Serum resistance and biofilm formation in clinical isolates of Acinetobacter baumannii

Resensitization to colistin results in rapid and stable recovery of adherence, serum resistance and ompW in Acinetobacter baumannii

BACKGROUND

Colistin resistance in Acinetobacter baumannii is an emerging problem that limits antimicrobial therapy options.

MATERIALS & METHODS

We isolated two pairs of colistin susceptible and colistin-resistant A. baumannii (K1007/K1006 and K408/K409) from two patients diagnosed with carbapenem-resistant A. baumannii infection. Colistin susceptible isolates were exposed to in vitro colistin induction for 50 generations. The selected cell populations were subjected to DNA and RNA sequencing and phenotypic assays.

RESULTS

In the in vitro induction assay, K408 gained colistin resistance on the corresponding day of clinical resistance (K408-G25) and got resensitized to colistin in the consecutive generation (K408-G26). A significant upregulation of ompW, ata, adeFGH genes on K408-G25 was followed by a downregulation upon resensitization to colistin (G26). Despite the upregulation of the ompW gene in transcriptomic analysis, the ompW protein disappeared on K408-G25 and recovered in the resensitized generation (G26). In parallel, disrupted cell membrane integrity recovered in K408-G26. In the K408-G25, downregulation of pbpG and upregulation of pbp1a/pbp3 genes decreased serum-resistance which was reversed in the resensitized generation (G26). The K1007 did not gain colistin resistance amongst 50-generations, however, the generation corresponding to clinical resistance day (K1007-G9) had a similar trend with K408-G25. The clinical colistin-resistant K409 and K1006 had SNPs on pmrA and pmrB genes.

CONCLUSION

In this study, we observed that A. baumannii regulates adhesion, efflux pumps and serum-resistance associated genes as an early response to colistin stress. Besides, the ompW protein disappears in the cell membrane of colistin resistant cells which recovers after resensitization to colistin. The lack of ompW protein in colistin-resistant cells should be taken into consideration for escape mutants in development of antivirulence vaccination or treatment options.

Presence and Persistence of ESKAPEE Bacteria before and after Hospital Wastewater Treatment

The metagenomic surveillance of antimicrobial resistance in wastewater has been suggested as a methodological tool to characterize the distribution, status, and trends of antibiotic-resistant bacteria. In this study, a cross-sectional collection of samples of hospital-associated raw and treated wastewater were obtained from February to March 2020. Shotgun metagenomic sequencing and bioinformatic analysis were performed to characterize bacterial abundance and antimicrobial resistance gene analysis. The main bacterial phyla found in all the samples were as follows: Proteobacteria, Bacteroides, Firmicutes, and Actinobacteria. At the species level, ESKAPEE bacteria such as E. coli relative abundance decreased between raw and treated wastewater, but S. aureus, A. baumannii, and P. aeruginosa increased, as did the persistence of K. pneumoniae in both raw and treated wastewater. A total of 172 different ARGs were detected; blaOXA, blaVEB, blaKPC, blaGES, mphE, mef, erm, msrE, AAC(6'), ant(3″), aadS, lnu, PBP-2, dfrA, vanA-G, tet, and sul were found at the highest abundance and persistence. This study demonstrates the ability of ESKAPEE bacteria to survive tertiary treatment processes of hospital wastewater, as well as the persistence of clinically important antimicrobial resistance genes that are spreading in the environment.

Nano-Selenium inhibited antibiotic resistance genes and virulence factors by suppressing bacterial selenocompound metabolism and chemotaxis pathways in animal manure

Numerous antibiotic resistance genes (ARGs) and virulence factors (VFs) found in animal manure pose significant risks to human health. However, the effects of graphene sodium selenite (GSSe), a novel chemical nano-Selenium, and biological nano-Selenium (BNSSe), a new bioaugmentation nano-Se, on bacterial Se metabolism, chemotaxis, ARGs, and VFs in animal manure remain unknown. In this study, we investigated the effects of GSSe and BNSSe on ARGs and VFs expression in broiler manure using high-throughput sequencing. Results showed that BNSSe reduced Se pressure during anaerobic fermentation by inhibiting bacterial selenocompound metabolism pathways, thereby lowering manure Selenium pollution. Additionally, the expression levels of ARGs and VFs were lower in the BNSSe group compared to the Sodium Selenite and GSSe groups, as BNSSe inhibited bacterial chemotaxis pathways. Co-occurrence network analysis identified ARGs and VFs within the following phyla Bacteroidetes (genera Butyricimonas, Odoribacter, Paraprevotella, and Rikenella), Firmicutes (genera Lactobacillus, Candidatus_Borkfalkia, Merdimonas, Oscillibacter, Intestinimonas, and Megamonas), and Proteobacteria (genera Desulfovibrio). The expression and abundance of ARGs and VFs genes were found to be associated with ARGs-VFs coexistence. Moreover, BNSSe disruption of bacterial selenocompound metabolism and chemotaxis pathways resulted in less frequent transfer of ARGs and VFs. These findings indicate that BNSSe can reduce ARGs and VFs expression in animal manure by suppressing bacterial selenocompound metabolism and chemotaxis pathways.

