Human serum albumin alters specific genes that can play a role in survival and persistence in Acinetobacter baumannii

Preserving the efficacy of antibiotics to tackle antibiotic resistance

Different international agencies recognize that antibiotic resistance is one of the most severe human health problems that humankind is facing. Traditionally, the introduction of new antibiotics solved this problem but various scientific and economic reasons have led to a shortage of novel antibiotics at the pipeline. This situation makes mandatory the implementation of approaches to preserve the efficacy of current antibiotics. The concept is not novel, but the only action taken for such preservation had been the 'prudent' use of antibiotics, trying to reduce the selection pressure by reducing the amount of antibiotics. However, even if antibiotics are used only when needed, this will be insufficient because resistance is the inescapable outcome of antibiotics' use. A deeper understanding of the alterations in the bacterial physiology upon acquisition of resistance and during infection will help to design improved strategies to treat bacterial infections. In this article, we discuss the interconnection between antibiotic resistance (and antibiotic activity) and bacterial metabolism, particularly in vivo, when bacteria are causing infection. We discuss as well how understanding evolutionary trade-offs, as collateral sensitivity, associated with the acquisition of resistance may help to define evolution-based therapeutic strategies to fight antibiotic resistance and to preserve currently used antibiotics.

Carbapenem-resistant Acinetobacter baumannii (CRAB): metabolic adaptation and transcriptional response to human urine (HU)

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 modifications 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 expanding knowledge on bacterial responses to distinct human fluids containing different concentrations of human serum albumin (HSA).

Fever-like temperature impacts on Staphylococcus aureus and Pseudomonas aeruginosa interaction, physiology, and virulence both in vitro and in vivo

BACKGROUND

Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) cause a wide variety of bacterial infections and coinfections, showing a complex interaction that involves the production of different metabolites and metabolic changes. Temperature is a key factor for bacterial survival and virulence and within the host, bacteria could be exposed to an increment in temperature during fever development. We analyzed the previously unexplored effect of fever-like temperatures (39 °C) on S. aureus USA300 and P. aeruginosa PAO1 microaerobic mono- and co-cultures compared with 37 °C, by using RNAseq and physiological assays including in vivo experiments.

RESULTS

In general terms both temperature and co-culturing had a strong impact on both PA and SA with the exception of the temperature response of monocultured PA. We studied metabolic and virulence changes in both species. Altered metabolic features at 39 °C included arginine biosynthesis and the periplasmic glucose oxidation in S. aureus and P. aeruginosa monocultures respectively. When PA co-cultures were exposed at 39 °C, they upregulated ethanol oxidation-related genes along with an increment in organic acid accumulation. Regarding virulence factors, monocultured SA showed an increase in the mRNA expression of the agr operon and hld, pmsα, and pmsβ genes at 39 °C. Supported by mRNA data, we performed physiological experiments and detected and increment in hemolysis, staphyloxantin production, and a decrease in biofilm formation at 39 °C. On the side of PA monocultures, we observed an increase in extracellular lipase and protease and biofilm formation at 39 °C along with a decrease in the motility in correlation with changes observed at mRNA abundance. Additionally, we assessed host-pathogen interaction both in vitro and in vivo. S. aureus monocultured at 39οC showed a decrease in cellular invasion and an increase in IL-8-but not in IL-6-production by A549 cell line. PA also decreased its cellular invasion when monocultured at 39 °C and did not induce any change in IL-8 or IL-6 production. PA strongly increased cellular invasion when co-cultured at 37 and 39 °C. Finally, we observed increased lethality in mice intranasally inoculated with S. aureus monocultures pre-incubated at 39 °C and even higher levels when inoculated with co-cultures. The bacterial burden for P. aeruginosa was higher in liver when the mice were infected with co-cultures previously incubated at 39 °C comparing with 37 °C.

CONCLUSIONS

Our results highlight a relevant change in the virulence of bacterial opportunistic pathogens exposed to fever-like temperatures in presence of competitors, opening new questions related to bacteria-bacteria and host-pathogen interactions and coevolution.

Evidence of Mycobacterium avium subsp. paratuberculosis binding to albumin: technical and biological implications

Albumin binding ability is a well-characterized feature of many bacteria. To the best of our knowledge, there are no previous reports about this ability among mycobacteria, even when bovine serum albumin (BSA) is a common component of supplements used for the enrichment of synthetic media for mycobacterial growth in vitro and also of buffers used in laboratory techniques. In this work we explored the albumin binding ability of Mycobacterium avium subsp. paratuberculosis (MAP), a pathogenic bacterium causing a known and relevant ruminant disease worldwide, by immunizing rabbits with MAP (grown in media containing or not BSA) or BSA and conducting ELISA and immunoblot experiments with the obtained sera. As a result, we found that MAP can bind BSA when cultured in a conventional BSA-containing medium and when incubated for a short time in the presence of the protein. We also evaluated the host specificity of MAP interaction with albumin and found a preference for the protein of bovine origin when compared with its horse and rabbit homologs. Considerations about its technical and biological implications are discussed.

