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Mupirocin susceptibility testing of Staphylococcus aureus has become more important as mupirocin is used more widely to suppress or eliminate S. aureus colonization and prevent subsequent health care- and community-associated infections. The present multicenter study evaluated two susceptibility testing screening methods to detect mupirocin high-level resistance (HLR), broth microdilution (BMD) MICs of >or=512 microg/ml, and a 6-mm zone diameter for a disk diffusion (DD) test with a 200-microg disk. Initial testing indicated that with Clinical and Laboratory Standards Institute methods for BMD and DD testing, the optimal conditions for the detection of mupirocin HLR were 24 h of incubation and reading of the DD zone diameters with transmitted light. Using the presence or absence of mupA as the "gold standard" for HLR, the sensitivity and specificity of a single-well 256 microg/ml BMD test were 97 and 99%, respectively, and those for the 200-microg disk test were 98 and 99%, respectively. Testing with two disks, 200 microg and 5 microg, was evaluated for its ability to distinguish HLR isolates (MICs >or= 512 microg/ml), low-level-resistant (LLR) isolates (MICs = 8 to 256 microg/ml), and susceptible isolates (MICs

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We compared the results obtained with six commercial MIC test systems (Etest, MicroScan, Phoenix, Sensititre, Vitek Legacy, and Vitek 2 systems) and three reference methods (agar dilution, disk diffusion, and vancomycin [VA] agar screen [VScr]) with the results obtained by the Clinical and Laboratory Standards Institute broth microdilution (BMD) reference method for the detection of VA-intermediate Staphylococcus aureus (VISA). A total of 129 S. aureus isolates (VA MICs by previous BMD tests,

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This report describes the results of an 11-laboratory study to determine if a cefoxitin broth microdilution MIC test could predict the presence of mecA in staphylococci. Using breakpoints of < or = 4 microg/ml for mecA-negative and > or = 6 or 8 microg/ml for mecA-positive isolates, sensitivity and specificity based on mecA or presumed mecA for Staphylococcus aureus at 18 h of incubation were 99.7 to 100% in three cation-adjusted Mueller-Hinton broths tested. For coagulase-negative strains at 24 h of incubation, breakpoints of < or = 2 microg/ml for mecA-negative and > or = 4 microg/ml for mecA-positive isolates gave sensitivity and specificity of 94 to 99% and 69 to 80%, respectively.

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A study conducted by 11 laboratories investigated the ability of four combinations of erythromycin (ERY) and clindamycin (CC) (ERY and CC at 4 and 0.5, 6 and 1, 8 and 1.5, and 0.5 and 2 microg/ml) in a single well of a broth microdilution panel to predict the presence of inducible CC resistance. Each laboratory tested approximately 30 Staphylococcus aureus isolates and 20 coagulase-negative staphylococcus (CoNS) isolates in a panel using cation-adjusted Mueller-Hinton broth from three different manufacturers. Only the strains resistant to ERY and those susceptible or intermediate to CC were included in the analysis (S. aureus, n = 333; CoNS, n = 97). Results of the D-zone test were used as the gold standard. After an 18-h incubation, the combination of 4 microg/ml ERY and 0.5 microg/ml CC performed the best, with 98 to 100% sensitivity and 100% specificity for both organism groups. After a 24-h incubation, the ERY-CC combinations of 4 and 0.5, 6 and 1, and 8 and 1.5 microg/ml correlated well with the D-zone test.

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We reevaluated Enterobacteriaceae disk diffusion breakpoints for the tetracyclines published in the Clinical and Laboratory Standards Institute (CLSI) document M100-S16, which were (susceptible/resistant) >or=19 mm/or=16 mm/or=19 mm/or=15 mm/or=14 mm/or=16 mm/

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Phenotypic methods for detecting mecA-mediated resistance in Staphylococcus aureus include both oxacillin and cefoxitin susceptibility tests; many laboratories perform multiple tests. Conflicting oxacillin and cefoxitin susceptibility results are most likely to occur for isolates that either have reduced susceptibility to oxacillin by a non-mecA-mediated mechanism or are mecA positive but are very heteroresistant. To understand the performance of oxacillin and cefoxitin tests for such isolates, we tested 135 S. aureus isolates using either cefoxitin or oxacillin and compared the results with mecA polymerase chain reaction. These strains either expressed borderline oxacillin MICs (1-4 microg/mL) and had undetermined mecA status or were mecA positive but were not detected by oxacillin broth microdilution (BMD) or disk diffusion (DD) in original testing. For 24-h readings, performance of cefoxitin tests (sensitivity/specificity) were DD (99/100), Etest using < or =6 microg/mL as susceptible (99/98), and Phoenix MIC using < or =4 microg/mL as susceptible (98/100). Using 6 microg/mL of cefoxitin as a screen test in both BMD and agar dilution also worked well (98/98-100). Sensitivity/specificity of oxacillin methods were oxacillin agar screen (BBL: 80/86; Remel, Lenexa, KS: 85/50), DD (91/59), BMD (85/88), MicroScan (89/96), VITEK Legacy (82/93), VITEK 2 (91/73), and Phoenix, (67/96). These results suggest that a cefoxitin test can be used alone to predict mecA-mediated resistance in S. aureus.