Trehalose-Guanosine Glycopolymer Hydrogels Direct Adaptive Glia Responses in CNS Injury

Neural tissue damaged after central nervous system (CNS) injury does not naturally regenerate but is instead replaced by non-neural fibrotic scar tissue that serves no neurological function. Scar-free repair to create a more permissive environment for regeneration requires altering the natural injury responses of glial cells. In this work, glycopolymer-based supramolecular hydrogels are synthesized to direct adaptive glia repair after CNS injury. Combining poly(trehalose-co-guanosine) (pTreGuo) glycopolymers with free guanosine (fGuo) generates shear-thinning hydrogels through stabilized formation of long-range G-quadruplex secondary structures. Hydrogels with smooth or granular microstructures and mechanical properties spanning three orders of magnitude are produced through facile control of pTreGuo hydrogel composition. Injection of pTreGuo hydrogels into healthy mouse brains elicits minimal stromal cell infiltration and peripherally derived inflammation that is comparable to a bioinert methyl cellulose benchmarking material. pTreGuo hydrogels alter astrocyte borders and recruit microglia to infiltrate and resorb the hydrogel bulk over 7 d. Injections of pTreGuo hydrogels into ischemic stroke alter the natural responses of glial cells after injury to reduce the size of lesions and increase axon regrowth into lesion core environments. These results support the use of pTreGuo hydrogels as part of neural regeneration strategies to activate endogenous glia repair mechanisms.

Acinetobacter Baumannii Phages: Past, Present and Future

Acinetobacter baumannii (A. baumannii) is one of the most common clinical pathogens and a typical multi-drug resistant (MDR) bacterium. With the increase of drug-resistant A. baumannii infections, it is urgent to find some new treatment strategies, such as phage therapy. In this paper, we described the different drug resistances of A. baumannii and some basic properties of A. baumannii phages, analyzed the interaction between phages and their hosts, and focused on A. baumannii phage therapies. Finally, we discussed the chance and challenge of phage therapy. This paper aims to provide a more comprehensive understanding of A. baumannii phages and theoretical support for the clinical application of A. baumannii phages.

Insights into Acinetobacter baumannii protective immunity

Acinetobacter baumannii is a nosocomic opportunistic Gram-negative bacteria known for its extensive drug-resistant phenotype. A. baumannii hospital-acquired infections are major contributors to increased costs and mortality observed during the COVID-19 pandemic. With few effective antimicrobials available for treatment of this pathogen, immune-based therapy becomes an attractive strategy to combat multi-drug resistant Acinetobacter infection. Immunotherapeutics is a field of growing interest with advances in vaccines and monoclonal antibodies providing insight into the protective immune response required to successfully combat this pathogen. This review focuses on current knowledge describing the adaptive immune response to A. baumannii, the importance of antibody-mediated protection, developments in cell-mediated protection, and their respective therapeutic application going forward. With A. baumannii’s increasing resistance to most current antimicrobials, elucidating an effective host adaptive immune response is paramount in the guidance of future immunotherapeutic development.

Genomic and phenotypic analyses of diverse non-clinical Acinetobacter baumannii strains reveals strain-specific virulence and resistance capacity

Acinetobacter baumannii is a critically important pathogen known for its widespread antibiotic resistance and ability to persist in hospital-associated environments. Whilst the majority of A. baumannii infections are hospital-acquired, infections from outside the hospital have been reported with high mortality. Despite this, little is known about the natural environmental reservoir(s) of A. baumannii and the virulence potential underlying non-clinical strains. Here, we report the complete genome sequences of six diverse strains isolated from environments such as river, soil, and industrial sites around the world. Phylogenetic analyses showed that four of these strains were unrelated to representative nosocomial strains and do not share a monophyletic origin, whereas two had sequence types belonging to the global clone lineages GC1 and GC2. Further, the majority of these strains harboured genes linked to virulence and stress protection in nosocomial strains. These genotypic properties correlated well with in vitro virulence phenotypic assays testing resistance to abiotic stresses, serum survival, and capsule formation. Virulence potential was confirmed in vivo, with most environmental strains able to effectively kill Galleria mellonella greater wax moth larvae. Using phenomic arrays and antibiotic resistance profiling, environmental and nosocomial strains were shown to have similar substrate utilisation patterns although environmental strains were distinctly more sensitive to antibiotics. Taken together, these features of environmental A. baumannii strains suggest the existence of a strain-specific distinct gene pools for niche specific adaptation. Furthermore, environmental strains appear to be equally virulent as contemporary nosocomial strains but remain largely antibiotic sensitive.

