Carbapenem-resistant Acinetobacter baumannii (CRAB) is a serious threat, and resistance is emerging to cefiderocol, one of the few remaining treatment options. However, how host micronutrients drive bacterial adaptation under antibiotic pressure is poorly defined. Using the AB5075 and CDC AR Bank #0033 (M27835) CRAB isolates, we combined time-kill assays, whole-genome sequencing, gene expression analysis, and phenotyping to test how vitamin B12 promotes small-colony variants (SCVs) -like adaptive derivatives and alters cefiderocol response. Vitamin B12 reproducibly selected slow-growing SCV-like adaptive derivatives that showed incomplete cefiderocol killing and frequent regrowth across drug concentrations in both genetic backgrounds. These derivatives carried genetic alterations affecting cell-surface structures, nutrient transport and TonB-dependent iron uptake, and energy balance, with capsule/biofilm remodeling and altered aminoglycoside killing. Co-exposure to vitamin B12 and cefiderocol further enriched SCV-like adaptive derivatives with changes in siderophore receptors and the BfmRS sensor BfmS. These findings identify vitamin B12 as an environmental cue that promotes the emergency of SCV-like adaptive derivatives and modulates cefiderocol response in CRAB, highlight nutrient availability as an underrecognized factor influencing antibiotic efficacy.

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a global health threat with few effective treatment options remaining. Cefiderocol, a last-resort siderophore-cephalosporin antibiotic, exploits bacterial iron transport systems via TonB-dependent receptors (TBDRs) to gain cellular entry. However, treatment failures and the emergence of resistance highlight concerns with in vivo efficacy. In this study, we report an unanticipated cefiderocol resistance mechanism where vitamin B12, a micronutrient supplement, modulates cefiderocol susceptibility. Our work revealed that vitamin B12 (methylcobalamin) affects and interacts with TBDRs and other metabolic and adaptation processes that contribute to increased cefiderocol MIC levels and the emergence of persistence phenotypes. We demonstrate that vitamin B12 supplementation elicits strain-specific transcriptomic responses in the AB5075 and AMA17 CRAB strains, characterized by the downregulation of genes encoding siderophore-mediated iron acquisition functions, stress responses, metabolic reprogramming, and biofilm biogenesis. Structural modeling and molecular docking reveal overlapping binding sites for vitamin B12 and cefiderocol within TBDRs such as CirA and PirA, suggesting competitive inhibition. Additionally, vitamin B12 exposure increases cefiderocol MICs across a panel of A. baumannii clinical and reference strains, enhances survival in time-kill assays, and promotes the emergence of small-colony variants displaying persistent phenotypes. Notably, this effect is stable, dose dependent, and further enhanced in the presence of host-derived fluids. These findings describe a previously unrecognized host-pathogen-drug interaction with potential clinical implications, suggesting that vitamin B12 exposure could contribute to cefiderocol treatment failure. Our results underscore the urgent need to consider the potential contribution of vitamin supplements to antimicrobial therapy and management strategies for CRAB infections.IMPORTANCECefiderocol, a last-line antibiotic for treating carbapenem-resistant Acinetobacter baumannii (CRAB) infections, uses iron-uptake receptors to enter bacterial cells. Our work demonstrates that vitamin B12, a common supplement in outpatients and hospitalized adults, can antagonize cefiderocol by affecting TonB-dependent receptor expression and competing at shared entry sites. As a result, cefiderocol MICs are raised, thus promoting persistent small-colony variants. This dose-dependent, strain-specific effect is amplified by host fluids, revealing a clinically plausible pathway leading to treatment failure that current susceptibility testing assays do not consider. Recognizing vitamin B12 exposure and incorporating physiological B12/iron conditions into antimicrobial susceptibility testing and models could improve decision-making for treatment regimens. More broadly, our findings highlight nutrient-antibiotic interactions as overlooked drivers of CRAB's resistance and persistence.

