In intensive care units, up to 30% of A. baumannii clinical isolates are resistant to at least three classes of antibiotics, often including fluoroquinolones and carbapenems. Now Mark Adams of Case Western Reserve University et al. show in this pathogen that multidrug resistance (MDR) is a diverse phenomenon. "A resistance island (RI) with several resistance genes is present in at least some MDR isolates, but there are additional resistance genes located outside the RI, so the RI is not the

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... e story," says Adams. "For example, there are five beta-lactamase genes in one MDR strain that we sequenced, and only one is located in the RI. Extensive selective pressure seems to have resulted in a diverse gene set for inactivating beta-lactam drugs. We also found 475 genes that are shared among six clinical isolates, but are absent from a related, nonpathogenic Acinetobacter species, including a significant fraction of the transcription factors and transporters in A. baumannii." Studying these, he says, could help define what makes this bug a pathogen.