Comparative in vitro studies of nalidixic acid, oxolinic acid, and a new analog with thieno (2,3-b) pyridine structure

Abstract

Bacterial susceptibility to a new organic acid related to nalidixic acid and oxolinic acid was tested. The in vitro susceptibility pattem of the new compound was found to be very similar to that of nalidixic acid.In our laboratory we synthesized a series of 4-oxo-4,7-dihydrothieno (2,3-b) pyridine-5-carboxylic acids. These acids are structurally related to nalidixic acid (NAL) and oxolinic acid (OA), which are well-known agents against gram-negative organisms (1-7). A preliminary bacteriological test showed that 7-ethyl-2-methyl-4-oxo-4,7-dihydrothieno (2,3-b) pyridine-5-carboxylic acid (I) (Fig. 1) was the most promising compound of the series. The present study evaluates the in vitro susceptibility of different organisms to this new compound and to NAL and OA as well as the potential for development ofbacterial resistance to these drugs.The following 60 organisms were used in this study: 9 strains of Pseudomonas aeruginosa, 4 Klebsiella pneumoniae, 2 Proteus morganii, 2 Proteus rettgeri, 4 Staphylococcus aureus, 6 Enterococci, 2 Serratia marcescens, and 31 Escherichia coli. All strains, except one E. coli (ATCC 25922), were isolates from a clinical laboratory (Institute for Tropical Medicine, Antwerp).Antibacterial activity was determined with the agar-dilution method in Mueller-Hinton agar. Twofold dilutions in water of stock solutions, containing 2,000 jig of the drugs per ml of 0.1 N NaOH, were added to the medium to obtain a final concentration range from 128 to 0.25 ,tg of drug per ml. A replicating device (8) was used. Overnight cultures of the bacteria to be tested were diluted so that the inocula contained 104 colony-forming units. The minimal inhibitory concentration (MIC) was defined as the lowest concentration of antibacterial agent allowing the growth ofno more than five colonies at the site of inoculation after overnight incubation at 370C.Results are represented in Table 1. Proteus sp. and E. coli were the most susceptible species, with a mean MIC of 1 and 2 ug/ml, respectively, for both I and NAL, and 0.25 jug/ml for OA. One strain of E. coli was resistant to both I and NAL (MIC > 128 ,ug/ml), with a MIC of 2 lag of OA per ml. All gram-positive strains and Pseudomonas aeruginosa were resistant to 64 ,ug of I or NAL per ml. All Enterococci strains were resistant to I and NAL. In general, the MICs of I and NAL were similar. However, certain differences were noted. For instance, Serratia marcescens strains were inhibited by 64 ug of I per ml, whereas they were resistant to more than 128 ,ug of NAL per ml. Otherwise, some of the Kleb-FIG. 1. Chemical structure ofI (7-ethyl-2-methyl-4-oxo-4,7-dihydrothieno (2,3-b) pyridine-5-carboxylic acid). siella sp. were more susceptible to NAL than to I. Without exception, the MICs of OA were lower than those of I or NAL. One K. pneumoniae strain was resistant to both I and NAL (MIC > 128 ,ug/ml); the MIC of OA was 64 itg/ml.