Human Albumin Alter Specific Genes that Can Play a Role in Survival and Persistence in Acinetobacter Baumannii

In 2017, the World Health Organization compiled a list of “priority pathogens”, this list included 12 antibiotic-resistant bacteria. Acinetobacter baumannii made the slot for “priority one pathogen for antibiotic research and development. ” A. baumannii infects patients in critical care units, demonstrates multi-drug-resistance and can survive “desiccation, nutrient starvation, and high concentrations of antimicrobials. ” The scientists performed the study to, “illustrate how human products, in particular, HSA, may play a significant role in both survival and persistence of A. baumannii. ” Acinetobacter baumannii has the ability to not only survive extreme conditions but, also to perform gene transfer, or “acquire genetic material. ” Due to these traits, A. baumannii has become a prominent bacterium, causing hospital-acquired infections. This observation led Quinn et al. (2018) to “perform transcriptomic analysis of strain A118 under HSA induction to identify genes that are altered by HSA. ” Their results showed “statistically significant differential expression of 296 protein-coding genes, including those associated with motility, biofilm formation, metabolism, efflux pumps, capsule synthesis, and transcriptional regulation. ” The team used phenotypic analysis for all these traits and found “an increase in surface-associated motility, a decrease in biofilm formation, reduced activity of a citric acid cycle associated enzyme, and increased survival associated with zinc availability. ” They also found alterations in the genes known to be part of the bacteria’s antibiotic resistance and pathogenicity.

The major results

RNA-Seq technology was used to find genes altered under HSA conditions. The analysis showed a significant alteration in the expression of 296 coding genes. It was found that 23 genes exhibited up-regulation and 273 genes exhibited down-regulation. There was a change in the expression of genes involved with motility and persistence. Transcriptomic analysis showed 4 genes (pilQ, yebC, h-ns, exbD), involved in motility, demonstrating altered expression under the HSA treatment. Motility assays were performed to determine the effect that HSA had on “surface-associated motility. ” Cells grown in the presence of HSA showed an average increase of 7. 28mm in motility diameter, compared to those cells grown in control media. PilQ was confirmed to show statistically significant upregulation by 1PCR analysis, under HSA induction. Quinn et al. (2018) examined “additional type IV pilus biogenesis and functions genes”, they found that “22 genes were also up-regulated and only two genes were down-regulated under HSA induction. ” Yeb-C and h-ns, transcriptional regulators that may play a role in motility under HSA induction, were found to have altered expression under HSA treatment. NCBI BLAST and gene ontology analysis was used to identify yebC as “putative transcriptional regulator that likely has sequence-specific DNA binding activity. ”

YebC, along with quorum quenching gene aidA, was notably downregulated upon induction of HSA. Global transcriptional repressor h-ns was also found to be down-regulated. H-ns was observed to be part of the regulation of gene expression of different virulence traits. Exb-D is believed to possibly play a role in Type IV pilus assembly and twitching motility. The gene was “shown to be statistically significant and differentially expressed. ” Down-regulation of exb-D was found. Two genes related to biofilm formation were identified, fimA and ompA. Both were statistically differentially expressed, under HSA induction. With cells grown in the presence of HSA, a 1. 4-fold decrease in biofilm formation was observed. K locus, which is associated with A. baumannii capsule synthesis, were upregulated in the presence of HSA. Global transcriptional regulators have roles in the expression of virulence features so plasma survival assays were used “in the presence of 25 nM and 25 uM zinc, respectively, to quantify potential delay of immune clearance. Supplementing plasma samples with 25 nM zinc resulted in a 2. 01-fold increase in survival at 90 minutes and a 2. 10-fold increase in survival at 120 minutes. ”

“Bacterial hydrolases that use zinc as a cofactor have been shown to be an important part of pathogenesis, encoding different metalloproteases and beta-lactamases that respond in various survival threatening conditions, including: beta-lactam presence, host barriers, neutrophil secretion, cytokine/interleukin signaling, immunoglobulin action, and other immunological responses to pathogen invasion. ” In this study, the multiple beta-lactam resistance mechanisms of A. baumannii were “differentially expressed in the presence of HSA, ” upregulation was seen. Outer membrane protein CarO showed significant down-regulation and per the authors of the study, it is known that “the loss of CarO is A. baumannii 4 related to carbapenem resistance. ” When it was determined that CarO showed down-regulation and two beta lactamases on the A118 genome were upregulated, susceptibility assays were performed. The only notable change was a slight decrease in the “halo inhibition of IMP. ” This all led to the conclusion that “HSA can have a role in the expression of genes related with resistance to beta-lactams, ” HSA was also found to have the ability to influence the expression of genes related with metabolism. Transcriptome analysis revealed 61 genes statistically significant and differentially expressed under HSA treatment involved in metabolism and nutrient acquisition. Out of these 61 genes, it was found that only 6 were upregulated, the other 55 were all down-regulated. Mdh and dctA are two of those down-regulated genes and are known to have direct links to “central metabolism”.

It was determined that A. baumannii adjusts and uses alternate metabolic pathways when needed, so no reduced growth pattern was observed. Metal ions are essential for A. baumannii survival. A. baumannii has evolved a mechanism that to “more efficiently sequester and take up iron and zinc” in response to host ability to deplete these ions to limit pathogens. The ferric acid regulator (FUR) was down-regulated under HSA induction.

In summary different relevant phenotypes associated with the pathoadaptability and pathogenesis of this pathogen were studied. It was identified how HSA, an essential human protein, can play a role in the survival and persistence of a pathogen. Among the 296 genes that showed statistically significant differential expression, the authors, focused on genes associated with motility, biofilm formation, antibiotic resistance, virulence, metabolic processes, and transcriptional regulation. Phenotypic analysis of these traits under HSA induction revealed an increase in surface-associated motility, a decrease in biofilm formation, a slight increase in antibiotic resistance against beta-lactam antibiotics, and a decrease in malate dehydrogenase activity.

Significance and further future studies

In other words, why does this article matter and what should or could be done next? The results of this study are significant because they provide critical information on the mechanisms A. baumannii employs for survival and persistence. The observed increase in motility, together with the increase rate in survivability, its ability to switch metabolism, as well as, an increase antibiotic resistance and virulence under HSA induction, can contribute to the success of A. baumannii using a multifactorial approach. Further studies could be done to uncover further mechanisms of survival for the pathogen, as well as to determine actions we could use to undermine these mechanisms. This may lead to increased susceptibility of the bacterium and the potential decrease in A. baumannii success.

Future studies can help uncover A. baumannii mechanisms of survival and persistence within the host, as well as, explain the utilization of albumin in A. baumannii success as a nosocomial pathogen. A. baumannii.

Conclusion

I chose this article after many searches and attempts to narrow down my choices. After some frustration, I started looking up anything related to bacteria, just trying to find a blip that would catch my eye, and basically stumbled upon Acinetobacter baumannii. I punched it into PubMed, scanned the articles and came across this one. The idea that something already in our blood can be used to alter such a dangerous bacterium fascinated me. I did have to read the article several times, partly because there is so much information, but also because I knew I would need to use my own words to discuss the article and they had already put it so perfectly, that it is hard to reword. It is interesting, informative, and clearly written. It led me to do further research, which I believe is highly indicative of a well laid out article, it grabbed my attention, answered questions, and it led to more questions.