Gerrit Brandis

Bacterial chromosomes are typically organized into two similarly sized replichores to synchronize bidirectional replication and fusion within a Tus/ ter- defined termination zone. Previous studies have suggested that replichore balance in both Salmonella and E. coli is maintained by selection for efficient and properly coordinated replication termination and chromosome segregation. However, there is a lack of understanding of the causal relationship between replichore asymmetry, cellular fitness, and replication-fork convergence, as well as how the termination machinery behaves when the two replichores differ substantially in length. Here we dissect the impact of replichore asymmetry by engineering 41 isogenic Salmonella inversion mutants spanning progressive imbalances, complemented by duplication-based asymmetries. Our constructed strains show that even cells with severely imbalanced chromosomes, where one replichore is 11-fold longer in size than the other (0.38 Mb vs. 4.48 Mb) are viable. However, relative fitness declined linearly with replichore size difference at approximately -0.17 per Mb. Additionally, highly imbalanced strains exhibited substantial division defects with severe cell elongation and lysis. Replication profiling showed that, even under extreme replichore asymmetry, forks consistently converge at the innermost ter sites of the shorter replichore and that two sequential Tus/ ter barriers were sufficient for complete replication fork arrest. Together, these results establish a quantitative rule linking replichore asymmetry to fitness, define the termination position under imbalance, and reveal a dual Tus/ ter barrier that safeguards termination when one replichore reaches the terminus region ahead of the other. Collectively these results connect chromosome architecture to replication mechanics and cell-division outcomes.

Ribosomal RNAs contain numerous conserved nucleotide modifications, yet the functional importance of most of these modifications remains unclear. In Escherichia coli , deletion of individual 16S rRNA modification enzymes generally produces only minor phenotypes, raising questions about their biological significance. Here, we generated a comprehensive collection of deletion mutants lacking individual and combined 16S rRNA modifications, culminating in a strain lacking all known 30S ribosomal subunit modifications. Despite the absence of all known 16S rRNA modifications, cells remained viable, exhibiting a fitness defect of ∼30% at 37 °C that increased to ∼50% at 20°C, consistent with impaired ribosome biogenesis. We identified strong epistatic interactions between modifications in the 3ʹ major and 3ʹ minor domains of 16S rRNA, resulting in disproportionately large effects on both fitness and antibiotic susceptibility. Live-cell single-molecule tracking revealed a marked increase in the fraction of non-translating ribosomes and a prolonged time required to enter productive translation, whereas translational elongation by actively engaged 70S ribosomes remained largely unaffected. In addition, fluorescence-based measurements showed that unmodified ribosomes exhibited increased stringency during translation initiation, reducing utilization of near-cognate start codons. Together, these findings demonstrate that 16S rRNA modifications are not essential for viability but collectively enhance the efficiency, robustness, and fidelity of ribosome assembly and translational initiation.

The rise of antibiotic resistance among pathogenic bacteria necessitates innovative therapeutic strategies, such as collateral sensitivity where resistance to one antibiotic increases susceptibility to another. We explored the clinical relevance of collateral sensitivity through genetic engineering and experimental evolution in the six critical bacterial pathogens Escherichia coli , Salmonella Typhimurium , Klebsiella pneumoniae , Acinetobacter baumannii , Pseudomonas aeruginosa , and Staphylococcus aureus . Our analysis of E. coli showed that clinically relevant resistance mutations did not confer collateral sensitivity, but we were able to identify at least three novel classes of ciprofloxacin-resistance mutations that cause collateral sensitivity to multiple antibiotics. Further analysis showed that the development of collateral effects differed between the tested species. Collateral sensitivity to gentamicin developed during ciprofloxacin-resistance evolution in almost all tested species, but A. baumannii developed collateral resistance instead. P. aeruginosa consistently developed collateral sensitivity, suggesting its potential for targeted treatment strategies. These findings highlight the complexity of collateral effects and support its therapeutic potential in designing more effective antibiotic regimens.

The rise of antibiotic resistance among pathogenic bacteria necessitates innovative therapeutic strategies. A promising technique is the use of collateral sensitivity where resistance to one antibiotic increases susceptibility to another. In this study, we explored the clinical relevance of collateral sensitivity through experimental evolution and genetic engineering in six critical bacterial pathogens using 23 distinct antibiotics. Our in-depth analysis of Escherichia coli showed that clinically relevant resistance mutations did not confer collateral sensitivity to the tested antibiotics. We were able to identify at least three new classes of ciprofloxacin-resistance mutations that cause collateral sensitivity to multiple antibiotics. However, these mutations incur significant fitness costs and are absent in ciprofloxacin-resistant clinical isolates. Our further analysis showed that the development of collateral effects differed significantly between the tested species. Most species showed development of collateral sensitivity to gentamicin during ciprofloxacin-resistance evolution but Acinetobacter baumanii developed collateral resistance instead. Overall, Pseudomonas aeruginosa showed the most consistent development of collateral sensitivity among the tested species, highlighting it as a promising candidate for the use of collateral-sensitivity-based treatment strategies. Our findings provide insights into the potential of collateral sensitivity as a therapeutic strategy and contribute to the development of more effective antibiotic treatment regimens.

