A Light-Regulated Type I Pilus Contributes to Acinetobacter baumannii Biofilm, Motility, and Virulence Functions

Motility of Acinetobacter baumannii: regulatory systems and controlling strategies

Acinetobacter baumannii is a Gram-negative opportunistic zoonotic pathogenic bacterium that causes nosocomial infections ranging from minor to life-threatening. The clinical importance of this zoonotic pathogen is rapidly increasing due to the development of multiple resistance mechanisms and the synthesis of numerous virulence factors. Although no flagellum-mediated motility exists, it may move through twitching or surface-associated motility. Twitching motility is a coordinated multicellular movement caused by the extension, attachment, and retraction of type IV pili, which are involved in surface adherence and biofilm formation. Surface-associated motility is a kind of movement that does not need appendages and is most likely driven by the release of extra polymeric molecules. This kind of motility is linked to the production of 1,3-diaminopropane, lipooligosaccharide formation, natural competence, and efflux pump proteins. Since A. baumannii's virulence qualities are directly tied to motility, it is possible that its motility may be used as a specialized preventative or therapeutic measure. The current review detailed the signaling mechanism and involvement of various proteins in controlling A. baumannii motility. As a result, we have thoroughly addressed the role of natural and synthetic compounds that impede A. baumannii motility, as well as the underlying action mechanisms. Understanding the regulatory mechanisms behind A. baumannii's motility features will aid in the development of therapeutic drugs to control its infection. KEY POINTS: • Acinetobacter baumannii exhibits multiple resistance mechanisms. • A. baumannii can move owing to twitching and surface-associated motility. • Natural and synthetic compounds can attenuate A. baumannii motility.

Virulence Factors and Pathogenicity Mechanisms of Acinetobacter baumannii in Respiratory Infectious Diseases

Acinetobacter baumannii (A. baumannii) has become a notorious pathogen causing nosocomial and community-acquired infections, especially ventilator-associated pneumonia. This opportunistic pathogen is found to possess powerful genomic plasticity and numerous virulence factors that facilitate its success in the infectious process. Although the interactions between A. baumannii and the pulmonary epitheliums have been extensively studied, a complete and specific description of its overall pathogenic process is lacking. In this review, we summarize the current knowledge of the antibiotic resistance and virulence factors of A. baumannii, specifically focusing on the pathogenic mechanisms of this detrimental pathogen in respiratory infectious diseases. An expansion of the knowledge regarding A. baumannii pathogenesis will contribute to the development of effective therapies based on immunopathology or intracellular signaling pathways to eliminate this harmful pathogen during infections.

Light regulation in critical human pathogens of clinical relevance such as Acinetobacter baumannii, Staphylococcus aureus and Pseudomonas aeruginosa

It is now clearly recognized that light modulates the physiology of many bacterial chemotrophs, either directly or indirectly. An interesting case are bacterial pathogens of clinical relevance. This work summarizes, discusses, and provides novel complementary information to what is currently known about light sensing and responses in critical human pathogens such as Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus aureus. These pathogens are associated with severe hospital and community infections difficult to treat due to resistance to multiple drugs. Moreover, light responses in Brucella abortus, an important animal and human pathogen, are also compiled. Evidence recovered so far indicates that light modulates aspects related to pathogenesis, persistence, and antibiotic susceptibility in these pathogens; such as motility, biofilm formation, iron uptake, tolerance to antibiotics, hemolysis and virulence. The pathogens elicit differential responses to light depending likely on their pathophysiology, ability to cause disease and characteristics of the host. The response to light is not restricted to discrete physiological traits but is global. In higher organisms, light provides spatial and temporal information. Then, it is crucial to understand what information light is providing in these bacterial pathogens. Our current hypothesis postulates that light serves as a signal that allows these pathogens to synchronize their behavior to the circadian rhythm of the host, to optimize infection. Advances on the molecular mechanism of light signal transduction and physiological responses to light, as well as in the relation between light and bacterial infection, would not only enlarge our understanding of bacterial pathogenesis but also could potentially provide alternative treatment options for infectious illnesses.

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.

Deciphering the virulence factors, regulation, and immune response to Acinetobacter baumannii infection

Deciphering the virulence factors, regulation, and immune response to Acinetobacter baumannii infectionAcinetobacter baumannii is a gram-negative multidrug-resistant nosocomial pathogen and a major cause of hospital acquired infetions. Carbapenem resistant A. baumannii has been categorised as a Priority1 critial pathogen by the World Health Organisation. A. baumannii is responsible for infections in hospital settings, clinical sectors, ventilator-associated pneumonia, and bloodstream infections with a mortality rates up to 35%. With the development of advanced genome sequencing, molecular mechanisms of manipulating bacterial genomes, and animal infection studies, it has become more convenient to identify the factors that play a major role in A. baumannii infection and its persistence. In the present review, we have explored the mechanism of infection, virulence factors, and various other factors associated with the pathogenesis of this organism. Additionally, the role of the innate and adaptive immune response, and the current progress in the development of innovative strategies to combat this multidrug-resistant pathogen is also discussed.

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.

