Reliability of the MicroScan WalkAway PC21 panel in identifying and detecting oxacillin resistance in clinical coagulase-negative staphylococci strains

The purpose of this study was to determine the reliability of the MicroScan WalkAway PosCombo21 (PC21) system for the identification of coagulase-negative staphylococci (CNS) strains and the detection of oxacillin resistance. Using molecular and phenotypic methods, 196 clinical strains were evaluated. The automated system demonstrated 100 % reliability for the identification of the clinical strains Staphylococcus haemolyticus, Staphylococcus hominis and Staphylococcus cohnii; 98.03 % reliability for the identification of Staphylococcus epidermidis; 70 % reliability for the identification of Staphylococcus lugdunensis; 40 % reliability for the identification of Staphylococcus warneri; and 28.57 % reliability for the identification of Staphylococcus capitis, but no reliability for the identification of Staphylococcus auricularis, Staphylococcus simulans and Staphylococcus xylosus. We concluded that the automated system provides accurate results for the more common CNS species but often fails to accurately identify less prevalent species. For the detection of oxacillin resistance, the automated system showed 100 % specificity and 90.22 % sensitivity. Thus, the PC21 panel detects oxacillin-resistant strains, but is limited by the heteroresistance that is observed when using most phenotypic methods.     

Practical disk diffusion method for detection of inducible clindamycin resistance in Staphylococcus aureus and coagulase-negative staphylococci

Resistance to macrolides in staphylococci may be due to active efflux (encoded by msrA) or ribosomal target modification (macrolide-lincosamide-streptogramin B [MLSB] resistance; usually encoded by ermA or ermC). MLSB resistance is either constitutive or inducible following exposure to a macrolide. Induction tests utilize closely approximated erythromycin and clindamycin disks; the flattening of the clindamycin zone adjacent to the erythromycin disk indicates inducible MLSB resistance. The present study reassessed the reliability of placing erythromycin and clindamycin disks in adjacent positions (26 to 28 mm apart) in a standard disk dispenser, compared to distances of 15 or 20 mm. A group of 130 clinical isolates of Staphylococcus aureus and 100 isolates of erythromycin-resistant coagulase-negative staphylococci (CNS) were examined by disk approximation; all CNS isolates and a subset of S. aureus isolates were examined by PCR for ermA, ermC, and msrA. Of 114 erythromycin-resistant S. aureus isolates, 39 demonstrated constitutive resistance to clindamycin, while 33 showed inducible resistance by disk approximation at all three distances. Only one isolate failed to clearly demonstrate induction at 26 mm. Of 82 erythromycin-resistant CNS isolates that contained ermA or ermC, 57 demonstrated constitutive clindamycin resistance, and 25 demonstrated inducible resistance, at 20 and 26 mm. None of the 42 S. aureus isolates or 18 CNS isolates containing only msrA and none of the erythromycin-susceptible isolates yielded positive disk approximation tests. Simple placement of erythromycin and clindamycin disks at a distance achieved with a standard disk dispenser allowed detection of 97% of S. aureus strains and 100% of CNS strains with inducible MLSB resistance in this study.     

Correlation between the VITEK2 system and cefoxitin disk diffusion for the daily detection of oxacillin resistance in a large number of clinical Staphylococcus aureus isolates

The aim of the present study was to compare the performance of the new VITEK2 AST-P551 card with the cefoxitin disk diffusion method for the daily detection of methicillin resistance with a high number of Staphylococcus aureus clinical isolates. Detection of the PBP2a protein or mecA gene was performed for each discordant case. Seventy (3.3%) isolates out of 2,107 clinical strains showed discordant results, two very major errors, four major errors and 64 minor errors. Fifty-nine (84%) discordant results were resolved, with a final overall agreement of 99.5%. Eleven (0.5%) strains remained discordant (minor error [mE]). Four of 370 MRSA strains were misclassified as susceptible in daily practice by the cefoxitin disk diffusion method. All of these strains were resistant to aminoglycosides and/or fluoroquinolones. The VITEK2 system is highly reliable for methicillin resistance detection at the routine level. Oxacillin-susceptible classified clinical strains with associated resistance patterns required attention.     