Antimicrobial activity of the Lacticaseibacillus rhamnosus CRL 2244 and its impact on the phenotypic and transcriptional responses in carbapenem resistant Acinetobacter baumannii

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a recognized nosocomial pathogen with limited antibiotic treatment options. Lactic acid bacteria (LAB) constitute a promising therapeutic alternative. Here we studied the antibacterial properties of a collection of LAB strains using phenotypic and transcriptomic analysis against A. baumannii clinical strains. One strain, Lacticaseibacillus rhamnosus CRL 2244, demonstrated a potent inhibitory capacity on A. baumannii with a significant killing activity. Scanning electron microscopy images showed changes in the morphology of A. baumannii with an increased formation of outer membrane vesicles. Significant changes in the expression levels of a wide variety of genes were also observed. Interestingly, most of the modified genes were involved in a metabolic pathway known to be associated with the survival of A. baumannii. The paa operon, Hut system, and fatty acid degradation were some of the pathways that were induced. The analysis reveals the impact of Lcb. rhamnosus CRL 2244 on A. baumannii response, resulting in bacterial stress and subsequent cell death. These findings highlight the antibacterial properties of Lcb. rhamnosus CRL 2244 and its potential as an alternative or complementary strategy for treating infections. Further exploration and development of LAB as a treatment option could provide valuable alternatives for combating CRAB infections.

Biofilm formation in drug-resistant Acinetobacter baumannii and Acinetobacter nosocomialis isolates obtained from a university hospital in Pelotas, RS, Brazil

This study aimed to investigate the antibiotic resistance and biofilm formation of Acinetobacter calcoaceticus-A. baumannii (ACB) complex isolates recovered from a university hospital in Pelotas, RS, Brazil. The species were confirmed using gyrB multiplex and blaOXA-51-like genes PCR. The presence of the bfmRS virulence gene was evaluated by the PCR, and the isolates were classified based on their biofilm-forming ability on polystyrene (PO) and glass surfaces (TM). Out of 50 ACB complex isolates evaluated, 41 were identified as A. baumannii and nine as A. nosocomialis. The bfmRS gene was detected in 97.6% (40/41) of A. baumannii and 33.3% (3/9) of A. nosocomialis species. Forty-nine isolates exhibited a multidrug-resistant (MDR) profile, while one A. nosocomialis isolate presented an extensively drug-resistant (XDR) profile. All isolates were able of forming biofilms on PO surfaces and 98% (49/50) on TM surfaces. A significant correlation was observed between biofilm production on PO and TM surfaces (P < 0.05). However, no correlation was found between biofilms forming and the presence of the bfmRS gene or displaying a certain antibiotic resistance profile. In conclusion, A. baumannii and A. nosocomialis are frequent species causing nosocomial infections in a hospital in Pelotas, RS, Brazil, and both are capable of forming biofilms.

Phenotypic and transcriptional analysis of the antimicrobial effect of lactic acid bacteria on carbapenem-resistant Acinetobacter baumannii: Lacticaseibacillus rhamnosus CRL 2244 an alternative strategy to overcome resistance?"

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a recognized nosocomial pathogen with limited antibiotic treatment options. Lactic acid bacteria (LAB) constitute a promising therapeutic alternative. Here we studied the antibacterial properties of a collection of LAB strains using phenotypic and transcriptomic analysis against A. baumannii clinical strains. One strain, Lacticaseibacillus rhamnosus CRL 2244, demonstrated a potent inhibitory capacity on A. baumannii with a significant killing activity. Scanning electron microscopy images showed changes in the morphology of A. baumannii with an increased formation of outer membrane vesicles. Significant changes in the expression levels of a wide variety of genes were also observed. Interestingly, most of the modified genes were involved in a metabolic pathway known to be associated with the survival of A. baumannii . The paa operon, Hut system, and fatty acid degradation were some of the pathways that were induced. The analysis reveals the impact of Lcb. rhamnosus CRL 2244 on A. baumannii response, resulting in bacterial stress and subsequent cell death. These findings highlight the antibacterial properties of Lcb. rhamnosus CRL 2244 and its potential as an alternative or complementary strategy for treating infections. Further exploration and development of LAB as a treatment option could provide valuable alternatives for combating CRAB infections.

Induced Heteroresistance in Carbapenem-Resistant Acinetobacter baumannii (CRAB) via Exposure to Human Pleural Fluid (HPF) and Its Impact on Cefiderocol Susceptibility

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.

Genomic Comparative Analysis of Two Multi-Drug Resistance (MDR) Acinetobacter baumannii Clinical Strains Assigned to International Clonal Lineage II Recovered Pre- and Post-COVID-19 Pandemic

BACKGROUND

After the emergence of COVID-19, numerous cases of A. baumannii/SARS-CoV-2 co-infection were reported. Whether the co-infecting A. baumannii strains have distinctive characteristics remains unknown.

METHODS AND RESULTS

A. baumannii AMA_NO was isolated in 2021 from a patient with COVID-19. AMA166 was isolated from a mini-BAL used on a patient with pneumonia in 2016. Both genomes were similar, but they possessed 337 (AMA_NO) and 93 (AMA166) unique genes that were associated with biofilm formation, flagellar assembly, antibiotic resistance, secretion systems, and other functions. The antibiotic resistance genes were found within mobile genetic elements. While both strains harbored the carbapenemase-coding gene blaOXA-23, only the strain AMA_NO carried blaNDM-1. Representative functions coded for by virulence genes are the synthesis of the outer core of lipooligosaccharide (OCL5), biosynthesis and export of the capsular polysaccharide (KL2 cluster), high-efficiency iron uptake systems (acinetobactin and baumannoferrin), adherence, and quorum sensing. A comparative phylogenetic analysis including 239 additional sequence type (ST) 2 representative genomes showed high similarity to A. baumannii ABBL141. Since the degree of similarity that was observed between A. baumannii AMA_NO and AMA166 is higher than that found among other ST2 strains, we propose that they derive from a unique background based on core-genome phylogeny and comparative genome analysis.

CONCLUSIONS

Acquisition or shedding of specific genes could increase the ability of A. baumannii to infect patients with COVID-19.

The Iron Content of Human Serum Albumin Modulates the Susceptibility of Acinetobacter baumannii to Cefiderocol

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.

