Rapid Antimicrobial Susceptibility Test Using Tetrazolium Reduction

Abstract

A rapid method of antimicrobial susceptibility testing has been developed, which uses a modified microdilution procedure and an inoculum of 107 bacteria per ml. Results are determined within 4 h with an indicator conng of 2(piodophenyl)-3(p-nitrophenyl)-5-phenyltetrazolium chloride. The precipitation of a red fonnazan by bacteria uninhibited by antimicrobials is accelerated by the addition of phenazine methosulfate. Isolates are clified as resistant, indeterminate, or susceptible, based on growth in up to two antimicrobial concentrations which conform closely to concentrations which correlate with the millimeter breakpoints used in the Bauer-Kirby method. Results of testing 10 antimicrobial agents against 1,126 isolates were compared with results obtained when the Bauer-Kirby method and the agar dilution procedure were used as reference methods. Enterococci were excluded because of false resistance. Discrepancies were classified as very major (false susceptibility), major (false reLsitance), and minor (combinations of susceptibility or reistance with indeterminate results). The rapid method versus the agar dilution method yielded 2.3% very major, 0.7% major, and 2.9% minor discrepancies, for a total of 6.0%. Of 58 organim-antimicrobial agent combinations tested, 23 displayed 1% very major discrepancies between the rapid method and the agar dilution method. Six were not therapeutically important. The remainder involved Staphylococcus aureus, Staphylococcus epidermnidis, Acinetobacter op., and most organisms tested with chloramphenicol. It is suggested that adjustments in antibiotic concentrations and/or inoculum size may eliminate these discrepancies. The rapid method appeared economical when compared with Autobac 1 and the Bauer-Kirby procedure.

Efficacy of Disc Diffusion and Agar Dilution Methods in Evaluating Helicobacter pylori Susceptibility to Antibiotics

“…25 Despite being cost-effective and easy to perform, the DD method has been considered unreliable for susceptibility testing of slow-growing H. pylori by some investigators because they believe results should be recorded within 18-24 hours. [26][27][28] In this study susceptibility/resistance rate of 20 H. pylori isolates to currently used antibiotics was determined by DD and AD (ADB/ADT) methods. The susceptibility (S) or resistance (R) rates of H. pylori isolates to MTZ determined by DD (S: 65%, R: 35%) and AD (S: 30%/40%, R: 70%/60%) methods showed a considerable difference between the results of two methods (P < 0.05).…”

Antibacterial Susceptibility of Bovine‐mastitis Pathogens Tested Directly in Milk from Infected Quarters

“…The data were analyzed for discrepancies between the TTC-mastitic milk-test and the other methods by classifying the discrepancies into three categories (BARTLETT and MAZENS 1979): false susceptibility, false resistance and combinations of susceptibility or resistance with intermediate results. False susceptibility was considered to be a major discrepancy i.e., the strain was resistant in TTC-mastitic milk-test but susceptible in another test.…”

Improvement of mupirocin E-test for susceptibility testing of Staphylococcus aureus

“…Following incubation at 35 8C for 18 h, E-test MIC values were read by two independent operators, as the point of intersection of the zone of inhibition with the E-test strip. Immediately, 100 ìl of a dye-indicator solution containing 1 mg 2(p-iodophenyl)-3(p-nitrophenyl)-5-phenyltetrazolium chloride and 60 ìg phenazine methosulfate ml À1 (Sigma) (Bartlett & Mazens, 1979) was added on the inhibition ellipse edge along each E-test strip. After 30 min, each plate was examined for colour development and the intersection-point of the zone of inhibition was again read by two independent operators.…”

“…However, Staphylococcus strains tend to give lower MICs in the mupirocin E-test than in the corresponding mupirocin agar dilution (Simpson et al, 1995). It has been shown that the disc diffusion method can be improved by adding the indicator dye tetrazolium-phenazine methosulfate (Bartlett & Mazens, 1979). Furthermore, the E-test for antimicrobial susceptibility of Helicobacter pylori was more accurate when tetrazolium/egg-yolk agar was employed (Valdez et al, 1998).…”