A procedure for evaluation and documentation of susceptibility test methods using the susceptibility of Klebsiella pneumoniae to ciprofloxacin as a model

First comprehensive evaluation of the M.I.C. evaluator device compared to Etest and CLSI broth microdilution for MIC testing of aerobic Gram-positive and Gram-negative bacterial species

The M.I.C. Evaluator strip (Thermo Fisher Scientific, Basingstoke, United Kingdom) uses a methodology similar to that of Etest. In this first assessment of the M.I.C. Evaluator device, 409 strains of aerobic Gram-positive bacteria (staphylococci, streptococci, and enterococci) and 325 strains of Enterobacteriaceae, Pseudomonas species, and Acinetobacter species were tested by M.I.C. Evaluator strip, Etest, and broth microdilution as a reference standard. The Gram-positive bacteria included staphylococci (methicillin-resistant Staphylococcus aureus, methicillin-susceptible S. aureus, and coagulase-negative staphylococci), Streptococcus pneumoniae, beta-hemolytic streptococci and viridians group strains, vancomycin-resistant enterococci, and other enterococci. The Gram-negative bacteria included 250 strains of 60 Enterobacteriaceae species plus 50 Pseudomonas and 25 Acinetobacter species. A total of 14 antimicrobial agents (depending on the species) were included. The same methodology and reading format were used for M.I.C. Evaluator strips and Etest. Broth microdilution methodology was performed according to CLSI document M07-A8. For the clinical strains, >95% of results were plus or minus one doubling dilution for all species. There were fewer than 5% minor errors, fewer than 3% major errors, and fewer than 1% very major errors. M.I.C. Evaluator strips and Etest often reported higher MICs than the reference broth microdilution method. The M.I.C. Evaluator strips provided results comparable to those of the predicate Etest device and are of value for the accurate testing of MICs for these important pathogens.

Evaluation of disk diffusion methods to detect low-level β-lactamase-negative ampicillin-resistant Haemophilus influenzae

We evaluated the efficacy of disk diffusion methods for detection of low-level β-lactamase-negative ampicillin-resistant (low-BLNAR) Haemophilus influenzae. Four hundred and seventy unselected, recent clinical isolates were tested with ampicillin (10 μg), cefaclor (30 μg) and cefuroxime (30 μg) on iso-Sensitest agar enriched with nicotinamide adenine dinucleotide (NAD) and horse blood [ST agar; Swedish Reference Group for Antibiotics (SRGA) guidelines], and on chocolate agar (in-house guidelines). Selected isolates (n = 147) were subjected to partial sequencing of the ftsI gene. Forty-seven strains (10.0%) were genotypically identified as low-BLNAR, which was confirmed by determination of minimal inhibitory concentration (MIC) using microbroth dilution method: only low level resistance to ampicillin was detected [MIC ≤1 μg/mL; MIC(50) = 0.5 μg/mL, implying susceptibility by Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antibiotic Susceptibility Testing (EUCAST) interpretative criteria]. The MIC of cefuroxime varied between 1 and 4 μg/mL (MIC(50) = 2 μg/mL), indicating susceptibility to cefuroxime by CLSI but not by EUCAST guidelines. Disk diffusion methods were able to discriminate low-BLNAR H. influenzae from the wild-type population with sensitivities ranging from 87% to 98% and specificities from 96% to 99%. Cefaclor was found to be superior to cefuroxime and ampicillin. Cefaclor zone diameter breakpoints of 30/29 and 23/22 mm are suggested for ST agar and chocolate agar, respectively.

Assessment of accuracy of disk diffusion tests for the determination of antimicrobial susceptibility of common bovine mastitis pathogens: a novel approach

A novel approach was used to assess disk diffusion accuracy for determination of antibiotic susceptibility of various bovine mastitis pathogens (Escherichia coli, Staphylococcus aureus, Staphylococcus chromogenes, and Streptococcus dysgalactiae). MIC and disk diffusion diameters were compared for 587 bovine mastitis bacterial isolates collected in Israel and 3,186 drug-organism combinations. Results were analyzed by ROC curves, Bayesian statistics, and standard descriptive methods. Low correlation was observed between results of disk diffusion and MIC for S. dysgalactiae and all antimicrobial agents, S. aureus and erythromycin and neomycin, and E. coli and gentamicin, neomycin, and polymyxin B. On a few occasions in which correlation was satisfactory, accepted susceptibility breakpoints to some of the antimicrobial agents resulted in high discrepancies with MIC results and new breakpoints were suggested-e.g., 21 mm for S. aureus susceptibility to penicillin G instead of 29 mm recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and <21 mm, resistant, 21-25 mm, intermediate, and >25 mm, susceptible for susceptibility of E. coli to trimethoprim/sulfamethoxazole. Thus, this approach enabled determination of the most accurate breakpoints that best fitted the specific prevalence of susceptibility in Israel. Thus, we suggest its adoption by microbiology diagnostic laboratories for the provision of accurate antimicrobial susceptibility results when using the disk diffusion test.