Surviving the host: Microbial metabolic genes required for growth of Pseudomonas aeruginosa in physiologically-relevant conditions

Pseudomonas aeruginosa, like other pathogens, adapts to the limiting nutritional environment of the host by altering patterns of gene expression and utilizing alternative pathways required for survival. Understanding the genes essential for survival in the host gives insight into pathways that this organism requires during infection and has the potential to identify better ways to treat infections. Here, we used a saturated transposon insertion mutant pool of P. aeruginosa strain PAO1 and transposon insertion sequencing (Tn-Seq), to identify genes conditionally important for survival under conditions mimicking the environment of a nosocomial infection. Conditions tested included tissue culture medium with and without human serum, a murine abscess model, and a human skin organoid model. Genes known to be upregulated during infections, as well as those involved in nucleotide metabolism, and cobalamin (vitamin B₁₂) biosynthesis, etc., were required for survival in vivo- and in host mimicking conditions, but not in nutrient rich lab medium, Mueller Hinton broth (MHB). Correspondingly, mutants in genes encoding proteins of nucleotide and cobalamin metabolism pathways were shown to have growth defects under physiologically-relevant media conditions, in vivo, and in vivo-like models, and were downregulated in expression under these conditions, when compared to MHB. This study provides evidence for the relevance of studying P. aeruginosa fitness in physiologically-relevant host mimicking conditions and identified metabolic pathways that represent potential novel targets for alternative therapies.

Iron Acquisition Mechanisms and Their Role in the Virulence of Acinetobacter baumannii

Iron is an essential element for survival of most organisms. One mechanism of host defense is to tightly chelate iron to several proteins to limit its extracellular availability. This has forced pathogens such as Acinetobacter baumannii to adapt mechanisms for the acquisition and utilization of iron even in iron-limiting conditions. A. baumannii uses a variety of iron acquisition strategies to meet its iron requirements. It can lyse erythrocytes to harvest the heme molecules, use iron-chelating siderophores, and use outer membrane vesicles to acquire iron. Iron acquisition pathways, in general, have been seen to affect many other virulence factors such as cell adherence, cell motility, and biofilm formation. The knowledge gained from research on iron acquisition led to the synthesis of the antibiotic cefiderocol, which uses iron uptake pathways for entry into the cell with some success as a novel cephalosporin. Understanding the mechanisms of iron acquisition of A. baumannii allows for insight into clinical infections and offer potential targets for novel antibiotics or potentiators of current drugs.

Interactions between host epithelial cells and Acinetobacter baumannii promote the emergence of highly antibiotic resistant and highly mucoid strains

Acinetobacter baumannii is an important nosocomial pathogen. Upon colonizing a host, A. baumannii are subjected to selective pressure by immune defenses as they adapt to the host environment. However, the mechanism of this pathoadaptation is unknown. Here, we established an in vitro system to evolve A. baumannii driven by the continuous selective pressure exerted by epithelial cells, and we used a combination of experimental evolution, phenotypic characterization and multi-omics analysis to address the underlying mechanism. When continuously exposed to selective pressure by pulmonary epithelial cells, A. baumannii showed ptk mutation-mediated mucoid conversion (reduced adhesion and increased anti-phagocytic ability) by enhancement of capsular exopolysaccharide chain length; rsmG mutation-mediated deficiency of 7-methylguanosine modification in the 524th nucleotide of 16S rRNA, which increased ribosome translation efficiency; and rnaseI mutation-mediated changes in outer membrane permeability and efflux pump expression. Together, these mutations altered susceptibility to a variety of antimicrobial agents, including the novel antibiotic cefiderocol, by regulating siderophore and siderophore-receptor biosynthesis. In conclusion, pulmonary epithelial cells modulate A. baumannii pathoadaptation, implicating the host–microbe interaction in the survival and persistence of A. baumannii.

Making sense of drug-efflux transporters in the physiological environment

Bacterial drug-efflux transporters act synergistically with diffusion barriers of cellular membranes and other resistance mechanisms to protect cells from antibiotics and toxic metabolites. Their critical roles in clinical antibiotic and multidrug resistance are well established. In addition, a large body of evidence has been accumulated in support of their important contributions to bacterial growth and proliferation during infections. However, how these diverse functions of drug transporters are integrated at the level of bacterial cell physiology remains unclear. This opinion briefly summarizes the current understanding of substrate specificities and physiological roles of drug-efflux pumps from Resistance-Nodulation-Division (RND) superfamily of proteins in two ESKAPE pathogens Pseudomonas aeruginosa and Acinetobacter baumannii. Based on the analysis of phenotypic and transcriptomic studies in vitro and in vivo, we propose that RND pumps of Gram-negative bacteria fall into three categories: constitutively expressed, regulated, and silent. These three categories of efflux pumps participate in different physiological programs, which are not involved in the central metabolism and bacterial growth.

