Multilaboratory evaluation of disk diffusion antimicrobial susceptibility testing of Neisseria meningitidis isolates

Comparison of Disc Diffusion, Etest, and a Modified CLSI Broth Microdilution Method for In Vitro Susceptibility Testing of Itraconazole, Posaconazole, and Voriconazole against Madurella mycetomatis

For many fungal infections, in vitro susceptibility testing is used to predict if an isolate is resistant or susceptible to the antifungal agent used to treat the infection. For Madurella mycetomatis, the main causative agent of mycetoma, in vitro susceptibility testing currently is not performed on a routine basis. The current in vitro susceptibility testing method is labor-intensive, and sonication must be done to generate a hyphal inoculum. For endpoint visualization, expensive viability dyes are needed. Here, we investigated if the currently used in vitro susceptibility method could be adapted to make it amendable for use in a routine setting which can be used in low-income countries, where mycetoma is endemic. First, we developed a methodology in which hyphal fragments can be generated without the need for sonication, by comparing different bead beating methodologies. Next, in vitro susceptibility was assessed using standard broth microdilution assays as well as disc diffusion, Etest, and VIPcheck methodologies. We demonstrate that after a hyphal suspension is generated by glass bead beating, disc diffusion, Etest, and VIPcheck can be used to determine susceptibility of Madurella mycetomatis to itraconazole, posaconazole, and voriconazole. The MICs found with Etest were comparable to those obtained with our modified CLSI-based broth microdilution in vitro susceptibility assay for itraconazole and posaconazole. Furthermore, we found an inverse relationship between the zones of inhibition and MICs obtained with the Etest and those obtained by the modified CLSI broth microdilution technique.

Comparing the disk-diffusion and agar dilution tests for Neisseria gonorrhoeae antimicrobial susceptibility testing

BACKGROUND

We assessed the validity of testing for antimicrobial susceptibility of clinical and mutant Neisseria gonorrhoeae (GC) isolates by disk diffusion in comparison to agar dilution, and Etest® (bioMerieux, France), respectively, for three third generation extended spectrum cephalosporins (ESC): ceftriaxone (CRO), cefixime (CFX), and cefpodoxime (CPD).

METHODS

One hundred and five clinical isolates and ten laboratory-mutants were tested following Clinical Laboratory Standard Institute (CLSI) and manufacturer's standards for each of the three methods. The measured diameters by the disk diffusion method were tested for correlation with the MIC values by agar dilution. In addition, comparisons with the Etest® were made. Categorical results for concordance, based on standard CLSI cutoffs, between the disk diffusion and the other two methods, respectively, were tested using the Chi-square statistics. Reproducibility was tested for CFX across a 6-month interval by repeated disk tests.

RESULTS

Across all 115 specimens, the disk diffusion tests produced good categorical agreements, exhibiting concordance of 93.1%, 92.1%, and 90.4% with agar dilution and 93.0%, 92.1%, and 90.4% with Etest®, for CRO, CFX, and CPD, respectively. Pearson correlations between disk-diffusion diameters and agar dilution MIC's were -0.59, -0.67, and -0.81 for CRO, CFX, and CPD, respectively. The correlations between disk diffusion and Etest® were -0.58, -0.73, and -0.49. Pearson correlation between the CFX disk readings over a 6-month interval was 91%.

CONCLUSIONS

Disk diffusion tests remain to be a useful, reliable and fast screening method for qualitative antimicrobial susceptibility testing for ceftriaxone, cefixime, and cefpodoxime.

Accurate assessment of antibiotic susceptibility and screening resistant strains of a bacterial population by linear gradient plate

