Inhibitor of Antibiotics in Bacteriologic Agar ∗

Abstract

Summary

A series of antibiotics have been tested for in vitro activity in various media. A component in routine bacteriologic agar, not present in agarose and which is blocked by addition of protamine or toluidine blue has been found to interfere with diffusion of the polymyxins, aminoglycoside and some other antibiotics. This is thought to be due to acidic sulfate groups present in bacteriologic agar. The relation of this effect to possible in vivo binding to tissue components and their significance for antimicrobial sensitivity tests has been briefly discussed.

Addendum An enhancement of infectivity of respiratory syncytial virus by diethylaminoethyl dextran was recently described by Nomura (Nomura, S., Proc. Soc. Exptl. Biol. Med. 128, 163, 1968). This agent (mol. wt. 2 × 10⁶, Pharmacia, Uppsala, Sweden) was tested in bacteriologic agar to determine whether or not it would increase the size of zones of inhibition of polymyxin B. A marked enhancing effect was found with as little as 0.03 mg per ml. Concentrations in medium 199 and 2% agar of 1 mg per ml of DEAE dextran increased the zone size of polymyxin disks against E. coli from 10.7 mm to 16.3 mm, corresponding to the zone size found with agarose gel using medium 199 and the same test organism. Thus, DEAE dextran has been shown to have an effect in agar as well as on virus infectivity.