Human serum albumin (HSA) regulates the expression of histone-like nucleoid structure protein (H-NS) in Acinetobacter baumannii

According to the Centers for Disease Control and Prevention, Acinetobacter baumannii is listed among the most threatening pathogens. A. baumannii is mainly a nosocomial pathogen with a distinctive ability to survive in multiple environments. These characteristics together with this bacterium's ability to acquire antibiotic resistance determinants make it a notorious pathogen. The presence of human serum albumin (HSA) is associated with modification of expression levels in numerous genes. The presence of HSA in the culture medium is also correlated with a reduction in levels of the global suppressor histone-like nucleoid structure protein, H-NS. Comparative transcriptome analysis of the wild type and isogenic Δhns strains cultured in lysogeny broth (LB) in the presence or absence of HSA revealed that the expression of a subset of eleven genes are modified in the Δhns cultured in LB and the wild-type strain in the presence of HSA, pointing out these genes as candidates to be regulated by the presence of HSA through H-NS. Six and five of these genes were up- or down-regulated, respectively. Three of these genes have functions in quorum sensing (acdA, kar and fadD), one in quorum quenching (aidA), two in stress response (katE, ywrO), three in metabolism (phaC, yedL1, and yedL2), one in biofilm formation (csuAB), and one in β-oxidation of fatty acids (fadA). The regulation of these genes was assessed by: (i) transcriptional analysis and qPCR at the transcriptional level; and (ii) by determining the phenotypic characteristics of each function. The results of these studies support the hypothesis that HSA-mediated reduction of H-NS levels may be one very important regulatory circuit utilized by A. baumannii to adapt to selected environments, such as those where HSA-containing human fluids are abundant.

Gram-negative multidrug-resistant organisms were dominant in neurorehabilitation ward patients in a general hospital in southwest China

This study aimed to investigate the prevalence of and risk factors for multidrug-resistant organism (MDRO) infection in the rehabilitation ward of a general hospital in Southwest China. We analyzed rehabilitation patients with nosocomial infections caused by MDROs from June 2016 to June 2020. MDRO infection pathogens and associated antibiotic resistance were calculated. Possible risk factors for MDRO-related infection in the neurorehabilitation ward were analyzed using chi-square, and logistic regression. A total of 112 strains of MDRO were found positive from 96 patients. The MDRO test-positive rate was 16.70% (96/575). Ninety-five MDRO strains were detected in sputum, of which 84.82% (95/112) were gram-negative bacteria. Acinetobacter baumannii (A. Baumannii), Pseudomonas aeruginosa (P. aeruginosa), and Klebsiella pneumonia (K. pneumonia) were the most frequently isolated MDRO strains. The logistic regression model and multifactorial analysis showed that long-term (≥ 7 days) antibiotic use (OR 6.901), history of tracheotomy (OR 4.458), and a low albumin level (< 40 g/L) (OR 2.749) were independent risk factors for the development of MDRO infection in patients in the rehabilitation ward (all P < 0.05). Gram-negative MRDOs were dominant in rehabilitation ward patients. Low albumin, history of a tracheostomy, and long-term use of antibiotics were independent risk factors for MRDO infection and are worthy of attention.

The Phenylacetic Acid Catabolic Pathway Regulates Antibiotic and Oxidative Stress Responses in Acinetobacter

The opportunistic pathogen Acinetobacter baumannii is responsible for a wide range of infections that are becoming increasingly difficult to treat due to extremely high rates of multidrug resistance. Acinetobacter's pathogenic potential is thought to rely on a "persist and resist" strategy that facilitates its remarkable ability to survive under a variety of harsh conditions. The paa operon is involved in the catabolism of phenylacetic acid (PAA), an intermediate in phenylalanine degradation, and is the most differentially regulated pathway under many environmental conditions. We found that, under subinhibitory concentrations of antibiotics, A. baumannii upregulates expression of the paa operon while simultaneously repressing chaperone-usher Csu pilus expression and biofilm formation. These phenotypes are reverted either by exogenous addition of PAA and its nonmetabolizable derivative 4-fluoro-PAA or by a mutation that blocks PAA degradation. Interference with PAA degradation increases susceptibility to antibiotics and hydrogen peroxide treatment. Transcriptomic and proteomic analyses identified a subset of genes and proteins whose expression is affected by addition of PAA or disruption of the paa pathway. Finally, we demonstrated that blocking PAA catabolism results in attenuated virulence in a murine catheter-associated urinary tract infection (CAUTI) model. We conclude that the paa operon is part of a regulatory network that responds to antibiotic and oxidative stress and is important for virulence. PAA has known regulatory functions in plants, and our experiments suggest that PAA is a cross-kingdom signaling molecule. Interference with this pathway may lead, in the future, to novel therapeutic strategies against A. baumannii infections. IMPORTANCE Acinetobacter baumannii causes a wide range of infections that are difficult to treat due to increasing rates of multidrug resistance; however, the mechanisms that this pathogen uses to respond to stress are poorly understood. Here, we describe a new mechanism of stress signaling in Acinetobacter that is mediated by the metabolite phenylacetic acid (PAA). We found that disrupting PAA catabolism interfered with A. baumannii's ability to adapt to stress, leading to decreased antibiotic tolerance and hydrogen peroxide resistance. We propose that investigating this stress response could lead to the development of novel therapeutics. In fact, PAA derivatives constitute a group of FDA-approved nonsteroidal anti-inflammatory drugs that could potentially be repurposed as antivirulence therapies to target multidrug-resistant Acinetobacter infections.

Acinetobacter baumannii response to cefiderocol challenge in human urine

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.

The Impact of Natural Transformation on the Acquisition of Antibiotic Resistance Determinants

Carbapenem and multidrug-resistant (MDR) Acinetobacter baumannii leads the World Health Organization's list of priority pathogens and represents an unmet medical need. Understanding the mechanisms underpinning the acquisition of antibiotic resistance in this pathogen is fundamental to the development of novel therapeutics as well as to infection prevention and antibiotic stewardship strategies designed to limit its spread. In their investigation, "Interbacterial Transfer of Carbapenem Resistance and Large Antibiotic Resistance Islands by Natural Transformation in Pathogenic Acinetobacter," Anne-Sophie Godeux and colleagues (mBio 13:e0263121, 2022, https://doi.org/10.1128/mBio.02631-21) delineate the unsuspected extent and circumstances under which natural transformation as a mechanism of intraspecies and interspecies exchange of genetic material occurs in Acinetobacter spp. This study offers key insights into how this notorious pathogen may have accelerated the development of its MDR phenotype via an unexpectedly robust and unnervingly casual approach to the acquisition of antibiotic resistance determinants through natural transformation.

