Studies on Antibiotic Resistance and the Nucleic Acid Content of Bacteria

Abstract

Data presented recently (Smolens and Vogt, 1952) showed that Haemophilus pertussi grown on Bordet-Gengou medium contained a higher percentage of desoxyribonucleic acid than ribonucleic acid. Considerable increase in ribonucleic acid content could be effected by growing the same strains in a modified Cohen-Wheeler medium (Cohen and Wheeler, 1946). We have found since that H. perussis grown in the two different media mentioned above shows a marked difference in resistance to antibiotics. Similar results are demonstrated by Micrococcus pyogenes var. aureus. This communication is concerned with the relation of nucleic acid to the emergence of resistance of bacteria to antibiotics. Also, the results of studies on the effect of environment on the susceptibility and resistance of bacteria to antibiotics will be presented. MATERIALS AND MEODS Cultures. Two strains of Haemophilus pertussis were used: no. 18323, which is one of the standard cultures of the National Institutes of Health, and Tillman which was isolated from a patient in the Children's Hospital of Philadelphia. These strains were grown on Bordet-Gengou medium containing 16 per cent defbrinated horse blood and dried from the frozen state in a large number of vials which were flame-sealed. A fresh vial was opened when required. M. pyogenes var. aureu8, strain 3A, streptomycin sensitive and resistant were also employed; these cultures were supplied through the courtesy of Dr. M. G. Sevag. Media. Two types of media were employed: freshly prepared Bordet-Gengou medium and the medium described by Cohen and Wheeler (1946) plus 2 per cent agar and 0.4 per cent Norit A as suggested by Powell, Culbertson, and Ensminger (1951). Antibiotics. Aureomycin, chloramphenicol, crystalline G penicillin, streptomycin sulfate, and terramycin were utilized. Dilutions were made in 0.85 per cent NaCl. Measurement of antibiotic acvy. Both the paper disc and the plate methods were used. The discs were 6.5 mm in diameter.