Light Regulates Acinetobacter baumannii Chromosomal and pAB3 Plasmid Genes at 37°C

The opportunistic pathogen A. baumannii has a remarkable capacity to persist in the hospital environment and cause devastating human infections. This capacity can be attributed partly to the sensing and regulatory systems that enable this pathogen to modify its physiology based on environmental cues. One of the signals that A. baumannii senses and responds to is light through the sensing and regulatory roles of the BlsA photoreceptor protein in cells cultured at temperatures below 30°C. This report presents evidence that a light stimulon is operational at 37°C, a condition at which the BlsA production and activity are drastically impaired. Global transcriptional analysis showed that the 37°C light stimulon includes the differential expression of chromosomal genes encoding a wide range of functions that are known to be involved in the adaptation to different metabolic conditions, as well as virulence and persistence in the host and the medical environment. Unexpectedly, the 37°C light stimulon also includes the differential expression of conjugation functions encoded by pAB3 plasmid genes. Our work further demonstrates that the TetR1 and H-NS regulators encoded by this conjugative plasmid control the expression of H₂O₂ resistance and surface motility, respectively. Furthermore, our data showed that pAB3 has an overall negative effect on the expression of these phenotypes and plays no significant virulence role. Although the nature of the bacterial factors and the mechanisms by which the regulation is attained at 37°C remain unknown, taken together, our work expands the current knowledge about light sensing and gene regulation in A. baumannii. IMPORTANCE 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. Although this plasmid affects the expression of virulence traits when tested under laboratory conditions, it does not have a significant impact when tested using ex vivo and in vivo experimental models. These findings provide a better understanding of the interplay between light regulation and plasmid persistence in the pathobiology of A. baumannii.

The Influence of Blue Light and the BlsA Photoreceptor on the Oxidative Stress Resistance Mechanisms of Acinetobacter 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.

Methods to Evaluate Bacterial Motility and Its Role in Bacterial–Host Interactions

Bacterial motility is a widespread characteristic that can provide several advantages for the cell, allowing it to move towards more favorable conditions and enabling host-associated processes such as colonization. There are different bacterial motility types, and their expression is highly regulated by the environmental conditions. Because of this, methods for studying motility under realistic experimental conditions are required. A wide variety of approaches have been developed to study bacterial motility. Here, we present the most common techniques and recent advances and discuss their strengths as well as their limitations. We classify them as macroscopic or microscopic and highlight the advantages of three-dimensional imaging in microscopic approaches. Lastly, we discuss methods suited for studying motility in bacterial–host interactions, including the use of the zebrafish model.

Factors mediating Acinetobacter baumannii biofilm formation: Opportunities for developing therapeutics

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A. baumannii rapidly acquires antimicrobial resistance and causes biofilm associated infections.

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Strategies to target intrinsic factors mediating A. baumannii biofilm formation offer therapeutic prospects.

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Antimicrobial polymers and coating medical devices with antibiofilm agents may prevent biofilm associated infections.

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Biofilm matrix or regulatory mechanisms such as quorum sensing are potential targets for treating chronic infections.

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Phage therapy, photodynamic therapy and nanoparticle therapy are novel promising approaches for treating biofilm associated infections.

Acinetobacter baumannii has notably become a superbug due to its mounting risk of infection and escalating rates of antimicrobial resistance, including colistin, the last-resort antibiotic. Its propensity to form biofilm on biotic and abiotic surfaces has contributed to the majority of nosocomial infections. Bacterial cells in biofilms are resistant to antibiotics and host immune response, and pose challenges in treatment. Therefore current scenario urgently requires the development of novel therapeutic strategies for successful treatment outcomes. This article provides a holistic understanding of sequential events and regulatory mechanisms directing A. baumannii biofilm formation. Understanding the key factors functioning and regulating the biofilm machinery of A. baumannii will provide us insight to develop novel approaches to combat A. baumannii infections. Further, the review article deliberates promising strategies for the prevention of biofilm formation on medically relevant substances and potential therapeutic strategies for the eradication of preformed biofilms which can help tackle biofilm-associated A. baumannii infections. Advances in emerging therapeutic opportunities such as phage therapy, nanoparticle therapy and photodynamic therapy are also discussed to comprehend the current scenario and future outlook for the development of successful treatment against biofilm-associated A. baumannii infections.

Identification of Two Variants of Acinetobacter baumannii Strain ATCC 17978 with Distinct Genotypes and Phenotypes

Acinetobacter baumannii is a nosocomial pathogen that exhibits substantial genomic plasticity. Here, the identification of two variants of A. baumannii ATCC 17978 that differ based on the presence of a 44-kb accessory locus, named AbaAL44 (A. baumannii accessory locus 44 kb), is described. Analyses of existing deposited data suggest that both variants are found in published studies of A. baumannii ATCC 17978 and that American Type Culture Collection (ATCC)-derived laboratory stocks comprise a mix of these two variants. Yet, each variant exhibits distinct interactions with the host in vitro and in vivo. Infection with the variant that harbors AbaAL44 (A. baumannii 17978 UN) results in decreased bacterial burdens and increased neutrophilic lung inflammation in a mouse model of pneumonia, and affects the production of interleukin 1 beta (IL-1β) and IL-10 by infected macrophages. AbaAL44 harbors putative pathogenesis genes, including those predicted to encode a type I pilus cluster, a catalase, and a cardiolipin synthase. The accessory catalase increases A. baumannii resistance to oxidative stress and neutrophil-mediated killing in vitro. The accessory cardiolipin synthase plays a dichotomous role by promoting bacterial uptake and increasing IL-1β production by macrophages, but also by enhancing bacterial resistance to cell envelope stress. Collectively, these findings highlight the phenotypic consequences of the genomic dynamism of A. baumannii through the evolution of two variants of a common type strain with distinct infection-related attributes.

Galleria mellonella as a Good Model to Study Acinetobacter baumannii Pathogenesis

The invertebrate model, Galleria mellonella, has been widely used to study host-pathogen interactions due to its cheapness, ease of handling, and similar mammalian innate immune system. G. mellonella larvae have been proven to be useful and a reliable model for analyzing pathogenesis mechanisms of multidrug resistant Acinetobacter baumannii, an opportunistic pathogen difficult to kill. This review describes the detailed experimental design of G. mellonella/A. baumannii models, and provides a comprehensive comparison of various virulence factors and therapy strategies using the G. mellonella host. These investigations highlight the importance of this host-pathogen model for in vivo pathogen virulence studies. On the long term, further development of the G. mellonella/A. baumannii model will offer promising insights for clinical treatments of A. baumannii infection.