Clinical Isolates of Acinetobacter spp. Are Highly Serum Resistant Despite Efficient Recognition by the Complement System

Gram-negative bacteria from the genus Acinetobacter are responsible for life-threating hospital-related infections such as pneumonia, septicemia, and meningitis, especially in immunocompromised patients. Worryingly, Acinetobacter have become multi- and extensively drug resistant (MDR/XDR) over the last few decades. The complement system is the first line of defense against microbes, thus it is highly important to increase our understanding of evasion mechanisms used by Acinetobacter spp. Here, we studied clinical isolates of Acinetobacter spp. (n=50), aiming to characterize their recognition by the complement system. Most isolates tested survived 1 h incubation in 30% serum, and only 8 isolates had a lower survival rate, yet none of those isolates were fully killed. Intriguingly, four isolates survived in human whole blood containing all cell component. Their survival was, however, significantly reduced. Flow cytometry analyses revealed that most of the isolates were detected by human IgG and IgM. Interestingly, we could not detect any significant concentration of deposited C1q, despite observing C4b deposition that was abolished in C1q-deficient serum, indicating transient binding of C1q to bacteria. Moreover, several isolates were recognized by MBL, with C4b deposition abolished in MBL-deficient serum. C3b was deposited on most isolates, but this was not, however, seen with respect to C5b and formation of the membrane attack complex (MAC), indicating that many isolates could avoid complement-mediated lysis. India ink staining showed that isolates were capsulated, and capsule thickness varied significantly between isolates. Studies performed on a wild-type strain and capsule mutant strains, demonstrated that the production of a capsular polysaccharide is one mechanism that mediates resistance to complement-mediated bactericidal activity by preventing MAC deposition and lysis. Our data showed that most clinical Acinetobacter spp. isolates are highly serum resistant despite being efficiently recognized by the complement system.

Phenotypic detection, antimicrobial susceptibility and virulence profile of staphylococci in the pig production setting, Makurdi, Nigeria

Livestock, particularly pigs, have increasingly been recognized as important reservoirs for zoonotic transmission of pathogenic bacteria, including staphylococci. Livestock production systems in developing countries of sub-Saharan Africa, including Nigeria, are characterized by high misuse/abuse of antimicrobials and a close association between humans and these animals, which promotes the emergence and transmission of resistant and potentially virulent bacteria. In the present study, we investigated the occurrence and characteristics (species distribution, virulence and resistance profile) of staphylococci from smallholder backyard pig farms, slaughter slabs and pig handlers in Makurdi, Nigeria. A total of 330 nasal swabs originating from 300 pigs and 30 in-contact humans were collected and processed. One hundred and thirteen samples [34.2 %; 95 % confidence interval (CI): 29.1–39.6] comprising 103 (34.3 %; 95 % CI: 29.0–40.0) and 10 (33.3 %; 95 % CI: 17.3–52.8 %) samples from pigs and humans, respectively, were positive for staphylococci, yielding 120 isolates (pigs n=110, humans n=10). The 120 isolates were distributed into 15 species with Staphylococcus aureus (n=25) followed by Staphylococcus cohnii (n=19) and Staphylococcus sciuri (n=14) occurring more frequently. All isolates were resistant to β-lactam (100 %) antibiotics. Resistance to some critical antimicrobials, including linezolid (22 %), vancomycin (19.2 %), gentamicin (7.5%) and the fluoroquinolones ciprofloxacin (75.8 %) and enrofloxacin (66.7 %), was also observed. Majority (99.2 %) of the isolates displayed a multidrug resistance phenotype with the AMP-C-CIP-E-ENR-FOX-OX-P-S-SXT-TE phenotype being predominant. Overall, 70 % of the isolates expressed the methicillin resistance phenotype, out of which 20 % (n=17) were MRSA. Resistance to serum bactericidal activity and biofilm production were respectively observed in 45 (100 %) and 5 (11.3 %) of the coagulase-positive staphylococci. Our findings demonstrated the occurrence of a high diversity of staphylococci expressing multidrug resistance and potentially virulent phenotypes among healthy swine and pig handlers in small-scale backyard farms in North-Central Nigeria. These findings underscore the potential role of pig production settings in the emergence and dissemination of potentially virulent staphylococci and the importance of the development of antimicrobial resistance monitoring systems/implementation of control measures in developing countries. Proper hygienic practices and control of indiscriminate use and misuse of antibiotics are recommended.

Strain Specific Variations in Acinetobacter baumannii Complement Sensitivity

The complement system is required for innate immunity against Acinetobacter baumannii, an important cause of antibiotic resistant systemic infections. A. baumannii strains differ in their susceptibility to the membrane attack complex (MAC) formed from terminal complement pathway proteins, but the reasons for this variation remain poorly understood. We have characterized in detail the complement sensitivity phenotypes of nine A. baumannii clinical strains and some of the factors that might influence differences between strains. Using A. baumannii laboratory strains and flow cytometry assays, we first reconfirmed that both opsonization with the complement proteins C3b/iC3b and MAC formation were inhibited by the capsule. There were marked differences in C3b/iC3b and MAC binding between the nine clinical A. baumannii strains, but this variation was partially independent of capsule composition or size. Opsonization with C3b/iC3b improved neutrophil phagocytosis of most strains. Importantly, although C3b/iC3b binding and MAC formation on the bacterial surface correlated closely, MAC formation did not correlate with variations between A. baumannii strains in their levels of serum resistance. Genomic analysis identified only limited differences between strains in the distribution of genes required for serum resistance, but RNAseq data identified three complement-resistance genes that were differentially regulated between a MAC resistant and two MAC intermediate resistant strains when cultured in serum. These data demonstrate that clinical A. baumannii strains vary in their sensitivity to different aspects of the complement system, and that the serum resistance phenotype was influenced by factors in addition to the amount of MAC forming on the bacterial surface.