Evaluation of new Vitek 2 card and disk diffusion method for determining susceptibility of Staphylococcus aureus to oxacillin

Detection of methicillin resistance in Staphylococcus aureus is a challenge, especially low-level resistance, which is often misdiagnosed. The aim of this study was to compare the diagnostic accuracies of the automated Vitek 2 system and disk diffusion tests, using cefoxitin and moxalactam, for the detection of methicillin resistance in S. aureus strains. Four sets of genotypically diverse isolates were selected from a national reference collection, including mecA-negative S. aureus isolates (n = 56), hospital-acquired (n = 88) and community-acquired (n = 40) S. aureus isolates, and heterogeneous methicillin-resistant S. aureus isolates (n = 29). Oxacillin susceptibility was tested by the Vitek 2 system with the AST P549 card and by disk diffusion methods using 10, 30, and 60 microg cefoxitin and 30 microg moxalactam. Oxacillin resistance was confirmed by PCR for the mecA gene. The overall sensitivities for oxacillin resistance detection were 97.5% for the Vitek 2 automated system, 98.7% for 60-microg cefoxitin and moxalactam disk diffusion, and 99.6% for 10- and 30-microg cefoxitin disks, respectively. Methicillin-susceptible S. aureus isolates were correctly reported as susceptible by all methods. The median times for methicillin testing were 7 h for the Vitek 2 system versus 24 h for disk diffusion methods. In conclusion, the cefoxitin and moxalactam disk diffusion methods and the Vitek 2 automated system are highly accurate methods for methicillin resistance detection, including a range of representative Belgian methicillin-resistant S. aureus strains and unusual strains exhibiting cryptic or low-level oxacillin resistance.     

Performance of different methods of oxacillin resistance detection in atypic strains of Staphylococcus aureus

Seventy-three of aminoglycoside-susceptible methicillin-resistant Staphylococcus aureus (AS-MRSA) and 12 kanamycin-tobramycin-resistant methicillin-susceptible S. aureus (KTR-MSSA) isolates were phenotypically and genotypically examined for methicillin susceptibility. The AS-MRSA profile represents 8.3% of MRSA strains and the KTR-MSSA profile represents 1.38% of MSSA strains. The diffusion method using the 5 microg oxacillin and 30 microg cefoxitin discs on Mueller-Hinton Agar (MHA) with and without NaCl, the incubation at 35 degrees C or 30 degrees C for 24 or 48 hours respectively, and the determining oxacillin MICs by E-test (AES, Combourg, France) were performed and used as phenotypic methods. We also used the mecA gene PCR which was considered as the "gold standard" for methicillin resistance detection, and the Slidex MRSA Detection (bioMérieux) that detect the presence of mecA gene product (PBP 2a). To increase the level of PBP 2a expression, the 30 microg cefoxitin disc was used as an inducer. All the AS-MRSA strains (100%) were detected by the cefoxitin disc in all conditions and by the oxacillin disc on MHA with 2% of NaCl at 35 degrees C. Without NaCl, the sensitivity fell to 97,2% by oxacillin disc. The oxacillin MICs for these isolates ranged from 2 to 128 mg/l. The mecA gene determinant and its product PBP 2a were detected in all AS-MRSA strains. All KTR-MSSA strains were phenotypically methicillin-susceptible and oxacillin MICs were below or borderline of breakpoint (< or =2 mg/l). The mecA gene determinant and its product were detected in one strain which was considered to be the most heterogeneous of those tested.     

Performance of MicroScan WalkAway and Vitek 2 for Detection of Oxacillin Resistance in a Set of Methicillin-Resistant Staphylococcus aureus Isolates with Diverse Genetic Backgrounds

