Relationship between the AdeABC efflux system gene content, netilmicin susceptibility and multidrug resistance in a genotypically diverse collection of Acinetobacter baumannii strains

Objectives: To assess the occurrence of the genes of the AdeABC efflux system and their association with antimicrobial resistance in Acinetobacter baumannii. Methods: A set of 116 strains selected for their diversity both in genotypic properties and geographic origin was investigated for the presence of the structural (adeA, adeB and adeC) and regulatory (adeR and adeS) genes of the AdeABC system by PCR, for resistance to 11 antimicrobials by disc diffusion, for MIC of netilmicin and for the presence of aacC2 and aacA4, encoding netilmicin-modifying enzymes. Results: Ninety-five strains were positive for adeA, adeB, adeR and adeS, 10 were positive for 1 to 3 of these genes and 11 were negative for all genes. The adeC gene was found in 49 strains with one or more of the other genes. Forty-one strains were resistant to a maximum of one agent and 75 strains to two or more agents. Netilmicin MICs showed an almost bimodal distribution with respective peaks of 0.5-1 and 8 mg/L; aacC2 or aacA4 was found in six strains with netilmicin MIC of >= 64 mg/L. All 61 strains with netilmicin MICs >= 4mg/L were both adeABRS-positive and resistant to two or more agents, whereas netilmicin MICs <= 2mg/L (n = 51) were found for all strains resistant to a maximum of one agent and those negative for one or more of the adeABRS genes. Conclusions: The AdeABC genes are common in A. baumannii, but may be absent in some, mostly fully susceptible strains. Decreased susceptibility to netilmicin (MIC 4-32 mg/L) is associated with both the presence of these genes and multidrug resistance and may be indicative of AdeABC overexpression.