Systematic investigation of recipient cell genetic requirements reveals important surface receptors for conjugative transfer of IncI2 plasmids

Intra- and Interspecies Conjugal Transfer of Plasmids in Gram-Negative Bacteria

Background/Objectives: Plasmid-mediated resistance is a significant mechanism that contributes to the gradual decrease in the efficacy of antibiotics from various classes, including carbapenems. The aim of this study is to investigate the frequency of transfer of carbapenemase-encoding plasmids from K. pneumoniae to E. coli and P. aeruginosa. Methods: Matings were performed on agar with subsequent isolation of transconjugant, recipient, and donor colonies. The frequency of conjugation (CF) and minimum inhibitory concentrations (MICs) of meropenem were determined for the PCR-confirmed transconjugants. A pharmacodynamic study was conducted using a hollow-fiber infection model on E. coli transconjugant in order to evaluate its viability in the presence of therapeutic concentrations of meropenem. Results: CF for K. pneumoniae-K. pneumoniae was similar to that for K. pneumoniae-E. coli and was higher the higher was meropenem MIC of the K. pneumoniae donor. The meropenem MICs for K. pneumoniae and E. coli transconjugants were higher (0.25-4 μg/mL) compared to recipients (0.03-0.06 μg/mL). P. aeruginosa did not acquire plasmids from K. pneumoniae. In pharmacodynamic experiments, an E. coli transconjugant with MIC of 2 mg/L within the "susceptibility range", failed to respond to meropenem treatment. Conclusions: The frequency of conjugation between K. pneumoniae and E. coli falls within a similar range. A higher permissiveness of K. pneumoniae for plasmids from K. pneumoniae, i.e., within the same species, was observed. Conjugation did not occur between K. pneumoniae and P. aeruginosa. The transconjugants with meropenem MICs with borderline susceptibility may pose a potential threat to the efficacy of meropenem.

Variation in plasmid conjugation among nontyphoidal Salmonella enterica serovars

Conjugation is a complex phenomenon involving multiple plasmid, bacterial, and environmental factors. Here we describe an IncI1 plasmid encoding multidrug antibiotic resistance to aminoglycosides, sulfonamides, and third-generation cephalosporins. This plasmid is widespread geographically among animal, human, and environmental sectors. We present data on the transmissibility of this plasmid from Salmonella enterica ser. Kentucky into 40 strains of S. enterica (10 strains each from serovars Enteritidis, Heidelberg, Infantis, and Typhimurium). Thirty seven out of 40 strains were able to take up the plasmid. Rates of conjugation were variable between strains ranging from 10-8 to 10-4. Overall, serovars Enteritidis and Typhimurium demonstrated the highest rates of conjugation, followed by Heidelberg, and then Infantis. No relationships were observed between the recipient cell surface and rate of conjugation. Recipient cell numbers correlated positively with conjugation rate and strains with high conjugation rates had marginally but significantly higher growth parameters compared to strains that took up the plasmid at lower frequencies. Environmental conditions known to impact cell growth, such as temperature, nutrient availability, and the presence of antibiotics, had a modulating effect on conjugation. Collectively, these results will further understanding of plasmid transmission dynamics in Salmonella, which is a critical first step towards the development of mitigation strategies.

Expanding the diversity of origin of transfer-containing sequences in mobilizable plasmids

Conjugative plasmids are important drivers of bacterial evolution. Most plasmids lack genes for conjugation and characterized origins of transfer (oriT), which has hampered our understanding of plasmid mobility. Here we used bioinformatic analyses to characterize occurrences of known oriT families across 38,057 plasmids, confirming that most conjugative and mobilizable plasmids lack identifiable oriTs. Recognizable oriT sequences tend to be intergenic, upstream of relaxase genes and specifically associated with relaxase types. We used these criteria to develop a computational method to search for and identify 21 additional families of oriT-containing sequences in plasmids from the pathogens Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii. Sequence analyses found 3,072 occurrences of these oriT-containing sequences across 2,976 plasmids, many of which encoded antimicrobial resistance genes. Six candidate oriT-containing sequences were validated experimentally and were shown to facilitate conjugation in E. coli. These findings expand our understanding of plasmid mobility.

Detergents with Scalable Properties Identify Noncanonical Lipopolysaccharide Binding to Bacterial Inner Membrane Proteins

Lipopolysaccharide (LPS) is vital for maintaining the outer membrane barrier in Gram-negative bacteria. LPS is also frequently obtained in complex with the inner membrane proteins after detergent purification. The question of whether or not LPS binding to inner membrane proteins not involved in outer membrane biogenesis reflects native lipid environments remains unclear. Here, we leverage the control of the hydrophilic-lipophilic balance and packing parameter concepts to chemically tune detergents that can be used to qualitatively differentiate the degree to which proteins copurify with phospholipids (PLs) and/or LPS. Given the scalable properties of these detergents, we demonstrate a detergent fine-tuning that enables the facile investigation of intact proteins and their complexes with lipids by native mass spectrometry (nMS). We conclude that LPS, a lipid that is believed to be important for outer membranes, can also affect the activity of membrane proteins that are currently not assigned to be involved in outer membrane biogenesis. Our results deliver a scalable detergent chemistry for a streamlined biophysical characterization of protein-lipid interactions, provide a rationale for the high affinity of LPS-protein binding, and identify noncanonical associations between LPS and inner membrane proteins with relevance for membrane biology and antibiotic research.