Update on Acinetobacter Species: Mechanisms of Antimicrobial Resistance and Contemporary In Vitro Activity of Minocycline and Other Treatment Options

Among Acinetobacter species, A. baumannii and other closely related species are commonly implicated in nos-ocomial infections. These organisms are usually multidrug resistant (MDR), and therapeutic options to treat A. baumannii infections are very limited. Clinicians have been resorting to older antimicrobial agents to treat in-fections caused by MDR A. baumannii, and some of these agents have documented toxicity and/or are not op-timized for the infection type to be treated. Recent clinical experience supported by antimicrobial susceptibility data suggests that minocycline has greater activity than other tetracyclines and glycylcyclines against various MDR pathogens that have limited therapeutic options available, including Acinetobacter species. An intrave-nous formulation of minocycline has recently become available for clinical use, and in contrast to most older tetracyclines, minocycline has high activity against Acinetobacter species. In this report, we summarized some of the characteristics of the tetracycline class, and quantified the minocycline activity against contemporary (2007– 2011) isolates and its potential therapeutic role against a collection of 5477 A. baumannii and other relevant gram-negative organisms when compared directly with tetracycline, doxycycline, and other broad-spectrum an-timicrobial agents. Acinetobacter baumannii strains were highly resistant to all agents tested, with the exception of minocycline (79.1 % susceptible) and colistin (98.8 % susceptible). Minocycline (minimum inhibitory concen-tration that inhibits 50 % and 90 % of the isolates [MIC50/90]: 1/8 µg/mL) displayed greater activity than doxycy