Sir

Jean-Marc Ghigo in his interesting Letter (Nature 412, 442–445; 2001) states the oft-repeated canard that “most natural bacterial populations are found associated with environmental surfaces”. This is not true.

Bacteria are the most numerous organisms in the biosphere. A quick look at a sample from the oceans, the largest habitat on Earth, would reveal individual bacterial cells (when stained for their DNA) scattered across the darkened field, like stars on a clear night.

Of course there are bacteria in biofilms, but in the biosphere these are the exception rather than the rule. Their more numerous free-living brethren may tell us more about why these bacteria live in such an unnatural state.