Human serum albumin (HSA), the predominant plasma protein, influences bacterial physiology and antibiotic susceptibility. We previously showed that HSA exposure alters Acinetobacter baumannii transcriptional profiles, reducing biofilm formation and modulating the expression of β-lactam resistance genes, while decreasing cefiderocol (FDC) activity through downregulation of TonB-dependent receptors (TBDRs). Since FDC uptake relies on siderophore-mediated transport, we hypothesised that albumin's iron content could affect bacterial susceptibility to FDC. However, its effect on FDC activity across carbapenem-resistant Gram-negative pathogens remains underexplored. Here, we evaluated FDC bactericidal efficacy in the presence of physiological HSA concentrations and various human biological fluids against five different species of carbapenem-resistant strains. Time-kill assays revealed that HSA markedly attenuates FDC activity, permitting bacterial regrowth in species including A. baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, and Achromobacter xylosoxidans. Human fluids with high protein content similarly reduced FDC efficacy. Transcriptional analyses indicated downregulation of iron uptake genes, suggesting that albumin-mediated iron availability modulates siderophore-dependent drug uptake. These findings highlight the significant role of host proteins and fluid environments in modulating FDC activity, underscoring the need to consider such factors when interpreting susceptibility information and optimising treatment strategies for infections caused by carbapenem-resistant pathogens.

Acinetobacter baumannii is an opportunistic pathogen that has a propensity to cause infections in immunocompromised patients. With the increasing incidence of antibiotic-resistant isolates, Acinetobacter baumannii has emerged as one of the most problematic pathogens in recent decades with the World Health Organization urging prioritization for additional research focusing on this nosocomial pathogen. The following protocol outlines the process of culturing and polarizing A549 human alveolar epithelial cells for infection with Acinetobacter baumannii to study biofilm formation. The protocol additionally outlines the process of fixing the infected A549 cells for downstream applications.

A growing increase in the number of serious infections caused by multidrug resistant bacteria (MDR) is challenging our society. Despite efforts to discover novel therapeutic options, few antibiotics targeting MDR have been approved by the Food and Drug Administration (FDA). Lactic acid bacteria have emerged as a promising therapeutic alternative due to their demonstrated ability to combat MDR pathogens in vitro. Our previous co-culture studies showed Lacticaseibacillus rhamnosus CRL 2244 as having a potent killing effect against carbapenem-resistant Acinetobacter baumannii (CRAB) strains. Here we report that cell-free conditioned media (CFCM) samples obtained from Lcb. rhamnosus CRL 2244 cultures incubated at different times display antimicrobial activity against 43 different pathogens, including CRAB, methicillin-resistant Staphylococcus aureus (MRSA) and carbapenemase Klebsiella pneumoniae (KPC)-positive strains. Furthermore, transwell and ultrafiltration analyses together with physical and chemical/biochemical tests showed that Lcb. rhamnosus CRL 2244 secretes a <3 kDa metabolite(s) whose antimicrobial activity is not significantly impaired by mild changes in pH, temperature and various enzymatic treatments. Furthermore, sensitivity and time-kill assays showed that the bactericidal activity of the Lcb. rhamnosus CRL 2244 metabolite(s) enhances the activity of some current FDA approved antibiotics. We hypothesize that this observation could be due to the effects of Lcb. rhamnosus CRL 2244 metabolite(s) on cell morphology and the enhanced transcriptional expression of genes coding for the phenylacetate (PAA) and histidine catabolic Hut pathways, metal acquisition and biofilm formation, all of which are associated with bacterial virulence. Interestingly, the extracellular presence of Lcb. rhamnosus CRL 2244 induced the transcription of the gene coding for the CidA/LgrA protein, which is involved in programmed cell death in some bacteria. Overall, the findings presented in this report underscore the promising potential of the compound(s) released by Lcb. rhamnosus CRL2244 as an alternative and/or complementary option to treat infections caused by A. baumannii as well as other MDR bacterial pathogens.

