Methicillin-resistant Staphylococcus aureus.
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Community-Acquired Infections↗
Biomedical subjects
Fred C Tenover
Publications and source records attributed to Fred C Tenover.
Genetic analysis of a high-level vancomycin-resistant isolate of Staphylococcus aureus.
Vancomycin is usually reserved for treatment of serious infections, including those caused by multidrug-resistant Staphylococcus aureus. A clinical isolate of S. aureus with high-level resistance to vancomycin (minimal inhibitory concentration = 1024 microg/ml) was isolated in June 2002. This isolate harbored a 57.9-kilobase multiresistance conjugative plasmid within which Tn1546 (vanA) was integrated. Additional elements on the plasmid encoded resistance to trimethoprim (dfrA), beta-lactams (blaZ), aminoglycosides (aacA-aphD), and disinfectants (qacC). Genetic analyses suggest that the long-anticipated transfer of vancomycin resistance to a methicillin-resistant S. aureus occurred in vivo by interspecies transfer of Tn1546 from a co-isolate of Enterococcus faecalis.
Anti-Bacterial Agents↗
Vancomycin-intermediate Staphylococcus aureus with phenotypic susceptibility to methicillin in a patient with recurrent bacteremia.
Vancomycin-intermediate Staphylococcus aureus (VISA) are an emerging problem. We observed a statistically significant inverse relationship in the MICs of vancomycin and oxacillin in S. aureus isolates from a patient undergoing hemodialysis who received 26 weeks of treatment with vancomycin during November 1999 through April 2000. All isolates were mecA positive and were indistinguishable by pulsed-field gel electrophoresis. The evolving susceptibility patterns of this strain highlight the challenges of detecting and treating VISA infections.
Bacteremia↗
Infection with vancomycin-resistant Staphylococcus aureus containing the vanA resistance gene.
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Adult↗
Epidemiological and microbiological characterization of infections caused by Staphylococcus aureus with reduced susceptibility to vancomycin, United States, 1997-2001.
Infections caused by Staphylococcus aureus with reduced vancomycin susceptibility (SA-RVS; minimum inhibitory concentration [MIC], >or=4 microg/mL), including vancomycin-intermediate S. aureus (VISA; MIC, 8 microg/mL), are a new clinical and public health dilemma. Prospective surveillance and a nested case-control study of patients in the United States infected with SA-RVS was conduced from March 1999 through December 2000. Control patients were persons infected with oxacillin-resistant S. aureus (MIC of vancomycin, <or=2 microg/mL). Among 19 case patients, 4 infections were due to VISA and 15 were due to non-VISA SA-RVS. Case patients with and those without VISA infection had similar clinical presentations and outcomes; the overall attributable mortality rate was 63%. Isolates recovered from case patients had heterogeneous pulsed-field gel electrophoresis banding patterns, regardless of the MIC of vancomycin. Neither dialysis nor chronic renal failure were predictive of case status compared with control status. Independent risk factors for being a case patient included antecedent vancomycin use and prior oxacillin-resistant S. aureus infection 2 or 3 months before the current infection.
Adult↗
Macrolide-resistant pneumococcal endocarditis and epidural abscess that develop during erythromycin therapy.
Suppurative complications of Streptococcus pneumoniae infections have become uncommon in the antibiotic era. We report a case of pneumococcal bacteremia and pneumonia complicated with epidural abscess and endocarditis in which macrolide resistance (the MLS(B) phenotype) emerged during erythromycin therapy. Genetic determinants known to mediate the most common mechanisms of macrolide resistance (methylation of the 23S rRNA and antibiotic efflux) were not detected by polymerase chain reaction or DNA hybridization. Sequence analysis of the DNA encoding the 23S rRNA of the macrolide-resistant isolate from the patient demonstrated the replacement of adenine by thymine at position 2058 (A2058T) in 2 of 4 alleles. Clinicians should be alert to the possibility of the emergence of resistance during macrolide therapy for community-acquired pneumonia, particularly if suppurative complications of pneumococcal infection are suspected.
Anti-Bacterial Agents↗
Detection of antimicrobial resistance by small rural hospital microbiology laboratories: comparison of survey responses with current NCCLS laboratory standards.
