Communal Living by Bacteria and the Pathogenesis of Urinary Tract Infections

I n this issue of PLoS Medicine, Rosen and colleagues [1] present convincing evidence that intracellular communities of Escherichia coli commonly exist and are likely of to be of clinical significance in uncomplicated bacterial cystitis. This novel finding is remarkable in many respects, not the least of which is the circuitous route by which our understanding about bacterial communities has evolved in the pathogenesis of infectious diseases. Since the inception of the germ theory of disease over 150 years ago, it has generally been assumed that potential pathogens invade the host essentially as solitary microorganisms (" lone soldiers "). Successful pathogens must find a susceptible host and then gain access to host tissues through a defect in epithelial barriers. The pathogen must replicate rapidly, and either overwhelms the host's innate and adaptive immune system, or successfully evades antimicrobial defenses by avoiding host recognition and clearance mechanisms [2]. This traditional view of microbial pathogenesis was turned on its head about 30 years ago when three microbiologists, Nealson, Platt, and Hastings [3], made the seemingly innocuous discovery that some marine bacteria actually have the capacity to communicate with each other and coordinate their activities to their mutual benefit. They investigated a halophilic (see Glossary) bacterium known as Vibrio fischeri, which forms an unusual symbiotic relationship with the Hawaiian bobtail squid (Euprymna scolopes). The bioluminescent V. fischeri is taken up by light organs along the body of the squid and, when high concentrations of bacteria are attained, the bacterium induces its luciferase genes to generate visible light. The bacteria benefit from this mutualistic relationship, as the squid provides a source of nutrients for the bacteria. Meanwhile, the squid benefits from the microbial light source, which provides a unique form of camouflage for the squid during nighttime mating rituals. The adult squid populations congregate in large numbers along the water's surface for mating. On moonlit nights, the squid's body is silhouetted against a starlit sky and is thus readily detectable from below by predatory fish. Light organs on the underside of the squid provide a " starry sky camouflage " by their association with the light-generating clusters of V. fischeri. The bioluminescent potential of this Vibrio species is only activated with high bacterial densities. But how do individual bacterial cells know their relative concentration within the light organ of the squid? The answer was discovered when V. fischeri was found to produce a soluble …