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a major human pathogen and a research priority for developing new antimicrobial agents. CRAB is a causative agent of a variety of infections in different body sites. One of the manifestations is catheter-associated urinary tract infection, which exposes the bacteria to the host's urine, creating a particular environment. Exposure of two CRAB clinical isolates, AB5075 and AMA40, to human urine (HU) resulted in the differential expression levels of 264 and 455 genes, respectively, of which 112 were common to both strains. Genes within this group play roles in metabolic pathways such as phenylacetic acid (PAA) catabolism, the Hut system, the tricarboxylic acid (TCA) cycle, and other processes like quorum sensing and biofilm formation. These results indicate that the presence of HU induces numerous adaptive changes in gene expression of the infecting bacteria. These changes presumably help bacteria establish and thrive in the hostile conditions in the urinary tract. These analyses advance our understanding of CRAB's metabolic adaptations to human fluids, as well as expand knowledge on bacterial responses to distinct human fluids containing different concentrations of human serum albumin (HSA).

The mortality rates of patients infected with Acinetobacter baumannii who were treated with cefiderocol (CFDC) were not as favorable as those receiving the best available treatment for pulmonary and bloodstream infections. Previous studies showed that the presence of human serum albumin (HSA) or HSA-containing fluids, such as human serum (HS) or human pleural fluid (HPF), in the growth medium is correlated with a decrease in the expression of genes associated with high-affinity siderophore-mediated iron uptake systems. These observations may explain the complexities of the observed clinical performance of CFDC in pulmonary and bloodstream infections, because ferric siderophore transporters enhance the penetration of CFDC into the bacterial cell. The removal of HSA from HS or HPF resulted in a reduction in the minimal inhibitory concentration (MIC) of CFDC. Concomitant with these results, an enhancement in the expression of TonB-dependent transporters known to play a crucial role in transporting iron was observed. In addition to inducing modifications in iron-uptake gene expression, the removal of HSA also decreased the expression of β-lactamases genes. Taken together, these observations suggest that environmental HSA has a role in the expression levels of select A. baumannii genes. Furthermore, the removal of iron from HSA had the same effect as the removal of HSA upon the expression of genes associated with iron uptake systems, also suggesting that at least one of the mechanisms by which HSA regulates the expression of certain genes is through acting as an iron source.

BACKGROUND:Acinetobacter baumannii causes a wide range of dangerous infections due to the emergence of pandrug-resistant strains. Therefore, there is a need for alternative therapeutics to treat these infections, including those targeting the host immune responses. However, immune responses, especially the humoral response against this pathogen, are poorly understood.METHODS:This study investigated the lymphocyte-mediated innate immune resistance to A. baumannii AB5075 pulmonary infection using B- and T-cell-deficient (Rag2-/-) mice, the protective effect of natural antibodies (NAbs), and the expression of complement-mediated responses using a mouse pneumonia model.RESULTS:Our results showed that intranasally infected Rag2-/- mice are impaired in clearing bacteria from lung, liver, and spleen at 24 hours postinfection compared to wildtype mice. Animal pretreatment with normal mouse serum or purified antibodies from naive mice rescued Rag2-/- mice from infection. Analysis of C3 complement protein binding demonstrated that NAbs increased C3 protein deposition on A. baumannii cells, indicating the activation of the classical complement pathway by NAbs.CONCLUSIONS:Overall, our study shows that NAbs mediate innate immune resistance against A. baumannii, a finding that may lead to the development of effective therapies against human infections caused by this antibiotic-resistant A. baumannii.

