False resistance to metronidazole of anaerobic bacteria using the E test

Fifty years devoted to anaerobes: historical, lessons, and highlights

Renew interest and enthusiasm for anaerobes stem from both technological improvements (culture media, production of an adequate anaerobic atmosphere, identification methods) and greater awareness on the part of clinicians. Anaerobic infections were historically treated empirically, targeting the species known to be involved in each type of infection. Prevotella, fusobacteria, and Gram-positive cocci (GPAC) were considered responsible for infections above the diaphragm whereas for intra-abdominal infections, Bacteroides of the fragilis group (BFG), GPAC and clostridia were predominantly implicated. The antibiotic susceptibility of anaerobes was only taken into consideration by the clinician in the event of treatment failure or when faced with infections by multidrug-resistant bacteria (MDR). The evolution of antibiotic resistance together with clinical failures due to the absence of detection of hetero-resistant clones has resulted in a greater need for accessible antibiotic susceptibility testing (AST) and disc diffusion method. Improved isolation and identification of anaerobes, along with the availability of accessible and robust methods for performing AST, will ensure that treatment, whether empirical or guided by an antibiogram, will lead to better outcomes for anaerobic infections.

Three independent yearly analyses of the spectrum and potency of metronidazole: a multicenter study of 1,108 contemporary anaerobic clinical isolates

To comply with ongoing United States Food and Drug Administration (FDA) recommendations about the validation of prescribing information, in each of three years (1994, 1996, 1997) at six medical centers, metronidazole was tested against > or = 50 strains of a range of contemporary anaerobic clinical bacteria. Species having > or = 90% susceptibility (MIC, < or = 8 microg/ml) to metronidazole (fulfilling FDA requirements) included Bacteroides fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron, B. vulgatus, Fusobacterium spp. and Clostridium spp. Only Eubacterium spp. and anaerobic Gram-negative cocci failed to achieve the required proportion of susceptibility throughout the 3 year period. Metronidazole appears to remain highly active versus anaerobic species associated with strict anaerobic organisms.

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.