Microbiology laboratory personnel from 77 rural hospitals in Idaho, Nevada, Utah, and eastern Washington were surveyed in July 2000 regarding their routine practices for detecting antimicrobial resistance. Their self-reported responses were compared to recommended laboratory practices. Most hospitals reported performing onsite bacterial identification and susceptibility testing. Many reported detecting targeted antimicrobial resistant organisms. While only 5/61 hospitals (8%) described using screening tests capable of detecting all 8 targeted types of resistance, most (57/61, 93%) were capable of accurately screening for at least 6 types. Conversely, most hospitals (58/61, 95%) reported confirmatory testing capable of identifying only 3 or fewer resistance types with high-level penicillin resistance among pneumococci, methicillin and vancomycin resistance among staphylococci and enterococci, and extended spectrum beta-lactamase production by Gram-negative bacilli presenting the greatest difficulties. Furthermore, only 50% of hospitals compiled annual antibiogram reports to help physicians choose initial therapy for suspected infectious illnesses. This survey suggests that the antimicrobial susceptibility testing in many rural hospitals may be unreliable.
Anti-Bacterial Agents↗
Antimicrobial proficiency testing of National Nosocomial Infections Surveillance System hospital laboratories.
OBJECTIVE: The National Nosocomial Infections Surveillance (NNIS) System personnel report trends in antimicrobial-resistant pathogens. To validate select antimicrobial susceptibility testing results and to identify test methods that tend to produce errors, we conducted proficiency testing among NNIS System hospital laboratories. SETTING: NNIS System hospital laboratories in the United States. METHODS: Each laboratory received five organisms (ie, an imipenem-resistant Serratia marcescens, an oxacillin-resistant Staphylococcus aureus, a vancomycin-resistant Enterococcus faecalis, a vancomycin-intermediate Staphylococcus epidermidis, and an extended-spectrum beta-lactamase (ESbetaL)-producing Klebsiella pneumoniae). Testing results were compared with reference testing results from the Centers for Disease Control and Prevention. RESULTS: Of 138 laboratories testing imipenem against the Serratia marcescens strain, 110 (80%) correctly reported minimum inhibitory concentrations (MICs) or zone sizes in the resistant range. All 193 participating laboratories correctly reported the Staphylococcus aureus strain as oxacillin resistant Of the 193 laboratories, 169 (88%) reported correct MICs or zone sizes for the vancomycin-resistant Enterococcus faecalis. One hundred sixty-two (84%) of 193 laboratories demonstrated the ability to detect a vancomycin-intermediate strain of Staphylococcus epidermidis, however, disk diffusion performed poorly when testing both staphylococci and enterococci with vancomycin. Although laboratory personnel correctly reported nonsusceptible extended-spectrum cephalosporins and aztreonam results for K. pneumoniae, only 98 (51%) of 193 correctly reported this organism as an ESbetaL producer. CONCLUSION: Overall, NNIS System hospital laboratory personnel detected most emerging resistance patterns. Disk diffusion continues to be unreliable for vancomycin testing of staphylococci and must be used cautiously for enterococci. Further education on the processing of ESbetaL-producing organisms is warranted.
Bacteria↗
In vitro activities of Daptomycin, Linezolid, and Quinupristin-Dalfopristin against a challenge panel of Staphylococci and Enterococci, including vancomycin-intermediate staphylococcus aureus and vancomycin-resistant Enterococcus faecium.
We assessed the in vitro activities of daptomycin, linezolid, and quinupristin-dalfopristin (QD) against a contemporary challenge panel of 88 staphylococcal and 90 enterococcal isolates. The staphylococci selected included vancomycin-intermediate Staphylococcus aureus (VISA), methicillin-resistant S. aureus, and coagulasenegative staphylococci. Enterococcal isolates included vancomycin-resistant Enterococcus faecium (VREF) containing either vanA, vanB1, or vanD. The MICs of daptomycin, linezolid, and QD were determined using commercial broth microdilution panels. All three VISA isolates were susceptible to daptomycin, linezolid, and QD. QD was the most active agent against staphylococcal isolates (MIC50 < or = 0.5 microg/ml and MIC90 = 1 microg/ml), including those with decreased susceptibility to vancomycin. QD was also the most active agent against VREF (MIC90 < or = 0.5 microg/ml). No differences were seen for susceptibility of vanA, vanB1, and vanD VREF strains for daptomycin, linezolid, or QD. Daptomycin was the most effective against E. faecalis. On the basis of manufacturer-suggested interpretive criteria, 92% of isolates were susceptible (MIC90 = 4 microg/ml). All isolates tested were susceptible to at least one antimicrobial agent for which interpretive criteria have been defined. Population analysis of three S. aureus isolates for which the daptomycin MICs were 8 microg/ml showed a pattern of homogeneous resistance.