Staphylococcus aureus α-Toxin Effect on Acinetobacter baumannii Behavior

Polymicrobial infections are more challenging to treat and are recognized as responsible for significant morbidity and mortality. It has been demonstrated that multiple Gram-negative organisms take advantage of the effects of Staphylococcus aureus α-toxin on mucosal host defense, resulting in proliferation and dissemination of the co-infecting pathogens. Through phenotypic approaches, we observed a decrease in the motility of A. baumannii A118 after exposure to cell-free conditioned media (CFCM) of S. aureus strains, USA300 and LS1. However, the motility of A. baumannii A118 was increased after exposure to the CFCM of S. aureus strains USA300 Δhla and S. aureus LSI ΔagrA. Hemolytic activity was seen in A118, in the presence of CFCM of S. aureus LS1. Further, A. baumannii A118 showed an increase in biofilm formation and antibiotic resistance to tetracycline, in the presence of CFCM of S. aureus USA300. Transcriptomic analysis of A. baumannii A118, with the addition of CFCM from S. aureus USA300, was carried out to study A. baumannii response to S. aureus’ released molecules. The RNA-seq data analysis showed a total of 463 differentially expressed genes, associated with a wide variety of functions, such as biofilm formation, virulence, and antibiotic susceptibility, among others. The present results showed that A. baumannii can sense and respond to molecules secreted by S. aureus. These findings demonstrate that A. baumannii may perceive and respond to changes in its environment; specifically, when in the presence of CFCM from S. aureus.

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

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.

Effect of Serum Albumin, a Component of Human Pleural Fluid, on Transcriptional and Phenotypic Changes on Acinetobacter baumannii A118

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.

Histone-like nucleoid-structuring protein (H-NS) regulatory role in antibiotic resistance in Acinetobacter baumannii

In the multidrug resistant (MDR) pathogen Acinetobacter baumannii the global repressor H-NS was shown to modulate the expression of genes involved in pathogenesis and stress response. In addition, H-NS inactivation results in an increased resistance to colistin, and in a hypermotile phenotype an altered stress response. To further contribute to the knowledge of this key transcriptional regulator in A. baumannii behavior, we studied the role of H-NS in antimicrobial resistance. Using two well characterized A. baumannii model strains with distinctive resistance profile and pathogenicity traits (AB5075 and A118), complementary transcriptomic and phenotypic approaches were used to study the role of H-NS in antimicrobial resistance, biofilm and quorum sensing gene expression. An increased expression of genes associated with β-lactam resistance, aminoglycosides, quinolones, chloramphenicol, trimethoprim and sulfonamides resistance in the Δhns mutant background was observed. Genes codifying for efflux pumps were also up-regulated, with the exception of adeFGH. The wild-type transcriptional level was restored in the complemented strain. In addition, the expression of biofilm related genes and biofilm production was lowered when the transcriptional repressor was absent. The quorum network genes aidA, abaI, kar and fadD were up-regulated in Δhns mutant strains. Overall, our results showed the complexity and scope of the regulatory network control by H-NS (genes involved in antibiotic resistance and persistence). These observations brings us one step closer to understanding the regulatory role of hns to combat A. baumannii infections.

Involvement of the Histone-Like Nucleoid Structuring Protein (H-NS) in Acinetobacter baumannii's Natural Transformation

Most Acinetobacter baumannii strains are naturally competent. Although some information is available about factors that enhance or reduce the frequency of the transformation of this bacterium, the regulatory elements and mechanisms are barely understood. In this article, we describe studies on the role of the histone-like nucleoid structuring protein, H-NS, in the regulation of the expression of genes related to natural competency and the ability to uptake foreign DNA. The expression levels of the natural transformation-related genes pilA, pilT, pilQ, comEA, comEC, comF, and drpA significantly increased in a Δhns derivative of A. baumannii A118. The complementation of the mutant with a recombinant plasmid harboring hns restored the expression levels of six of these genes (pilT remained expressed at high levels) to those of the wild-type strain. The transformation frequency of the A. baumannii A118 Δhns strain was significantly higher than that of the wild-type. Similar, albeit not identical, there were consequences when hns was deleted from the hypervirulent A. baumannii AB5075 strain. In the AB5075 complemented strain, the reduction in gene expression in a few cases was not so pronounced that it reached wild-type levels, and the expression of comEA was enhanced further. In conclusion, the expression of all seven transformation-related genes was enhanced after deleting hns in A. baumannii A118 and AB5075, and these modifications were accompanied by an increase in the cells' transformability. The results highlight a role of H-NS in A. baumannii's natural competence.

Testing physiologically relevant conditions in minimal inhibitory concentration assays

The minimal inhibitory concentration (MIC) assay uses agar or broth dilution methods to measure, under defined test conditions, the lowest effective concentration of an antimicrobial agent that inhibits visible growth of a bacterium of interest. This assay is used to test the susceptibilities of bacterial isolates and of novel antimicrobial drugs, and is typically done in nutrient-rich laboratory media that have little relevance to in vivo conditions. As an extension to our original protocol on MIC assays (also published in Nature Protocols), here we describe the application of the MIC broth microdilution assay to test antimicrobial susceptibility in conditions that are more physiologically relevant to infections observed in the clinic. Specifically, we describe a platform that can be applied to the preparation of medium that mimics lung and wound exudate or blood conditions for the growth and susceptibility testing of bacteria, including ESKAPE pathogens. This protocol can also be applied to most physiologically relevant liquid medium and aerobic pathogens, and takes 3-4 d to complete.