Adherence ability and serum resistance of different hospital clusters of Acinetobacter baumannii

The role of mechanical ventilation and catheters in favouring Acinetobacter baumannii infections needs to be better understood. This study evaluated the adherence of 19 isolates of different hospital clusters of A. baumannii to abiotic surfaces and epithelial cells (HEp-2). Of the hydrophobic isolates, 80% adhered to polystyrene, indicating a close relationship between hydrophobicity and adherence. All isolates adhered to epithelial cells to different degrees, and 73·7% showed an aggregated pattern. Analysis of the serum resistance of catheter-tip isolates showed that all were resistant. These worrisome results showed that the high capacity of A. baumannii to adhere to surfaces and survive in human serum could hinder treatment and control of this pathogen.

Host Innate Immune Responses to Acinetobacter baumannii Infection

Acinetobacter baumannii has emerged as a major threat to global public health and is one of the key human pathogens in healthcare (nosocomial and community-acquired)-associated infections. Moreover, A. baumannii rapidly develops resistance to multiple antibiotics and is now globally regarded as a serious multidrug resistant pathogen. There is an urgent need to develop novel vaccines and immunotherapeutics as alternatives to antibiotics for clinical management of A. baumannii infection. However, our knowledge of host immune responses to A. baumannii infection and the identification of novel therapeutic targets are significantly lacking. This review highlights the recent advances and critical gaps in our understanding how A. baumannii interacts with the host innate pattern-recognition receptors, induces a cascade of inflammatory cytokine and chemokine responses, and recruits innate immune effectors (such as neutrophils and macrophages) to the site of infection for effective control of the infection. Such knowledge will facilitate the identification of new targets for the design and development of effective therapeutics and vaccines to fight this emerging threat.

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.

Genomic and Phenotypic Analyses of Acinetobacter baumannii Isolates From Three Tertiary Care Hospitals in Thailand

Antibiotic resistant strains of Acinetobacter baumannii are responsible for a large and increasing burden of nosocomial infections in Thailand and other countries of Southeast Asia. New approaches to their control and treatment are urgently needed and an attractive strategy is to remove the bacterial polysaccharide capsule, and thus the protection from the host's immune system. To examine phylogenetic relationships, distribution of capsule chemotypes, acquired antibiotic resistance determinants, susceptibility to complement and other traits associated with systemic infection, we sequenced 191 isolates from three tertiary referral hospitals in Thailand and used phenotypic assays to characterize key aspects of infectivity. Several distinct lineages were circulating in three hospitals and the majority belonged to global clonal group 2 (GC2). Very high levels of resistance to carbapenems and other front-line antibiotics were found, as were a number of widespread plasmid replicons. A high diversity of capsule genotypes was encountered, with only three of these (KL6, KL10, and KL47) showing more than 10% frequency. Almost 90% of GC2 isolates belonged to the most common capsule genotypes and were fully resistant to the bactericidal action of human serum complement, most likely protected by their polysaccharide capsule, which represents a key determinant of virulence for systemic infection. Our study further highlights the importance to develop therapeutic strategies to remove the polysaccharide capsule from extensively drug-resistant A. baumanii during the course of systemic infection.

Outer membrane protein A (OmpA) as a potential therapeutic target for Acinetobacter baumannii infection

Acinetobacter baumannii (A. baumannii) is an important opportunistic pathogen causing serious nosocomial infections, which is considered as the most threatening Gram-negative bacteria (GNB). Outer membrane protein A (OmpA), a major component of outer membrane proteins (OMPs) in GNB, is a key virulence factor which mediates bacterial biofilm formation, eukaryotic cell infection, antibiotic resistance and immunomodulation. The characteristics of OmpA in Escherichia coli (E. coli) have been extensively studied since 1974, but only in recent years researchers started to clarify the functions of OmpA in A. baumannii. In this review, we summarized the structure and functions of OmpA in A. baumannii (AbOmpA), collected novel therapeutic strategies against it for treating A. baumannii infection, and emphasized the feasibility of using AbOmpA as a potential therapeutic target.

Molecular Analyses of Biofilm-Producing Clinical Acinetobacter baumannii Isolates from a South Indian Tertiary Care Hospital

OBJECTIVES

The aim of the study was to determine the presence of antimicrobial-resistance (AMR) genes, virulence genes, and mobile genetic elements (MGEs) in 14 biofilm-producing clinical isolates of Acinetobacter baumannii.

MATERIALS AND METHODS

PCR amplification was performed to analyse the prevalence of genes associated with antibiotic resistance (extended-spectrum β-lactamases [ESBLs] and metallo-β-lactamases [MBLs]), virulence factors, MGEs (class 1 integron, Tn1213, and A. baumannii antibiotic resistance [AbaR]), and comM among the study isolates. Random amplified polymorphic DNA (RAPD) PCR was then deployed to understand their phylogenetic relationship. All the isolates were investigated for biofilm production.

