To get around host defenses, Salmonella enterica serovars that specialize in a single type of host are known to shuffle their genomic decks by recombining their chromosomes, rearrangements that result in inversions, translocations, duplications, or deletions of various sizes.
But what drives these rearrangements, exactly? Scientists have proposed two possible explanations: either horizontal gene transfer creates an imbalance in the Salmonella genome (called a replichore imbalance) that must be evened out by shuffling pieces of the genome, or the rearrangements are a consequence of the host-specific lifestyle, which either stimulates or promotes tolerance of such rearrangements.
Matthews et al. have new evidence that the host-specific lifestyle, not a replichore imbalance, is at the root of these rearrangements. The team examined the arrangement of rRNA operons in the genomes of Salmonella enterica serovar Typhi isolates taken from asymptomatic individuals over a 23-year period to capture changes in chromosome structure during long-term infections. Strains isolated from the same person at different time points often had different chromosomal arrangements, indicating that rearrangements took place over time within a carrier. Matthews et al. also estimated the replichore balance of these strains over time. The rearrangements apparently did not alleviate imbalances, and some of the rearrangements even decreased replichore balance, suggesting that it is something inherent to the the restricted lifestyle of host-specific Salmonella that is responsible for frequent chromosomal rearrangements in these serovars.