Modification of interpretive breakpoints for netilmicin disk susceptibility tests with Pseudomonas aeruginosa

Evaluation of Aminoglycoside and Carbapenem Resistance in a Collection of Drug-Resistant Pseudomonas aeruginosa Clinical Isolates

Pseudomonas aeruginosa, a Gram-negative bacterium, is a member of the ESKAPE pathogens and one of the leading causes of healthcare-associated infections worldwide. Aminoglycosides (AGs) are recognized for their efficacy against P. aeruginosa. The most common resistance mechanism against AGs is the acquisition of AG-modifying enzymes (AMEs) by the bacteria, including AG N-acetyltransferases (AACs), AG O-phosphotransferases (APHs), and AG O-nucleotidyltransferases (ANTs). In this study, we obtained 122 multidrug-resistant P. aeruginosa clinical isolates and evaluated the antibacterial effects of six AGs and two carbapenems alone against all clinical isolates, and in combination against eight selected strains. We further probed for four representatives of the most common AME genes [aac(6')-Ib, aac(3)-IV, ant(2")-Ia, and aph(3')-Ia] by polymerase chain reaction (PCR) and compared the AME patterns of these 122 clinical isolates to their antibiotic resistance profile. Among the diverse antibiotics resistance profile displayed by these clinical isolates, we found correlations between the resistance to various AGs as well as between the resistance to one AG and the resistance to carbapenems. PCR results revealed that the presence of aac(6')-Ib renders these isolates more resistant to a variety of antibiotics. The correlation between resistance to various AGs and carbapenems partially reflects the complex resistance strategies adapted in these pathogens and encourages the development of strategic treatment for each P. aeruginosa infection by considering the genetic information of each isolated bacteria.

Antibiotic disk diffusion testing revisited. Single strain regression analysis. Review article

The standardized (NCCLS, ICS, DIN etc.) disk diffusion method is the most widespread technique for antibiotic susceptibility testing. Interpretive zone breakpoints are calculated from the regular regression line between minimum inhibitory concentrations (MIC) of bacterial isolates and the corresponding inhibition zone diameters around the disk containing the antibiotic. Studies of the regression line has revealed marked differences between different bacterial species. A newly described equation, the single strain regression analysis (SRA) equation, can be used to determine the regression line constants for individual strains. This method was applied to ciprofloxacin and S. aureus, E. faecalis, E. coli, P. mirabilis, P. aeruginosa, and P. maltophilia. The slope and intercept constants were determined for all 40 strains and showed a strong similarity within each species. A close similarity was also observed between the two Pseudomonas species and between S. aureus and E. faecalis. When the regression lines calculated by SRA for individual strains were extrapolated towards higher MIC values, the lines obtained for the more susceptible strains predicted the zones of more resistant strains within the species. The applications of SRA to several other antibiotics and bacterial species in earlier studies were reviewed. One exception to the predictive power of SRA has been detected earlier, H. influenzae and erythromycin. This led to the formulation of the standard curve regression analysis (SCA) equation which requires the use of two or more strains. Methodological aspects of SRA/SCA applications were presented. Three areas are particularly well suited for the use of SRA/SCA: 1. Calculation of interpretive zone breakpoints corresponding to recommended MIC limits in the individual laboratory. 2. Analysis of the effects of various disk contents of antibiotic on the resulting inhibition zones for various bacteria when new antibiotics are introduced. 3. Analytical tool as part of external quality control programmes.

Types of interpretive errors in susceptibility testing. Zone breakpoints for norfloxacin disk diffusion testing

A total of 548 strains of the eleven most common urinary tract pathogens were investigated for possible errors in norfloxacin susceptibility tests comparing MIC determinations with disk diffusion assays. Most strains were found to be sensitive with MIC-90 values below 1.0 for the Enterobacteriaceae while the classical nalidixic acid resistant species, the gram-positive bacteria and Pseudomonas aeruginosa, were less susceptible to norfloxacin with MIC-90 above 1.0 mg/l. MIC-values close to interpretive MIC-limits were recorded for S. faecalis and S. agalactiae using the recommendations of the national Committee for Clinical Laboratory Standards (NCCLS) (susceptible, S less than or equal to 4.0) and for P. aeruginosa and S. aureus using the Swedish Reference Group for Antibiotics (SRGA) standards (S less than or equal to 1.0). Susceptibility interpretations for these species showed a lack of accuracy consistent with methodological problems of reproducibility, an error called type I. The changes in the MIC-limits required for these strains to correct the error would be S less than or equal to 4 for P. aeruginosa and S. aureus, S less than or equal to 8 for S. agalactiae and S less than or equal to 0.5 for S. faecalis. A type II error, occurring when a bacterial species shows a regression line different from the regular line, was also identified for S. saprophyticus. The use of breakpoints derived from single strains regression analysis corrected this error and also reduced the frequency of similar misinterpretations in other species. The term "species-specific MIC-limits" should be introduced along with the established concept of "species-specific interpretive zone breakpoints" to allow for the correction of type I interpretive errors. Type II errors can be corrected by using species-specific interpretive breakpoints, either issued by reference laboratories or derived by calculations from single-strain regression analysis in the individual laboratory.