Phosphorylation of Extracellular Proteins in Acinetobacter baumannii in Sessile Mode of Growth

Acinetobacter baumannii is a problematic nosocomial pathogen owing to its increasing resistance to antibiotics and its great ability to survive in the hospital environment, which is linked to its capacity to form biofilms. Structural and functional investigations of post-translational modifications, such as phosphorylations, may lead to identification of candidates for therapeutic targets against this pathogen. Here, we present the first S/T/Y phosphosecretome of two A. baumannii strains, the reference strain ATCC 17978 and the virulent multi-drug resistant strain AB0057, cultured in two modes of growth (planktonic and biofilm) using TiO2 chromatography followed by high resolution mass spectrometry. In ATCC 17978, we detected a total of 137 (97 phosphoproteins) and 52 (33 phosphoproteins) phosphosites in biofilm and planktonic modes of growth, respectively. Similarly, in AB0057, 155 (119 phosphoproteins) and 102 (74 phosphoproteins) phosphosites in biofilm and planktonic modes of growth were identified, respectively. Both strains in the biofilm mode of growth showed a higher number of phosphosites and phosphoproteins compared to planktonic growth. Several phosphorylated sites are localized in key regions of proteins involved in either drug resistance (β-lactamases), adhesion to host tissues (pilins), or protein secretion (Hcp). Site-directed mutagenesis of the Hcp protein, essential for type VI secretion system-mediated interbacterial competition, showed that four of the modified residues are essential for type VI secretion system activity.

ppGpp signaling plays a critical role in virulence of Acinetobacter baumannii

Acinetobacter baumannii, a major nosocomial pathogen, survives in diverse hospital environments, and its multidrug resistance is a major concern. The ppGpp-dependent stringent response mediates the reprogramming of genes with diverse functions in several bacteria. We investigated whether ppGpp is involved in A. baumannii’s pathogenesis by examining biofilm formation, surface motility, adhesion, invasion, and mouse infection studies. Transcriptome analysis of early stationary phase cultures revealed 498 differentially-expressed genes (≥ 2-fold change) in a ppGpp-deficient A. baumannii strain; 220 and 278 genes were up and downregulated, respectively. Csu operon expression, important in pilus biosynthesis during early biofilm formation, was significantly reduced in the ppGpp-deficient strain. Our findings suggest that ppGpp signaling influences A. baumannii biofilm formation, surface motility, adherence, and virulence. We showed the association between ppGpp and pathogenicity in A. baumannii for the first time; ppGpp may be a novel antivirulence target in A. baumannii.

Acinetobacter defence mechanisms against biological aggressors and their use as alternative therapeutic applications

Several Acinetobacter strains are important nosocomial pathogens, with Acinetobacter baumannii being the species of greatest worldwide concern due to its multi-drug resistance and the recent appearance of hyper-virulent strains in the clinical setting. Colonisation of this environment is associated with a multitude of bacterial factors, and the molecular features that promote environmental persistence in abiotic surfaces, including intrinsic desiccation resistance, biofilm formation and motility, have been previously addressed. On the contrary, mechanisms enabling Acinetobacter spp. survival when faced against other biological competitors are starting to be characterised. Among them, secretion systems (SS) of different types, such as the T5bSS (Contact-dependent inhibition systems) and the T6SS, confer adaptive advantages against bacterial aggressors. Regarding mechanisms of defence against bacteriophages, such as toxin-antitoxin, restriction-modification, Crispr-Cas and CBASS, among others, have been identified but remain poorly characterised. In view of this, we aimed to summarise the present knowledge on defence mechanisms that enable niche establishment in members of the Acinetobacter genus. Different proposals are also described for the use of some components of these systems as molecular tools to treat Acinetobacter infections.

Mapping Global Prevalence of Acinetobacter baumannii and Recent Vaccine Development to Tackle It

Acinetobacter baumannii is a leading cause of nosocomial infections that severely threaten public health. The formidable adaptability and resistance of this opportunistic pathogen have hampered the development of antimicrobial therapies which consequently leads to very limited treatment options. We mapped the global prevalence of multidrug-resistant A. baumannii and showed that carbapenem-resistant A. baumannii is widespread throughout Asia and the Americas. Moreover, when antimicrobial resistance rates of Acinetobacter spp. exceed a threshold level, the proportion of A. baumannii isolates from clinical samples surges. Therefore, vaccines represent a realistic alternative strategy to tackle this pathogen. Research into anti-A. baumannii vaccines have enhanced in the past decade and multiple antigens have been investigated preclinically with varying results. This review summarises the current knowledge of virulence factors relating to A. baumannii–host interactions and its implication in vaccine design, with a view to understanding the current state of A. baumannii vaccine development and the direction of future efforts.

