Susceptibility and resistance of plaque bacteria to minocycline

The in vitro susceptibility of 55 strains of subgingival plaque bacteria to minocycline was determined. A concentration of 1 microgram/ml minocycline was found to inhibit 85% of the strains tested and the MIC ranged from 0.03 to 32 micrograms/ml. For 71% of the strains tested the MBC was at least 4 times greater than the corresponding MIC, suggesting a bacteriostatic activity for minocycline. A concentration of 20 mg/ml of magnesium ions was capable of neutralizing 8 micrograms/ml of minocycline and was used to eliminate "carry-over" effects inherent in the experimental procedure. After 6 to 7 weeks exposure to sub-lethal concentrations of minocycline there was no appreciable increase in the MICs of most organisms with the exception of Actinobacillus actinomycetemcomitans NCTC 10981 and Campylobacter concisus NCTC 11485. Short term (6 hour exposure of bacteria to minocycline (8 micrograms/ml) markedly reduced the viability of a number of periodontopathogens but had little effect on the viability of Veillonella parvula NCTC 11456 and Fusobacterium nucleatum NCTC 11326. These in vitro investigations have demonstrated that minocycline is capable of inhibiting most of the periodontitis-associated bacteria tested and can kill some of these bacteria after a comparatively short exposure time. However, some of the organisms tested exhibited a low susceptibility to minocycline and others became less susceptible following exposure to low concentrations of the antibiotic for several weeks.