Capsules and their traits shape phage susceptibility and plasmid conjugation efficiency.

Bacterial evolution is affected by mobile genetic elements like phages and conjugative plasmids, offering new adaptive traits while incurring fitness costs. Their infection is affected by the bacterial capsule. Yet, its importance has been difficult to quantify because of the high diversity of confounding mechanisms in bacterial genomes such as anti-viral systems and surface receptor modifications. Swapping capsule loci between Klebsiella pneumoniae strains allowed us to quantify their impact on plasmid and phage infection independently of genetic background. Capsule swaps systematically invert phage susceptibility, revealing serotypes as key determinants of phage infection. Capsule types also influence conjugation efficiency in both donor and recipient cells, a mechanism shaped by capsule volume and conjugative pilus structure. Comparative genomics confirmed that more permissive serotypes in the lab correspond to the strains acquiring more conjugative plasmids in nature. The least capsule-sensitive pili (F-like) are the most frequent in the species' plasmids, and are the only ones associated with both antibiotic resistance and virulence factors, driving the convergence between virulence and antibiotics resistance in the population. These results show how traits of cellular envelopes define slow and fast lanes of infection by mobile genetic elements, with implications for population dynamics and horizontal gene transfer.

The absence of surface D-alanylation, localized on lipoteichoic acid, impacts the Clostridioides difficile way of life and antibiotic resistance.

Introduction: The dlt operon encodes proteins responsible for the esterification of positively charged D-alanine on the wall teichoic acids and lipoteichoic acids of Gram-positive bacteria. This structural modification of the bacterial anionic surface in several species has been described to alter the physicochemical properties of the cell-wall. In addition, it has been linked to reduced sensibilities to cationic antimicrobial peptides and antibiotics. Methods: We studied the D-alanylation of Clostridioides difficile polysaccharides with a complete deletion of the dltDABCoperon in the 630 strain. To look for D-alanylation location, surface polysaccharides were purified and analyzed by NMR. Properties of the dltDABCmutant and the parental strains, were determined for bacterial surface's hydrophobicity, motility, adhesion, antibiotic resistance. Results: We first confirmed the role of the dltDABCoperon in D-alanylation. Then, we established the exclusive esterification of D-alanine on C. difficile lipoteichoic acid. Our data also suggest that D-alanylation modifies the cell-wall's properties, affecting the bacterial surface's hydrophobicity, motility, adhesion to biotic and abiotic surfaces,and biofilm formation. In addition, our mutant exhibitedincreased sensibilities to antibiotics linked to the membrane, especially bacitracin. A specific inhibitor DLT-1 of DltA reduces the D-alanylation rate in C. difficile but the inhibition was not sufficient to decrease the antibiotic resistance against bacitracin and vancomycin. Conclusion: Our results suggest the D-alanylation of C. difficile as an interesting target to tackle C. difficile infections.