INTRODUCTION

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The current quality control isolate recommended by the National Committee for Clinical Laboratory Standards (NCCLS) for use when performing susceptibility testing with Haemophilus influenzae is a non--lactamase-producing strain, H. influenzae ATCC 49247 (6, 7). NCCLS guidelines for quality control testing of H. influenzae susceptibility do not include a -lactamase-producing strain and therefore do not provide a method for the detection of the presence of adequate amounts of the -lactamase inhibitor components of amoxicillin-clavulanic acid, ampicillin-sulbactam, and piperacillin-tazobactam. As the percentage of -lactamase-producing H. influenzae isolates increases, appropriate quality control for susceptibility testing becomes important. Among countries in Europe, variable percentages of -lactamase-producing H. influenzae isolates have been reported (3, 8, 9). In a U.S. multicenter surveillance study performed in 1994 and 1995, 36.4% of 1,537 clinical isolates of H. influenzae produced -lactamase (2). A U.S. surveillance study of respiratory tract pathogens performed in 1996 and 1997 reported 33.4% -lactamase producers among 1,558 H. influenzae isolates (10). Another U.S. study performed in 1997 showed that among 1,676 H. influenzae isolates, 41.6% produced -lactamase (4). With these percentages of -lactamase producers, it is critical that quality control be performed to ensure that adequate amounts of the -lactamase inhibitors are present.

The -lactamase-producing strains of H. influenzae are not suitable for use as quality control organisms in evaluations of the clavulanic acid content of amoxicillin-clavulanic acid. This is due to the high level of inhibition of the TEM-1 and ROB-1 -lactamases produced by H. influenzae provided by clavulanic acid. This was demonstrated in a study in which 43 -lactamase-producing strains of H. influenzae for which amoxicillin MICs ranged from 1 to >16 µg/ml were tested. Only 0.12 µg of clavulanic acid per ml was needed to restore amoxicillin activity to the NCCLS susceptibility breakpoint for amoxicillin-clavulanic acid (4 µg/ml) for all 43 isolates (1).

Escherichia coli ATCC 35218 is recommended by NCCLS (6, 7) as the quality control organism for the -lactam--lactamase inhibitor agents. However, current guidelines recommend the testing of E. coli ATCC 35218 with Mueller-Hinton (MH) medium. The primary objective of this study was to determine if comparable quality control results for E. coli ATCC 35218 would be produced for the three -lactam--lactamase inhibitor agents with MH medium and Haemophilus test medium (HTM). It would be preferable to test E. coli ATCC 35218 with HTM, since the test conditions for quality control isolates should be the same as those for patient isolates.

Another objective of this study was to determine if comparable results could be obtained with Mueller-Hinton chocolate (MHC) agar and MH agar when E. coli ATCC 35218 was tested with all three -lactam--lactamase inhibitor agents by the E-test and the disk diffusion methodologies. Prior to 1990, NCCLS recommended the use of MHC agar for disk diffusion tests with H. influenzae (5). Isolates were tested in triplicate with each medium type on each day of testing for 10 consecutive days by the broth microdilution, E-test, and disk diffusion methods. The results obtained with HTM were compared to the results obtained with MH medium and to the acceptable NCCLS ranges for E. coli ATCC 35218.

Additionally, in many clinical laboratories, quality control isolates are repeatedly subcultured and used for quality control testing. To determine if this practice would have an effect on the susceptibility results, an isolate was passaged consecutively on each of the 10 test days and was tested in triplicate on each day. The results were compared to the results obtained with a fresh isolate by the broth microdilution, E-test, and disk diffusion methods.

	MATERIALS AND METHODS

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Test organisms. E. coli ATCC 35218 and H. influenzae ATCC 49247 were used.

Isolate preparation. Prior to testing, E. coli ATCC 35218 and H. influenzae ATCC 49247 were subcultured twice on sheep blood agar plates and chocolate agar plates, respectively, from CULTI-LOOPS (Lake Charles, La.). The passaged E. coli ATCC 35218 isolate was subcultured three times. The passaged isolate was then subcultured consecutively on each day of the study up to day 10.

