SUSCEPTIBILITY RESPONSE OF CLINICALLY ISOLATED BACTERIAL STRAINS TO DOXYCYCLINE

Abstract

Antibacterial activity of doxycycline (DOTC, α-6-deoxyoxytetracycline) was compared with that of 3 tetracycline derivatives; chlortetracycline (CTC), oxytetracycline (OTC) and tetracycline (TC). The bacterial strains used were 27 gram-negative rod bacteria, 100 Shigella and 100 staphylococci, and were of clinical origin. Level of DOTC resistance in most of the gram-negative bacteria including Shigella strains was sim ilar to other TC derivatives. In TC resistant strains, however, DOTC was found to be slightly effective than OTC or CTC. Among the TC resistant Shigella strains, some strains showed high sensitivity to DOTC. In TC resistant staphylococci, the mean of DOTC was 8 times or more effective than 3 other TC derivatives. In TC sensitive staphylococci, DOTC and CTC were slightly effective than OTC and TC. In vitro acquisition of DOTC resistance was studied by cultivation on the plate c ontaining serial concentrations of the drug. Stepwise increase in drug-resistance took place but high degree of drug-resistance was not obtained so far as tested.