Evolution of Bacterial Interspecies Hybrids with Enlarged Chromosomes.

Conjugation driven by a chromosomally integrated F-plasmid (high frequency of recombination strain) can create bacteria with hybrid chromosomes. Previous studies of interspecies hybrids have focused on hybrids in which a region of donor chromosome replaces an orthologous region of recipient chromosome leaving chromosome size unchanged. Very little is known about hybrids with enlarged chromosomes, the mechanisms of their creation, or their subsequent trajectories of adaptative evolution. We addressed this by selecting 11 interspecies hybrids between Escherichia coli and Salmonella Typhimurium in which genome size was enlarged. In three cases, this occurred by the creation of an F'-plasmid while in the remaining eight, it was due to recombination of donor DNA into the recipient chromosome. Chromosome length increased by up to 33% and was associated in most cases with reduced growth fitness. Two hybrids, in which chromosome length was increased by the addition of 0.97 and 1.3 Mb, respectively, were evolved to study genetic pathways of fitness cost amelioration. In each case, relative fitness rapidly approached one and this was associated with large deletions involving recombination between repetitive DNA sequences. The locations of these repetitive sequences played a major role in determining the architecture of the evolved genotypes. Notably, in ten out of ten independent evolution experiments, deletions removed DNA of both species, creating high-fitness strains with hybrid chromosomes. In conclusion, we found that enlargement of a bacterial chromosome by acquisition of diverged orthologous DNA is followed by a period of rapid evolutionary adjustment frequently creating irreversibly hybrid chromosomes.

Genetic Architecture and Fitness of Bacterial Interspecies Hybrids.

Integration of a conjugative plasmid into a bacterial chromosome can promote the transfer of chromosomal DNA to other bacteria. Intraspecies chromosomal conjugation is believed responsible for creating the global pathogens Klebsiella pneumoniae ST258 and Escherichia coli ST1193. Interspecies conjugation is also possible but little is known about the genetic architecture or fitness of such hybrids. To study this, we generated by conjugation 14 hybrids of E. coli and Salmonella enterica. These species belong to different genera, diverged from a common ancestor >100 Ma, and share a conserved order of orthologous genes with ∼15% nucleotide divergence. Genomic analysis revealed that all but one hybrid had acquired a contiguous segment of donor E. coli DNA, replacing a homologous region of recipient Salmonella chromosome, and ranging in size from ∼100 to >4,000 kb. Recombination joints occurred in sequences with higher-than-average nucleotide identity. Most hybrid strains suffered a large reduction in growth rate, but the magnitude of this cost did not correlate with the length of foreign DNA. Compensatory evolution to ameliorate the cost of low-fitness hybrids pointed towards disruption of complex genetic networks as a cause. Most interestingly, 4 of the 14 hybrids, in which from 45% to 90% of the Salmonella chromosome was replaced with E. coli DNA, showed no significant reduction in growth fitness. These data suggest that the barriers to creating high-fitness interspecies hybrids may be significantly lower than generally appreciated with implications for the creation of novel species.

Mutant RNA polymerase can reduce susceptibility to antibiotics via ppGpp-independent induction of a stringent-like response.

BACKGROUND: Mutations in RNA polymerase (RNAP) can reduce susceptibility to ciprofloxacin in Escherichia coli, but the mechanism of transcriptional reprogramming responsible is unknown. Strains carrying ciprofloxacin-resistant (CipR) rpoB mutations have reduced growth fitness and their impact on clinical resistance development is unclear. OBJECTIVES: To assess the potential for CipRrpoB mutations to contribute to resistance development by estimating the number of distinct alleles. To identify fitness-compensatory mutations that ameliorate the fitness costs of CipRrpoB mutations. To understand how CipRrpoB mutations reprogramme RNAP. METHODS: E. coli strains carrying five different CipRrpoB alleles were evolved with selection for improved fitness and characterized for acquired mutations, relative fitness and MICCip. The effects of dksA mutations and a ppGpp0 background on growth and susceptibility phenotypes associated with CipRrpoB alleles were determined. RESULTS: The number of distinct CipRrpoB mutations was estimated to be >100. Mutations in RNAP genes and in dksA can compensate for the fitness cost of CipRrpoB mutations. Deletion of dksA reduced the MICCip for strains carrying CipRrpoB alleles. A ppGpp0 phenotype had no effect on drug susceptibility. CONCLUSIONS: CipRrpoB mutations induce an ppGpp-independent stringent-like response. Approximately half of the reduction in ciprofloxacin susceptibility is caused by an increased affinity of RNAP to DksA while the other half is independent of DksA. Stringent-like response activating mutations might be the most diverse class of mutations reducing susceptibility to antibiotics.