Ward-Specific Rates of Nasal Cocolonization with Methicillin-Susceptible and -Resistant Staphylococcus spp. and Potential Impact on Molecular Methicillin-Resistant Staphylococcus aureus Screening Tests

Abstract

eWe report that the rates of nasal cocolonization with methicillin-susceptible Staphylococcus aureus and methicillin-resistant coagulase-negative staphylococci can vary widely between patients admitted to different wards within a single hospital. Such cocolonization can greatly influence the performance of molecular methicillin-resistant S. aureus (MRSA) screening tests depending on the methods used and targets selected. Staphylococci, especially methicillin-resistant Staphylococcus aureus (MRSA) strains, are major pathogens implicated in nosocomial infections, and the link between S. aureus nasal colonization and staphylococcal disease has been well established (1), with nosocomial S. aureus bacteremia being three times more frequent in S. aureus carriers than in noncarriers (2). Moreover, Safdar and Bradley underlined the importance of MRSA carrier detection by showing that patients colonized with MRSA were four times more likely to develop invasive infections than patients colonized with methicillin-sensitive S. aureus (MSSA) (3). Finally, MRSA infections are associated with a high financial burden. For example, the cost of management for patients with an orthopedic device infection due to MRSA is estimated at $100,000 per case, representing a 50% additional cost compared to that of MSSA infections (4).In this context, several manufacturers have developed rapid molecular MRSA screening tests as an alternative to conventional culture methods (5-8). Staphylococcal methicillin resistance is encoded by the mecA gene, located on a mobile genetic element designated the staphylococcal cassette chromosome mec (SCCmec) acquired by horizontal transfer and chromosomic insertion. Different generations of MRSA molecular tests have been successively developed using different combinations of genetic targets. The first-generation tests rely on the combined amplification of the mecA gene and of an S. aureus-specific gene such as spa (Fig. 1) (9). As the mecA gene is present in both MRSA and methicillin-resistant coagulase-negative staphylococci (MRCoNS) (10), specificity of this first generation of MRSA screening tests in clinical specimens can be altered by MSSA and MRCoNS cocolonization (11). To cope with this issue, the second-generation MRSA screening tests used a set of primers targeting the junction sequence between SCCmec and the S. aureus chromosome. Specifically, one primer targets an S. aureus-specific sequence near the SCCmec insertion site located in the orfX gene, and another primer targets an SCCmecspecific sequence. Hence, the amplification detects the presence of the SCCmec element only when it is inserted in the S. aureus genome, thus eliminating interference due to MRCoNS. Nevertheless, it had been demonstrated that this detection method can still give a falsepositive result in the presence of S. aureus isolates harboring an SCC element lacking the mecA gene; such isolates are designated mecA dropout isolates (12, 13). These SCC-positive, methicillin-susceptible S. aureus (MSSA) isolates are misidentified as...

Colonização nasal por Staphylococcus sp. em pacientes internados

“…Although MRSA is more frequently studied, the mecA gene, responsible for its resistance to methicillin, can also be found in strains of methicillin-resistant coagulase-negative Staphylococcus (MR-CoNS). (7,8,14,15) The results of present study are in agreement with others (4,11,16) in which S. aureus was not the most prevalent. The nasal vestibules of about 20% of the healthy population were colonized by S. aureus, (4,5) an important pathogen that can spread throughout the community and has a high resistance potential.…”

“…In the present study, the prevalence of MRSA was almost 10%, a value similar to that obtained in other studies involving hospitalized patients or health professionals. (15,21) There are reports of nasal colonization of healthy individuals by MRSA (1-8%), which represent a potential risk factor for subsequent infection by S. aureus. (15,19) Although much attention has been directed to the resistance of S. aureus, CoNS should also be studied due to an increase in the resistant strains.…”

“…(15,21) There are reports of nasal colonization of healthy individuals by MRSA (1-8%), which represent a potential risk factor for subsequent infection by S. aureus. (15,19) Although much attention has been directed to the resistance of S. aureus, CoNS should also be studied due to an increase in the resistant strains. In the present study, the level of resistance to methicillin was relatively high, with similar values for both S. aureus and CoNS.…”

Comparative Evaluation of Two PCR-Based Methods for Detection of Methicillin-Resistant Staphylococcus aureus (MRSA): Xpert MRSA Gen 3 and BD-Max MRSA XT

“…With the onset of community-acquired MRSA (4), outpatients and emergency room patients became at risk for MRSA carriage, further increasing the need for determination of carriage of MRSA. More recently, livestock-associated MRSA carrying variant mecC genes further complicated the epidemiology of MRSA by increasing its molecular diversity (5). Microbiology laboratories usually define MRSA phenotypically, but the underlying genetic structures responsible for this phenotype are constantly changing (6)(7)(8), and the molecular assays designed several years back are not necessarily relevant to the epidemiology observed today (9)(10)(11).…”

Mupirocin Resistance in Isolates of Staphylococcus spp. from Nasal Swabs in a Tertiary Hospital in France

“…The patients included in this study were admitted to a 38-bed ICU, which includes 20 beds of medical care and 18 beds of surgical care, or to a 39-bed OS ward with single rooms. The nasal swabs were used to seed chromogenic ChromID medium (bioMérieux, Marcy l'Etoile, France), which supports the growth of all staphylococcal species, using the quadrant technique described previously (14). Identification of the different staphylococcal isolates present (on the basis of colony morphology) was performed using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) (bioMérieux) (15).…”