Laboratory- and species-specific interpretive breakpoints for disk diffusion tests of chloramphenicol susceptibility of Haemophilus influenzae

A total of 601 clinical isolates of Haemophilus influenzae isolated in six different regions of Sweden were tested for chloramphenicol susceptibility by using agar dilution MIC determinations and disk diffusion tests. For seven strains MICs were 4 micrograms/ml or higher, and for one strain the MIC was 2 micrograms/ml. All eight strains produced chloramphenicol acetyltransferase. For the remaining 593 strains, MICs were less than or equal to 1 microgram/ml, and the MICs for 50% and 90% of the strains were both 0.5 microgram/ml. Disk diffusion tests carried out by using revised interpretive criteria introduced in 1984 by the Swedish Reference Group for Antibiotics correctly identified the 593 strains as susceptible and the 8 strains as resistant. Quality assessments were performed in 29 clinical microbiology laboratories. The revised criteria for chloramphenicol disk diffusion testing gave rise to false resistance results in some laboratories. The interpretive accuracy improved when the interlaboratory variation was compensated for by using adjusted breakpoints. Such revision was possible through peak correction, single-strain regression analysis, and standard curve regression analysis. Peak-corrected breakpoints improved the accuracy from an overall incidence of false-resistant isolates of 4.4% to 2.3%. Single-strain regression analysis and standard curve regression analysis provided laboratory- and species-specific breakpoints which reduced false resistance rates of 0.14% and 0%, respectively.

Influence of cation supplements on activity of netilmicin against Pseudomonas aeruginosa in vitro and in vivo

In vitro studies were performed with 74 Pseudomonas aeruginosa isolates which were collected during a multicenter trial. The isolates were obtained from 70 patients who had been treated with netilmicin as the only antipseudomonal antibiotic. Clinically, 83% of the patients were cured or improved, and 64% of the Pseudomonas isolates were eliminated by chemotherapy. The 74 clinical isolates and 38 additional isolates with known mechanisms of aminoglycoside resistance were tested in three separate laboratories by disk diffusion methods and by microdilution tests with three broth media (Mueller-Hinton broth with full, half, and no cation supplements). Isolates that responded to netilmicin therapy and those that failed to respond were all susceptible by the disk test, and most were susceptible by microdilution tests with unsupplemented broth. However, over half of the clinical isolates appeared to be resistant when cations were added to the broth medium. Strains capable of producing enzymes that inactivate netilmicin were resistant by all methods tested. Broth dilution and agar dilution results were most comparable when half of the recommended cation supplements was added to Mueller-Hinton broth. Further consideration should be given to reducing the concentration of cations that are added to Mueller-Hinton broth when netilmicin susceptibility tests are being performed. However, additional studies with other aminoglycosides are needed before appropriate testing conditions can be standardized.

Netilmicin disk susceptibility tests: effect of cations on the MIC correlates

When testing Pseudomonas aeruginosa against netilmicin, MICs were markedly affected by the concentration of cations added to the test medium. A susceptible disk test result (zone greater than or equal to 15 mm) corresponded to MIC less than or equal to 4.0 micrograms/ml in unsupplemented broth, less than or equal to 12 micrograms/ml in broth with half the usual amount of cations and less than or equal to 32 micrograms/ml in broth with the recommended concentration of cations. Tests with 30 micrograms netilmicin disks best predicted susceptibility as determined by MICs in broth without added cations. When the MICs were determined in cation supplemented broth, the number of interpretive discrepancies increased to an unacceptably high level.

In vitro evaluations of amikacin: an assessment of the currently used methods of disk diffusion and dilution susceptibility, antimicrobial synergy, and the measurement of amikacin concentrations

The amikacin antimicrobial susceptibility tests were reviewed and found to be acceptable for clinical laboratory use. The early change from the 10-micrograms to the 30-micrograms diagnostic disk concentrations has resulted in reasonable accuracy, according to data from surveys of the College of American Pathologists, and acceptable discrimination between susceptible and resistant microorganisms. Similarly, standardized dilution susceptibility methods have proven acceptable, but great care must be exercised to select an agar medium in which performance was evaluated by the criteria of the National Committee for Clinical Laboratory Standards. Breakpoint concentrations selected as susceptible for amikacin (equal to or less than 16 micrograms/ml) were based on infected patient pharmacokinetics and previously correlated with patient bacteriologic outcome. Amikacin serum levels have been accurately measured by numerous procedures, including gas-liquid chromatography, radioimmunoassay, radioenzymatic assay, bioassay, and latex agglutination tests. Recent surveys of the College of American Pathologists support the earlier suspicions of lower accuracy and specificity with the bioassay method. Care must be taken to rapidly and appropriately process specimens from patients receiving concurrent high doses of antipseudomonal penicillins because of documented inactivation of some aminoglycosides by these penicillins. Amikacin is less affected by these beta-lactams. Evaluations of the antibacterial activity of amikacin in combination with other antimicrobial agents, principally the beta-lactams, continue to show high rates of enhanced killing or synergy. Although the methods for assessment of synergy have not been standardized, remarkably favorable and similar results between laboratories have been reported.

Reliability of a disk diffusion method using semiconfluent growth in the determination of aminoglycoside resistance

Susceptibility of 553 blood culture isolates to gentamicin, tobramycin, amikacin, and netilmicin was determined by a routine disk diffusion method with semiconfluent growth (AB Biodisk, Solna, Sweden) and a microtiter-method (Sensititre). The disk diffusion test gave false sensitive results in 9.4-71% of the 9-39 aminoglycoside resistant strains studied. False resistant results occurred in 1.0-7.6% of measurements among the over 500 aminoglycoside sensitive strains. Because of the frequent error rate, the disk method with semiconfluent growth should be replaced with other methods in the determination of aminoglycoside resistance in bacteriological laboratories.