Cation concentration variability of four distinct Mueller-Hinton agar brands influences polymyxin B susceptibility results.

Polymyxins have been the only alternative therapeutic option for the treatment of serious infections caused by multidrug-resistant Acinetobacter baumannii or Pseudomonas aeruginosa isolates. For this reason, it is of crucial importance that susceptibility tests provide accurate results when testing these drug-pathogen combinations. In this study, the effect of cation concentration variability found on different commercial brands of Mueller-Hinton agar (MHA) for testing polymyxin B susceptibility was evaluated. The polymyxin B susceptibilities determined using Etest and disk diffusion were compared to those determined by the CLSI reference broth microdilution method. In general, the polymyxin B MIC values were higher when determined by Etest than when determined by broth microdilution against both A. baumannii and P. aeruginosa isolates. A high very major error rate (10%) was observed, as well as a trend toward lower MICs, compared to those determined by broth microdilution when the Merck MHA was tested by Etest. Poor essential agreement rates (10 to 70%) were observed for P. aeruginosa when all MHA brands were tested by Etest. Although an excellent categorical agreement rate (100%) was seen between the disk diffusion and broth microdilution methods for P. aeruginosa, larger zones of inhibition were shown obtained using the Merck MHA. The high cation concentration variability found for the MHA brands tested correlated to the low accuracy, and discrepancies in the polymyxin B MICs were determined by Etest method, particularly for P. aeruginosa isolates.

Fusarium keratitis in Brazil: genotyping, in vitro susceptibilities, and clinical outcomes.

BACKGROUND: The purpose of this paper is to describe clinical characteristics and determine correlations between clinical outcomes and antifungal susceptibility among molecularly characterized ocular Fusarium isolates in Brazil. METHODS: Forty-one Fusarium isolates obtained from 41 eyes of 41 patients were retrieved from the ophthalmic microbiology laboratory at São Paulo Federal University and grown in pure culture. These isolates were genotyped and antifungal susceptibilities determined for each isolate using a broth microdilution method. The corresponding medical records were reviewed to determine clinical outcomes. RESULTS: The 41 isolates were genotypically classified as Fusarium solani species complex (36 isolates, 88%), Fusarium oxysporum species complex (two isolates, 5%), Fusarium dimerum species complex (one isolate, 2%) and two isolates that did not group into any of the species complexes. Final best corrected visual acuity varied from 20/20 to light perception and was on average 20/800 (logarithm of the minimum angle of resolution (LogMAR) 1.6). A history of trauma was the most common risk factor, being present in 21 patients (51%). Therapeutic penetrating keratoplasty was necessary in 22 patients (54%). Amphotericin B had the lowest minimum inhibitory concentration for 90% of isolates (MIC90) value (2 µg/mL) and voriconazole had the highest (16 µg/mL). There was an association between a higher natamycin MIC and need for therapeutic penetrating keratoplasty (Mann-Whitney test, P < 0.005). CONCLUSION: Trauma was the main risk factor, and therapeutic penetrating keratoplasty was necessary in 54% of patients. Amphotericin B had the lowest MIC90 (2 µg/mL) of the three antifungal agents tested. There was an association between higher natamycin MIC levels and corneal perforation, emphasizing the need for antifungal susceptibility testing and tailoring of antifungal strategies.