production by strains of Streptomyces, highlighting results from analysis of the recently sequenced Streptomyces coelicolor and Streptomyces avermitilis genomes.

We address this question: Why is multiple secondary metabolite production in Streptomyces species so commonplace? We argue that synergy or contingency in the action of individual metabolites against biological competitors may, in some cases, be a powerful driving force for the evolution of multiple secondary metabolite production. This argument is illustrated with examples of the coproduction of synergistically acting antibiotics and contingently acting siderophores: two well-known classes of secondary metabolite. We focus, in particular, on the coproduction of beta-lactam antibiotics and beta-lactamase inhibitors, the coproduction of type A and type B streptogramins, and the coregulated production and independent uptake of structurally distinct siderophores by species of Streptomyces.

Possible mechanisms for the evolution of multiple synergistic and contingent metabolite production in Streptomyces species are discussed. It is concluded that the production by Streptomyces species of two or more secondary metabolites that act synergistically or contingently against biological competitors may be far more common than has previously been recognized, and that synergy and contingency may be common driving forces for the evolution of multiple secondary metabolite production by these sessile saprophytes.

Proc Natl Acad Sci U S A. 2003 Nov 25;100 Suppl 2:14555-61. Epub 2003 Sep 11.

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