Transport of the lipophilic analog minocycline differs from that of tetracycline in susceptible and resistant Escherichia coli strains.

Plasmids which specify resistance to tetracycline offer much less resistance to its more lipophilic analog, minocycline. Resistance to minocycline varies for different plasmids. In the case of plasmid R222 (bearing the class B tetracycline resistance determinant on Tn10), minocycline resistance is comparatively high (10 microgram/ml, or 6% of the tetracycline resistance level). For plasmid pIP7 (bearing the class A determinant), minocycline resistance is only 1% of the tetracycline resistance level. To understand the basis for these differences, we compared the transport of the two tetracyclines by susceptible cells and by resistant cells. Uptake of minocycline by susceptible cells was 10 to 20 times more rapid than uptake of tetracycline and occurred largely via an energy-dependent route. This host-mediated energy-dependent uptake of both analogs was still present in tetracycline-resistant cells. In resistant cells, the same plasmid-mediated active efflux system previously described for tetracycline also exported minocycline. The 15-fold greater susceptibility of tetracycline-resistant R222-bearing cells to minocycline as compared with tetracycline could be explained at least in part by the more rapid influx of minocycline, which more easily overcame the efflux system. The particularly low minocycline resistance offered by pIP7 was due to a weak efflux for minocycline, 10-fold less effective than that mediated by R222. The rate-limiting step for uptake of both analogs appeared to be the outer membrane. That the lipophilic minocycline should cross this membrane more rapidly than tetracycline stands in contrast with other studies which show the outer membrane to be a barrier for entry of lipophilic substances.