Understanding the Role of Host Human Products on the Acquisition of Resistance Traits through Natural Transformation in Acinetobacter baumannii.

Background: Acinetobacter baumannii has been classified by the CDC as a serious threat, as drug resistant strains are becoming increasingly common. A. baumannii’s capacity to incorporate exogenous DNA, via horizontal genetic transfer (HGT), is a major contributor to the development of antimicrobial resistance (AMR). However, few studies have addressed this topic and factors inducing natural competence in this species are unknown. We recently observed that bovine serum albumin (BSA) and Ca²⁺ significantly enhance transformation frequency and increased expression levels of two competence genes (comEA and pilQ) in A. baumannii strains. With the aim to identify other relevant host products that can enhance competence during A. baumanniicolonization/infection, we tested various human proteins.

Methods: A. baumannii strains A118, ATCC 17978, A42 and ATCC 19606 and plasmid (pDSred) and genomic DNA were used. Cells were grown overnight (ON) in Luria Bertani (LB) broth or LB supplemented with human protein, such as human serum albumin (HSA, 0.2%), collagen type IV (115 ng/mL or 166 ng/mL) and hyaluronic acid (1 mg/mL). Fresh LB was inoculated with ON cultures and 200 ng of gDNA or pDSred and incubated for 1 hr. Transformations were plated on kanamycin (KAN) and colony forming units (CFU) were plated on LB agar. Experiments were performed in triplicate and statistical analysis was performed (GraphPad Prism). A p-value <0.05 was considered significant.

Results: Transformation experiments showed that HSA triggered an increase (Mean= 6.88E-08, SD=2.94E=08) in natural transformation in strain A118. Growth in HSA showed a significant increase of 8.13, 2.15 and 1.23 folds in A118, ATCC 17978 and ATCC 19606 respectively. Collagen IV and hyaluronic acid did not have a significant effect on transformation frequencies in any of the four strains used.

Conclusion: These results support our hypothesis that human products can trigger or enhance transformation in A. baumannii. This effect appears to be albumin specific, as both BSA and HSA have been identified as inducers of natural competence. Overall, these results suggest that human products eventually contribute to the emergence of AMR in this threatening pathogen.