[Comparison of the antibacterial efficacy of doxycycline and tetracycline (author's transl)].

In order to explain the mechanism of resistance within the tetracycline group, the minimal inhibitory concentration of tetracycline HCl was compared with that of doxycycline in the micro dilution test for bacterial collectives of 48 strains each from the groups of gram-negative and gram-positive cocci, resp. There was no significant difference between the inhibitory effect of doxycycline and tetracycline HCl for the bacterial collectives of E. coli, Klebsiella, Proteus and Staphylococcus aureus heamolyticus strains which were inhibited by a relatively low concentration of tetracycline HCl and doxycycline, respectively. On the other hand a significant difference in inhibitory effect was observed the calculation was made according to the Kolmogoroff-Smirnov test for the bacterial collectives of Enterococci and Staphylococcus aureus haemolyticus strains which were tetracycline resistant or only inhibited by higher concentrations. These results also have a practical importance, because a further testing against doxycycline can bring about a sensitivity to this substance in case of tetracycline resistant grampositive cocci. Possible explanations for the higher efficacy of doxycycline on these bacterial collectives will be discussed.