Of 104 genotypically diverse methicillin-resistant Staphylococcus aureus (MRSA) isolates tested with the MicroScan WalkAway (Pos MIC 24 panel) and Vitek 2 (AST-P549 card) systems, 7 and 6 isolates, respectively, showed an oxacillin MIC of ≤2mg/liter. Most of these MRSA isolates were community acquired. However, if the cefoxitin screen of AST-P549 was also considered, MRSA detection failed for only one isolate.The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) has increased over the last years. Reliable detection of MRSA is important since a false report of a patient''s isolate as methicillin susceptible would result in inadequate therapy with probably fatal consequences (2). Whereas MRSA infections formerly occurred almost exclusively in hospitalized patients, community-acquired MRSA (cMRSA) isolates have been reported recently in patients without any previous contact with the health care system (7).Many laboratories rely on automatic susceptibility testing methods that use oxacillin MIC testing, oxacillin breakpoint detection in the presence of salt, or cefoxitin MIC testing as markers for the presence of methicillin resistance. Many studies have investigated the detection of MRSA by the Vitek 2 system (3, 4, 8, 11, 12, 13, 15, 17); however, data for the performance of the MicroScan WalkAway system in MRSA detection are scarce (17).Most studies evaluating the performance of Vitek 2 used consecutive clinical strains (3, 8, 11, 12, 15), but this approach may be biased by the overrepresentation of locally predominant clones and may not predict performance in other geographical areas. We therefore used a collection of MRSA strains with distinct pulsed-field gel electrophoresis (PFGE) patterns to study MRSA detection using the MicroScan WalkAway and Vitek 2 systems.From 1998 to 2006, noncopy MRSA isolates (n = 1,516), initially identified by oxacillin screening agar or Vitek, from four hospitals in the Bochum area were collected and typed by PFGE as described previously (5). Of these, 120 isolates with different PFGE patterns were chosen. The patterns were interpreted according to the criteria of Tenover et al. (18), and isolates grouped into PFGE types and subtypes.For susceptibility tests, isolates from frozen storage were subcultured twice on Columbia blood agar at 37°C in 5% CO₂ before being tested with the Vitek 2 system using the AST-P549 card and the MicroScan WalkAway system using the Pos MIC 24 panel.Whenever results for oxacillin in the Vitek 2 or MicroScan WalkAway system or for the cefoxitin screen in the Vitek 2 system were not indicative of MRSA, a mecA PCR was performed from colonies growing on purity control plates of both automatic systems and a S. aureus-specific PCR for SA442 (16) was used as an internal positive control. In addition, the Panton-Valentine leukocidin (PVL)-coding genes lukS-PVL and lukF-PVL were detected by PCR (9). SCCmec typing (10) and spa typing (6) were performed as described previously.Loss of mecA during storage of isolates could be demonstrated in 16 of 120 isolates by PCR (14), a proportion that is similar to that described before (19). Of the remaining 104 true MRSA isolates, 95 were detected as MRSA with both automatic systems.An oxacillin MIC of ≤2 mg/liter was measured for six isolates with the Vitek 2 test and for seven isolates with the WalkAway test (Table ​(Table1);1); thus, those isolates would not have been detected as MRSA based on oxacillin MICs alone. Microdilution performed according to CLSI methods (1) showed resistant oxacillin MICs for all but one of these isolates, whereas by Etest on Mueller-Hinton agar with 2% NaCl, oxacillin MICs of ≥4 mg/liter were found for only two isolates. Microcolonies were the only indication for MRSA in most of the remaining strains, demonstrating the challenge of detecting MRSA in those isolates. The cefoxitin screen incorporated in the Vitek 2 AST-P549 card was positive for five of six isolates not detected by oxacillin MIC. Thus, cefoxitin testing together with oxacillin MIC testing clearly leads to better MRSA detection. Cefoxitin MICs of ≥16 mg/liter and ≥4 mg/liter were also found by microdilution and Etest.

TABLE 1.

All test results for MRSA isolates with negative cefoxitin screen in Vitek 2 or oxacillin MIC of ≤2 mg/liter in Vitek 2 or MicroScan WalkAway assay^a

Agar dilution method for detection of inducible clindamycin resistance in Staphylococcus spp

We describe the development and validation of an agar dilution method for the detection of inducible clindamycin resistance by using 227 previously characterized erythromycin-resistant, clindamycin-susceptible Staphylococcus sp. isolates. Mueller-Hinton agar with defibrinated horse blood containing a range of erythromycin concentrations (1 to 8 mg/liter) combined with clindamycin at 0.5 mg/liter was used to determine the optimal concentration that produced growth of inducible isolates while inhibiting that of isolates without the inducible phenotype. A concentration of clindamycin of 0.5 mg/liter with erythromycin at 1 mg/liter was the optimal combination for detection of inducible resistance and resulted in a sensitivity of 100% (95% confidence interval [CI], 97.9 to 100) and a specificity of 100% (95% CI, 93.0 to 100). Attention must be paid to ensuring that a sufficient inoculum has been used, since an inoculum below the standard 10(7) bacteria/ml may result in false-negative results. This method has been incorporated into routine use in our laboratory.     

