J Med Microbiol 53 (2004), 1195-1199; DOI: 10.1099/jmm.0.45697-0
© 2004 Society for General Microbiology
ISSN 0022-2615
Evaluation of phenotypic methods for methicillin resistance characterization in coagulase-negative staphylococci (CNS)
Juliana Caierão1,
Maiara Musskopf1,
Silvana Superti2,
Eliane Roesch3,
Cícero G Dias1,2 and
Pedro A d'Azevedo1
1Fundação Faculdade Federal de Ciências Médicas de Porto Alegre, Rua Sarmento Leite 245/211, Porto Alegre, RS 90050-170, Brazil 2Hospital Mãe de Deus, Porto Alegre, Brazil 3Laboratório Central de Análises Clínicas da Irmandade Santa Casa de Misericórdia de Porto Alegre, RS, Brazil
Correspondence Pedro A. d'Azevedo pedroaze{at}fffcmpa.tche.br
Received April 9, 2004
Accepted August 5, 2004
Coagulase-negative staphylococci (CNS) are the major cause of nosocomial infections. Methicillin-resistant strains are particularly important because they narrow therapeutic options. Detecting methicillin resistance among CNS has been a challenge for years. The objective of this study was to determine the accuracy of an agar screening test (0.6 and 4 µg oxacillin ml–1), disc diffusion and the automated MicroScan system to characterize methicillin resistance among CNS. One hundred and seventy five strains were analysed: 41.1 % Staphylococcus epidermidis and 59.9 % other species; 69.1 % were mecA-positive. The results showed that the methods have optimal correlation with the detection of mecA gene for S. epidermidis, Staphylococcus hominis and Staphylococcus haemolyticus. However, accuracy of the tests is impaired when less common species are analysed. The only 100 % accurate test was agar screening with 4 µg oxacillin ml–1.
Abbreviations: CNS, coagulase-negative staphylococci; PBP, penicillin-binding protein.
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Introduction
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Coagulase-negative staphylococci (CNS) have been the major cause of nosocomial infections (Marshall et al., 1998; Yamazumi et al., 2001). Methicillin resistance among CNS is particularly important due to cross-resistance to virtually all ß-lactam agents and other antimicrobial classes. As a result, therapeutic approaches are restricted to glycopeptide and new antimicrobial agents as linezolid (Woods et al., 2002). Therefore, an accurate analysis of methicillin resistance may allow the provision of better antimicrobial therapy and avoid the selection of vancomycin-resistant strains.
The rates of methicillin resistance have increased in the last two decades, according to the National Nosocomial Infectious Surveillance System (NNIS, 2001). In Brazil, a multi-centre study showed that 87.7 % of CNS isolated from blood cultures were resistant to methicillin (Sader et al., 1999). The most common mechanism of methicillin resistance is the production of a penicillin-binding protein (PBP) with low affinity for ß-lactams, PBP2a. PBP2a is coded by the mecA gene and can be detected by PCR, which is considered the gold-standard test to identify methicillin resistance (Chambers, 1997; Swenson et al., 2001).
The detection of methicillin resistance among CNS isolates has been a challenge for years (Gradelski et al., 2001) because these micro-organisms present an hetero-resistant pattern (Chambers, 1997). Although recent breakpoints (NCCLS, 1999) have shown an acceptable correlation with the presence or absence of the mecA gene in Staphylococcus epidermidis, Staphylococcus hominis and Staphylococcus haemolyticus isolates, species with borderline MICs (Hussain et al., 2000; Gradelski et al., 2001; Ferreira et al., 2003) and less common species impair test accuracy. Thus, the aim of this study was to verify the accuracy of the agar screening test (0.6 and 4 mg oxacillin ml–1), disc diffusion and the automated MicroScan system in the detection of methicillin resistance among CNS isolates.
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Methods
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This study included 175 consecutive CNS strains collected from hospitalized patients in southern Brazil. The samples were identified through MicroScan, panel Positive Combo 13 (Dade Behring). Methicillin resistance was characterized through an agar screening test, disc diffusion (NCCLS, 2003) and automated MicroScan system. For the agar screening test, strains were plated on tryptic soy agar with 5 % sheep blood and a 0.5 McFarland standard suspension was prepared for each sample. Mueller–Hinton agar plates (Oxoid) supplemented with 4 % (w/v) NaCl were used and two oxacillin concentrations were tested: 0.6 µg ml–1 (Kohner et al., 1999) and 4 µg ml–1 (Ferreira et al., 2003). The plates were inoculated using Steer's replicator, incubated at 35 °C and screened after 24 and 48 h incubation. The growth of more than one colony on the plates indicated oxacillin-resistant strains.
Disc-diffusion test was performed according to NCCLS (2003) guidelines. Methicillin susceptibility testing with MicroScan was performed according to the manufacturers’ instructions. The NCCLS (2003) breakpoints and MICs for methicillin susceptibility were used to analyse resistance through disc diffusion and MicroScan.
The results were compared to PCR for the detection of the mecA gene (Santos et al., 1999). Staphylococcus aureus ATCC 33591 (methicillin-resistant) and S. aureus ATCC 25923 (methicillin-susceptible) were included as control organisms.
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Results and Discussion
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Table 1 shows the CNS species distribution and the methicillin susceptibility profile, considering the presence or absence of mecA gene. According to MicroScan identification 72 (41.1 %) were S. epidermidis, 36 (20.6 %) S. hominis subspecies hominis, 6 (3.4 %) S. hominis subspecies novobiosepticus, 41 (23.4 %) S. haemolyticus, 5 (2.9 %) Staphylococcus warneri, 5 (2.9 %) Staphylococcus simulans, 3 (1.7 %) Staphylococcus capitis subspecies urealyticus, 2 (1.1 %) Staphylococcus auricularis, 2 (1.1 %) Staphylococcus saprophyticus, 1 (0.6 %) Staphylococcus sciuri, 1 (0.6 %) Staphylococcus cohnii subspecies cohnii and 1 micro-organism not identified by MicroScan. The presence of the mecA gene was detected in 69.1 % of strains (121/175).
