Interplay between Meropenem and Human Serum Albumin on Expression of Carbapenem Resistance Genes and Natural Competence in Acinetobacter baumannii

Abstract

Acinetobacter baumannii A118, a mostly susceptible strain and AB5075, carbapenem-resistant, were cultured in Lysogeny broth (LB) or LB with different supplements: 3.5% human serum albumin (HSA), human serum (HS), meropenem, or meropenem plus 3.5% HSA. Natural transformation levels were enhanced in A. baumannii A118 and AB5075 cultured in medium supplemented with 3.5% HSA. Addition of meropenem plus 3.5% HSA caused synergistic enhancement of natural transformation in A. baumannii A118. Medium containing 3.5% HSA or meropenem enhanced the expression levels of the competence and type IV pilus associated genes. The combination meropenem plus 3.5% HSA produced a synergistic enhancement in the expression levels of many of these genes. The addition of HS, which has a high content of HSA, was also an inducer of these genes. Cultures grown in medium supplemented with HS or 3.5% HSA also affected resistance genes, which were expressed at higher or lower levels depending on the modification required to enhance resistance. The inducing or repressing activity of these modulators also occurred in three more carbapenem-resistant strains tested. An exception was the A. baumannii AMA16 bla NDM-1 gene, which was repressed in the presence of 3.5% HSA. In conclusion, HSA produces an enhancement of natural transformation and a modification in expression levels of competence genes and antibiotic resistance. Furthermore, when HSA is combined with carbapenems, which may increase the stress response, the expression of genes involved in natural competence is increased in A. baumannii . This process may favor the acquisition of foreign DNA and accelerate evolution. Importance Acinetobacter baumannii causes a variety of nosocomial- and community-infections that are usually resistant to multiple antimicrobial agents. As new strains acquire more resistance genes, these infections become more difficult to treat and mortality can reach up to 39%. The high genomic plasticity exhibited by A. baumannii must be the consequence of numerous mechanisms that include acquiring foreign DNA and recombination. Here, we describe the ability of A. baumannii to induce competence genes when exposed to environments that resemble those found in the human body during untreated infection or after administration of carbapenems. In this latter scenario expression of genes related to resistance also modify their expression levels such that resistance is increased. The contributions of this article are two-fold. Firstly, when A. baumannii is exposed to products present during infection, it responds, augmenting the ability to capture DNA and accelerate evolution. Secondly, in those conditions, the bacterium also modifies the expression of resistance genes to increase its resistance levels. In summary, recognition of substances that are naturally (e.g., HSA) or artificially (treatment with carbapenems) induces A. baumannii to enhance expression of resistance determinants and genes regulating competence.

Human Serum Proteins and Susceptibility of Acinetobacter baumannii to Cefiderocol: Role of Iron Transport

“…We previously observed that genes associated with β-lactam resistance, such as bla OXA-51-like , bla OXA-23 , pbp1, and pbp3, were up-regulated in A. baumannii AB5075, AMA16, and AB0057 when cultured in the presence of 3.5% HSA (51,52). To further examine if changes in genes associated with β-lactam resistance were affected by HS or HPF in the CRAB strains, qRT-PCR was used to assess the gene expression of bla OXA-51-like , bla OXA-23 , bla NDM-1 , pbp1, pbp3, and carO in cultures of the strains AMA16 and AB0057 containing HS or HPF.…”

“…baumannii senses components of human fluids and responds by modifying its transcriptional and phenotypic profiles [46][47][48][49][50]. Human serum albumin (HSA), as well as human pleural fluid (HPF), modulates the expression of genes associated with iron uptake systems, biofilm formation, antibiotic resistance, and DNA acquisition, among others [47,51,52]. In previous studies, we observed that CRAB AB5075 genes associated with iron uptake systems were down-regulated when exposed to HPF and 0.2% HSA [47], while genes associated with β-lactam resistance were up-regulated in the presence of physiological concentrations of HSA and human serum (HS) [51,52].…”

“…Human serum albumin (HSA), as well as human pleural fluid (HPF), modulates the expression of genes associated with iron uptake systems, biofilm formation, antibiotic resistance, and DNA acquisition, among others [47,51,52]. In previous studies, we observed that CRAB AB5075 genes associated with iron uptake systems were down-regulated when exposed to HPF and 0.2% HSA [47], while genes associated with β-lactam resistance were up-regulated in the presence of physiological concentrations of HSA and human serum (HS) [51,52]. Understanding the modification in the expression of iron uptake systems and β-lactam resistance genes in A. baumannii exposed to biological fluids could allow future strategies to protect antibiotics from a possible deleterious in vivo efficacy.…”

Involvement of H-NS in <em>Acinetobacter baumannii’s</em> Natural Transformation

“…Natural transformation assays were performed as were previously described [25,26]. Briefly, 20 ul of A. baumannii cells grown overnight in LB medium at 37ºC were mixed with 1 μg of pMBLe-OA-ArK (apramycin resistance) plasmid and the mixture was spotted onto twitching motility plates (10).…”

“…A. baumannii transformation frequency increases in the presence of human pleural fluid and human serum albumin [13,20,25]. Furthermore, A. baumannii DNA uptake by A. baumannii occurs while moving across wet surfaces [12].…”