Acetamides↗
Carbapenem-resistant strain of Klebsiella oxytoca harboring carbapenem-hydrolyzing beta-lactamase KPC-2.
We investigated a Klebsiella oxytoca isolate demonstrating resistance to imipenem, meropenem, extended-spectrum cephalosporins, and aztreonam. The MICs of both imipenem and meropenem were 32 microg/ml. The beta-lactamase activity against imipenem and meropenem was inhibited in the presence of clavulanic acid. Isoelectric focusing studies demonstrated five beta-lactamases with pIs of 8.2 (SHV-46), 6.7 (KPC-2), 6.5 (unknown), 6.4 (probable OXY-2), and 5.4 (TEM-1). The presence of the bla(SHV) and bla(TEM) genes was confirmed by specific PCR assays and DNA sequence analysis. Transformation and conjugation studies with Escherichia coli showed that the beta-lactamase with a pI of 6.7, Klebsiella pneumoniae carbapenemase-2 (KPC-2), was encoded on an approximately 70-kb conjugative plasmid that also carried SHV-46, TEM-1, and the beta-lactamase with a pI of 6.5. The bla(KPC-2) determinant was cloned in E. coli and conferred resistance to imipenem, meropenem, extended-spectrum cephalosporins, and aztreonam. The amino acid sequence of KPC-2 showed a single amino acid difference, S174G, when compared with KPC-1, another carbapenem-hydrolyzing beta-lactamase from K. pneumoniae 1534. Hydrolysis studies showed that purified KPC-2 hydrolyzed not only carbapenems but also penicillins, cephalosporins, and aztreonam. KPC-2 had the highest affinity for meropenem. The kinetic studies revealed that KPC-2 was inhibited by clavulanic acid and tazobactam. An examination of the outer membrane proteins of the parent K. oxytoca strain demonstrated that it expressed detectable levels of OmpK36 (the homolog of OmpC) and a higher-molecular-weight OmpK35 (the homolog of OmpF). Thus, carbapenem resistance in K. oxytoca 3127 is due to production of the Bush group 2f, class A, carbapenem-hydrolyzing beta-lactamase KPC-2. This beta-lactamase is likely located on a transposon that is part of a conjugative plasmid and thus has a very high potential for dissemination.
Amino Acid Sequence↗
Beta-lactamase genes of the penicillin-susceptible Bacillus anthracis Sterne strain.
Susceptibility to penicillin and other beta-lactam-containing compounds is a common trait of Bacillus anthracis. Beta-lactam agents, particularly penicillin, have been used worldwide to treat anthrax in humans. Nonetheless, surveys of clinical and soil-derived strains reveal penicillin G resistance in 2 to 16% of isolates tested. Bacterial resistance to beta-lactam agents is often mediated by production of one or more types of beta-lactamases that hydrolyze the beta-lactam ring, inactivating the antimicrobial agent. Here, we report the presence of two beta-lactamase (bla) genes in the penicillin-susceptible Sterne strain of B. anthracis. We identified bla1 by functional cloning with Escherichia coli. bla1 is a 927-nucleotide (nt) gene predicted to encode a protein with 93.8% identity to the type I beta-lactamase gene of Bacillus cereus. A second gene, bla2, was identified by searching the unfinished B. anthracis chromosome sequence database of The Institute for Genome Research for open reading frames (ORFs) predicted to encode beta-lactamases. We found a partial ORF predicted to encode a protein with significant similarity to the carboxy-terminal end of the type II beta-lactamase of B. cereus. DNA adjacent to the 5' end of the partial ORF was cloned using inverse PCR. bla2 is a 768-nt gene predicted to encode a protein with 92% identity to the B. cereus type II enzyme. The bla1 and bla2 genes confer ampicillin resistance to E. coli and Bacillus subtilis when cloned individually in these species. The MICs of various antimicrobial agents for the E. coli clones indicate that the two beta-lactamase genes confer different susceptibility profiles to E. coli; bla1 is a penicillinase, while bla2 appears to be a cephalosporinase. The beta-galactosidase activities of B. cereus group species harboring bla promoter-lacZ transcriptional fusions indicate that bla1 is poorly transcribed in B. anthracis, B. cereus, and B. thuringiensis. The bla2 gene is strongly expressed in B. cereus and B. thuringiensis and weakly expressed in B. anthracis. Taken together, these data indicate that the bla1 and bla2 genes of the B. anthracis Sterne strain encode functional beta-lactamases of different types, but gene expression is usually not sufficient to confer resistance to beta-lactam agents.
