Sir

In his Essay 'Microbial awakenings' (Nature 457, 1083; 2009), Slava Epstein proposes that microbial cells stochastically revert from a state of dormancy to a growing state, allowing a clonal population of dormant cells to exploit rare and transient conditions in an environment with unpredictable periods of feast and famine. This 'scout' strategy is compatible with some — but not all — explanations for the general unculturability of environmental microbes.

Credit: DAVID PARKINS

Other explanations include oxidative stress and substrate-accelerated death on transfer to the laboratory environment, and slow-growth strategies. If most microbes in energy-limited and spatially heterogeneous environments are slow-growing species ('K-strategists'), they may be permanently prepared for a very slow but steady existence under nutrient limitation. Rapid growth won't occur because the cells would not then be ready for the end of a brief nutrient flush. Microbes using the K strategy are always prepared for resource exhaustion. As a result, they grow only slowly in laboratory culture, as noted for many 'unculturable' soil bacteria.

Other microbes, rarer in nutrient-poor environments, respond rapidly to nutrient flushes and grow at much higher rates when resources are plentiful. However, these microbes ('r-strategists') must re-enter a dormant state before such a nutrient flush is exhausted. This transition is an active and energy-requiring process, and cell death is rapid if the transition cannot be completed. Once such cells are in a dormant state, they wait for the next nutrient flush while slowly losing viability.

Therefore, Epstein's scout theory could apply to opportunistic r-strategists, but not so easily to K-strategists.

See also: Dormant microbes: time to revive some old ideas