Influence of disk separation distance on accuracy of the disk approximation test for detection of inducible clindamycin resistance in Staphylococcus spp

We undertook this study to assess the accuracy of the clindamycin-erythromycin disk approximation test (D-test) for detection of inducible clindamycin resistance in Staphylococcus spp. One hundred sixty-three Staphylococcus aureus and 68 coagulase-negative Staphylococcus (CoNS) spp. which were erythromycin nonsusceptible but clindamycin susceptible were tested using the D-test performed at both 15-mm and 22-mm disk separations and compared with genotyping as the "gold standard." The rate of inducible clindamycin resistance was 96.3% for S. aureus and 33.8% for CoNS spp. The sensitivities of the D-tests performed at 15 mm and 22 mm were 100% and 87.7%, respectively, and specificities were 100% for both. The use of 22-mm disk separation for the D-test to detect inducible clindamycin resistance results in an unacceptably high very major error rate (12.3%). All isolates with false-negative results harbored the ermA gene, and the majority were methicillin-resistant Staphylococcus aureus. False-negative results were associated with smaller clindamycin zone sizes and double-edged zones. We recommend using a disk separation distance of 相似文献   

Methods for the detection of resistance to oxacillin in strains of Staphylococcus aureus

Methods suitable for detection of resistance of staphylococci to oxacillin were tested in a group of 77 strains of Staphylococcus aureus (39 strains sensitive and 38 strains resistant to oxacillin). The influence of the composition of the medium, the growth phase of the inoculum and time of incubation on detection of resistant strains was investigated. By none of the methods resistance to oxacillin was proved in the control group of sensitive strains. For evidence of oxacillin resistance after 24 hours' incubation the standard micro-method is suitable and the diffuse method with 1 microgram disc of oxacillin which both detected 100% of resistant strains, and the screening method with a yield of 97% strains. The growth phase of the inoculum and the incubation period do not influence the results of these methods. The micromethod in MH broth, the diffuse method with a 10 micrograms oxacillin disc and the dilution plate method were influenced to a considerable extent by the composition of the medium, the growth phase of the inoculum, the incubation period and they revealed a small number of resistant strains.     

Two new colorimetric methods for early detection of vancomycin and oxacillin resistance in Staphylococcus aureus

We developed two colorimetric methods for the detection of vancomycin- and oxacillin-resistant Staphylococcus aureus in 相似文献   

Optimal inoculation methods and quality control for the NCCLS oxacillin agar screen test for detection of oxacillin resistance in Staphylococcus aureus

To define more precisely the inoculation methods to be used in the oxacillin screen test for Staphylococcus aureus, we tested agar screen plates prepared in house with 6 microg of oxacillin/ml and 4% NaCl using the four different inoculation methods that would most likely be used by clinical laboratories. The organisms selected for testing were 19 heteroresistant mecA-producing strains and 41 non-mecA-producing strains for which oxacillin MICs were near the susceptible breakpoint. The inoculation method that was preferred by all four readers and that resulted in the best combination of sensitivity and specificity was a 1-microl loopful of a 0.5 McFarland suspension. A second objective of the study was to then use this method to inoculate plates from five different manufacturers of commercially prepared media. Although all commercial media performed with acceptable sensitivity compared to the reference lot, one of the commercial lots demonstrated a lack of specificity. Those lots of oxacillin screen medium that fail to grow heteroresistant strains can be detected by using S. aureus ATCC 43300 as a positive control in the test and by using transmitted light to carefully examine the plates for any growth. However, lack of specificity with commercial lots may be difficult to detect using any of the current quality control organisms.     