Ampicillin Resistance↗
Antimicrobial susceptibility testing of carbapenems: multicenter validity testing and accuracy levels of five antimicrobial test methods for detecting resistance in Enterobacteriaceae and Pseudomonas aeruginosa isolates.
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.
Carbapenems↗
Cell wall thickening is a common feature of vancomycin resistance in Staphylococcus aureus.
We have previously shown that a thickened cell wall is responsible for the vancomycin resistance of vancomycin-resistant Staphylococcus aureus (VRSA) (equivalent to vancomycin-intermediate S. aureus and glycopeptide-intermediate S. aureus) strain Mu50 (L. Cui, H. Murakami, K. Kuwahara-Arai, H. Hanaki, and K. Hiramatsu, Antimicrob. Agents Chemother. 44:2276-2285, 2000). However, the mechanism of vancomycin resistance in other VRSA strains remained unclear. In this study, 16 clinical VRSA strains from seven countries were subjected to serial daily passage in drug-free medium. After 10 to 84 days of passage in the nonselective medium, passage-derived strains with decreased MICs of vancomycin (MIC, <4 mg/liter) were obtained. However, all of the passage-derived strains except one (15 of 16) still possessed subpopulations that were resistant to vancomycin as judged by population analysis, and vancomycin-resistant mutant strains were selected from the passage-derived strains by one-step vancomycin selection with a frequency of 4.25 x 10(-6) to 1.64 x 10(-3). The data indicated that vancomycin-resistant cells are frequently generated from the passage-derived strains even after vancomycin selective pressure is lifted. Cell wall thicknesses and MICs of glycopeptides (vancomycin and teicoplanin) and beta-lactams (imipenem and oxacillin) were determined for a total of 48 strains, including 15 sets of three strains: the clinical VRSA strain, the passage-derived strain, and the vancomycin-resistant mutant strain obtained from the passage-derived strain. No simple correlation between glycopeptide and beta-lactam MICs was seen, while significant correlations between MICs of vancomycin and teicoplanin (r = 0.679; P < 0.001) and between MICs of imipenem and oxacillin (r = 0.787; P < 0.001) were recognized. Moreover, all of the VRSA strains had significantly thickened cell walls, which became thinner with the loss of vancomycin resistance during drug-free passages and again became thick in the resistant mutant strains. The data showed that cell wall thickness had high correlation with the MICs of the two glycopeptides (correlation coefficients, 0.908 for vancomycin and 0.655 for teicoplanin) but not with those of the beta-lactam antibiotics tested. These results together with coupled changes of cell wall thickness and vancomycin MICs in 16 isogenic sets of strains indicate that thickening of the cell wall is a common phenotype of clinical VRSA strains and may be a phenotypic determinant for vancomycin resistance in S. aureus.
Anti-Bacterial Agents↗
Comparison of PCR assay to culture for surveillance detection of vancomycin-resistant enterococci.
Direct multiplex PCR assay using vanA and vanB primers, which provides rapid results, was more sensitive than culture on selective media for samples collected by rectal swab (20 of 46 versus 8 of 46; P < 0.001) or perianal swab (17 of 58 versus 12 of 58; P = 0.059) for the detection of gastrointestinal colonization by vancomycin-resistant enterococci.