Carbapenem-resistant Pseudomonas aeruginosa (CRPA) is a global threat, but the mechanism of non-carbapenemase carbapenem resistance is still unclear. In the current study, we investigated the contributions of point mutations in mexR , oprD , and ftsI to carbapenem resistance in P. aeruginosa during in vivo evolution studies with consecutive clinical isolates. Real-time qPCR and Electrophoretic Mobility Shift Assay demonstrated that MexR (Gln55Pro) mutation increased MexAB efflux pump genes expression by altering MexR’s binding capacity, leading to a four- to eight-fold increase in meropenem MIC in the Pae d1 Green ∆ mexR and PAO1∆ mexR mutants. The OprD (Trp415*) truncation affected porin structure, and the constructed mutant Pae d1 Green oprD Trp415* increased meropenem MIC by 16-fold (from 0.25 to 4 µg/mL). The contribution of ftsI mutation to meropenem resistance was confirmed by clinical linkage analysis and was estimated to cause a two-fold increase in meropenem MIC by comparing the resistant clinical isolate with the Pae d1 Green oprD Trp415*∆ mexR double mutant. The study found that the oprD Trp415* allele alone accounts for the imipenem MIC in clinical isolates, while the ∆ mexR and ftsI Arg504Cys alleles do not contribute to imipenem resistance. In conclusion, we identified and explored the contributions of mexR , oprD, and ftsI mutations to high level non-carbapenemase carbapenem resistance in P. aeruginosa . These findings highlight the interplay of different mutations in causing non-carbapenemase carbapenem-resistance in P. aeruginosa . IMPORTANCE: The emergence of carbapenem-resistant Pseudomonas aeruginosa (CRPA) poses a significant global health threat, complicating treatment options for infections caused by this pathogen. Understanding the mechanisms behind non-carbapenemase carbapenem resistance is critical for developing effective therapeutic strategies. This study provides crucial insights into how specific point mutations in key genes- mexR , oprD , and ftsI -contribute to carbapenem resistance, particularly the MexR (Gln55Pro) mutation’s effect on efflux pump expression and the OprD (Trp415*) truncation’s impact on porin structure. The findings elucidate the complex interplay of these mutations, highlighting their roles in conferring high-level resistance, and underscore the imperative for continued research to inform therapeutic strategies against CRPA infections.

The rise of antibiotic-resistant bacterial infections poses a global threat. Antibiotic resistance development is generally studied in batch cultures which conceals the heterogeneity in cellular responses. Using single-cell imaging, we studied the growth response of Escherichia coli to sub-inhibitory and inhibitory concentrations of nine antibiotics. We found that the heterogeneity in growth increases more than what is expected from growth rate reduction for three out of the nine antibiotics tested. For two antibiotics (rifampicin and nitrofurantoin), we found that sub-populations were able to maintain growth at lethal antibiotic concentrations for up to 10 generations. This perseverance of growth increased the population size and led to an up to 40-fold increase in the frequency of antibiotic resistance mutations in gram-negative and gram-positive species. We conclude that antibiotic perseverance is a common phenomenon that has the potential to impact antibiotic resistance development across pathogenic bacteria.

The rise of antibiotic-resistant bacterial infections poses a global threat. Antibiotic resistance development is generally studied in batch cultures which conceals the heterogeneity in cellular responses. Using single-cell imaging, we studied the growth response of Escherichia coli to sub-inhibitory and inhibitory concentrations of nine antibiotics. We found that the heterogeneity in growth increases more than what is expected from growth rate reduction for five out of the nine antibiotics tested. For two antibiotics (rifampicin and nitrofurantoin), we found that sub-populations were able to maintain growth at lethal antibiotic concentrations for up to 10 generations. This perseverance of growth increased the effective population size and led to an up to 40-fold increase in antibiotic resistance development in Gram-negative and Gram-positive species. We conclude that antibiotic perseverance is a common phenomenon across the bacterial kingdom that has the potential to impact antibiotic resistance development.

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.

Analysis of bacterial genomes shows that, whereas diverse species share many genes in common, their linear order on the chromosome is often not conserved. Whereas rearrangements in gene order could occur by genetic drift, an alternative hypothesis is rearrangement driven by positive selection during niche adaptation (SNAP). Here, we provide the first experimental support for the SNAP hypothesis. We evolved Salmonella to adapt to growth on malate as the sole carbon source and followed the evolutionary trajectories. The initial adaptation to growth in the new environment involved the duplication of 1.66 Mb, corresponding to one-third of the Salmonella chromosome. This duplication is selected to increase the copy number of a single gene, dctA, involved in the uptake of malate. Continuing selection led to the rapid loss or mutation of duplicate genes from either copy of the duplicated region. After 2000 generations, only 31% of the originally duplicated genes remained intact and the gene order within the Salmonella chromosome has been significantly and irreversibly altered. These results experientially validate predictions made by the SNAP hypothesis and show that SNAP can be a strong driving force for rearrangements in chromosomal gene order.

Bacterial chromosomes frequently carry multiple copies of genes at separate chromosomal locations. In Salmonella , these include the 7 rrn operons and the duplicate tuf genes.