The Importance of Bacterial Replichore Balance

Sammanfattning: In most bacterial pathogens, the genome is comprised within a single circular chromosome which is typically organized by the origin-to-terminus axis that divides the chromosome into equally-sized arms of replication (replichores). This similarity in length is presumed to be required for the synchronization of the two replication forks to meet at the terminus for efficient chromosome segregation. Transfer of genes between organisms, different from the route of parent to offspring, is called horizontal gene transfer (HGT). Acquiring foreign DNA through HGT is an important factor for the evolution of virulence in bacteria since it provides access to new features such as new toxins and antibiotic resistance genes. Chromosomes of many pathogenic bacteria such as Salmonella spp. carry such horizontally-transferred DNA fragments called pathogenicity islands. However, after such HGT events, the existing organization of chromosome can be disrupted and an imbalance between the two halves of the circular chromosome might occur. The predicted outcome of a replichore imbalance is the retardation of growth which in turn might result in the out-competition by other faster-growing bacteria in the environment. For that reason, we have investigated the association of the fitness cost and the replichore imbalance with isogenic strains with varying degrees of inter-replichore inversions. Our results showed that there is a correlation between the magnitude of replichore imbalance and fitness cost, for example 2.49-fold imbalance (one replichore 2.49-fold longer than the other) resulted in 11% reduction of fitness in comparison with balanced replichores. Therefore, our data suggest that the replichore imbalance could be utilized to predict the fitness cost of HGT events.