Bacterial Proteins↗
Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: establishing a national database.
Oxacillin-resistant Staphylococcus aureus (ORSA) is a virulent pathogen responsible for both health care-associated and community onset disease. We used SmaI-digested genomic DNA separated by pulsed-field gel electrophoresis (PFGE) to characterize 957 S. aureus isolates and establish a database of PFGE patterns. In addition to PFGE patterns of U.S. strains, the database contains patterns of representative epidemic-type strains from the United Kingdom, Canada, and Australia; previously described ORSA clonal-type isolates; 13 vancomycin-intermediate S. aureus (VISA) isolates, and two high-level vancomycin-resistant, vanA-positive strains (VRSA). Among the isolates from the United States, we identified eight lineages, designated as pulsed-field types (PFTs) USA100 through USA800, seven of which included both ORSA and oxacillin-susceptible S. aureus isolates. With the exception of the PFT pairs USA100 and USA800, and USA300 and USA500, each of the PFTs had a unique multilocus sequence type and spa type motif. The USA100 PFT, previously designated as the New York/Tokyo clone, was the most common PFT in the database, representing 44% of the ORSA isolates. USA100 isolates were typically multiresistant and included all but one of the U.S. VISA strains and both VRSA isolates. Multiresistant ORSA isolates from the USA200, -500, and -600 PFTs have PFGE patterns similar to those of previously described epidemic strains from Europe and Australia. The USA300 and -400 PFTs contained community isolates resistant only to beta-lactam drugs and erythromycin. Noticeably absent from the U.S. database were isolates with the previously described Brazilian and EMRSA15 PFGE patterns. These data suggest that there are a limited number of ORSA genotypes present in the United States.
Databases, Factual↗
The World Health Organization's External Quality Assurance System Proficiency Testing Program has improved the accuracy of antimicrobial susceptibility testing and reporting among participating laboratories using NCCLS methods.
A total of 150 laboratories in 33 countries that followed the NCCLS testing procedures participated in the World Health Organization's External Quality Assurance System for Antimicrobial Susceptibility Testing (EQAS-AST) from January 1998 through March 2001. Laboratories tested seven bacterial isolates for antimicrobial resistance and reported the results to the Centers for Disease Control and Prevention (CDC) in Atlanta, Ga. The results were compared to the results generated at the CDC with the NCCLS broth microdilution and disk diffusion reference methods. Although there were few testing errors with Salmonella enterica subsp. enterica serovar Enteritidis, drugs that are not appropriate for therapy of Salmonella infections were tested and reported by 136 (91%) of 150 laboratories. In addition, 29 (20%) of 150 laboratories used the Staphylococcus aureus breakpoints to report oxacillin results for Staphylococcus saprophyticus. For a vanB-containing Enterococcus faecalis strain, 124 (83%) of 150 laboratories correctly reported vancomycin results that were +/-1 doubling dilution from the reference MIC or +/-3 mm from the reference disk diffusion result. Of the laboratories that tested Streptococcus agalactiae by disk diffusion, 17% reported nonsusceptible results for penicillin in error. While 110 laboratories (73%) tested the S. pneumoniae challenge isolate against a fluoroquinolone, 83% tested it against ciprofloxacin, for which there are no NCCLS interpretive criteria. Ten of 12 laboratories testing levofloxacin and 4 of 4 laboratories testing ofloxacin by an MIC method correctly reported resistant results for the isolate. Feedback letters sent to participating laboratories highlighted areas of susceptibility testing in individual laboratories that needed improvement. The positive impact of the feedback letters and the overall effectiveness of the EQAS program were documented in repeat testing challenges with pneumococci and staphylococci. The 31 and 19% increases in the numbers of laboratories using appropriate testing methods for pneumococci and staphylococci, respectively, in 2000 versus 1998 indicate that laboratory performance is improving.
Anti-Bacterial Agents↗
Optimization of computer software settings improves accuracy of pulsed-field gel electrophoresis macrorestriction fragment pattern analysis.
