Evaluation of the E-test for susceptibility testing of theBacteroides fragilisgroup

Antibiotic susceptibility of blood culture isolates of anaerobic bacteria at a Norwegian university hospital

In the present study the susceptibility of 200 blood culture isolates of anaerobic bacteria to benzylpenicillin, clindamycin, metronidazole, imipenem and piperacillin-tazobactam was examined. Metronidazole, imipenem, and piperacillin-tazobactam showed the highest activity, with 98.5% of the isolates being fully susceptible to either agent. A high rate of reduced susceptibility to clindamycin among both Bacteroides spp. (37%) and Clostridium spp. (28%) was found. Almost all Bacteroides isolates were resistant to penicillin, and only 60% of Prevotella spp. were susceptible to this agent. The antibiotic susceptibility of anaerobic bacteria in Norway should be surveyed regularly.

MIC determination of fusidic acid and of ciprofloxacin using multidisk diffusion tests

OBJECTIVE

To investigate the possibility of estimating the MICs of fusidic acid and ciprofloxacin for bacterial isolates using series of antibiotic disk concentrations in diffusion tests, so-called M-tests.

METHODS

Thirty Staphylococcus aureus and S. epidermidis strains were tested for fusidic acid susceptibility. Sixty-one clinical isolates of eight bacterial species were tested for ciprofloxacin susceptibility. Disk diffusion was standardized according to the Swedish reference group for antibiotics (SRGA). For fusidic acid, a series of disks (1.5, 5.0, 15, 50 and 150 microg) was used. Ciprofloxacin was applied in four different diffusion sources (1, 3, 10 and 30 microg) on a single strip, the M-strip, and used. True MIC values were determined using the standardized agar dilution method according to the SRGA. Single-strain regression analysis (SRA) was employed to calculate critical concentration equivalents (Qzero).

RESULTS

Fusidic acid and ciprofloxacin critical concentrations were determined for the bacterial isolates. The mean conversion factors for Qzero to yield the true MIC were 2.06 (range 0.34-8.9) for fusidic acid and 2.05 (range 0.37-8.1) for ciprofloxacin. There was a correlation between true MIC values (all MICs expressed as 2 log + 9) and the calculated MIC values (Qzero x conversion factor) for both fusidic acid (R = 0.9822) and ciprofloxacin (R = 0.9696).

CONCLUSIONS

MIC values of clinical isolates can be estimated using SRA calculations on zone measurements in disk tests with several concentrations of the antibiotic in diffusion sources.

Calibration of the disk diffusion test for trovafloxacin susceptibility testing of four anaerobic species

OBJECTIVES

To study trovafloxacin susceptibility among clinical isolates of four anaerobic bacterial species using minimum inhibitory concentrations (MIC) determinations, E test assays and disk diffusion test results and to calibrate the disk diffusion method for these species using single strain regression analysis (SRA).

METHODS

One-hundred and eighty-seven clinical isolates of four anaerobic bacterial species were included. Trovafloxacin MIC determinations were performed using the agar dilution technique and MIC estimations using the E test. The disk diffusion test was performed according to Swedish Reference Group for Antibiotics standardization. NCCLS limits for susceptibility categories were applied. SRA was performed using 1, 3, 10, 30, and 100 microg trovafloxacin disk contents and ATCC control strains. The regression lines obtained permitted the calculation of zone equivalents to MIC limits as well as an evaluation of various disk potencies.

RESULTS

Trovafloxacin susceptibility (S + I) was noted in 98.9, 100, 100, and 97% of Bacteroides fragilis, Bacteroides thetaiotaomicron, Clostridium perfringens, and Peptostreptococcus magnus strains, respectively, as judged by MIC determinations. Agar dilution and E test estimations gave the same results, but E test values were consistently lower than MIC values by the reference method. Regression lines calculated for the four species using SRA showed different equation constants indicating species-related differences. Interpretive zone diameter breakpoints were calculated for the four species and used for the interpretation of susceptibility.

CONCLUSIONS

The disk diffusion test was successfully calibrated for trovafloxacin susceptibility testing of four anaerobic species using single strain regression analysis, SRA. There was a good agreement between the results of MIC-tests and disk testing. Interpretive errors of type I are prone to occur among Bacteroides isolates and might require species-related MIC limits. SRA calculations permitted the testing of the effect of different disk potencies on inhibition zones produced at the interpretive MIC limits. Criteria for the selection of a minimal disk content showed that 5 microg trovafloxacin is sufficient, but a 10 microg disk will safeguard against residual laboratory variation without producing too large inhibition zones for very susceptible strains.