Role of the NtrC family response regulator in nitrogen metabolism of Acinetobacter baumannii

Abstract

Acinetobacter baumannii is an important Gram-negative nosocomial pathogen that causes opportunistic infections and employs different mechanisms to survive in the presence of antibiotics in the host. Nutrient limitation is one of the important defense mechanisms of the mammalian immune system to fight against the colonization of pathogens like A. baumannii. The present study describes an NtrC-type Response Regulator (RR) A1S_1978 involved in modulating the metabolism and cell morphology of A. baumannii via a two-component system. This RR was found to be highly conserved in the Acinetobacter and other important Gram-negative pathogens. Sequence analysis reveals that this RR contains an HTH_8 DNA-binding domain. It is also observed that deletion of this RR resulted in elongated cell phenotype and altered colony morphology of A. baumannii. We showed that the ability of A. baumannii to form biofilm and pellicle is partly abolished upon deletion of this response regulator. We showed that mutant strains lacking RR A1S_1978 have diminished growth in the absence of the nitrogen source. The transcriptome analysis of the A1S_1978 deletion mutant revealed that 253 genes were differentially expressed, including 80 genes that were upregulated by at least 2-fold and 173 genes that were down regulated in the ΔA1S_1978 strain. The transcriptome data showed an association between the A1S_1978 RR and key genes related to various nitrogen and amino acid metabolism processes, which was further confirmed by real time PCR analysis. The deletion of this RR leads to a reduction in persister cell formation against ciprofloxacin antibiotic. Taken together the results of this investigation provide significant evidence that the RR A1S_1978 is a global regulator in A. baumannii.