Computer-assisted analysis of pulsed-field gel electrophoresis (PFGE) libraries can facilitate comparisons of fragment patterns present on multiple gels. We evaluated the ability of the Advanced Analysis (version 4.01) and Database (version 1.12) modules of the Phoretix gel analysis software package (Nonlinear USA, Inc., Durham, N.C.) to accurately match DNA fragment patterns. Two gels containing 38 lanes of SmaI-digested Enterococcus faecalis OG1RF DNA were analyzed to assess the impact of (i) varying the lane position of the standards, (ii) using gel plugs made at different times, and (iii) normalizing the fragment patterns by using molecular weight (MW) algorithms versus retardation factor (R(f)) algorithms. Two sets of PFGE libraries (one containing SmaI restriction patterns from 62 Enterococcus faecium isolates and the other containing SmaI restriction patterns of 89 Staphylococcus aureus isolates) were analyzed to assess the impact of varying the matching tolerance algorithm (designated as the vector box setting [VBS]) in the Phoretix software. Varying the lane position of standards on a gel and using gel plugs made on different days resulted in different VBSs, although it was not possible to judge whether those differences were statistically significant. Normalization of E. faecalis OG1RF fragment patterns by R(f) and MW methodology yielded no statistically significant differences in variability between the same fragment on different lanes. Suboptimal VBSs decreased the specificity with which related isolates were grouped together in dendrograms. The optimal VBS for analysis of PFGE fragment patterns from E. faecalis isolates differed from that for S. aureus isolates and sometimes was not that recommended by the manufacturer. Thus, computer-assisted analysis of PFGE patterns seemed to compensate for the intra- and intergel variation evaluated in the present study, and optimizing the software for the species to be tested was a critical preliminary step before further PFGE library analysis.
Algorithms↗
Evaluation of the NCCLS extended-spectrum beta-lactamase confirmation methods for Escherichia coli with isolates collected during Project ICARE.
To determine whether confirmatory tests for extended-spectrum beta-lactamase (ESBL) production in Escherichia coli are necessary, we selected 131 E. coli isolates that met the National Committee for Clinical Laboratory Standards (NCCLS) screening criteria for potential ESBL production from the Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) strain collection. For all 131 isolates, the broth microdilution (BMD) MIC of at least one extended-spectrum cephalosporin was >/=2 micro g/ml. For 21 of 131 (16%) isolates, the ESBL confirmatory test was positive; i.e., the BMD MICs of ceftazidime or cefotaxime decreased by >/=3 doubling dilutions in the presence of clavulanic acid (CA) or the disk diffusion zone diameters increased by >/=5 mm around ceftazidime or cefotaxime disks in the presence of CA. All 21 isolates were shown by PCR to contain at least one of the genes bla(TEM), bla(SHV), and bla(OXA), and in isoelectric focusing (IEF) tests, all isolates demonstrated at least one beta-lactamase band consistent with a TEM, SHV, or OXA enzyme. Of the 21 isolates, 3 showed a CA effect for cefotaxime by BMD but not by disk diffusion testing. A total of 59 (45%) of the 131 isolates demonstrated decreased susceptibility to cefpodoxime alone (MIC = 2 to 4 micro g/ml), and none had a positive ESBL confirmatory test result. These were classified as false positives according to ESBL screen test results. For the remaining 51 (39%) isolates, the cefpodoxime MICs ranged from 16 to >128 micro g/ml and the MICs for the other extended-spectrum cephalosporins were highly variable. All 51 isolates gave negative ESBL confirmatory test results. Most showed IEF profiles consistent with production of both a TEM and an AmpC beta-lactamase, and representative isolates of several phenotypic groups showed changes in porin profiles; these 51 isolates were considered true negatives. In all, only 16% of 131 E. coli isolates identified as potential ESBL producers by the current NCCLS screening criteria were confirmed as ESBL producers. Thus, changing the interpretation of extended-spectrum cephalosporins and aztreonam results from the susceptible to the resistant category without confirming the presence of an ESBL phenotype would lead to a large percentage of false resistance results and is not recommended. However, by increasing the cefpodoxime MIC screening breakpoint to >/=8 micro g/ml, 45% of the false-positive results could be eliminated. NCCLS has incorporated this change in the cefpodoxime screening breakpoint in its recent documents.
Anti-Bacterial Agents↗