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Mol Microbiol. 2005 Jul;57(1):1-8.

Maintaining a healthy SPANC balance through regulatory and mutational adaptation.

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School of Molecular and Microbial Biosciences, The University of Sydney, NSW 2006, Australia.


Stress protection is an important but costly contributor to bacterial survival. Two distinct forms of environmental protection share a common cost and a significant species-wide variability. Porin-mediated outer membrane permeability and the RpoS-controlled general stress response both involve a trade-off between self-preservation and nutritional competence, called the SPANC balance. Interestingly, different Escherichia coli strains exhibit distinct settings of the SPANC balance. It is tilted towards high stress resistance and a restricted diet in some isolates whereas others have broader nutritional capability and better nutrient affinity but lower levels of resistance. Growth- or stress-related selective pressures working in opposite directions (antagonistic pleiotropy) result in polymorphisms affecting porins and RpoS. Consequently, these important cellular components are present at distinct concentrations in different isolates. A generalized hypothesis to explain bacterial adaptation, based on the SPANC investigations, is offered. A holistic approach to bacterial adaptation, involving a gamut of regulation and mutation, is likely to be the norm in broadening the capabilities of a species. Indeed, there is unlikely to be a standard regulatory setting typical for all members of a species. Gene regulation provides a limited fine control for maintaining the right level of adaptation in a particular niche but mutational changes provide the coarse control for adaptation between the species-wide environments of free-living bacteria.

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