User Experience with Sensititre® ARIS® 2X in Detecting MRSA
By Dr. Vincent LaBombardi, St. Vincent’s Hospital and Medical Center, New York City

Methicillin resistant S. aureus (MRSA) has been isolated in the hospitalized patient since 1961. In many metropolitan centers, MRSA is isolated more frequently than is methicillin susceptible S. aureus. MRSA has largely been a nosocomial problem. In the late 1980’s we saw a few outpatients, all of whom were IV drug users, from whom we isolated MRSA. Recently reports have appeared of patients, who had not been recently hospitalized, who presented with infections caused by MRSA. These community-acquired MRSA (CA-MRSA) have been reported in children in day care, adolescents and young adults playing contact sports including football and wrestling, among military recruits and in a prison population (1-4). The majority of these infections have been in skin and soft tissue. This organism has also been described as a cause of necrotizing pneumonia (5). This organism is apparently transmitted person to person by close contact. We first noticed these infections in our HIV+ population who presented with rather aggressive cutaneous infections.

These organisms have been reported worldwide and have some common features that separate them from nosocomially acquired MRSA. These organisms carry the mecA gene on a type IV staphylococcal chromosomal cassette (SCCmec type IV) (6). These organisms, in the large majority of cases, also contain bicomponent genes that produce Panton-Valentine leucocidin (PVL) (5,6). This is a virulence factor that is responsible for the death of leukocytes by causing the formation of pores in the cell membrane. This factor is responsible for the rather aggressive infections caused by these organisms. These CA-MRSA appear to be more susceptible, than are nosocomial MRSA, to non-beta lactam antibiotics. These organisms can be susceptible to the quinolones, erythromycin, clindamycin, trimethoprim/sulfamethoxazole and the tetracyclines. Since there are so virulent it is imperative that infections caused by these organisms be treated quickly and appropriately. We therefore need to perform susceptibility tests and report the results on antibiotics that we would normally not test or report for nosocomial MRSA. Therefore, any specimen from a patient seen in any of our clinics or the emergency room will be set up utilizing our ARIS system with Sensititre gram-positive panels. Since the origin of an infection may not be obvious to the laboratory, we will also utilize this test panel for any skin and soft tissue specimen that grows S. aureus. Currently we test and report oxacillin, penicillin, vancomycin, erythromycin, clarithromycin, clindamycin, ciprofloxacin, tetracycline, SXT, and linezolid. Daptomycin is not currently available on this panel but can be tested off-line on request.

Of major concern is the possibility that these CA-MRSA will start to combine with nosocomial strains and become multi-drug resistant. It is imperative that’s these isolates be rapidly identified and appropriate susceptibility tests performed and reported.

References:

1. Dietrich DW, Auld DB, Mermel LA. Community-acquired Methicillin resistant Staphylococcus aureus in southern New England. Pediatrics 2004; 113:347-52
2. Fergie JE, Purcell K. Community-acquired Methicillin-resistant Staphylococcus aureus infections in south Texas children. Pediatr Infect Dis J 2001; 20:860-3
3. Outbreaks of community-associated Methicillin-resistant Staphylococcus aureus skin infections-Los Angeles County California, 2002-2003. MMWR Feb 7 2003; 52:88.
4. Zinderman CE. Community-acquired Methicillin resistant Staphylococcus aureus among military recruits. Emerg Infect Dis 2004; 10:941-4
5. Gillet Y, Issartel B, Vanhems P, et al. Association between Staphylococcus aureus strains carrying the gene for Panton-Valentine leukocidin and highly lethal necrotizing pneumonia in young immunocompetent patients. Lancet 2002; 359:753-9
6. Diep BA, Sensabaugh GF, Somboona NS, et al. Widespread skin and soft-tissue infections due to two Methicillin-resistant Staphylococcus aureus strains harboring the genes for Panton-Valentine leucocidin. J Clin Microbiol 2004; 452:2080-4.

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