Interplay between meropenem and human serum albumin on expression of carbapenem resistance genes and natural competence in Acinetobacter baumannii

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.

Interaction of Acinetobacter baumannii with Human Serum Albumin: Does the Host Determine the Outcome?

Acinetobacter baumannii has become a serious threat to human health due to its extreme antibiotic resistance, environmental persistence, and capacity to survive within the host. Two A. baumannii strains, A118 and AB5075, commonly used as model systems, and three carbapenem-resistant strains, which are becoming ever more dangerous due to the multiple drugs they can resist, were exposed to 3.5% human serum albumin (HSA) and human serum (HS) to evaluate their response with respect to antimicrobial resistance, biofilm formation, and quorum sensing, all features responsible for increasing survival and persistence in the environment and human body. Expression levels of antibiotic resistance genes were modified differently when examined in different strains. The cmlA gene was upregulated or downregulated in conditions of exposure to 3.5% HSA or HS depending on the strain. Expression levels of pbp1 and pbp3 tended to be increased by the presence of HSA and HS, but the effect was not seen in all strains. A. baumannii A118 growing in the presence of HS did not experience increased expression of these genes. Aminoglycoside-modifying enzymes were also expressed at higher or lower levels in the presence of HSA or HS. Still, the response was not uniform; in some cases, expression was enhanced, and in other cases, it was tapered. While A. baumannii AB5075 became more susceptible to rifampicin in the presence of 3.5% HSA or HS, strain A118 did not show any changes. Expression of arr2, a gene involved in resistance to rifampicin present in A. baumannii AMA16, was expressed at higher levels when HS was present in the culture medium. HSA and HS reduced biofilm formation and production of N-Acyl Homoserine Lactone, a compound intimately associated with quorum sensing. In conclusion, HSA, the main component of HS, stimulates a variety of adaptative responses in infecting A. baumannii strains.

Albumin Neutralizes Hydrophobic Toxins and Modulates Candida albicans Pathogenicity

Albumin is abundant in serum but is also excreted at mucosal surfaces and enters tissues when inflammation increases vascular permeability. Host-associated opportunistic pathogens encounter albumin during commensalism and when causing infections. Considering the ubiquitous presence of albumin, we investigated its role in the pathogenesis of infections with the model human fungal pathogen, Candida albicans. Albumin was introduced in various in vitro models that mimic different stages of systemic or mucosal candidiasis, where it reduced the ability of C. albicans to damage host cells. The amphipathic toxin candidalysin mediates necrotic host cell damage induced by C. albicans. Using cellular and biophysical assays, we determined that albumin functions by neutralizing candidalysin through hydrophobic interactions. We discovered that albumin, similarly, can neutralize a variety of fungal (α-amanitin), bacterial (streptolysin O and staurosporin), and insect (melittin) hydrophobic toxins. These data suggest albumin as a defense mechanism against toxins, which can play a role in the pathogenesis of microbial infections. IMPORTANCE Albumin is the most abundant serum protein in humans. During inflammation, serum albumin levels decrease drastically, and low albumin levels are associated with poor patient outcome. Thus, albumin may have specific functions during infection. Here, we describe the ability of albumin to neutralize hydrophobic microbial toxins. We show that albumin can protect against damage induced by the pathogenic yeast C. albicans by neutralizing its cytolytic toxin candidalysin. These findings suggest that albumin is a toxin-neutralizing protein that may play a role during infections with toxin-producing microorganisms.

Update on Multidrug Resistance Efflux Pumps in Acinetobacter spp

Acinetobacter spp. have become of increased clinical importance as studies have shown the antimicrobial resistant potential of these species. Efflux pumps can lead to reduced susceptibility to a variety of antibiotics and are present in large number across Acinetobacter spp. There are six families of efflux pumps that have been shown to be of clinical relevance: the major facilitator superfamily (MFS), small multidrug resistance (SMR) family, ATP-binding cassette (ABC) family, multidrug and toxic compound extrusion (MATE) family, proteobacterial antimicrobial compound efflux (PACE) family, and the resistance-nodulation-division (RND) family. Much work has been done for understanding and characterizing the roles these efflux pumps play in relation to antimicrobial resistance and the physiology of these bacteria. RND efflux pumps, with their expansive substrate profiles, are a major component of Acinetobacter spp. antimicrobial resistance. New discoveries over the last decade have shed light on the complex regulation of these efflux pumps, leading to greater understanding and the potential of slowing the reduced susceptibility seen in these bacterial species.

Pathogenesis of Gram-Negative Bacteremia

Gram-negative bacteremia is a devastating public health threat, with high mortality in vulnerable populations and significant costs to the global economy. Concerningly, rates of both Gram-negative bacteremia and antimicrobial resistance in the causative species are increasing. Gram-negative bacteremia develops in three phases. First, bacteria invade or colonize initial sites of infection. Second, bacteria overcome host barriers, such as immune responses, and disseminate from initial body sites to the bloodstream. Third, bacteria adapt to survive in the blood and blood-filtering organs. To develop new therapies, it is critical to define species-specific and multispecies fitness factors required for bacteremia in model systems that are relevant to human infection. A small subset of species is responsible for the majority of Gram-negative bacteremia cases, including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii The few bacteremia fitness factors identified in these prominent Gram-negative species demonstrate shared and unique pathogenic mechanisms at each phase of bacteremia progression. Capsule production, adhesins, and metabolic flexibility are common mediators, whereas only some species utilize toxins. This review provides an overview of Gram-negative bacteremia, compares animal models for bacteremia, and discusses prevalent Gram-negative bacteremia species.

