Joy Wireman

Major families of multiresistant plasmids from geographically and epidemiologically diverse staphylococci.

Staphylococci are increasingly aggressive human pathogens suggesting that active evolution is spreading novel virulence and resistance phenotypes. Large staphylococcal plasmids commonly carry antibiotic resistances and virulence loci, but relatively few have been completely sequenced. We determined the plasmid content of 280 staphylococci isolated in diverse geographical regions from the 1940s to the 2000s and found that 79% of strains carried at least one large plasmid >20 kb and that 75% of these large plasmids were 20–30 kb. Using restriction fragment length polymorphism (RFLP) analysis, we grouped 43% of all large plasmids into three major families, showing remarkably conserved intercontinental spread of multiresistant staphylococcal plasmids over seven decades. In total, we sequenced 93 complete and 57 partial staphylococcal plasmids ranging in size from 1.3 kb to 64.9 kb, tripling the number of complete sequences for staphylococcal plasmids >20 kb in the NCBI RefSeq database. These plasmids typically carried multiple antimicrobial and metal resistances and virulence genes, transposases and recombinases. Remarkably, plasmids within each of the three main families were >98% identical, apart from insertions and deletions, despite being isolated from strains decades apart and on different continents. This suggests enormous selective pressure has optimized the content of certain plasmids despite their large size and complex organization.

Large plasmids of Escherichia coli and Salmonella encode highly diverse arrays of accessory genes on common replicon families

Plasmids are important in evolution and adaptation of host bacteria, yet we lack a comprehensive picture of their own natural variation. We used replicon typing and RFLP analysis to assess diversity and distribution of plasmids in the ECOR, SARA, SARB and SARC reference collections of Escherichia coli and Salmonella. Plasmids, especially large (≥30 kb) plasmids, are abundant in these collections. Host species and genotype clearly impact plasmid prevalence; plasmids are more abundant in ECOR than SAR, but, within ECOR, subgroup B2 strains have the fewest large plasmids. The majority of large plasmids have unique RFLP patterns, suggesting high variation, even within dominant replicon families IncF and IncI1. We found only four conserved plasmid types within ECOR, none of which are widely distributed. Within SAR, conserved plasmid types are primarily serovar-specific, including a pSLT-like plasmid in 13 Typhimurium strains. Conservation of pSLT contrasts with variability of other plasmids, suggesting evolution of serovar-specific virulence plasmids is distinct from that of most enterobacterial plasmids. We sequenced a conserved serovar Heidelberg plasmid but did not detect virulence or antibiotic resistance genes. Our data illustrate the high degree of natural variation in large plasmids of E. coli and Salmonella, even among plasmids sharing backbone genes.

New Policy for Titles of Letters to the Editor

mice (70 mg/kg of body weight) in a manner simulating the sustained release of an intramuscular injection in a human (maximum concentration at 3 h, half-life of 2.5 h) was as effective against P. aeruginosa as two 70-mg/kg injections simulating intravenous injection (maximum concentration at 15 min, half-life of 1.1 h). The emergence of d2-porin-deficient mutants of P. aeruginosa resistant to imipenem has been reported in many studies involving intermittent dosing in humans (6). The low concentrations in serum observed at the end of each dosing interval with intermittent dosing could contribute to the development of resistance. As mentioned previously, continuous infusion of imipenem (4,000 mg daily) should result in a constant concentration in serum in excess of 4 x the MIC breakpoint value. We agree that treatment of infections due to pathogens for which MICs are high may test the value of any dosing regimen adjustment and that combination therapy with an aminoglycoside is indicated. We have shown that for combination therapy with a 1-lactam antibiotic and an aminoglycoside, the greatest degree of synergy in vivo occurs when the daily dose of the 1-lactam is divided into small doses administered at frequent intervals (simulating continuous infusion) and the aminoglycoside is administered at infrequent intervals (2). In summary, dosing of carbapenems by continuous infusion should be no less effective than intermittent administration in the treatment of infections caused by susceptible organisms and may allow the use of smaller total daily doses. Unfortunately, the instability of aqueous solutions of imipenem-cilastatin after 4 h at room temperature largely eliminates the feasibility of continuous infusion of this agent in the clinical setting (1). REFERENCES