BACTERIOLOGICAL STUDY ON DOXYCYCLINE, A NEW TETRACYCLINE DERIVATIVE

Abstract

The new tetracycline derivative, doxycycline, has 'the same antibacterial spectrum as tetracycline but the in vitro antibacterial activity is 2 to 4 times more potent. Especially the interesting thing about the results of the sensitivity studies on approximately 100 clinical isolates of staphylococci is that approximately 60% of the strains showed high resistance to tetracycline with MIC of 100 mcg/ml and approximately 20% of those showed a somewhat sensitive tendency to doxycycline with MIC of 25-50 mcg/ml, and this tendency of sensitivity increase became marked by decreasing the inoculum size. The remaining 80% of the strains showed complete cross resistance, the MIC against doxycycline being also 100 mcg/ml.
On the other hand, doxycycline showed 2 to 4 times more activity against tetracycline sensitive, strains.
The same tendency for the change in sensitivity due to the addition of serum protein was seen as with 'tetracycline. Doxycycline was somewhat more stable in regard to the influence by the change of pH. In experimental infections in the mouse, therapeutic results with doxycycline in hemolytic streptococcal infections were superior to tetracycline but the results in E. coli infections were the same as tetracycline. There would be a need to conduct further studies on additional staphylococcal strains.