In Vivo Detection of Clonally Derived Methicillin-Resistant/Methicillin-Susceptible Staphylococcus aureus Strains Is Not a Rare Event

Daskalaki and colleagues (3) recently reported the identification of mixed clonally derived Staphylococcus aureus isolates that were responsible for recurrent bacteremia and showed important genetic rearrangements during the time frame of an infection. At our institution, in addition to systematically screening patients for methicillin-resistant S. aureus (MRSA) carriage, all patients from internal medicine wards were also tested for the carriage of methicillin-sensitive S. aureus (MSSA). Identification was based on analyzing cultures by using conventional and selective chromogenic media (2) as well as by using molecular assays (4). During a 3-month period in 2007, more than 1,000 tests were performed for a systematic screening of patients at admission and on discharge (ongoing study). Overall, 10% of the patients colonized by MRSA simultaneously showed the presence of MSSA.

Samples collected from one of these patients were studied further. This patient was initially identified as an MSSA carrier over several months by culture and PCR. The patient's status as an MSSA carrier was detected on 11 and 12 March (from blood cultures and from a superficial wound sample). PCR assays indicated a positive result for MRSA skin colonization, whereas cultures remained negative on 25 May. Meanwhile, the patient was decolonized for MRSA by treatment with mupirocin nasal ointment and whole-body chlorhexidine baths. Nasal and inguinal pooled swabs tested positive for MRSA by PCR only on 1 and 11 June. Then, both the cultures and the PCR assays turned out to be positive, allowing isolation of the MRSA strain on 26 June. Antibiotic susceptibility testing (with a SirScan disk diffusion method) yielded identical profiles, except for the oxacillin resistance conferred by the acquisition of a type IV staphylococcal chromosome cassette (SCCmec) element (5). Extensive genotyping experiments (Fig. 1) using multiple-locus variable-number tandem-repeat analysis (MLVA) and repetitive-element sequence-based PCR showed that the three isolates were clonal. In addition, MLVA allowed visualization of a specific product resulting from the SCCmec element. Using pulsed-field gel electrophoresis (PFGE), the two MSSA strains appeared to be clonal and closely related to the MRSA strain, which showed a single band difference, with a slight increase in molecular weight, probably resulting from the acquisition of the SCCmec element. Two weeks after the MRSA strain was isolated, the patient converted to a MRSA-free but persistently positive MSSA carrier status. Testing of all isolates yielded false-positive results, using a commercially available assay, IDI-GeneOhm, that relies on the identification of the orfX-SCCmec junction (6). Note that the mobility of the SCCmec element is known. The transfer to S. aureus (8) or its acquisition in the presence (7) or absence of antibiotic-selective pressure (1) was previously reported.

FIG. 1.

Genotyping of isolates. Extensive genotyping experiments were performed with the three isolates collected during 2007 in March (isolate 1), May (isolate 2), and July (isolate 3) from a single patient. MLVA (A) shows that the three strains are clonal but that isolate 2 harbors the mecA gene (indicated by an asterisk). Whereas the PFGE analysis (B) shows that strain 2 displays a single band difference (with an increased molecular weight [MW], likely due to the SCCmec element insertion) from the other two strains (indicated by a double asterisk), the repetitive-element sequence-based PCR (C) typing experiments (DiversiLab, bioMérieux, France) showed that the three isolates were clonal.

Several important issues derive from these observations and should be explored further. (i) Use a combined MSSA/MRSA detection strategy to assess the frequency of the loss/acquisition of the SCCmec element; (ii) assess the potential of molecular assays to detect MRSA carriage before cultures turn out positive; (iii) address whether smaller type IV or V SCCmec cassettes are transferred more frequently than larger type I, II, or III SCCmec cassettes; and (iv) assess the importance of such MSSA/MRSA carriage on MRSA detection performance using several molecular assays.

The transfer of the SCCmec element in a MSSA strain has been observed in several countries and under different epidemiological conditions. Further studies are now warranted to evaluate the frequency of this event and its impact on infection control strategies.