The vexing persistence of infections caused by microbial biofilms (4), even in the face of active immune defences and aggressive antibiotic therapy, continues to motivate the search for the fundamental mechanisms of biofilm reduced susceptibility. One of the most attractive hypotheses to explain the resistance of attached micro-organisms to antimicrobial agents suggests that freefloating and sessile cells inhabit distinct physiological states even when grown in the same medium (2, 3). We outline here a fresh version of this hypothesis that hinges on the spatial heterogeneity of physiological activity within the biofilm.

It has long been recognized that gradients in nutrient concentrations inside biofilms probably lead to corresponding gradients in growth rate or other physiological activities within the biofilm. Now fluorescent probe and reporter gene technologies have advanced to the point that these hypothesized physiological gradients can be directly visualized. Such experimental approaches demonstrate, often vividly, that cells of the same microbial species can inhabit extremely different physiological states, even though they may