Infections caused by Carbapenem-resistant Acinetobacter baumannii (CRAB) isolates, such as hospital-acquired pneumonia (HAP), bacteremia, and skin and soft tissue infections, among others, are particularly challenging to treat. Cefiderocol, a chlorocatechol-substituted siderophore antibiotic, was approved by the U.S. Food and Drug Administration (FDA) in 2019 and prescribed for the treatment of CRAB infections. Despite the initial positive treatment outcomes with this antimicrobial, recent studies reported a higher-than-average all-cause mortality rate in patients treated with cefiderocol compared to the best available therapy. The cause(s) behind these outcomes remains unconfirmed. A plausible hypothesis is heteroresistance, a phenotype characterized by the survival of a small proportion of cells in a population that is seemingly isogenic. Recent results have demonstrated that the addition of human fluids to CRAB cultures leads to cefiderocol heteroresistance. Here, we describe the molecular and phenotypic analyses of CRAB heteroresistant bacterial subpopulations to better understand the nature of the less-than-expected successful outcomes after cefiderocol treatment. Isolation of heteroresistant variants of the CRAB strain AMA40 was carried out in cultures supplemented with cefiderocol and human pleural fluid (HPF). Two AMA40 variants, AMA40 IHC1 and IHC2, were resistant to cefiderocol. To identify mutations and gene expression changes associated with cefiderocol heteroresistance, we subjected these variants to whole genome sequencing and global transcriptional analysis. We then assessed the impact of these mutations on the pharmacodynamic activity of cefiderocol via susceptibility testing, EDTA and boronic acid inhibition analysis, biofilm formation, and static time-kill assays. Heteroresistant variants AMA40 IHC1 and AMA40 IHC2 have 53 chromosomal mutations, of which 40 are common to both strains. None of the mutations occurred in genes associated with high affinity iron-uptake systems or β-lactam resistance. However, transcriptional analyses demonstrated significant modifications in levels of expression of genes associated with iron-uptake systems or β-lactam resistance. The blaNDM-1 and blaADC-2, as well as various iron-uptake system genes, were expressed at higher levels than the parental strain. On the other hand, the carO and ompA genes’ expression was reduced. One of the mutations common to both heteroresistant strains was mapped within ppiA, a gene associated with iron homeostasis in other species. Static time-kill assays demonstrated that supplementing cation-adjusted Mueller–Hinton broth with human serum albumin (HAS), the main protein component of HPF, considerably reduced cefiderocol killing activity for all three strains tested. Notably, collateral resistance to amikacin was observed in both variants. We conclude that exposing CRAB to fluids with high HSA concentrations facilitates the rise of heteroresistance associated with point mutations and transcriptional upregulation of genes coding for β-lactamases and biofilm formation. The findings from this study hold significant implications for understanding the emergence of CRAB resistance mechanisms against cefiderocol treatment. This understanding is vital for the development of treatment guidelines that can effectively address the challenges posed by CRAB infections.

Objectives: Carbapenem-resistant Acinetobacter baumannii (CRAB) isolates are one of the most difficult pathogens to treat. Cefiderocol, a chlorocatechol-substituted siderophore antibiotic, was approved by the U.S. Food and Drug Administration (FDA) in 2019 indicated for the treatment of infections due to CRAB infections. Despite the initial positive treatment outcomes with this antimicrobial, recent studies reported higher than all-cause mortality rate in patients treated with cefiderocol. The cause(s) behind these outcomes remains unconfirmed. A plausible hypothesis is heteroresistance, a phenotype characterized by the survival of a small proportion of cells in a population seemingly isogenic. Recent results have shown that the addition of human fluids to CRAB cultures leads to cefiderocol heteroresistance. Here we describe molecular and phenotypic analyses of CRAB heteroresistant bacterial subpopulations to better understand the nature of the less-than-expected successful outcomes after cefiderocol treatment. Methods: Isolation of heteroresistant variants of the CRAB strain AMA40 was carried out in cultures supplemented with cefiderocol and human pleural fluid (HPF). Two AMA40 variants, AMA40 IHC1 and IHC2, were subjected to whole genome sequencing and transcriptional analysis to identify mutations and expression changes associated with cefiderocol heteroresistance. The impact of these mutations on the pharmacodynamic activity of cefiderocol was assessed via susceptibility testing, EDTA and boronic acid inhibition analysis, biofilm formation, and static time-kill assays. Results: Heteroresistant variants AMA40 IHC1 and AMA40 IHC2 have 53 chromosomal mutations, of which 40 are common to both strains. None of the mutations occurred in genes associated with high affinity iron-uptake systems or β-lactam resistance. However, transcriptional analyses showed significant modifications in levels of expression of genes associated with these functions. The blaNDM-1 and blaADC-2, as well as various iron-uptake system genes, were expressed at higher levels than the parental strain. On the other hand, the carO and ompA genes’ expression was reduced. One of the mutations common to both heteroresistant strains mapped within pipA, a gene associated with iron homeostasis in other species. Static time-kill assays showed that supplementing cation-adjusted Mueller-Hinton broth with human serum albumin, the main protein component of HPF, considerably reduced cefiderocol killing activity for all three strains tested. Notably, collateral resistance to amikacin was observed in both variants. Conclusions: We conclude that exposing CRAB to fluids containing high HSA facilitates the rise of heteroresistance associated with point mutations and upregulation of genes coding for β-lactamases and biofilm formation.