RESULTS

Two isolates were antibiotic-sensitive (AS), 3 were multi-drug-resistant (MDR), and the remaining 9 were extensively drug-resistant (XDR). The majority of the isolates were found to be positive for biofilm production and were sensitive against tetracycline and colistin only. Ab14 and Ab11 were found to be resistant to minocycline and colistin, respectively. blaTEM, blaOXA, blaNDM, blaVIM, blaSIM, and blaPER-1; class 1 integron; composite transposon Tn1213; AbaR island, and virulence factor genes were detected among the isolates. These pathogens were found to have originated from multiple clonal lineages.

CONCLUSION

Biofilm-producing A. baumannii with multiple virulence and AMR genes pose serious clinical challenges. The presence of MGEs further compounds the situation as these isolates serve as potential reservoirs of AMR and virulence genes. Together with their capacity for natural competence, A. baumannii, if left unchecked, will lead to the spread of resistance determinants to previously sensitive bacteria and may aid in the emergence of untreatable pan-drug-resistant phenotypes.

The Mechanisms of Disease Caused by Acinetobacter baumannii

Acinetobacter baumannii is a Gram negative opportunistic pathogen that has demonstrated a significant insurgence in the prevalence of infections over recent decades. With only a limited number of “traditional” virulence factors, the mechanisms underlying the success of this pathogen remain of great interest. Major advances have been made in the tools, reagents, and models to study A. baumannii pathogenesis, and this has resulted in a substantial increase in knowledge. This article provides a comprehensive review of the bacterial virulence factors, the host immune responses, and animal models applicable for the study of this important human pathogen. Collating the most recent evidence characterizing bacterial virulence factors, their cellular targets and genetic regulation, we have encompassed numerous aspects important to the success of this pathogen, including membrane proteins and cell surface adaptations promoting immune evasion, mechanisms for nutrient acquisition and community interactions. The role of innate and adaptive immune responses is reviewed and areas of paucity in our understanding are highlighted. Finally, with the vast expansion of available animal models over recent years, we have evaluated those suitable for use in the study of Acinetobacter disease, discussing their advantages and limitations.

Sub-MIC effects of a proline-rich antibacterial peptide on clinical isolates of Acinetobacter baumannii

INTRODUCTION

Acinetobacter baumannii is one of the most important nosocomial pathogens, mainly due to its ability to accumulate antibiotic-resistances and to persist in the hospital environment - characteristics related to biofilm production. It is well-known that A. baumannii is inhibited by the proline-rich peptide Bac7(1-35), but its putative effects at sub-MICs were never considered.

AIMS

We examined the sub-MIC effect of Bac7(1-35) on the growth rate, resistance induction and some A. baumannii features linked to virulence.

METHODOLOGY

Growth kinetics in the presence of sub-MICs of Bac7(1-35) were evaluated spectrophotometrically. Peptide uptake was quantified by cytometric analysis. The ability of Bac7(1-35) to interfere with biofilm production was investigated by the crystal violet method and confocal microscopy. Bacterial motility was observed at the interphase between a layer of a semi-solid medium and the polystyrene bottom of a Petri dish. The induction of resistance was evaluated after serial passages with sub-MICs of the peptide.

RESULTS

Although the MIC of Bac7(1-35) was between 2-4 µM for all tested strains, its effect on the growth rate at sub-MICs was strain-dependent and correlated with the amount of peptide internalized by each strain. Sub-MICs of Bac7(1-35) induced a strongly strain-dependent effect on biofilm formation and reduced motility in almost all strains, but interestingly the peptide did not induce resistance.

CONCLUSION

Bac7(1-35) is internalized into A. baumannii and is able to inhibit biofilm formation and bacterial motility, without inducing resistance. This study stresses the importance of considering possible effects that antimicrobials could have at sub-MICs, mimicking a common condition during antibiotic treatment.

Tug of war between Acinetobacter baumannii and host immune responses

Acinetobacter baumannii is an emerging nosocomial, opportunistic pathogen with growing clinical significance. Acinetobacter baumannii has an exceptional ability to rapidly develop drug resistance and to adhere to abiotic surfaces, including medical equipment, significantly promoting bacterial spread and also limiting our ability to control A. baumannii infections. Consequently, A. baumannii is frequently responsible for ventilator-associated pneumonia in clinical settings. In order to develop an effective treatment strategy, understanding host-pathogen interactions during A. baumannii infection is crucial. Various A. baumannii virulence factors have been identified as targets of host innate pattern-recognition receptors, which leads to activation of downstream inflammasomes to develop inflammatory responses, and the recruitment of innate immune effectors against A. baumannii infection. To counteract host immune attack, A. baumannii regulates its expression of different virulence factors. This review summarizes the significance of mechanisms of host-bacteria interaction, as well as different bacteria and host defense mechanisms during A. baumannii infection.

