Potential Unreliability of Nitrofurantoin Disks in Susceptibility Testing

Abstract

The Kirby-Bauer disk diffusion antimicrobial susceptibility test was studied with three lots of commercial nitrofurantoin disks by using 50 strains of Proteus mirabilis. One lot of disks consistently gave zone diameters 3 to 6 mm larger than the other two and resulted in a substantial number of Proteus mirabilis strains being erroneously categorized as sensitive. Antimicrobial content in disks of the three lots did not account for these differences. The pH of eluate from the unreliable disks was higher than from disks of the other two and resulted in more rapid diffusion of the antimicrobial and consequently larger zones of inhibition.The disk diffusion method of Bauer et al. (2) is commonly used to determine the susceptibility of bacteria to antimicrobials. Many variables (1; A. W. Bauer, Proc. Third Int. Congr. Chemother., p. 466, 1964) which influence test results have been defined including inoculum size, pH, and ionic concentration of media, as well as the content of antibiotics in disks. During a circumscribed period of 1972, an inordinate number of Proteus species appeared to be susceptible to nitrofurantoin when tested in our laboratory. The present investigation was undertaken to discover the cause of these "falsely" large zones of inhibition. by overnight incubation at 37 C, and spectrophotometric absorption was measured at 400 nm with a Beckman DU spectrophotometer after nitromethane extraction and the addition of hyamine reagent (3). Complete elution of nitrofurantoin was checked by a repeat cycle of elution into fresh distilled water. The concentration of nitrofurantoin was then extrapolated from a standard plot of nitrofurantoin at various concentrations. Standards were prepared by dissolving nitrofurantoin powder in dimethyl formamide, followed by dilution in distilled water, pH 6.0. pH effect. Antimicrobial in disks of each lot was eluted in 1 ml of deionized water during 2 h of incubation at 37 C, and the pH was measured with a glass electrode potentiometer. To study the effect of pH on nitrofurantoin activity, the antimicrobial was eluted from lot I disks in 1 ml of distilled water, adjusted to pH values between 4 and 11 with either 0.01 N NaOH or 0.01 N HCl, and then reabsorbed onto the original disks by evaporation under nitrogen.

Development of hemiacetal esterified levofloxacin to prevent chelation with metal-containing drugs

“…This in vitro chelation potential has the following formation order: Fe 3+ > Al 3+ > Cu 2+ > Zn 2+ > Mg 2+ > Ca 2+ . [38] Several prodrug esters have been reported to improve the oral absorption of penicillin, [39][40][41] for example pivaloyloxymethyl ester, acetoxymethyl ester, propionyloxymethyl ester and phthalidyl ester. A transient masking of the penicillin carboxyl group improves those oral absorptions by increasing lipophilicity.…”

Comparison of antibiotic discs from different sources.

“…with similar discs from Oxoid. This may have been due to differences in drug content of the discs or to differences in the pH of the discs (Hoo and Drew, 1974), a higher pH resulting in more rapid diffusion of the drug content into the culture medium, and consequently a larger zone of inhibition.…”