Two Acinetobacter baumannii Isolates Obtained From a Fatal Necrotizing Fasciitis Infection Display Distinct Genomic and Phenotypic Characteristics in Comparison to Type Strains

Acinetobacter baumannii has been recognized as a critical pathogen that causes severe infections worldwide not only because of the emergence of extensively drug-resistant (XDR) derivatives, but also because of its ability to persist in medical environments and colonize compromised patients. While there are numerous reports describing the mechanisms by which this pathogen acquires resistance genes, little is known regarding A. baumannii’s virulence functions associated with rare manifestations of infection such as necrotizing fasciitis, making the determination and implementation of alternative therapeutic targets problematic. To address this knowledge gap, this report describes the analysis of the NFAb-1 and NFAb-2 XDR isolates, which were obtained at two time points during a fatal case of necrotizing fasciitis, at the genomic and functional levels. The comparative genomic analysis of these isolates with the ATCC 19606^T and ATCC 17978 strains showed that the NFAb-1 and NFAb-2 isolates are genetically different from each other as well as different from the ATCC 19606^T and ATCC 17978 clinical isolates. These genomic differences could be reflected in phenotypic differences observed in these NFAb isolates. Biofilm, cell viability and flow cytometry assays indicate that all tested strains caused significant decreases in A549 human alveolar epithelial cell viability with ATCC 17978, NFAb-1 and NFAb-2 producing significantly less biofilm and significantly more hemolysis and capacity for intracellular invasion than ATCC 19606^T. NFAb-1 and NFAb-2 also demonstrated negligible surface motility but significant twitching motility compared to ATCC 19606^T and ATCC 17978, likely due to the presence of pili exceeding 2 µm in length, which are significantly longer and different from those previously described in the ATCC 19606^T and ATCC 17978 strains. Interestingly, infection with cells of the NFAb-1 isolate, which were obtained from a premortem blood sample, lead to significantly higher mortality rates than NFAb-2 bacteria, which were obtained from postmortem tissue samples, when tested using the Galleria mellonella in vivo infection model. These observations suggest potential changes in the virulence phenotype of the A. baumannii necrotizing fasciitis isolates over the course of infection by mechanisms and cell processes that remain to be identified.

Acinetobacter baumannii LOS Regulate the Expression of Inflammatory Cytokine Genes and Proteins in Human Mast Cells

Herein, we investigated the effect of bacterial lipooligosaccharides (LOS), from Acinetobacter baumannii, on the expression of pro-inflammatory genes that play an essential role in bacterial clearance. LAD2 human mast cells were stimulated with LOS derived from two strains of A. baumannii—ATCC 19606 and MDRA T14. LOS exposure induced the expression of genes for pro-inflammatory mediators, including TNF-α, IL-8, LTC4S, CCL4, and TLR4. The mRNA expression levels of a majority of the pro-inflammatory genes, except TLR4, in A. baumannii-LOS stimulated mast cells were increased. Moreover, co-culture of neutrophils with the supernatant obtained from LOS (ATCC 19606 and MDRA T14)-induced LAD2 cells increased the transmigration of neutrophils, which plays a critical role in the early protection against bacterial infections. The results of the present study suggest that LOS could be involved in the pathogenicity of A. baumannii by inducing inflammatory responses via mast cells and that IL-8 is involved in recruiting neutrophils in response to bacterial invasion.

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.

Light Modulates Important Pathogenic Determinants and Virulence in ESKAPE Pathogens Acinetobacter baumannii, Pseudomonas aeruginosa, and Staphylococcus aureus

Light sensing has been extensively characterized in the human pathogen Acinetobacter baumannii at environmental temperatures. However, the influence of light on the physiology and pathogenicity of human bacterial pathogens at temperatures found in warm-blooded hosts is still poorly understand. In this work, we show that Staphylococcus aureus, Acinetobacter baumannii, and Pseudomonas aeruginosa (ESKAPE) priority pathogens, which have been recognized by the WHO and the CDC as critical, can also sense and respond to light at temperatures found in human hosts. Most interestingly, in these pathogens, light modulates important pathogenicity determinants as well as virulence in an epithelial infection model, which could have implications in human infections. In fact, we found that alpha-toxin-dependent hemolysis, motility, and growth under iron-deprived conditions are modulated by light in S. aureus Light also regulates persistence, metabolism, and the ability to kill competitors in some of these microorganisms. Finally, light exerts a profound effect on the virulence of these pathogens in an epithelial infection model, although the response is not the same in the different species; virulence was enhanced by light in A. baumannii and S. aureus, while in A. nosocomialis and P. aeruginosa it was reduced. Neither the BlsA photoreceptor nor the type VI secretion system (T6SS) is involved in virulence modulation by light in A. baumannii Overall, this fundamental knowledge highlights the potential use of light to control pathogen virulence, either directly or by manipulating the light regulatory switch toward the lowest virulence/persistence configuration.IMPORTANCE Pathogenic bacteria are microorganisms capable of producing disease. Dangerous bacterial pathogens, such as Staphylococcus aureus, Pseudomonas aeruginosa, and Acinetobacter baumannii, are responsible for serious intrahospital and community infections in humans. Therapeutics is often complicated due to resistance to multiple antibiotics, rendering them ineffective. In this work, we show that these pathogens sense natural light and respond to it by modulating aspects related to their ability to cause disease; in the presence of light, some of them become more aggressive, while others show an opposite response. Overall, we provide new understanding on the behavior of these pathogens, which could contribute to the control of infections caused by them. Since the response is distributed in diverse pathogens, this notion could prove a general concept.

An overview of Acinetobacter baumannii pathogenesis: Motility, adherence and biofilm formation

The Gram-negative opportunistic pathogen Acinetobacter baumannii has gain notoriety in recent decades, primarily due to its propensity to cause nosocomial infections in critically ill patients. Its global spread, multi-drug resistance features and plethora of virulence factors make it a serious threat to public health worldwide. Though much effort has been expended in uncovering its successes, it continues to confound researchers due to its highly adaptive nature, mutating to meet the needs of a given environment. Its persistence in the clinical setting allows it to be in close proximity to a potential host, where contact can be made facilitating infection and colonization. In this article, we aim to provide a current overview of the bacterial virulence factors, specifically focusing on factors involved in the initial stages of infection, highlighting the role of adaptation facilitated by two-component systems and biofilm formation. Finally, the study of host-pathogen interactions using available animal models, their suitability, notable findings and some perspectives moving forward are also discussed.