The dynamics of a bacterial population exposed to the minimum inhibitory concentration (MIC) of an antibiotic is an important issue in pharmacological research. Therefore, a novel antibiotic susceptibility test is urgently needed that can both precisely determine the MIC and accurately select antibiotic-resistant strains from clinical bacterial populations. For this purpose, we developed a method based on Fick's laws of diffusion using agar plates containing a linear gradient of antibiotic. The gradient plate contained two layers. The bottom layer consisted of 15 mL agar containing the appropriate concentration of enrofloxacin and allowed to harden in the form of a wedge with the plate slanted such that the entire bottom was just covered. The upper layer consisted of 15 mL plain nutrient agar added with the plate held in the horizontal position. After allowing vertical diffusion of the drug from the bottom agar layer for 12 h, the enrofloxacin concentration was diluted in proportion to the ratio of the agar layer thicknesses. The uniform linear concentration gradient was verified by measuring the enrofloxacin concentration on the agar surface. When heavy bacterial suspensions were spread on the agar surface and incubated for more than 12 h, only resistant cells were able to form colonies beyond the boundary of confluent growth of susceptible cells. In this way, the true MIC of enrofloxacin was determined. The MICs obtained using this linear gradient plate were consistent with those obtained using conventional antibiotic susceptibility tests. Discrete colonies were then spread onto a gradient plate with higher antibiotic concentrations; the boundary line increased significantly, and gene mutations conferring resistance were identified. This new method enables the rapid identification of resistant strains in the bacterial population. Use of the linear gradient plate can easily identify the precise MIC and reveal the dynamic differentiation of bacteria near the MIC. This method allows the study of genetic and physiological characteristics of individual strains, and may be useful for early warning of antibiotic resistance that may occur after use of certain antimicrobial agents, and guide clinical treatment.

Integrated Multilevel Surveillance of the World's Infecting Microbes and Their Resistance to Antimicrobial Agents

Microbial surveillance systems have varied in their source of support; type of laboratory reporting (patient care or reference); inclusiveness of reports filed; extent of microbial typing; whether single hospital, multihospital, or multicountry; proportion of total medical centers participating; and types, levels, integration across levels, and automation of analyses performed. These surveillance systems variably support the diagnosis and treatment of patients, local or regional infection control, local or national policies and guidelines, laboratory capacity building, sentinel surveillance, and patient safety. Overall, however, only a small fraction of available data are under any surveillance, and very few data are fully integrated and analyzed. Advancing informatics and genomics can make microbial surveillance far more efficient and effective at preventing infections and improving their outcomes. The world's microbiology laboratories should upload their reports each day to programs that detect events, trends, and epidemics in communities, hospitals, countries, and the world.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. As resistance continues to emerge among a wide range of clinically relevant bacteria, the complexity of this communication increases. This updated version provides an overview of the important susceptibility concerns for most commonly isolated bacterial pathogens.

Nalidixic acid disk for laboratory detection of ciprofloxacin resistance in Neisseria meningitidis

Recently the CLSI recommended a disk diffusion method and breakpoints for meningococci which include breakpoints derived for nalidixic acid which serve as surrogate markers for gyrase A mutations associated with diminished fluoroquinolone susceptibility. This study presents the application of this methodology to a panel of 57 meningococcal strains isolated in Spain that include all levels of susceptibility to ciprofloxacin. In conclusion, the most useful method to predict isolates with gyrA mutations that decrease the activity of fluoroquinolones is the use of 30-microg nalidixic acid disks.

Resistance testing of meningococci: the recommendations of the European Monitoring Group on Meningococci

One of the most controversial topics concerning Neisseria meningitidis is the definition of antimicrobial resistance. A process of harmonization, promoted by the European Monitoring Group on Meningococci (EMGM), is already in progress. There are several aspects of the MIC definition for meningococcal strains for which a consensus is needed, as follows. (1) What methods can be used? At present, both the Etest and agar dilution methods are recommended by the EMGM. Microdilution, also a recommended method, is not used across Europe. (2) What media produce the most reliable data? The use of Mueller-Hinton medium supplemented with 5% blood is recommended for the MIC determination of N. meningitidis by both Etest and agar dilution method. (3) What is the minimum set of antimicrobial agents to be tested? A minimum set should include rifampicin, ciprofloxacin (or another quinolone), penicillin, ampicillin and ceftriaxone. (4) What are the most suitable breakpoints for definition? There is no widely accepted robust evidence base for the definition of breakpoints. Although progress has been made in achieving a standardized methodology, we are still far from obtaining common breakpoints for antimicrobial resistance definition. These differences will imply inability for building a common epidemiologic figure about drug resistance.