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Table 1. Identification of CNS isolates and the methicillin susceptibility profile based on the presence or absence of mecA gene
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The results of phenotypic methods are shown in Table 2. The disc-diffusion test showed that 112 out of 121 strains were mecA-positive (92.5 % sensitive). On the other hand, 49 out of 54 mecA-negative strains were susceptible to methicillin (90.7 % specific) by disc-diffusion test.
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Table 2. Phenotypic test results relative to PCR for mecA gene to determine methicillin resistance R, Resistant; S, susceptible; Sens., sensitivity; Sp., specificity.
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The agar screening test containing 0.6 µg oxacillin ml–1 showed that 119 out of 121 mecA-positive strains were resistant (98.3 % sensitive). Only one S. saprophyticus mecA-negative strain was methicillin-resistant (98.1 % specific). The agar screening test containing 4 µg oxacillin ml–1 was totally accurate.
MicroScan characterized 120 out of 121 strains as methicillin-resistant. The system classified 46 out of 54 mecA-negative strains as susceptible (85.1 % specific). All strains but one S. epidermidis, described as false-positive, presented borderline MICs (Table 3).
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Table 3. Results of MICs obtained by the automated MicroScan system for false-positive strains compared to PCR for the mecA gene
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As a result of the difficulty of characterizing methicillin resistance among CNS, phenotypic methods have been constantly studied. Except for the agar screening test containing 4 µg oxacillin ml–1, all the other methods misclassified resistant and susceptible strains. The false-negative results may have occurred due to an extremely heterogeneous expression of resistance (Gerberding et al., 1991; Chambers, 1997).
On the other hand, the false-positive results may have been the result of overproduction of penicillinase, or super-expression/alterations of constitutive PBPs. It is known that penicillinase-resistant penicillins may show some degree of lysis when such enzymes are present. The super-expression and alterations of constitutive PBPs generate a higher concentration of free transpeptidase which will synthesize the bacteria's cell wall (Chambers, 1997).
Even though researchers have shown that an agar screening test containing 6 µg oxacillin ml–1 is not efficient towards CNS (Tenover et al., 1999), and it is no longer recommended for CNS (NCCLS, 1999), comparative studies to assess the agar screening test and disc-diffusion have pointed out that agar screening is more sensitive for identifying methicillin-resistant strains (York et al., 1996; Hussain et al., 1998; Frebourg et al., 1998; Kohner et al., 1999). For the agar screening test we used Mueller–Hinton agar plates supplemented with 4 % NaCl and incubated for 48 h because, under these conditions, methicillin resistance coded by the mecA gene is easily characterized (York et al., 1996).
The oxacillin concentration of 0.6 µg ml–1 proposed by Kohner et al. (1999) is based on the tenfold decrease in resistance breakpoints for CNS isolates. Rowe et al. (2002) showed that this concentration is 100 % accurate for the detection of the mecA gene in S. epidermidis but it is less accurate for all other species. We believe it is important to have accurate methods to detect methicillin resistance among less frequently found species, considering their increasing importance as opportunistic pathogens.
The agar screening test containing 4 µg oxacillin ml–1 was used based on results reached in a recent study (Ferreira et al., 2003) which analysed different antimicrobial concentrations (1, 2, 4 and 6 µg ml–1). In our study, agar screening with 4 µg oxacillin ml–1 did not present false-negative results, which makes its specificity superior to 0.6 µg oxacillin ml–1. Both concentrations correctly characterized all mecA-negative isolates, even considering less common species.
MicroScan was very specific but had low sensitivity, impairing its accuracy. The disc-diffusion test was the least accurate method, in agreement with previously reported data. Kolbert et al. (1995) showed that disc diffusion failed to characterize two mecA-positive and 13 mecA-negative strains. Three out of the false-negative isolates were ß-lactam hyper-producers. Ferreira et al. (2003) presented sensitivity and specificity comparable to the results we have obtained for disc diffusion: 94.2 and 91.8 %, respectively. In another study (Hussain et al., 1998), the researchers proved that the disc-diffusion test had low sensitivity, characterizing only 66 out of 99 mecA-positive CNS as resistant.
Finally, our results show that all phenotypic tests had excellent correlation with the genotypic characterization of resistance if the most common species, S. epidermidis, S. hominis and S. haemolyticus, are analysed. The performance of the methods was impaired when less common species were analysed, as demonstrated previously by Hussain et al. (2000) and Gradelski et al. (2001). However, even though when species other than the most frequent ones are taken into consideration, the agar screening test with 4 µg oxacillin ml–1 was totally accurate. Based on these results, and considering just the techniques we evaluated (which did not include detection of PBP2a or the recent indication of the NCCLS to use cefoxitin discs for the disc-diffusion test), we recommend this phenotypic method as the most reliable to characterize methicillin resistance among CNS isolates. Besides, it is a technically simple option for routine clinical laboratories.
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ACKNOWLEDGEMENTS
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The authors wish to thank Dr Lúcia Martins Teixeira, Instituto de Microbiologia Professor Paulo Góes, Universidade Federal do Rio de Janeiro, Brazil, for her cooperation. This study was supported in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Faculdade Federal de Ciências Médicas de Porto Alegre (FFFCMPA).
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Introduction
Methods