Antibiofilm and antivirulence efficacy of myrtenol enhances the antibiotic susceptibility of Acinetobacter baumannii

Acinetobacter baumannii (AB) is rising as a human pathogen of critical priority worldwide as it is the leading cause of chronic opportunistic infections in healthcare settings and the condition is ineradicable with antibiotic therapy. AB possesses the ability to form biofilm on abiotic as well as biotic surfaces which plays a major role in its pathogenesis and resistance in clinical settings. Hence, the demand for an alternative therapy to combat the biofilm-associated infections is increasing. The present study explored the antibiofilm potential of myrtenol, a bicyclic monoterpene present in various plants against reference and clinical strains of AB. Myrtenol (200 μg/mL) exhibited a strong antibiofilm activity without exerting any harmful effect on growth and metabolic viability of AB strains. Microscopic analyses confirmed the reduction in the biofilm thickness and surface coverage upon myrtenol treatment. Especially, myrtenol was found to be effective in disrupting the mature biofilms of tested AB strains. Furthermore, myrtenol inhibited the biofilm-associated virulence factors of AB strains such as extracellular polysaccharide, cell surface hydrophobicity, oxidant resistance, swarming and twitching motility. Transcriptional analysis unveiled the suppression of the biofilm-associated genes such as bfmR, csuA/B, bap, ompA, pgaA, pgaC, and katE by myrtenol. Notably, myrtenol improved the susceptibility of AB strains towards conventional antibiotics such as amikacin, ciprofloxacin, gentamicin and trimethoprim. Thus, the present study demonstrates the therapeutic potential of myrtenol against biofilm-associated infections of AB.

Structural Basis for the Regulation of Biofilm Formation and Iron Uptake in A. baumannii by the Blue-Light-Using Photoreceptor, BlsA

The opportunistic human pathogen, A. baumannii, senses and responds to light using the blue light sensing A (BlsA) photoreceptor protein. BlsA is a blue-light-using flavin adenine dinucleotide (BLUF) protein that is known to regulate a wide variety of cellular functions through interactions with different binding partners. Using immunoprecipitation of tagged BlsA in A. baumannii lysates, we observed a number of proteins that interact with BlsA, including several transcription factors. In addition to a known binding partner, the iron uptake regulator Fur, we identified the biofilm response regulator BfmR as a putative BlsA-binding partner. Using microscale thermophoresis, we determined that both BfmR and Fur bind to BlsA with nanomolar binding constants. To better understand how BlsA interacts with and regulates these transcription factors, we solved the X-ray crystal structures of BlsA in both a ground (dark) state and a photoactivated light state. Comparison of the light- and dark-state structures revealed that, upon photoactivation, the two α-helices comprising the variable domain of BlsA undergo a distinct conformational change. The flavin-binding site, however, remains largely unchanged from dark to light. These structures, along with docking studies of BlsA and Fur, reveal key mechanistic details about how BlsA propagates the photoactivation signal between protein domains and on to its binding partner. Taken together, our structural and biophysical data provide important insights into how BlsA controls signal transduction in A. baumannii and provides a likely mechanism for blue-light-dependent modulation of biofilm formation and iron uptake.

Quorum Sensing as a Target for Controlling Surface Associated Motility and Biofilm Formation in Acinetobacter baumannii ATCC® 17978TM

The important nosocomial pathogen Acinetobacter baumannii presents a quorum sensing (QS) system (abaI/abaR) mediated by acyl-homoserine-lactones (AHLs) and several quorum quenching (QQ) enzymes. However, the roles of this complex network in the control of the expression of important virulence-related phenotypes such as surface-associated motility and biofilm formation is not clear. Therefore, the effect of the mutation of the AHL synthase AbaI, and the exogenous addition of the QQ enzyme Aii20J on surface-associated motility and biofilm formation by A. baumannii ATCC^® 17978^TM was studied in detail. The effect of the enzyme on biofilm formation by several multidrug-resistant A. baumannii clinical isolates differing in their motility pattern was also tested. We provide evidence that a functional QS system is required for surface-associated motility and robust biofilm formation in A. baumannii ATCC^® 17978^TM. Important differences were found with the well-studied strain A. nosocomialis M2 regarding the relevance of the QS system depending on environmental conditions The in vitro biofilm-formation capacity of A. baumannii clinical strains was highly variable and was not related to the antibiotic resistance or surface-associated motility profiles. A high variability was also found in the sensitivity of the clinical strains to the action of the QQ enzyme, revealing important differences in virulence regulation between A. baumannii isolates and confirming that studies restricted to a single strain are not representative for the development of novel antimicrobial strategies. Extracellular DNA emerges as a key component of the extracellular matrix in A. baumannii biofilms since the combined action of the QQ enzyme Aii20J and DNase reduced biofilm formation in all tested strains. Results demonstrate that QQ strategies in combination with other enzymatic treatments such as DNase could represent an alternative approach for the prevention of A. baumannii colonization and survival on surfaces and the prevention and treatment of infections caused by this pathogen.