The effects of antimicrobial peptides WAM-1 and LL-37 on multidrug-resistant Acinetobacter baumannii

Increasing multidrug resistance (MDR) in Acinetobacter baumannii warrants therapeutic alternatives, and the bactericidal nature of antimicrobial peptides (AMPs) offers a possible approach. In this study, we examined the interaction of cathelicidin AMPs WAM-1, a marsupial AMP, and LL-37, a human AMP, with A. baumannii clinical isolates. We characterized the antibiotic resistance of the isolates, the bacteriostatic and bactericidal effects of these AMPs, synergistic activity with antibiotics, and their effects on biofilm formation and dispersal. All clinical isolates were resistant to commonly prescribed antibiotics, with four of seven isolates showing MDR. WAM-1 and LL-37 showed variable activity in clinical isolates, with WAM-1 having a stronger bacteriostatic effect than LL-37 and showing rapid bactericidal activity against clinical isolates. Furthermore, synergistic bactericidal activity was observed with WAM-1 and commonly prescribed antibiotics. Both peptides were able to inhibit biofilm formation in all clinical isolates at some concentrations, and WAM-1 dispersed mature biofilm in most isolates. LL-37 was unable to disperse mature biofilms in any strains. Further studies must be done to elucidate the true value of these alternative treatments, but these results suggest that MDR A. baumannii's susceptibility to AMPs may result in innovative therapeutics to prevent or treat these infections.

Genomic and physiological analyses of an indigenous strain, Enterococcus faecium 17OM39

The human gut microbiome plays a crucial role in human health and efforts need to be done for cultivation and characterisation of bacteria with potential health benefits. Here, we isolated a bacterium from a healthy Indian adult faeces and investigated its potential as probiotic. The cultured bacterial strain 17OM39 was identified as Enterococcus faecium by 16S rRNA gene sequencing. The strain 17OM39 exhibited tolerance to acidic pH, showed antimicrobial activity and displayed strong cell surface traits such as hydrophobicity and autoaggregation capacity. The strain was able to tolerate bile salts and showed bile salt hydrolytic (BSH) activity, exopolysaccharide production and adherence to human HT-29 cell line. Importantly, partial haemolytic activity was detected and the strain was susceptible to the human serum. Genomics investigation of strain 17OM39 revealed the presence of diverse genes encoding for proteolytic enzymes, stress response systems and the ability to produce essential amino acids, vitamins and antimicrobial compound Bacteriocin-A. No virulence factors and plasmids were found in this genome of the strain 17OM39. Collectively, these physiological and genomic features of 17OM39 confirm the potential of this strain as a candidate probiotic.

Effects of In vivo Emergent Tigecycline Resistance on the Pathogenic Potential of Acinetobacter baumannii

Multidrug-resistant lineages of Acinetobacter baumannii (MDRAB) are important nosocomial pathogens. As tigecycline remains active against most MDRAB we sought to investigate whether tigecycline resistance impacts biological fitness. The effects of treatment-emergent tigecycline resistance were investigated in vitro and in vivo using two pre- (AB210; W6976) and post-therapy (AB211; W7282) clinical pairs, recovered from individual patients, where tigecycline resistance was associated with up-regulated efflux activity. All isolates belonged to the same epidemic UK lineage. Significant differences were observed in end-point survival proportions between AB210 and AB211, but not between W6976 and W7282, using the Galleria mellonella infection model. Isolate AB211 outcompeted AB210 in vivo, in contrast to isolate W7282, which was outcompeted by its pre-therapy counterpart, W6972. Whole-genome sequencing of isolates W6976 and W7282 revealed a mutation in the adeABC regulatory gene, adeS in W7282; resulting in a Ser-8 → Arg substitution. Previous whole-genome comparison of AB210 and AB211 also identified a non-synonymous mutation in adeS, among several other lesions in genes involved in biofilm formation and DNA mismatch repair; consistent with the phenotypic differences described here. In conclusion, the differing effects on the wider phenotype were not predictable from the antibiograms or clonal lineage, despite a common mechanism of tigecycline resistance.

Cloning of ompA gene from Acinetobacter baumannii into the eukaryotic expression vector pBudCE4.1 as DNA vaccine

Antibiotic resistant features of Acinetobacter baumannii is partly due to the decreased outer membrane proteins (OMPs) permeability. The OmpA is one of the most conserved proteins among A. baumannii with a considerable antigenic potential to stimulate the multidimensional immune system responses. The present study was aimed to clone the ompA gene into the eukaryotic expression vector with potential as DNA vaccine. The ompA gene of A. baumannii was amplified using polymerase chain reaction (PCR). The target DNA was cloned and sub-cloned into the pTZ57R/T and pBudCE4.1 vectors, respectively. The recombinant vectors containing ompA were then validated using colony PCR, vector sequencing and double-digestion strategies. The pBudCE4.1-ompA recombinant plasmid was transfected into the human dermal fibroblast cells (HDF) and presence of ompA transcript and protein was evaluated using reverse transcribed-PCR (RT-PCR) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Our finding from colony PCR, sequencing and enzyme double digestion result confirmed that target gene has been successfully inserted into the pTZ57RT and pBudCE4.1. The presence of an expected band (1112 bp) in RT-PCR as wells as a ~ 38 kDa band during SDS-PAGE showed that the recombinant pBudCE4.1-ompA construct was efficiently transfected into the HDF cells and expressed. Altogether, our observation demonstrated that the recombinant pBudCE4.1-ompA construct was successfully produced although further experiments are needed.

