AbaM Regulates Quorum Sensing, Biofilm Formation, and Virulence in <i>Acinetobacter baumannii</i>

Abstract

<i>Acinetobacter baumannii</i> possesses a single divergent <i>luxR</i>/<i>luxRI</i>-type quorum-sensing (QS) locus named <i>abaR</i>/<i>abaI</i> This locus also contains a third gene located between <i>abaR</i> and <i>abaI</i>, which we term <i>abaM</i>, that codes for an uncharacterized member of the RsaM protein family known to regulate <i>N</i>-acylhomoserine lactone (AHL)-dependent QS in other beta- and gammaproteobacteria. Here, we show that disruption of <i>abaM</i> via a T26 insertion in <i>A. baumannii</i> strain AB5075 resulted in increased production of <i>N</i>-(3-hydroxydodecanoyl)-l-homoserine lactone and enhanced surface motility and biofilm formation. In contrast to the wild type and the <i>abaI</i>::T26 mutant, the virulence of the <i>abaM</i>::T26 mutant was completely attenuated in a <i>Galleria mellonella</i> infection model. Transcriptomic analysis of the <i>abaM</i>::T26 mutant revealed that AbaM differentially regulates at least 76 genes, including the <i>csu</i> pilus operon and the acinetin 505 lipopeptide biosynthetic operon, that are involved in surface adherence, biofilm formation and virulence. A comparison of the wild type, <i>abaM</i>::T26 and <i>abaI</i>::T26 transcriptomes, indicates that AbaM regulates ∼21% of the QS regulon including the <i>csu</i> operon. Moreover, the QS genes (<i>abaI</i> and <i>abaR</i>) were among the most upregulated in the <i>abaM</i>::T26 mutant. <i>A. baumannii</i><i>lux</i>-based <i>abaM</i> reporter gene fusions revealed that <i>abaM</i> expression is positively regulated by QS but negatively autoregulated. Overall, the data presented in this work demonstrates that AbaM plays a central role in regulating <i>A. baumannii</i> QS, virulence, surface motility, and biofilm formation.<b>IMPORTANCE</b><i>Acinetobacter baumannii</i> is a multiantibiotic-resistant pathogen of global health care importance. Understanding <i>Acinetobacter</i> virulence gene regulation could aid the development of novel anti-infective strategies. In <i>A. baumannii</i>, the <i>abaR</i> and <i>abaI</i> genes that code for the receptor and synthase components of an <i>N</i>-acylhomoserine (AHL) lactone-dependent quorum sensing system (QS) are separated by <i>abaM</i> Here, we show that although mutation of <i>abaM</i> increased AHL production, surface motility, and biofilm development, it resulted in the attenuation of virulence. AbaM was found to control both QS-dependent and QS-independent genes. The significance of this work lies in the identification of AbaM, an RsaM ortholog known to control virulence in plant pathogens, as a modulator of virulence in a human pathogen.