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Is MRSE the Source of MRSA?
Abstract & Commentary
By Dean L. Winslow, MD, FACP, FIDSA Chief, Division of AIDS Medicine, Santa Clara Valley Medical Center; Clinical Professor, Stanford University School of Medicine, Section Editor, HIV, is Associate Editor for Infectious Disease Alert.
Synopsis: Nasal carriage of methicillin-resistant coagulase negative staphylococci (MR-CoNS) was investigated in 291 adults upon hospital admission. MR-CoNS carriage was present in 19.2% of patients. SCCmec type IV was found in 22% of the Co-NS isolated, and sequencing revealed extensive structural homology between SCCmec IV in methicillin-resistant Staphylococcus epidermidis (MRSE) and methicillin-resistant Staphylococcus aureus (MRSA).
Source: Barbier F, et al. Methicillin-resistant coagulase-negative staphylococci in the community: High homology of SCCmec IVa between Staphylococcus epidermidis and major clones of methicillin-resistant Staphylococcus aureus. J Infect Dis. 2010; 202:270-281.
Nasal carriage of mr-cons was prospectively investigated in 291 adults upon hospital admission. The isolates were characterized by SCCmec typing, long-range PCR for SCCmec IV, and multiple-locus, variable-number, tandem-repeat analysis (MLVA) for MRSE strains. Three SCCmec IVa elements were fully sequenced.
The overall carriage rate was 19.2% (25.9% in patients with previous exposure to the health care system and 16.5% in patients without previous hospital exposure). Of the 83 MR-CoNS, nine carried SCCmec IVa, nine carried other SCCmec IV subtypes, 15 carried other SCCmec types, and 50 possessed nontypable SCCmec types. Long-range PCR analysis showed structural homology between SCCmec IV in MRSE and MRSA. Complete sequencing of SCCmec Iva from three MRSE strains revealed high homology to available sequences of CA-MRSA, including clones USA300 and USA400.
Staphylococci acquire methicillin resistance from the recombinase-mediated insertion of the staphylococcal chromosomal cassette mobile genetic element containing mec, a gene encoding a mutant penicillin binding protein 2 (PBP2a) with reduced affinity for virtually all beta lactam antibiotics. The most common clones of community-associated MRSA in both Europe and the United States (USA300, USA400, and ST80) all harbor SCCmec IVa. This SCC mec type also is increasing in prevalence among healthcare-associated MRSA.
Since the 1990s, CA-MRSA has, in many areas, replaced methicillin-susceptible Staphylococcus aureus (MSSA) as the most common isolate of S. aureus. At both the county hospitals in San Francisco (San Francisco General Hospital) and in San Jose (Santa Clara Valley Medical Center), more than 60% of S. aureus isolates from patients seeking care in the emergency departments are MRSA. In many respects, it has actually surprised me that it took so many years of use of antistaphylococcal penicillins and cephalosporins to select out MRSA, since penicillin-resistant beta lactamase-producing MSSA arose within a few years of the introduction of penicillin G in the 1940s.
Coagulase-negative staphylococci, in contrast, have been recognized for many years as being resistant to beta-lactam antibiotics, and the use of vancomycin has been recommended for their treatment for decades.
While this study does not provide definitive proof, the high prevalence of SCCmec IVa in MR-CoNS, and the high degree of homology between the SCCmec's obtained from MRSE and MRSA, strongly suggest that MRSE may have been the source of methicillin resistance in S. aureus. Previous evidence suggests that the original source of mec in Staphylococci was the animal pathogen S. aureus.