Mechanisms Protecting Acinetobacter baumannii against Multiple Stresses Triggered by the Host Immune Response, Antibiotics and Outside-Host Environment

Acinetobacter baumannii is considered one of the most persistent pathogens responsible for nosocomial infections. Due to the emergence of multidrug resistant strains, as well as high morbidity and mortality caused by this pathogen, A. baumannii was placed on the World Health Organization (WHO) drug-resistant bacteria and antimicrobial resistance research priority list. This review summarizes current studies on mechanisms that protect A. baumannii against multiple stresses caused by the host immune response, outside host environment, and antibiotic treatment. We particularly focus on the ability of A. baumannii to survive long-term desiccation on abiotic surfaces and the population heterogeneity in A. baumannii biofilms. Insight into these protective mechanisms may provide clues for the development of new strategies to fight multidrug resistant strains of A. baumannii.

Acinetobacter baumannii NCIMB8209: a Rare Environmental Strain Displaying Extensive Insertion Sequence-Mediated Genome Remodeling Resulting in the Loss of Exposed Cell Structures and Defensive Mechanisms

Acinetobacter baumannii is an ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) opportunistic pathogen, with poorly defined natural habitats/reservoirs outside the clinical setting. A. baumannii arose from the Acinetobacter calcoaceticus-A. baumannii complex as the result of a population bottleneck, followed by a recent population expansion from a few clinically relevant clones endowed with an arsenal of resistance and virulence genes. Still, the identification of virulence traits and the evolutionary paths leading to a pathogenic lifestyle has remained elusive, and thus, the study of nonclinical (“environmental”) A. baumannii isolates is necessary. We conducted here comparative genomic and virulence studies on A. baumannii NCMBI8209 isolated in 1943 from the microbiota responsible for the decomposition of guayule, and therefore well differentiated both temporally and epidemiologically from the multidrug-resistant strains that are predominant nowadays. Our work provides insights on the adaptive strategies used by A. baumannii to escape from host defenses and may help the adoption of measures aimed to limit its further dissemination.

Acinetobacter baumannii represents nowadays an important nosocomial pathogen of poorly defined reservoirs outside the clinical setting. Here, we conducted whole-genome sequencing analysis of the Acinetobacter sp. NCIMB8209 collection strain, isolated in 1943 from the aerobic degradation (retting) of desert guayule shrubs. Strain NCIMB8209 contained a 3.75-Mb chromosome and a plasmid of 134 kb. Phylogenetic analysis based on core genes indicated NCIMB8209 affiliation to A. baumannii, a result supported by the identification of a chromosomal bla_OXA-51-like gene. Seven genomic islands lacking antimicrobial resistance determinants, 5 regions encompassing phage-related genes, and notably, 93 insertion sequences (IS) were found in this genome. NCIMB8209 harbors most genes linked to persistence and virulence described in contemporary A. baumannii clinical strains, but many of the genes encoding components of surface structures are interrupted by IS. Moreover, defense genetic islands against biological aggressors such as type 6 secretion systems or CRISPR-cas are absent from this genome. These findings correlate with a low capacity of NCIMB8209 to form biofilm and pellicle, low motility on semisolid medium, and low virulence toward Galleria mellonella and Caenorhabditis elegans. Searching for catabolic genes and concomitant metabolic assays revealed the ability of NCIMB8209 to grow on a wide range of substances produced by plants, including aromatic acids and defense compounds against external aggressors. All the above features strongly suggest that NCIMB8209 has evolved specific adaptive features to a particular environmental niche. Moreover, they also revealed that the remarkable genetic plasticity identified in contemporary A. baumannii clinical strains represents an intrinsic characteristic of the species.

IMPORTANCEAcinetobacter baumannii is an ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) opportunistic pathogen, with poorly defined natural habitats/reservoirs outside the clinical setting. A. baumannii arose from the Acinetobacter calcoaceticus-A. baumannii complex as the result of a population bottleneck, followed by a recent population expansion from a few clinically relevant clones endowed with an arsenal of resistance and virulence genes. Still, the identification of virulence traits and the evolutionary paths leading to a pathogenic lifestyle has remained elusive, and thus, the study of nonclinical (“environmental”) A. baumannii isolates is necessary. We conducted here comparative genomic and virulence studies on A. baumannii NCMBI8209 isolated in 1943 from the microbiota responsible for the decomposition of guayule, and therefore well differentiated both temporally and epidemiologically from the multidrug-resistant strains that are predominant nowadays. Our work provides insights on the adaptive strategies used by A. baumannii to escape from host defenses and may help the adoption of measures aimed to limit its further dissemination.

Insights Into Mechanisms of Biofilm Formation in Acinetobacter baumannii and Implications for Uropathogenesis

Multidrug resistant Acinetobacter baumannii is a serious healthcare threat. In fact, the Center for Disease Control recently reported that carbapenem-resistant A. baumannii is responsible for more than 8,500 infections, 700 deaths, and $281 million in healthcare costs annually in the United States with few, if any, treatment options available, leading to its designation as a pathogen of urgent concern and a priority for novel antimicrobial development. It is hypothesized that biofilms are, at least in part, responsible for the high prevalence of A. baumannii nosocomial and recurrent infections because they frequently contaminate hospital surfaces and patient indwelling devices; therefore, there has been a recent push for mechanistic understanding of biofilm formation, maturation and dispersal. However, most research has focused on A. baumannii pneumonia and bloodstream infections, despite a recent retrospective study showing that 17.1% of A. baumannii isolates compiled from clinical studies over the last two decades were obtained from urinary samples. This highlights that A. baumannii is an underappreciated uropathogen. The following minireview will examine our current understanding of A. baumannii biofilm formation and how this influences urinary tract colonization and pathogenesis.