Acinetobacter baumanniiis a growing public health threat due to increasing incidence of carbapenem-resistant strains in U.S. hospitals. With widespread antibiotic resistance, there is an urgent need for alternative therapies to treat these infections, which are often acquired in hospital intensive care units. Antibody-based immunotherapies offer one potential treatment avenue, however, adaptive immune responses to A. baumanniihave not been well characterized. Pilot experiments, using a mouse pneumonia model to explore the role of adaptive immunity in clearing A. baumannii, uncovered an unexpected and severe defect in resistance to A. baumanniiinfection in Rag2−/-mice during the first 48 hours after infection. Subsequent experiments showed that intranasally infected Rag2−/−or B cell-deficient μMT mice are impaired compared to wild type mice in clearing bacteria from lung, liver, and spleen at 24 hours post infection. Prophylactic administration of normal mouse serum or purified immunoglobulins from naive mice demonstrates a protective effect of natural antibodies (NAbs) by rescuing Rag2−/−mice from severe infection. Analysis of C3 complement protein binding demonstrated that NAbs increased C3 protein deposition on A. baumanniicells, indicating the activation of the classical complement pathway as a potential mechanism of action. Overall, our study demonstrates a previously unknown role for natural antibodies in the innate resistance to Acinetobacterpneumonia, a finding that may lead to the development of effective therapies against human infections caused by this antibiotic-resistant pathogen.

Siderophores are iron-chelating molecules that solubilize Fe3+ for microbial utilization and facilitate colonization or infection of eukaryotes by liberating host iron for bacterial uptake. By fluorescently labeling membrane receptors and binding proteins, we created 20 sensors that detect, discriminate, and quantify apo- and ferric siderophores. The sensor proteins originated from TonB-dependent ligand-gated porins (LGPs) of Escherichia coli (Fiu, FepA, Cir, FhuA, IutA, BtuB), Klebsiella pneumoniae (IroN, FepA, FyuA), Acinetobacter baumannii (PiuA, FepA, PirA, BauA), Pseudomonas aeruginosa (FepA, FpvA), and Caulobacter crescentus (HutA) from a periplasmic E. coli binding protein (FepB) and from a human serum binding protein (siderocalin). They detected ferric catecholates (enterobactin, degraded enterobactin, glucosylated enterobactin, dihydroxybenzoate, dihydroxybenzoyl serine, cefidericol, MB-1), ferric hydroxamates (ferrichromes, aerobactin), mixed iron complexes (yersiniabactin, acinetobactin, pyoverdine), and porphyrins (hemin, vitamin B12). The sensors defined the specificities and corresponding affinities of the LGPs and binding proteins and monitored ferric siderophore and porphyrin transport by microbial pathogens. We also quantified, for the first time, broad recognition of diverse ferric complexes by some LGPs, as well as monospecificity for a single metal chelate by others. In addition to their primary ferric siderophore ligands, most LGPs bound the corresponding aposiderophore with ∼100-fold lower affinity. These sensors provide insights into ferric siderophore biosynthesis and uptake pathways in free-living, commensal, and pathogenic Gram-negative bacteria.

