The driving force behind non-genetic population differences Researchers have used a common bacterium to better understand how and why physical differences in a genetically identical population come about. Health © Shutterstock Phenotypic heterogeneity, or physical differences within a genetically identical population in the same environment, are common among asexual bacteria and have puzzled biologists for years. Scientists believe they are an evolutionary strategy to either preserve a population in an uncertain environment or help a population survive by dividing resources. The EU-funded BIOSIGHET (Exploring the biological significance of phenotypic heterogeneity in Pseudomonas) project aimed to test these hypotheses using Pseudomonas fluorescens, a commonly studied asexual bacterium. Populations of this species show phenotypic heterogeneity by expressing or not expressing a colanic acid capsule (Cap+ or Cap-, respectively). Researchers began by investigating whether various environmental conditions influenced capsule expression. They found that capsules were more frequently expressed at lower temperatures, and less so at higher temperatures. Next, BIOSIGHET looked to see whether the capsule provided a competitive advantage to bacteria that expressed it, by growing Cap+/Cap- populations with Cap- only populations. At lower temperatures, the Cap+/Cap- population was more successful, showing that the capsule could provide a competitive advantage. Lastly, researchers studied the genetics of capsule production, finding that bacteria with low levels of the pyrimidine nucleotide UTP delay cell division by producing capsules. This demonstrates that in the case of P. fluorescens, phenotypic heterogeneity helps a population survive difficult environmental conditions. The findings of BIOSIGHET have contributed to biologists' understanding of this phenomenon. This will have an impact on the treatment of infectious disease, and on industries that rely on microorganisms to break down or manufacture products. Keywords Phenotypic heterogeneity, evolutionary strategy, BIOSIGHET, Pseudomonas, colanic acid capsule