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Acinetobacter sp. isolates having multidrug resistance (MDR) patterns have become common in many medical centers worldwide, limiting therapeutic options. A five-center study tested 103 contemporary clinical Acinetobacter spp., including MDR strains, by reference broth microdilution and disk diffusion (15-mug disk content) methods against tigecycline. Applying U.S. Food and Drug Administration tigecycline breakpoint criteria for Enterobacteriaceae (susceptibility at < or =2 microg/ml [< or =1 microg/ml by the European Committee on Antimicrobial Susceptibility Testing]; disk diffusion breakpoints at > or =19 mm and < or =14 mm) to Acinetobacter spp. led to an unacceptable error rate (23.3%). However, an adjustment of tigecycline disk diffusion breakpoints (susceptible/resistant) to > or =16/ < or =12 mm reduced intermethod errors to an acceptable level (only 9.7%, all minor).

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The cefoxitin disk diffusion (DD) test for predicting mecA-mediated oxacillin resistance in staphylococci was assessed during a three-phase study. In phase 1, one laboratory tested 62 and 53 strains of Staphylococcus aureus and coagulase-negative staphylococci (CoNS), respectively. These data were used to choose the provisional cefoxitin DD breakpoints (resistant/susceptible) of < or =19 mm/> or =20 mm for S. aureus and < or =24 mm/> or =25 mm for CoNS for the next phase of testing. In phase 2, 10 laboratories each tested approximately 40 in-house strains of staphylococci (half of which were S. aureus) using Mueller-Hinton agar from different manufacturers. In this phase, the sensitivity and specificity, respectively, of the cefoxitin disk test were 98 and 100% for S. aureus and 99 and 96% for CoNS. The cefoxitin DD test performed equivalently to oxacillin broth microdilution (BMD) and to oxacillin DD tests among S. aureus and mecA-positive CoNS strains but gave better results than oxacillin BMD or oxacillin DD for mecA-negative strains of CoNS. The cefoxitin DD test also was much easier to read and did not require the use of transmitted light for detection of resistance. Based on data from the first two phases, the Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS) adopted the use of the cefoxitin DD test for predicting mecA-mediated oxacillin resistance in staphylococci and revised Table 2C in CLSI document M100-S14 to reflect the change. In the third phase, an additional 61 challenge strains of CoNS for which the oxacillin MICs were 0.5 to 2 microg/ml were tested in a single laboratory to determine the effectiveness of the cefoxitin DD test for this group of borderline-resistant isolates. These data were used to refine the description of the test in CLSI document M100-S15. The cefoxitin DD test is preferred over the oxacillin DD test for predicting mecA-mediated oxacillin resistance in S. aureus and CoNS.

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An eight-laboratory study addressed the urgent need for quality control (QC) ranges for susceptibility determination when testing colistin (polymyxin E) and polymyxin B, two polycationic peptide antimicrobial agents, against multidrug-resistant gram-negative bacilli. For Escherichia coli ATCC 25922l, the QC ranges were as follows: for colistin, 0.25 to 1 microg/ml (11 to 17 mm), and for polymyxin B, 0.25 to 2 microg/ml (13 to 19 mm). For Pseudomonas aeruginosa ATCC 27853, the QC ranges were as follows: for colistin, 0.25 to 2 microg/ml (11 to 17 mm), and for polymyxin B, 0.25 to 2 mug/ml (14 to 18 mm). More than 97% of all reported QC results were within these proposed ranges.

