[Comparative analysis of tetracycline, doxycycline and minocycline on chromosomal and plasmid tetracycline-resistant and -sensitive strains of Staphylococcus aureus using MIC, microcalorimetric effect and ultrastructural alterations].

A staphylococcus aureus strain of a patient with plasmidical tetracyclinresistance has first been cured from the plasmid and after that selected in nutritive broth by increasing the concentrations of tetracyclin. By this way we found three variants of the same strain: a tetracyclin-sensitive and two tetracyclin-resistant: an R-plasmid-mediated and the other as a result of mutation. These three variants have been tested against tetracyclin, minocyclin and doxycyclin on an agarmedium. Under influence of these three substances a microcalorimetrical investigation of the strains has taken place. We then tested the strains with the electronic microscope for alterations in the ultra-structure by the three substances. Following results have been obtained: 1. The sensitive strains were inhibited in the same extent by all of the three substances. Plasmidical resistant strains were only resistant against tetracyclin but not against minocyclin and even less against doxycyclin. 2. The microcalorimetrical tests confirmed the conclusions from the cultural test. 3. Alterations in ultra-structure were only found under tetracyclin-influence by strains in division. Comparing the three tetracyclines we did not find any significant difference in quality. 4. With all of the three investigation-methods we proved that R-plasmid-mediated tetracyclin-resistance of staphylococci is not effective for minocyclin and even less for doxycyclin.