Evaluation of different disk diffusion/media combinations for detection of methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci

In order to find a disk diffusion method with both high sensitivity and specificity for determination of methicillin resistance primarily for S. aureus but also for coagulase-negative staphylococci we screened several methodological variants using a material of 66 S. aureus comprising of 11 methicillin-susceptible, 18 borderline-resistant, and 37 methicillin-resistant strains. Only four of the combinations studied performed with both high sensitivity and specificity. Two of these, the Columbia agar +4.5% NaCl and Mueller Hinton agar +2% NaCl combined with a 5 microg oxacillin disk, confluent inoculum and 24 h incubation at 35 degrees C were further evaluated using 105 MRSA and 91 mecA-negative S. aureus and 193 clinical isolates of coagulase-negative staphylococci. The Columbia agar +4.5% NaCl performed excellently for both S. aureus and coagulase-negative staphylococci. For Columbia agar +4.5% NaCl using a 5 microg oxacillin disk we suggest an interpretive zone diameter of R < or =15 mm and S > or =16 mm for S. aureus and R < or =24 mm and S >or =26 mm for coagulase-negative staphylococci. The Mueller Hinton agar +2% NaCl performed well for coagulase-negative staphylococci but for S. aureus at least three (3%) very major errors were found, making this method less attractive.     

Phenotypic detection of methicillin resistance in Staphylococcus aureus by disk diffusion testing and Etest on Mueller-Hinton agar

Cefoxitin is increasingly recommended for detection of methicillin resistance in Staphylococcus aureus (MRSA) when using disk diffusion testing. In this study, 95 mecA-negative S. aureus isolates and a highly genetically diverse collection of mecA-positive S. aureus types (n=50) were used to investigate the influence of technical factors such as disk potency, incubation time, and temperature on Mueller-Hinton agar. The use of cefoxitin MIC testing by Etest for the same purpose was investigated under similar conditions. For disk diffusion, the accuracy was high at both 35 degrees C and 36 degrees C using overnight incubation, while incubation at 30 degrees C or 37 degrees C was associated with slightly lower accuracy. Increasing incubation times from 18 to 24 h did not improve accuracy at either temperature. Cefoxitin Etest MICs for mecA-positive strains were 6 mg/liter or higher, while cefoxitin Etest MICs for mecA-negative strains were 4 mg/liter, corresponding to S>or=22 mm and Ror=17 mm and R相似文献   

Spurious oxacillin resistance in Staphylococcus aureus because of defective oxacillin disks.

Clinical isolates of Staphylococcus aureus wee inappropriately categorized as intermediate or resistant to oxacillin on the basis of tests with two lots of oxacillin disks. The potency of one lot was tested and found to be below accepted limits. Routine quality control tests failed to detect the defective disks.     

Evaluation of a cefoxitin disk diffusion test for the routine detection of methicillin-resistant Staphylococcus aureus

Two oxacillin disk methods were compared with a cefoxitin disk diffusion test for detection of methicillin-resistant Staphylococcus aureus (MRSA), with PCR for mecA as the reference method. When tested with 115 MRSA and 350 methicillin-susceptible S. aureus isolates, the cefoxitin disk test (specificity 100%, sensitivity 96.5%) was superior to the oxacillin disk methods (specificity 99.1%, sensitivity 90.4%). Testing with both oxacillin and cefoxitin disks would give better sensitivity (100%) than the cefoxitin test alone, but at the expense of specificity (99.1%). The cefoxitin disk test required no special test conditions and would improve the reliability of routine tests for detection of MRSA.     

Staphylococcus aureus: new detection of intrinsic resistance using the diffusion method

Because of an heterogeneously-expressed resistance among methicillin-resistant Staphylococcus aureus strains the conditions for antibiogram determination had rapidly to be modified so as to improve their detection. The newly recommended conditions (incubation at +30 degrees C or on hypersalted agar medium) remain widely used at the moment, although they appear to be more and more often badly adapted, particularly because of the recently-observed renewed outbreak of wild strains with a weak in vitro phenotypic expression. It is the reason why we searched for a new and more reliable phenotypic method although still accessible for any laboratory. Sixty-five strains of Staphylococcus aureus entered the study. The absence or presence of mecA gene was previously investigated by gene amplification. These strains were of various origins and had often caused difficulties for the detection of intrinsic resistance to methicillin on the antibiogram. Our results confirm the failures of the classical methods (false negative results at +30 degrees C, false negative or positive results on hypersalted agar medium incubated at +37 degrees C). They also allow to propose a new method which relies on the determination of the susceptibility to cefoxitin using the usual conditions for antibiogram determination. In our series of strains, this new method proved to widely improve both the sensitivity and the susceptibility for the detection of methicillin-resistance by diffusion on the antibiogram.     