Antimicrobial agents. Powders of antimicrobial agents with known potencies were obtained as follows: amoxicillin and clavulanic acid, SmithKline Beecham, Collegeville, Pa.; ampicillin and piperacillin, Sigma Chemical Co., St. Louis, Mo.; sulbactam, Pfizer Inc., Groton, Conn.; and tazobactam, Wyeth-Ayerst, Pearl River, N.Y. Amoxicillin-clavulanic acid and ampicillin-sulbactam were tested in 2:1 ratios. Tazobactam was tested with a fixed 4-µg/ml concentration in each well.

Media. HTM and MHC medium from three different manufacturers were used. Prepared media were obtained as follows: MH broth and agar, Becton Dickinson; HTM broth and agar, Becton Dickinson, Remel (Lenexa, Kans.), PML Microbiologicals (Wilsonville, Oreg.); and MHC agar, Becton Dickinson, Remel, and Northeast Laboratories (Waterville, Mass.). One lot of each medium type from each manufacturer was tested.

Broth microdilution MIC assays. Broth microdilution MIC assays were performed by the NCCLS methodology (6) in cation-adjusted MH broth (CAMHB) and HTM broth. MICs were determined visually.

E-test MIC testing. E-test strips impregnated with amoxicillin-clavulanic acid, ampicillin-sulbactam, and piperacillin-tazobactam were obtained from AB Biodisk (Piscataway, N.J.). The E test was performed according to the recommendations of the manufacturer on MH, HTM, and MHC agars.

Disk diffusion susceptibility testing. Antibiotic-impregnated disks containing 20 µg of amoxicillin with 10 µg of clavulanic acid, 10 µg of ampicillin with 10 µg of sulbactam, and 100 µg of piperacillin with 10 µg of tazobactam were obtained from Becton Dickinson Microbiology Systems, Sparks, Md. Disk diffusion testing was performed by the NCCLS methodology (7) on MH, HTM, and MHC agars.

Analysis of results. Susceptibility testing with E. coli ATCC 35218 was performed on HTM broth and agar and MHC agar media from three different manufacturers of each medium. The isolates were tested in triplicate for 10 consecutive days. This generated a total of 90 test results for each medium by each methodology for each antimicrobial agent. Susceptibility testing with E. coli ATCC 35218 was performed on MH media in triplicate on each day of testing. The results of the broth microdilution, E-test, and disk diffusion methods with E. coli ATCC 35218 on MH media and HTM were compared for amoxicillin-clavulanic acid, ampicillin-sulbactam, and piperacillin-tazobactam. E-test and disk diffusion testing of E. coli ATCC 35218 with all three antimicrobial agents was performed on MHC agar. All results obtained with HTM and MHC media were compared to the acceptable ranges established by NCCLS for E. coli ATCC 35218 with MH media. The number of dilution differences between HTM and MH media was also compared for the broth microdilution and E-test MIC testing methods. E-test results that fell between the doubling dilutions seen by the broth microdilution method were rounded to the next higher dilution. The zone diameter size was measured in millimeters for disk diffusion testing. The diameter sizes obtained with HTM and MHC agars were compared to the diameter sizes obtained with MH agar. The results for matched E. coli ATCC 35218 and passaged E. coli ATCC 35218 isolates were compared for all of the media and antibiotic agents by each methodology. For the disk tests, straight-line regression analysis was used to evaluate the paired differences. For the MIC tests, confidence intervals based on a binomial model were used for the statistical assessment.

	RESULTS AND DISCUSSION

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Quality control. A total of 90 test results for each antimicrobial agent for each broth and agar were generated for E. coli ATCC 35218. Quality control results with CAMHB and MH agar for E. coli ATCC 35218 were in range each of the 10 days by all three methodologies. Quality control results for H. influenzae ATCC 49247 with HTM were in range for each day of testing with one exception. On day 4, there was contamination by the broth microdilution method only. Day 4 results were not included in the study for the broth microdilution method; therefore, the total number of tests for the microbroth dilution method was reduced to 81.

Broth microdilution method with HTM broth. The broth microdilution test results for E. coli ATCC 35218 with HTM broth were compared to the NCCLS reference ranges, which were established with CAMHB. Of the results for amoxicillin-clavulanic acid and piperacillin-tazobactam with HTM, 100% were within the acceptable NCCLS range. A total of 86% (74 of 81) of the results for ampicillin-sulbactam were within the NCCLS range. The seven results that were not acceptable for ampicillin-sulbactam were out of range by 1 dilution (Table 1). When the broth microdilution test results with HTM were compared to the results obtained with CAMHB, all results were within 1 dilution for all of the antimicrobial agents tested. When a difference was observed, the results were generally 1 doubling concentration higher in HTM broth than in CAMHB (Table 2).