Human Pleural Fluid and Human Serum Albumin Modulate the Behavior of a Hypervirulent and Multidrug-Resistant (MDR) Acinetobacter baumannii Representative Strain

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.

Cerebrospinal fluid (CSF) augments metabolism and virulence expression factors in Acinetobacter baumannii

In a recent report by the Centers for Disease Control and Prevention (CDC), multidrug resistant (MDR) Acinetobacter baumannii is a pathogen described as an "urgent threat." Infection with this bacterium manifests as different diseases such as community and nosocomial pneumonia, bloodstream infections, endocarditis, infections of the urinary tract, wound infections, burn infections, skin and soft tissue infections, and meningitis. In particular, nosocomial meningitis, an unwelcome complication of neurosurgery caused by extensively-drug resistant (XDR) A. baumannii, is extremely challenging to manage. Therefore, understanding how A. baumannii adapts to different host environments, such as cerebrospinal fluid (CSF) that may trigger changes in expression of virulence factors that are associated with the successful establishment and progress of this infection is necessary. The present in-vitro work describes, the genetic changes that occur during A. baumannii infiltration into CSF and displays A. baumannii's expansive versatility to persist in a nutrient limited environment while enhancing several virulence factors to survive and persist. While a hypervirulent A. baumannii strain did not show changes in its transcriptome when incubated in the presence of CSF, a low-virulence isolate showed significant differences in gene expression and phenotypic traits. Exposure to 4% CSF caused increased expression of virulence factors such as fimbriae, pilins, and iron chelators, and other virulence determinants that was confirmed in various model systems. Furthermore, although CSF's presence did not enhance bacterial growth, an increase of expression of genes encoding transcription, translation, and the ATP synthesis machinery was observed. This work also explores A. baumannii's response to an essential component, human serum albumin (HSA), within CSF to trigger the differential expression of genes associated with its pathoadaptibility in this environment.

Proteomic and Systematic Functional Profiling Unveils Citral Targeting Antibiotic Resistance, Antioxidant Defense, and Biofilm-Associated Two-Component Systems of Acinetobacter baumannii To Encumber Biofilm and Virulence Traits

Acinetobacter baumannii is a nosocomial-infection-causing bacterium and also possesses multidrug resistance to a wide range of conventional antibiotics. The biofilm-forming ability of A. baumannii plays a major role in its resistance and persistence. There is an alarming need for novel treatment strategies to control A. baumannii biofilm-associated issues. The present study demonstrated the strong antibiofilm and antivirulence efficacy of citral against A. baumannii. In addition, proteomic analysis revealed the multitarget potential of citral against A. baumannii. Furthermore, citral treatment enhances the susceptibility of A. baumannii to the host innate immune system and reactive oxygen species (ROS). Cytotoxicity analysis revealed the nonfatal effect of citral on human PBMCs. Therefore, citral could be the safest therapeutic compound and can be taken for further clinical evaluation for the treatment of biofilm-associated infections by A. baumannii.

Acinetobacter baumannii has been reported as a multidrug-resistant bacterium due to biofilms and antimicrobial resistance mechanisms. Hence, novel therapeutic strategies are necessary to overcome A. baumannii infections. This study revealed that citral at 200 μg/ml attenuated A. baumannii biofilms by up to 90% without affecting viability. Furthermore, microscopic analyses and in vitro assays confirmed the antibiofilm efficacy of citral. The global effect of citral on A. baumannii was evaluated by proteomic, transcriptional, and in silico approaches. Two-dimensional (2D) gel electrophoresis and matrix-assisted laser desorption ionization–time of flight/time of flight (MALDI-TOF/TOF) analyses were used to assess the effect of citral on the A. baumannii cellular proteome. Quantitative real-time PCR (qPCR) analysis was done to validate the proteomic data and identify the differentially expressed A. baumannii genes. Protein-protein interactions, gene enrichment, and comparative gene network analyses were performed to explore the interactions and functional attributes of differentially expressed proteins of A. baumannii. Global omics-based analyses revealed that citral targeted various mechanisms such as biofilm formation, antibiotic resistance, antioxidant defense, iron acquisition, and type II and type IV secretion systems. The results of antioxidant analyses and antibiotic sensitivity, blood survival, lipase, and hemolysis assays validated the proteomic results. Cytotoxicity analysis showed a nontoxic effect of citral on peripheral blood mononuclear cells (PBMCs). Overall, the current study unveiled that citral has multitarget efficacy to inhibit the biofilm formation and virulence of A. baumannii.

IMPORTANCEAcinetobacter baumannii is a nosocomial-infection-causing bacterium and also possesses multidrug resistance to a wide range of conventional antibiotics. The biofilm-forming ability of A. baumannii plays a major role in its resistance and persistence. There is an alarming need for novel treatment strategies to control A. baumannii biofilm-associated issues. The present study demonstrated the strong antibiofilm and antivirulence efficacy of citral against A. baumannii. In addition, proteomic analysis revealed the multitarget potential of citral against A. baumannii. Furthermore, citral treatment enhances the susceptibility of A. baumannii to the host innate immune system and reactive oxygen species (ROS). Cytotoxicity analysis revealed the nonfatal effect of citral on human PBMCs. Therefore, citral could be the safest therapeutic compound and can be taken for further clinical evaluation for the treatment of biofilm-associated infections by A. baumannii.

