The Clinical and Translational Science Collaborative of Cleveland and National Institutes of Health.
The class A mec gene complex also carries the psm-mec gene. The psm-mec gene product can be detected as a peak near 2415m/z using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The correlation of a 2415m/z peak with methicillin resistance (mecA carriage) in consecutive staphylococcal blood culture isolates was evaluated. A 2415±2.00m/z peak was observed in 37% (51/137) of methicillin-resistant Staphylococcus aureus, in none (0/146) of the methicillin-susceptible S. aureus, in 6% (10/180) of methicillin-resistant S. epidermidis, and in 2% (1/63) of methicillin-susceptible S. epidermidis isolates. Although the sensitivity of the 2415m/z peak for mecA carriage in S. aureus and S. epidermidis was low (37% and 6%, respectively), the specificity was high (≥98%). This peak can be identified during routine MALDI-TOF MS clinical testing, and the analysis adds no reagent cost and requires minimal time expenditure.
The population structure of health care-associated pathogens reflects patterns of diversification, selection, and dispersal over time. Empirical data detailing the long-term population dynamics of nosocomial pathogens provide information about how pathogens adapt in the face of exposure to diverse antimicrobial agents and other host and environmental pressures and can inform infection control priorities. Extensive sequencing of clinical isolates from one hospital spanning a decade and a second hospital in the Cleveland, OH, metropolitan area over a 3-year time period provided high-resolution genomic analysis of the Acinetobacter baumannii metapopulation. Genomic analysis demonstrated an almost complete replacement of the predominant strain groups with a new, genetically distinct strain group during the study period. The new group, termed clade F, differs from other global clone 2 (GC2) strains of A. baumannii in several ways, including its antibiotic resistance and lipooligosaccharide biosynthesis genes. Clade F strains are part of a large phylogenetic group with broad geographic representation. Phylogenetic analysis of single-nucleotide variants in core genome regions showed that although the Cleveland strains are phylogenetically distinct from those isolated from other locations, extensive intermixing of strains from the two hospital systems was apparent, suggesting either substantial exchange of strains or a shared, but geographically restricted, external pool from which infectious isolates were drawn. These findings document the rapid evolution of A. baumannii strains in two hospitals, with replacement of the predominant clade by a new clade with altered lipooligosaccharide loci and resistance gene repertoires. IMPORTANCE Multidrug-resistant (MDR) A. baumannii is a difficult-to-treat health care-associated pathogen. Knowing the resistance genes present in isolates causing infection aids in empirical treatment selection. Furthermore, knowledge of the genetic background can assist in tracking patterns of transmission to limit the spread of infections in hospitals. The appearance of a new genetic background in A. baumannii strains with a different set of resistance genes and cell surface structures suggests that strong selective pressures exist, even in highly MDR pathogens. Because the new strains have levels of antimicrobial resistance similar to those of the strains that were displaced, we hypothesize that other features, including host colonization and infection, may confer additional selective advantages and contribute to their increased prevalence.
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