E test with HTM agar. When the results obtained by the E test with HTM agar for E. coli ATCC 35218 were compared with the NCCLS established reference range, 100% of the results for amoxicillin-clavulanic acid and piperacillin-tazobactam were within the acceptable range. However, only 77% (69 of 90) of the results for ampicillin-sulbactam were in range. All 21 results that were out of range for ampicillin-sulbactam were within 1 dilution of the acceptable range (Table 1). When E-test results with HTM agar were compared to the results obtained with MH agar, all results were within 1 doubling dilution for all of the antimicrobial agents tested. When a difference was observed, the results were generally 1 doubling concentration higher with HTM agar than with MH agar (Table 2).

Disk diffusion testing with HTM agar. The disk diffusion testing results for E. coli ATCC 35218 with HTM agar were compared to the NCCLS reference range. Of the results for ampicillin-sulbactam and piperacillin-tazobactam with HTM agar, 100% were within the acceptable NCCLS range. For amoxicillin-clavulanic acid, 82% (74 of 90) of the results were in range. All 16 results that were out of range for amoxicillin-clavulanic acid were out of range by 1 mm (Table 3). Ninety-six percent (86 of 90) of the results for amoxicillin-clavulanic acid, 99% (89 of 90) of the results for ampicillin-sulbactam, and 98% (88 of 90) of the results for piperacillin-tazobactam with HTM agar were within 2 mm of the results with MH agar. When a difference was observed between the agars, zone sizes were generally smaller with HTM agar than with MH agar (Table 2).

E test with MHC agar. E-test results for E. coli ATCC 35218 with MHC agar were compared to the NCCLS established reference range for E. coli ATCC 35218 with MH agar. Of the results for amoxicillin-clavulanic acid and piperacillin-tazobactam with MHC agar, 100% were within the acceptable NCCLS range. However, only 27% (24 of 90) of the results for ampicillin-sulbactam were within range. The results for 66 tests on MHC agar that were out of the acceptable NCCLS range for ampicillin-sulbactam were 1 to 4 doubling dilutions higher than the results for tests on MH agar (Table 1). When the results of the E test with MHC agar were compared to the results of the E test with MH agar, all results were within 1 dilution for amoxicillin-clavulanic acid and piperacillin-tazobactam. Of the results for ampicillin-sulbactam with MHC agar, a 2-doubling dilution difference was seen for 28% (25 of 90) of the results. When a difference was observed, the results were generally higher with MHC agar.

Disk diffusion testing with MHC agar. The results obtained by disk diffusion testing of E. coli ATCC 35218 with MHC agar were compared to the NCCLS reference range. Complete agreement with established NCCLS ranges was not seen for any of the antimicrobial agents. Of the results for piperacillin-tazobactam with MHC agar, 98% were within the acceptable NCCLS range. However, only 47% of the results for amoxicillin-clavulanic acid agar with MHC and 42% of the results for ampicillin-sulbactam with MHC agar were within the acceptable NCCLS range (Table 3). When the disk diffusion testing results obtained with MHC agar were compared to those obtained with MH agar, up to a 5-mm difference was observed for all antimicrobial agents tested. When a difference was observed between agars, the zone sizes were generally smaller with MHC agar than with MH agar. Theoretically, the differences observed between the media may be due to better growth in the richer medium. Another reason for the difference could be that the components in the enriched medium may be affecting the antimicrobial agents.

Passaged isolates. The same procedures that were performed with the E. coli ATCC 35218 isolates were performed with the passaged E. coli ATCC 35218 isolates. Quality control results for the passaged E. coli ATCC 35218 isolates were in range on each of the 10 days on CAMHB and MH agar by all three methodologies and with all three antimicrobial agents. There were no statistically significant differences between the results obtained with the fresh and passaged isolates for any of the media, antimicrobial agents, or methodologies tested. P values were greater than 0.05 in all cases.

View this table: [in this window] [in a new window]   TABLE 4.   Antimicrobial agent-methodology results for quality control testing of the -lactam--lactamase inhibitors with E. coli ATCC 35218 with HTM and MHC agar