The O-specific polysaccharide lyase from the phage LKA1 tailspike reduces Pseudomonas virulence

Pseudomonas phage LKA1 of the subfamily Autographivirinae encodes a tailspike protein (LKA1gp49) which binds and cleaves B-band LPS (O-specific antigen, OSA) of Pseudomonas aeruginosa PAO1. The crystal structure of LKA1gp49 catalytic domain consists of a beta-helix, an insertion domain and a C-terminal discoidin-like domain. The putative substrate binding and processing site is located on the face of the beta-helix whereas the C-terminal domain is likely involved in carbohydrates binding. NMR spectroscopy and mass spectrometry analyses of degraded LPS (OSA) fragments show an O5 serotype-specific polysaccharide lyase specificity. LKA1gp49 reduces virulence in an in vivo Galleria mellonella infection model and sensitizes P. aeruginosa to serum complement activity. This enzyme causes biofilm degradation and does not affect the activity of ciprofloxacin and gentamicin. This is the first comprehensive report on LPS-degrading lyase derived from a Pseudomonas phage. Biological properties reveal a potential towards its applications in antimicrobial design and as a microbiological or biotechnological tool.

Molecular characterization of β-lactamase genes in clinical isolates of carbapenem-resistant Acinetobacter baumannii

Background

Acinetobacter baumannii is a nosocomial pathogen which is establishing as a major cause of morbidity and mortality within the healthcare community. The success of this pathogen is largely due to its ability to rapidly gain resistance to antimicrobial therapies and its capability to persist in an abiotic environment through the production of a biofilm. Our tertiary-care hospital has showed high incidence of carbapenem-resistant Acinetobacter baumannii (CRAB) isolates.

Methods

In this study we explore both genotypic and phenotypic properties of 26 CRAB isolates: 16 isolates were collected from January 2010 to March 2011, and 10 were collected between February and May 2015.

Results

We determined that all 26 CRAB isolates possessed multiple β-lactamase genes, including genes from Groups A, C, and D. Specifically, 42% of the isolates possesses the potentially plasmid-borne genes of OXA-23-like or OXA-40-like β-lactamase. The presence of mobile gene element integron cassettes and/or integrases in 88% of the isolates suggests a possible mechanism of dissemination of antibiotic resistance genes. Additionally, the location of insertion sequence (IS) ISAba1 in promotor region of of the OXA-51-like, ADC-7, and ampC genes was confirmed. Multilocus sequence typing (MLST) demonstrated that all 26 CRAB isolates were either sequence type (ST)-229 or ST-2. Interestingly, ST-2 went from being the minority CRAB strain in the 2010–2011 isolates to the predominant strain in the 2015 isolates (from 32 to 90%). We show that the ST-2 strains have an enhanced ability to produce biofilms in comparison to the ST-229 strains, and this fact has potentially led to more successful colonization of the clinical environment over time.

Conclusions

This study provides a longitudinal genetic and phenotypic survey of two CRAB sequence types, and suggests how their differing phenotypes may interact with the selective pressures of a hospital setting effecting strain dominance over a 5-year period.

Electronic supplementary material

The online version of this article (10.1186/s12941-017-0248-3) contains supplementary material, which is available to authorized users.

In vitro effect of subminimal inhibitory concentrations of antibiotics on the biofilm formation ability of Acinetobacter baumannii clinical isolates

The ability of A cinetobacter baumannii strains to form biofilm is one of the most important virulence factor which enables bacterial survival in a harsh environment and decreases antibiotic concentration as well. Subminimal inhibitory concentrations (subMICs) of antibiotics may change bacterial ultrastructure or have an influence on some different molecular mechanisms resulting in morphological or physiological changes in bacteria itself. The aim of this study was to determine effects of 1/2, 1/4, 1/8 and 1/16 minimal inhibitory concentrationsof imipenem, ampicillin-sulbactam, azithromycin, rifampicin and colistin on biofilm formation ability of 22 biofilm non-producing and 46 biofilm producing A. baumannii strains (30 weak producing strains and 16 moderate producing strains). Results of this study indicate that 1/2-1/16 MICs of imipenem, azithromycin, and rifampicin can reduce bacterial biofilm formation ability in moderate producing strains (p < 0.05), whereas 1/16 MIC of imipenem and 1/4-1/8 MICs of rifampicin reduce the biofilm formation in weak producing strains (p < 0.05). Statisticaly significant effect was detected among biofilm non-producing strains after their exposure to 1/16 MIC of azithromycin (p = 0.039). SubMICs of ampicillin-sulbactam and colistin did not have any significant effect on biofilm formation among tested A. baumannii strains.

In-silico screening and experimental validation reveal L-Adrenaline as anti-biofilm molecule against biofilm-associated protein (Bap) producing Acinetobacter baumannii

Acinetobacter baumannii, an ESKAPE pathogen, causes various nosocomial infections and has capacity to produce biofilm. Biofilm produced by this bacterium is highly tolerant to environmental factors and different antibiotics. Biofilm-associated protein (Bap) plays a significant role in the biofilm formation by A. baumannii and found in the extra cellular matrix of the biofilm. Therefore, it becomes essential to find a potential drug against Bap that has capacity to inhibit biofilm formation by A. baumannii. In-silico screening, molecular mechanics and molecular dynamics studies identified ZINC00039089 (L-Adrenaline) as an inhibitor for Bap of A. baumannii. Recently, it is reported that Bap can form amyloid like structure; hence we have created dimer of Bap protein. This inhibitor can bind to dimeric Bap with good affinity. It confirms that ZINC00039089 (L-Adrenaline) can bind with Bap monomer as well as oligomeric Bap, responsible for amyloid formation and biofilm formation. Hence, we have tested Adrenaline as an anti-biofilm molecule and determined its IC₅₀ value against biofilm. The result showed Adrenaline has anti-biofilm activity with IC₅₀ value of 75μg/ml. Therefore; our finding suggests that L-Adrenaline can be developed to inhibit biofilm formation by carbapenem resistant strain of Acinetobacter baumannii.