Acinetobacter baumannii is a common nosocomial pathogen that utilizes numerous mechanisms to aid its survival in both the environment and the host. Coordination of such mechanisms requires an intricate regulatory network. We report here that A. baumannii can directly regulate several stress-related pathways via the two-component regulatory system BfmRS. Similar to previous studies, results from transcriptomic analysis showed that mutation of the BfmR response regulator causes dysregulation of genes required for the oxidative stress response, the osmotic stress response, the misfolded protein/heat shock response, Csu pilus/fimbria production, and capsular polysaccharide biosynthesis. We also found that the BfmRS system is involved in controlling siderophore biosynthesis and transport, and type IV pili production. We provide evidence that BfmR binds to various stress-related promoter regions and show that BfmR alone can directly activate transcription of some stress-related genes. Additionally, we show that the BfmS sensor kinase acts as a BfmR phosphatase to negatively regulate BfmR activity. This work highlights the importance of the BfmRS system in promoting survival of A. baumannii. IMPORTANCE Acinetobacter baumannii is a nosocomial pathogen that has extremely high rates of multidrug resistance. This organism's ability to endure stressful conditions is a key part of its ability to spread in the hospital environment and cause infections. Unlike other members of the gammaproteobacteria, A. baumannii does not encode a homolog of the RpoS sigma factor to coordinate its stress response. Here, we demonstrate that the BfmRS two-component system directly controls the expression of multiple stress resistance genes. Our findings suggest that BfmRS is central to a unique scheme of general stress response regulation by A. baumannii.

As a facultative pathogen, Acinetobacter baumannii persists in various environments by sensing different environmental cues, including light. This report provides evidence of light-dependent regulation at 37°C of the expression of genes coding for a wide range of functions, including those involved in the conjugation of the pAB3 plasmid.

Cefiderocol, a recently introduced antibiotic, has a chemical structure that includes a cephalosporin that targets cell wall synthesis and a chlorocatechol siderophore moiety that facilitates cell penetration by active iron transporters. Analysis of the effect that human serum, human serum albumin, and human pleural fluid had on growing Acinetobacter baumannii showed that genes related to iron uptake were down-regulated. At the same time, β-lactamase genes were expressed at higher levels. The minimum inhibitory concentrations of this antimicrobial in A. baumannii cells growing in the presence of human serum, human serum albumin, or human pleural fluid were higher than those measured when these fluids were absent from the culture medium. These results correlate with increased expression levels of β-lactamase genes and the down-regulation of iron uptake-related genes in cultures containing human serum, human serum albumin, or human pleural fluid. These modifications in gene expression could explain the less-than-ideal clinical response observed in patients with pulmonary or bloodstream A. baumannii infections. The exposure of the infecting cells to the host's fluids could cause reduced cefiderocol transport capabilities and increased resistance to β-lactams. The regulation of genes that could impact the A. baumannii susceptibility to cefiderocol, or other antibacterials, is an understudied phenomenon that merits further investigation.

Cefiderocol (CFDC) is a novel chlorocatechol-substituted siderophore antibiotic approved to treat complicated urinary tract infections (cUTI) and hospital-acquired and ventilator-acquired pneumonia (HAP/VAP). Previous work determined that albumin-rich human fluids increase the minimum inhibitory concentration (MICs) of Acinetobacter baumannii against CFDC and reduce the expression of genes related to iron uptake systems. This latter effect may contribute to the need for higher concentrations of CFDC to inhibit growth. The presence of human urine (HU), which contains low albumin concentrations, did not modify MIC values of two carbapenem-resistant A. baumannii. Levels of resistance to CFDC were not modified by HU in strain AMA40 but were reduced in strain AB5075. Expanding the studies to other carbapenem-resistant A. baumannii isolates showed that the presence of HU resulted in unmodified or reduced MIC of CDFC values. The expression of piuA, pirA, bauA, and bfnH determined by qRT-PCR was enhanced in A. baumannii AMA40 and AB5075 by the presence of HU in the culture medium. All four tested genes code for functions related to recognition and transport of ferric-siderophore complexes. The effect of HU on expression of pbp1, pbp3, blaOXA-51-like, blaADC, and blaNDM-1, genes associated with resistance to β-lactams, as well as genes coding for efflux pumps and porins was variable, showing dependence with the strain analyzed. We conclude that the lack of significant concentrations of albumin and free iron in HU makes this fluid behave differently from others we tested. Unlike other albumin rich fluids, the presence of HU does not impact the antibacterial activity of CFDC when tested against A. baumannii.