Carbapenemases: Transforming Acinetobacter baumannii into a Yet More Dangerous Menace

Acinetobacter baumannii is a common cause of serious nosocomial infections. Although community-acquired infections are observed, the vast majority occur in people with preexisting comorbidities. A. baumannii emerged as a problematic pathogen in the 1980s when an increase in virulence, difficulty in treatment due to drug resistance, and opportunities for infection turned it into one of the most important threats to human health. Some of the clinical manifestations of A. baumannii nosocomial infection are pneumonia; bloodstream infections; lower respiratory tract, urinary tract, and wound infections; burn infections; skin and soft tissue infections (including necrotizing fasciitis); meningitis; osteomyelitis; and endocarditis. A. baumannii has an extraordinary genetic plasticity that results in a high capacity to acquire antimicrobial resistance traits. In particular, acquisition of resistance to carbapenems, which are among the antimicrobials of last resort for treatment of multidrug infections, is increasing among A. baumannii strains compounding the problem of nosocomial infections caused by this pathogen. It is not uncommon to find multidrug-resistant (MDR, resistance to at least three classes of antimicrobials), extensively drug-resistant (XDR, MDR plus resistance to carbapenems), and pan-drug-resistant (PDR, XDR plus resistance to polymyxins) nosocomial isolates that are hard to treat with the currently available drugs. In this article we review the acquired resistance to carbapenems by A. baumannii. We describe the enzymes within the OXA, NDM, VIM, IMP, and KPC groups of carbapenemases and the coding genes found in A. baumannii clinical isolates.

Mutation of the Carboxy-Terminal Processing Protease in Acinetobacter baumannii Affects Motility, Leads to Loss of Membrane Integrity, and Reduces Virulence

Motility plays an essential role in the host–parasite relationship of pathogenic bacteria, and is often associated with virulence. While many pathogenic bacteria use flagella for locomotion, Acinetobacter baumannii strains do not have flagella, but have other features that aid in their motility. To study the genes involved in motility, transposon mutagenesis was performed to construct A. baumannii mutant strains. Mutant strain MR14 was found to have reduced motility, compared to wild-type ATCC 17978. NCBI BLAST analysis revealed that the Tn10 transposon in the MR14 genome is integrated into the gene that encodes for carboxy-terminal processing protease (Ctp). Additionally, MR14 exhibits a mucoidy, sticky phenotype as the result of increased extracellular DNA (eDNA) caused by bacterial autolysis. Transmission and scanning electron microscopy revealed cytoplasmic content leaving the cell and multiple cell membrane depressions, respectively. MR14 showed higher sensitivity to environmental stressors. Mutation of the ctp gene reduced invasion and adhesion of A. baumannii to airway epithelial cells, potentially due to increased hydrophobicity. In the zebrafish model of infection, MR14 increased the survival rate by 40% compared to the wild-type. Taken together, the ctp gene in A. baumannii has a pivotal role in maintaining membrane integrity, adaptation to environmental stress, and controlling virulence.

Through the eyes of a pathogen: light perception and signal transduction in Acinetobacter baumannii

Sunlight is a ubiquitous environmental stimulus for the great majority of living organisms on Earth; therefore it is logical to expect the development of "seeing mechanisms" which lead them to successfully adapt to particular ecological niches. Although these mechanisms were recognized in photosynthetic organisms, it was not until recent years that the scientific community found out about light perception in chemotrophic ones. In this review we summarize the current knowledge about the mechanism of light sensing through the blue light receptor BlsA in Acinetobacter baumannii. We highlight its function as a global regulator that pleiotropically modulates a large number of physiological processes, many of which are linked to the ability of this opportunist pathogen to persist in adverse intrahospital environments. Moreover, we describe with some specific examples the molecular basis of how this photoregulator senses blue light and translates this physical signal by modulating gene expression of target regulons. Finally, we discuss the possible course of these investigations needed to dissect this complex regulatory network, which ultimately will help us better understand the A. baumannii physiology.

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.

BlsA Is a Low to Moderate Temperature Blue Light Photoreceptor in the Human Pathogen Acinetobacter baumannii

Light is an environmental signal that produces extensive effects on the physiology of the human pathogen Acinetobacter baumannii. Many of the bacterial responses to light depend on BlsA, a bluelight using FAD (BLUF)-type photoreceptor, which also integrates temperature signals. In this work, we disclose novel mechanistic aspects of the function of BlsA. First, we show that light modulation of motility occurs only at temperatures lower than 24°C, a phenotype depending on BlsA. Second, blsA transcript levels were significantly reduced at temperatures higher than 25°C, in agreement with BlsA protein levels in the cell which were undetectable at 26°C and higher temperatures. Also, quantum yield of photo-activation of BlsA (lBlsA) between 14 and 37°C, showed that BlsA photoactivity is greatly compromised at 25°C and absent above 28°C. Fluorescence emission and anisotropy of the cofactor together with the intrinsic protein fluorescence studies suggest that the FAD binding site is more susceptible to structural changes caused by increments in temperature than other regions of the protein. Moreover, BlsA itself gains structural instability and strongly aggregates at temperatures above 30°C. Overall, BlsA is a low to moderate temperature photoreceptor, whose functioning is highly regulated in the cell, with control points at expression of the cognate gene as well as photoactivity.

