In vitro activity of minocycline alone or in combination in multidrug-resistant Acinetobacter baumannii isolates

Minocycline (MIN) usually shows good activity against Acinetobacter baumannii strains. The reintroduction to the market of intravenous MIN provides an additional agent to the limited options for the treatment of A. baumannii infections. The activity of MIN as a single agent and in combination with rifampicin (RIF), colistin (COL) or imipenem (IMI) was evaluated by means of killing curves and 24 h-time-kill curves in five A. baumannii isolates which were selected on the basis of different antimicrobial resistance profiles. MIN showed bacteriostatic activity in three isolates (2 × or 16 × MIC) and bactericidal activity in the other isolates (64 × MIC). In isolates harbouring the tetB gene, the associations studied were always indifferent. However, in isolates not harbouring tetB, the use of MIN in combination showed a rapid synergistic effect (at 4 h) in four out of nine combinations (two with RIF and one each with IMI and COL). At 24 h, this effect was observed in six out of nine combinations (two in each association). MIN in combination with RIF, IMI and COL showed bactericidal synergy in most of the isolates which did not harbour the tetB gene, but the combinations were not synergistic in tetB-positive isolates.