Human pleural fluid triggers global changes in the transcriptional landscape of Acinetobacter baumannii as an adaptive response to stress

Acinetobacter baumannii is a feared, drug-resistant pathogen, characterized by its ability to resist extreme environmental and nutrient-deprived conditions. Previously, we showed that human serum albumin (HSA) can increase foreign DNA acquisition specifically and alter the expression of genes associated with pathogenicity. Moreover, in a recent genome-wide transcriptomic study, we observed that pleural fluid (PF), an HSA-containing fluid, increases DNA acquisition, can modulate cytotoxicity, and control immune responses by eliciting changes in the A. baumannii metabolic profile. In the present work, using more stringent criteria and focusing on the analysis of genes related to pathogenicity and response to stress, we analyzed our previous RNA-seq data and performed phenotypic assays to further explore the impact of PF on A. baumannii's microbial behavior and the strategies used to overcome environmental stress. We observed that PF triggered differential expression of genes associated with motility, efflux pumps, antimicrobial resistance, biofilm formation, two-component systems (TCSs), capsule synthesis, osmotic stress, and DNA-damage response, among other categories. Phenotypic assays of A. baumannii A118 and two other clinical A. baumannii strains, revealed differences in their responses to PF in motility, biofilm formation, antibiotic susceptibility, osmotic stress, and outer membrane vesicle (OMV) production, suggesting that these changes are strain specific. We conclude that A. baumannii's pathoadaptive responses is induced by HSA-containing fluids and must be part of this bacterium armamentarium to persist in hostile environments.

Acinetobacter baumannii: Envelope Determinants That Control Drug Resistance, Virulence, and Surface Variability

Acinetobacter baumannii has emerged as an important nosocomial pathogen, particularly for patients in intensive care units and with invasive indwelling devices. The most recent clinical isolates are resistant to several classes of clinically important antibiotics, greatly restricting the ability to effectively treat critically ill patients. The bacterial envelope is an important driver of A. baumannii disease, both at the level of battling against antibiotic therapy and at the level of protecting from host innate immune function. This review provides a comprehensive overview of key features of the envelope that interface with both the host and antimicrobial therapies. Carbohydrate structures that contribute to protecting from the host are detailed, and mutations that alter these structures, resulting in increased antimicrobial resistance, are explored. In addition, protein complexes involved in both intermicrobial and host-microbe interactions are described. Finally we discuss regulatory mechanisms that control the nature of the cell envelope and its impact on host innate immune function.

Identification of Potential Virulence Factors in the Model Strain Acinetobacter baumannii A118

Acinetobacter baumannii A118, a strain isolated from the blood of an infected patient, is naturally competent and unlike most clinical strains, is susceptible to a variety of different antibiotics including those usually used for selection in genetic manipulations. These characteristics make strain A118 a convenient model for genetic studies of A. baumannii. To identify potential virulence factors, its complete genome was analyzed and compared to other A. baumannii genomes. A. baumannii A118 includes gene clusters coding for the acinetobactin and baumannoferrin iron acquisition systems. Iron-regulated expression of the BauA outer membrane receptor for ferric-acinetobactin complexes was confirmed as well as the utilization of acinetobactin. A. baumannii A118 also possesses the feoABC genes, which code for the main bacterial ferrous uptake system. The functionality of baumannoferrin was suggested by the ability of A. baumannii A118 culture supernatants to cross feed an indicator BauA-deficient strain plated on iron-limiting media. A. baumannii A118 behaved as non-motile but included the csuA/BABCDE chaperone-usher pilus assembly operon and produced biofilms on polystyrene and glass surfaces. While a known capsular polysaccharide (K) locus was identified, the outer core polysaccharide (OC) locus, which belongs to group B, showed differences with available sequences. Our results show that despite being susceptible to most antibiotics, strain A118 conserves known virulence-related traits enhancing its value as model to study A. baumannii pathogenicity.

Human Pleural Fluid Elicits Pyruvate and Phenylalanine Metabolism in Acinetobacter baumannii to Enhance Cytotoxicity and Immune Evasion

Acinetobacter baumannii (Ab) is one of the most treacherous pathogens among those causing hospital-acquired pneumonia (HAP). A. baumannii possesses an adaptable physiology, seen not only in its antibiotic resistance and virulence phenotypes but also in its metabolic versatility. In this study, we observed that A. baumannii undergoes global transcriptional changes in response to human pleural fluid (PF), a key host-derived environmental signal. Differential gene expression analyses combined with experimental approaches revealed changes in A. baumannii metabolism, affecting cytotoxicity, persistence, bacterial killing, and chemotaxis. Over 1,220 genes representing 55% of the differentially expressed transcriptomic data corresponded to metabolic processes, including the upregulation of glutamate, short chain fatty acid, and styrene metabolism. We observed an upregulation by 1.83- and 2.61-fold of the pyruvate dehydrogenase complex subunits E3 and E2, respectively. We also found that pyruvate (PYR), in conjunction with PF, triggers an A. baumannii pathogenic behavior that adversely impacts human epithelial cell viability. Interestingly, PF also amplified A. baumannii cytotoxicity against murine macrophages, suggesting an immune evasion strategy implemented by A. baumannii. Moreover, we uncovered opposing metabolic strategies dependent on the degree of pathogenicity of the strains, where less pathogenic strains demonstrated greater utilization of PYR to promote persister formation in the presence of PF. Additionally, our transcriptomic analysis and growth studies of A. baumannii suggest the existence of an alternative phenylalanine (PA) catabolic route independent of the phenylacetic acid pathway, which converts PA to phenylpyruvate (PP) and shuttles intermediates into styrene metabolism. This alternative route promoted a neutrophil-evasive state, as PF-induced degradation of PP significantly reduced overall human neutrophil chemotaxis in ex vivo chemotactic assays. Taken together, these data highlight A. baumannii pathoadaptabililty in response to host signals and provide further insight into the role of bacterial metabolism in virulence traits, antibiotic persistence strategies, and host innate immune evasion.