Evolutionary novelties have been important in the history of life, but their origins are usually difficult to examine in detail. We previously described the evolution of a novel trait, aerobic citrate utilization (Cit+), in an experimental population of Escherichia coli. Here we analyze genome sequences to investigate the history and genetic basis of this trait. At least three distinct clades coexisted for more than 10,000 generations prior to its emergence. The Cit+ trait originated in one clade by a tandem duplication that captured an aerobically-expressed promoter for the expression of a previously silent citrate transporter. The clades varied in their propensity to evolve this novel trait, although genotypes able to do so existed in all three clades, implying that multiple potentiating mutations arose during the population’s history. Our findings illustrate the importance of promoter capture and altered gene regulation in mediating the exaptation events that often underlie evolutionary innovations.

The upper respiratory tract (URT) is a crucial site for host defense, as it is home to bacterial communities that both modulate host immune defense and serve as a reservoir of potential pathogens. Young children are at high risk of respiratory illness, yet the composition of their URT microbiota is not well understood. Microbial profiling of the respiratory tract has traditionally focused on culturing common respiratory pathogens, whereas recent culture-independent microbiome profiling can only report the relative abundance of bacterial populations. In the current study, we used both molecular profiling of the bacterial 16S rRNA gene and laboratory culture to examine the bacterial diversity from the oropharynx and nasopharynx of 51 healthy children with a median age of 1.1 years (range 1-4.5 years) along with 19 accompanying parents. The resulting profiles suggest that in young children the nasopharyngeal microbiota, much like the gastrointestinal tract microbiome, changes from an immature state, where it is colonized by a few dominant taxa, to a more diverse state as it matures to resemble the adult microbiota. Importantly, this difference in bacterial diversity between adults and children accompanies a change in bacterial load of three orders of magnitude. This indicates that the bacterial communities in the nasopharynx of young children have a fundamentally different structure from those in adults and suggests that maturation of this community occurs sometime during the first few years of life, a period that includes ages at which children are at the highest risk for respiratory disease.

While traditionally microbiologists have examined bacterial behavior averaged over large populations, increasingly we are becoming aware that bacterial populations can be composed of phenotypically diverse individuals generated by a variety of mechanisms. Though the results of different mechanisms, the phenomena of bistability, persistence, variation in chemotactic response, and phase and antigenic variation are all strategies to develop population-level diversity. The understanding of individuality in bacteria requires an appreciation of their environmental and ecological context, and thus evolutionary theory regarding adaptations to time-variable environments is becoming more applicable to these problems. In particular, the application of game and information theory to bacterial individuality has addressed some interesting problems of bacterial behavior. In this review we discuss the mechanisms of generating population-level variability, and the application of evolutionary theory to problems of individuality in bacteria.

Bacteria live almost exclusively in communities with other microorganisms, and often in association with multicellular hosts. These communities are capable of maintaining complex structural and functional stability over time, and exhibit fascinating properties of resiliency in response to environmental changes. This is a result of interactions between microbes and the environment and amongst members of the community. A multitude of chemical interactions occur in microbial communities where primary and secondary metabolites contribute to a wealth of interactions between organisms. The chemicals include a variety of nutrients, toxic or neutral metabolic byproducts, antibiotics, and cell-cell signaling molecules. These chemical and physical signals facilitate microbial relationship that can be competitive, cooperative or neutral, and thus are responsible for determining community structure. In turn, the surrounding community changes the microenvironment of individual cells who respond to chemical and environmental cues in a combinatorial manner. Current laboratory understanding of the genetics and mechanisms of interactions between microbes has the power to help us understand how complex microbial communities behave in the natural environment. In this chapter we review the current understanding of microbial communication, from the genetic and molecular aspects, to our current understanding of their ecological role.

Rapid evolution of microbes under laboratory conditions can lead to domestication of environmental or clinical strains. In this work, we show that domestication due to laboratory passage in rich medium is extremely rapid. Passaging of wild-type Salmonella in rich medium led to diversification of genotypes contributing to the loss of a spatial phenotype, called the rdar morphotype, within days. Gene expression analysis of the rdar regulatory network demonstrated that mutations were primarily within rpoS, indicating that the selection pressure for scavenging during stationary phase had the secondary effect of impairing this highly conserved phenotype. If stationary phase was omitted from the experiment, radiation of genotypes and loss of the rdar morphotype was also demonstrated, but due to mutations within the cellulose biosynthesis pathway and also in an unknown upstream regulator. Thus regardless of the selection pressure, rapid regulatory changes can be observed on laboratory timescales. The speed of accumulation of rpoS mutations during daily passaging could not be explained by measured fitness and mutation rates. A model of mutation accumulation suggests that to generate the observed accumulation of r 38 mutations, this locus must experience a mutation rate of approximately 10 À4 mutations/gene/generation. Sequencing and gene expression of population isolates indicated that there were a wide variety of r 38 phenotypes within each population. This suggests that the rpoS locus is highly mutable by an unknown pathway, and that these mutations accumulate rapidly under common laboratory conditions.