Acinetobacter baumannii is a catalase-positive Gram-negative bacterial pathogen that causes severe infections among compromised patients. Among its noteworthy regulatory mechanisms, this microorganism regulates its lifestyle through the blue light using flavin (BLUF) protein BlsA. This protein regulates a diverse set of cellular processes that include, but are not limited to, motility, biofilm formation, phenylacetic acid metabolism, iron uptake, and catalase activity. We set out to determine how A. baumannii regulates catalase activity and other related oxidative stress phenotypes in response to light. Notably, because A. baumannii ATCC 17978 encodes four catalase homologs - which we refer to as KatA, KatE, KatE2, and KatG - we also aimed to show which of these enzymes exhibit light- and BlsA-dependent activity. Our work not only provides insight into the general function of all four catalase homologs and the impact of light on these functions, but also directly identifies KatE as a BlsA-regulated enzyme. We further demonstrate that the regulation of KatE by BlsA is dependent on a lysine residue that we previously demonstrated to be necessary for the regulation of surface motility. Furthermore, we show that BlsA's five most-C-terminal residues - previously considered dispensable for BlsA's overall function - are necessary for the light-independent and light-dependent regulation of catalase and superoxide dismutase activities, respectively. We hypothesize that these identified critical residues are necessary for BlsA's interaction with protein partners including the transcriptional regulators Fur and BfmR. Together these data expand the understanding regarding how A. baumannii uses light as a signal to control oxidative stress resistance mechanisms that are critical for its pathophysiology.

Acinetobacter baumannii is a nosocomial pathogen capable of causing serious infections associated with high rates of morbidity and mortality. Due to its antimicrobial drug resistance profile, A. baumannii is categorized as an urgent priority pathogen by the Centers for Disease Control and Prevention in the United States and a priority group 1 critical microorganism by the World Health Organization. Understanding how A. baumannii adapts to different host environments may provide critical insights into strategically targeting this pathogen with novel antimicrobial and biological therapeutics. Exposure to human fluids was previously shown to alter the gene expression profile of a highly drug-susceptible A. baumannii strain A118 leading to persistence and survival of this pathogen. Herein, we explore the impact of human pleural fluid (HPF) and human serum albumin (HSA) on the gene expression profile of a highly multi-drug-resistant strain of A. baumannii AB5075. Differential expression was observed for ~30 genes, whose products are involved in quorum sensing, quorum quenching, iron acquisition, fatty acid metabolism, biofilm formation, secretion systems, and type IV pilus formation. Phenotypic and further transcriptomic analysis using quantitative RT-PCR confirmed RNA-seq data and demonstrated a distinctive role of HSA as the molecule involved in A. baumannii's response.

Acinetobacter baumannii is a multidrug-resistant pathogen that causes numerous infections associated with high mortality rates. Exposure to human body fluids, such as human pleural fluid (HPF) and human serum, modulates gene expression in A. baumannii, leading to changes in its pathogenic behavior. Diverse degrees of effects at the transcriptional level were observed in susceptible and carbapenem-resistant strains. The transcriptional analysis of AB5075, a hyper-virulent and extensively drug-resistant strain showed changes in genes associated with quorum sensing, quorum quenching, fatty acids metabolism, and high-efficient iron uptake systems. In addition, the distinctive role of human serum albumin (HSA) as a critical component of HPF was evidenced. In the present work, we used model strain to analyze more deeply into the contribution of HSA in triggering A. baumannii’s response. By qRT-PCR analysis, changes in the expression level of genes associated with quorum sensing, biofilm formation, and phenylacetic acid pathway were observed. Phenotypic approaches confirmed the transcriptional response. HSA, a predominant component of HPF, can modulate the expression and behavior of genes not only in a hyper-virulent and extensively drug-resistant A. baumannii model, but also in other strains with a different degree of susceptibility and pathogenicity.