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Although both broth microdilution (BMD) and disk diffusion (DD) are listed by NCCLS as acceptable methods for testing Acinetobacter spp. for antimicrobial susceptibility, few studies have compared the results generated by the two methods. We tested 196 isolates of Acinetobacter spp. from nine U.S. hospitals and from the Centers for Disease Control culture collection by using BMD and DD and clinically appropriate antimicrobial agents. Categorical results for amikacin, ciprofloxacin, gatifloxacin, gentamicin, imipenem, levofloxacin, meropenem, tobramycin, and trimethoprim-sulfamethoxazole were comparable for the two methods: there was only one very major (VM) error, with tobramycin, and only one major (M) error, with meropenem, when DD results were compared with BMD results. However, VM errors were frequent with the beta-lactams and beta-lactam-beta-lactam inhibitor combinations, while M errors were often observed with tetracyclines. For BMD, tests frequently exhibited subtle growth patterns that were difficult to interpret, especially for beta-lactams. If subtle growth (i.e., granular, small button, or "starry" growth) was considered positive, error rates between BMD and DD were unacceptably high for ampicillin-sulbactam (VM error, 9.8%; minor [m] error, 16.1%), piperacillin (VM error, 5.7%; m error, 13.5%), piperacillin-tazobactam (VM error, 9.3%; m error, 12.9%), ceftazidime (VM error, 6.2%; m error, 11.4%), cefepime (VM error, 6.2%; m error, 13.0%), cefotaxime (m error, 21.2%), ceftriaxone (m error, 23.3%), tetracycline (M error, 11.4%; m error, 32.1%), and doxycycline (M error, 2.6%). When subtle growth patterns were ignored, the agreement still did not achieve acceptable levels. To determine if the problems with BMD testing occurred in other laboratories, we sent frozen BMD panels containing beta-lactam drugs and nine isolates to six labs with experience in performing BMD and DD. Among these laboratories, cefepime MICs ranged from < or =8 to > or =32 microg/ml for four of the nine strains, confirming the problem in interpreting BMD results. Discrepancies between the categorical interpretations of BMD and DD tests were noted primarily with cefepime and piperacillin, for which the BMD results were typically more resistant. Clinical laboratories should be aware of these discrepancies. At present, there are no data to indicate which method provides more clinically relevant information.

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From January 1996 to May 1999, Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) received 448 nonduplicate clinical isolates of Enterobacteriaceae and Pseudomonas aeruginosa that were reported to be imipenem intermediate or resistant. However, broth microdilution (BMD) confirmatory testing at the Project ICARE central laboratory confirmed this result in only 11 of 123 (8.9%) Enterobacteriaceae isolates and 241 of 325 (74.2%) P. aeruginosa isolates. To investigate this overdetection of imipenem resistance, we tested 204 selected isolates from the Project ICARE collection plus five imipenem-resistant challenge strains at the Centers for Disease Control and Prevention against imipenem and meropenem by agar dilution, disk diffusion, Etest (AB BIODISK North America, Inc., Piscataway, N.J.), two MicroScan WalkAway conventional panels (Neg MIC Plus 3 and Neg Urine Combo 3) (Dade MicroScan, Inc., West Sacramento, Calif.), and two Vitek cards (GNS-116 containing meropenem and GNS-F7 containing imipenem) (bioMérieux Vitek, Inc., Durham, N.C.). The results of each test method were compared to the results of BMD testing using in-house-prepared panels. Seven imipenem-resistant and five meropenem-resistant isolates of Enterobacteriaceae and 43 imipenem-resistant and 21 meropenem-resistant isolates of P. aeruginosa were identified by BMD. For Enterobacteriaceae, the imipenem and meropenem test methods produced low numbers of very major and major errors. All test systems in the study produced low numbers of very major and major errors when P. aeruginosa was tested against imipenem and meropenem, except for Vitek testing (major error rate for imipenem, 20%). Further testing conducted in 11 of the participating ICARE hospital laboratories failed to pinpoint the factors responsible for the initial overdetection of imipenem resistance. However, this study demonstrated that carbapenem testing difficulties do exist and that laboratories should consider using a second, independent antimicrobial susceptibility testing method to validate carbapenem-intermediate and -resistant results.

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The need for new antimicrobial agents with activity against Gram-positive organisms has become increasingly important because of emerging resistance. We compared the activity of a new b-lactam antimicrobial agent, RWJ-54428 (MC-02 479), with representatives of other classes of antimicrobial agents against 76 Staphylococcus aureus (including four glycopeptide- intermediate strains), 50 coagulase-negative staphylococci, 20 Enterococcus faecalis, 20 Enterococcus faecium, 10 Enterococcus gallinarum/Enterococcus casseliflavus, 54 Streptococcus pneumoniae and 22 viridans streptococcal isolates. The MIC(90) of RWJ-54,428 was < or = 2 mg/L for all groups of bacteria tested except E. faecium. The activity against four strains of glycopeptide-intermediate S. aureus was similar to that for other methicillin-resistant S. aureus isolates (range 0.5-2.0 mg/L).

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