Structural and functional analysis of the Acinetobacter baumannii BlsA photoreceptor and regulatory protein

The Acinetobacter baumannii BlsA photoreceptor has an N-terminal (NT) BLUF domain and a C-terminal (CT) amino acid sequence with no significant homology to characterized bacterial proteins. In this study, we tested the biological role of specific residues located in these BlsA regions. Site-directed mutagenesis, surface motility assays at 24°C and protein overexpression showed that residues Y7, Q51 and W92 are essential for not only light-regulated motility, but also BlsA's solubility when overexpressed in a heterologous host. In contrast, residues A29 and F32, the latter representing a difference when compared with other BLUF-containing photoreceptors, do not play a major role in BlsA's biological functions. Analysis of the CT region showed that the deletion of the last five BlsA residues has no significant effect on the protein's light-sensing and motility regulatory functions, but the deletion of the last 14 residues as well as K144E and K145E substitutions significantly alter light-regulated motility responses. In contrast to the NT mutants, these CT derivatives were overexpressed and purified to homogeneity to demonstrate that although these mutations do not significantly affect flavin binding and photocycling, they do affect BlsA's photodynamic properties. Notably, these mutations map within a potential fifth α-helical component that could play a role in predicted interactions between regulatory partners and BlsA, which could function as a monomer according to gel filtration data. All these observations indicate that although BlsA shares common structural and functional properties with unrelated photoreceptors, it also exhibits unique features that make it a distinct BLUF photoreceptor.

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.

Thioredoxin-A is a virulence factor and mediator of the type IV pilus system in Acinetobacter baumannii

The Gram-negative pathogen, Acinetobacter baumannii has emerged as a global nosocomial health threat affecting the majority of hospitals in the U.S. and abroad. The redox protein thioredoxin has been shown to play several roles in modulation of cellular functions affecting various virulence factors in Gram-negative pathogens. This study aims to explore the role of thioredoxin-A protein (TrxA) in A. baumannii virulence. We determined that deletion of the TrxA gene did not significantly affect resistance to environmental stressors such as temperature, salt, and pH. However, TrxA was critical for survival in the presence of elevated levels of hydrogen peroxide. Lack of TrxA was associated with decreased expression of type IV pili related genes and an inability to undergo normal twitching motility. Interestingly, the TrxA-null mutant was able to form biofilms better than the wildtype (WT) and was observed to be significantly less virulent than the WT in a pulmonary infection model. These results are supportive of thioredoxin playing a key role in A. baumannii virulence.

Urinary tract colonization is enhanced by a plasmid that regulates uropathogenic Acinetobacter baumannii chromosomal genes

Multidrug resistant (MDR) Acinetobacter baumannii poses a growing threat to global health. Research on Acinetobacter pathogenesis has primarily focused on pneumonia and bloodstream infections, even though one in five A. baumannii strains are isolated from urinary sites. In this study, we highlight the role of A. baumannii as a uropathogen. We develop the first A. baumannii catheter-associated urinary tract infection (CAUTI) murine model using UPAB1, a recent MDR urinary isolate. UPAB1 carries the plasmid pAB5, a member of the family of large conjugative plasmids that represses the type VI secretion system (T6SS) in multiple Acinetobacter strains. pAB5 confers niche specificity, as its carriage improves UPAB1 survival in a CAUTI model and decreases virulence in a pneumonia model. Comparative proteomic and transcriptomic analyses show that pAB5 regulates the expression of multiple chromosomally-encoded virulence factors besides T6SS. Our results demonstrate that plasmids can impact bacterial infections by controlling the expression of chromosomal genes.

Acinetobacter baumannii is generally considered an opportunistic pathogen. Here, Di Venanzio et al. develop a mouse model of catheter-associated urinary tract infection and show that a plasmid confers niche specificity to an A. baumannii urinary isolate by regulating the expression of chromosomal genes.

QSAR-based investigation on antibiotics facilitating emergence and dissemination of antibiotic resistance genes: A case study of sulfonamides against mutation and conjugative transfer in Escherichia coli

Antibiotic resistance genes (ARGs), which are emerging environmental contaminants, have posed great threats to global public health. Although extensive efforts have been undertaken to investigate ARG pollution, little attention has been paid to the structural information of antibiotics when exploring their impact on the emergence and dissemination of ARGs. In this study, setting Escherichia coli (E. coli) as the test organism, the effects of sulfonamides (SAs) on growth, mutation frequency and conjugative transfer frequency were tested, and quantitative structure-activity relationship (QSAR) was used to quantitatively analyze the promotion of SAs on these biological effects and explore their possible mechanism. The constructed QSAR models reveal that SAs may increase expression of the FtsZ protein and pili in E. coli via binding to the SdiA protein, ultimately leading to SAs facilitation of growth, mutation frequency and conjugative transfer frequency. The results indicate that SAs can produce selective pressure on E. coli to promote the emergence and dissemination of ARGs. This study provides reference data for further investigation of the emergence and dissemination of ARGs under antibiotic exposure and a new perspective for the mechanistic exploration of ARG pollution.

Recent perspectives on the virulent factors and treatment options for multidrug-resistant Acinetobacter baumannii

Acinetobacter baumannii (AB) is one of the most notorious and opportunistic pathogens, which caused high morbidity and mortality rate and World Health Organization (WHO) declared this bacterium as priority-1 pathogen in 2017. The current antibacterial agents, such as colistins, carbapenems, and tigecyclines have limited applications, which necessitate novel and alternative therapeutic remedies. Thus, the understanding of recent perspectives on the virulent factors and antibiotic resistance mechanism exhibited by the bacteria are extremely important. In addition to many combinatorial therapies of antibacterial, there is several natural compounds demonstrated significant antibacterial potential towards these bacteria. The computational systems biology and high throughput screening approaches provide crucial insights in identifying novel drug targets and lead molecules with therapeutics potential. Hence, this review provides profound insight on the recent aspects of the virulent factors associated with AB, role of biofilm formation in drug resistance and the mechanisms of multidrug resistance. This review further illustrates the status of current therapeutic agents, scope, and applications of natural therapeutics, such as herbal medicines and role of computational biology, immunoinformatics and virtual screening in novel lead developments. Thus, this review provides novel insight on latest developments in drug-resistance mechanism of multidrug-resistant A. baumannii (MDRAB) and discovery of probable therapeutic interventions.