Acinetobacter baumannii is a Gram-negative, opportunistic pathogen that has become widespread in clinical settings. The emergence of strains that are multi-, extreme- and pan-drug resistant has created a significant healthcare burden due to the limitation of treatment options available, leading the WHO to declare the development of new drugs to treat this pathogen a critical priority. The study of key virulence factors and their impact on the immune response may help to identify new targets for novel therapeutics.
It is established that neutrophils play crucial role in the defence against A. baumannii infections. However, the bacterial factors that modulate the process of neutrophil chemotaxis toward the site of infection remain largely unknown. To identify novel bacterial-derived neutrophil chemoattractants, an A. baumannii transposon mutant library of a contemporary clinical isolate, AB5075-UW was screened using a high throughout human neutrophil chemotaxis assay. A total of 942 A. baumannii mutants were screened, and of these, 24 and 23 mutants induced an increase and reduction in chemotaxis, respectively. Further ex vivo and in vivo validation assays, identified two mutants (ddc, ABUW_2564 and nfa, ABUW_3555) that resulted in reduced neutrophil chemotaxis and the phenotype was restored to wild type level by supplying the genes in trans. The gene ddc (ABUW_2564) encodes a putative D-alanyl-D-alanine carboxypeptidase (DDCpase), which is thought to involve in peptidoglycan (PG) maturation and recycling. PG is a potent stimulator of host immune system that activates a protective inflammatory response. It was postulated that in the absence of DDCpase, there is a decrease in PG, which in turn decreases neutrophil chemotaxis. The gene nfa, (ABUW_3555) encodes for putative purine and pyrimidine nucleoside phosphorylase (PpnP), an enzyme of the nucleoside salvage pathway. It was hypothesised that the absence of PpnP may alter nucleotide metabolism, which in turn may impact biosynthetic pathways requiring nucleotide triphosphates, such as PG biosynthesis. This study confirmed the role of two novel A. baumannii genes as drivers of neutrophil chemotaxis, with further biochemical characterisation being a key future direction.