Evaluation of a disk method for detection of hippurate hydrolysis by Campylobacter spp

A disk method for detecting hippurate hydrolysis by Campylobacter spp. was evaluated and compared with the conventional tube test used at the Centers for Disease Control (CDC) (G.K. Morris and C.M. Patton, p. 302-308, in E.H. Lennette, A. Balows, W.J. Hausler, Jr., and H.J. Shadomy, ed., Manual of Clinical Microbiology, 4th ed., 1985) and high-performance liquid chromatography (HPLC). A total of 118 Campylobacter strains were tested. Eighty-seven strains (74%) were hippurate positive by the HPLC method, and the remaining 31 (26%) were found to be hippurate negative. By using HPLC as the reference technique, the CDC method showed a sensitivity of 80% and a specificity of 81%; the disk method showed a sensitivity of 93% and a specificity of 94%. The disk method can be performed with a small inoculum of bacteria, did not present problems of interpretation, and showed better results than the CDC method (P = 0.015).     

Performance of eight methods, including two new rapid methods, for detection of oxacillin resistance in a challenge set of Staphylococcus aureus organisms

Using a set of 55 Staphylococcus aureus challenge organisms, we evaluated six routine methods (broth microdilution, disk diffusion, oxacillin agar screen, MicroScan conventional panels, MicroScan rapid panels, and Vitek cards) currently used in many clinical laboratories and two new rapid methods, Velogene and the MRSA-Screen, that require less than a day to determine the susceptibility of S. aureus to oxacillin. The methods were evaluated by using the presence of the mecA gene, as detected by PCR, as the "gold standard." The strains included 19 mecA-positive heterogeneously resistant strains of expression class 1 or 2 (demonstrating oxacillin MICs of 4 to >16 microg/ml) and 36 mecA-negative strains. The oxacillin MICs of the latter strains were 0.25 to 4 microg/ml when tested by broth microdilution with 2% NaCl-supplemented cation-adjusted Mueller-Hinton broth as specified by the NCCLS. However, when tested by agar dilution with 4% salt (the conditions used in the oxacillin agar screen method), the oxacillin MICs of 16 of the mecA-negative strains increased to 4 to 8 microg/ml. On initial testing, the percentages of correct results (% sensitivity/% specificity) were as follows: broth microdilution, 100/100; Velogene, 100/100; Vitek, 95/97; oxacillin agar screen, 90/92; disk diffusion, 100/89; MicroScan rapid panels, 90/86; MRSA-Screen, 90/100; and MicroScan conventional, 74/97. The MRSA-Screen sensitivity improved to 100% if agglutination reactions were read at 15 min. Repeat testing improved the performance of some but not all of the systems.     

Performance of CHROMagar selective medium and oxacillin resistance screening agar base for identifying Staphylococcus aureus and detecting methicillin resistance

Two new selective media, oxacillin resistance screening agar base (ORSAB) and CHROMagar Staph aureus (CSA), were evaluated for identification of Staphylococcus aureus and for screening of methicillin resistance by addition of antimicrobial agents to these media. A well-defined collection consisting of 1,140 staphylococci was used. A total of 624 were S. aureus, of which 358 were methicillin susceptible and 266 were methicillin resistant, and 516 were coagulase-negative staphylococci. The methicillin-resistant S. aureus (MRSA) strains were selected based on the results of phage typing; 247 different types were included in the analysis. For identification of S. aureus, both media performed better after 24 h than after 48 h. The sensitivities at 24 h were comparable (CSA, 98.6%; ORSAB, 97.1%), but the specificity of CSA was significantly higher (CSA, 97.1%; ORSAB, 92.1%). For screening of methicillin resistance, antibiotic supplements were added to both media. The sensitivity was lower after 24 h (CSA, 58.6%; ORSAB, 84.2%) and increased significantly after 48 h (CSA, 77.5%; ORSAB, 91.4%). At both time intervals ORSAB was significantly more sensitive than CSA. However, the specificities of both media were high after 24 h (CSA, 99.1%; ORSAB, 98.3%) and decreased significantly after 48 h of incubation (CSA, 94.7%; ORSAB, 95.5%). In conclusion, for identification of S. aureus, CSA is more accurate than ORSAB because of a significantly higher specificity. For screening of MRSA, ORSAB performs better than CSA, but the usefulness in clinical practice is limited because a significant number of strains are not detected.