Complexity of Complement Resistance Factors Expressed by Acinetobacter baumannii Needed for Survival in Human Serum

Acinetobacter baumannii is a bacterial pathogen with increasing impact in healthcare settings, due in part to this organism's resistance to many antimicrobial agents, with pneumonia and bacteremia as the most common manifestations of disease. A significant proportion of clinically relevant A. baumannii strains are resistant to killing by normal human serum (NHS), an observation supported in this study by showing that 12 out of 15 genetically diverse strains of A. baumannii are resistant to NHS killing. To expand our understanding of the genetic basis of A. baumannii serum resistance, a transposon (Tn) sequencing (Tn-seq) approach was used to identify genes contributing to this trait. An ordered Tn library in strain AB5075 with insertions in every nonessential gene was subjected to selection in NHS. We identified 50 genes essential for the survival of A. baumannii in NHS, including already known serum resistance factors, and many novel genes not previously associated with serum resistance. This latter group included the maintenance of lipid asymmetry genetic pathway as a key determinant in protecting A. baumannii from the bactericidal activity of NHS via the alternative complement pathway. Follow-up studies validated the role of eight additional genes identified by Tn-seq in A. baumannii resistance to killing by NHS but not by normal mouse serum, highlighting the human species specificity of A. baumannii serum resistance. The identification of a large number of genes essential for serum resistance in A. baumannii indicates the degree of complexity needed for this phenotype, which might reflect a general pattern that pathogens rely on to cause serious infections.

Virulence profiles and innate immune responses against highly lethal, multidrug-resistant nosocomial isolates of Acinetobacter baumannii from a tertiary care hospital in Mexico

Virulence profiles and innate immune responses were studied in Acinetobacter baumannii from nosocomial infections collected over one year in a tertiary care hospital in Mexico. A. baumannii were identified by VITEK 2 System followed by susceptibility tests. Carbapenemase genes, active efflux mechanism to imipenem and meropenem and outer membrane proteins profile were analyzed to evaluate their role on the activity of carbapenem resistance. All isolates were genotyped by pulsed field gel electrophoresis. The ability to form biofilm was determined on a polystyrene surface. The resistance to complement was determined with a pooled human normal serum and TNFα release by infected macrophages was determined by ELISA. The 112 isolates from this study were associated with a 52% of mortality. All were resistance to β-lactams, fluoroquinolones, and trimethroprim-sulfamethoxal, 96 and 90% were resistant to meropenem and imipenem, respectively, but with high susceptibility to polymyxin B, colistin and tigecyclin. Isolates were classified in 11 different clones. Most isolates, 88% (99/112), were metallo-β-lactamases and carbapenemases producers, associated in 95% with the presence of bla_OXA-72 gene. Only 4/99 and 1/99 of the carbapenem-resistant isolates were related to efflux mechanism to meropenem or imipenem resistance, respectively. The loss of expression of 22, 29, and/or 33-36-kDa proteins was detected in 8/11 of the clinical isolates with resistance to carbapenem. More than 96% (108/112) of the isolates were high producers of biofilms on biotic surfaces. Finally, all isolates showed variable resistance to normal human serum activity and were high inductors of TNFα release by macrophages. In summary, these results suggest that multidrug-resistant A. baumannii can persist in the hospital environment through its ability to form biofilms. The high mortality observed was due to their ability to survive normal human serum activity and capability to induce potent inflammatory immune response making this nosocomial pathogen a serious threat to hospitalized patients.

Microbiological features and clinical impact of the type VI secretion system (T6SS) in Acinetobacter baumannii isolates causing bacteremia

We investigated the genetic background and microbiological features of T6SS-positive Acinetobacter baumannii isolates and clinical impact of the T6SS in patients with A. baumannii bacteremia. One hundred and 62 A. baumannii isolates from patients with bacteremia in 2 tertiary-care hospitals in Korea were included in this study. Approximately one-third (51/162, 31.5%) of the A. baumannii clinical isolates possessed the hcp gene, and the hcp-positive isolates were found in several genotypes in multilocus sequence typing. The expression and secretion of Hcp protein varied among the clinical isolates. A. baumannii isolates with detectable Hcp secretion (T6SS+) could better outcompete Escherichia coli compared with T6SS- isolates, including hcp-negative and inactivated hcp-positive isolates. In addition, T6SS+ isolates showed higher biofilm-forming activity and better survival in the presence of normal human serum than the T6SS- isolates. T6SS+ isolates were more frequently detected in patients with catheter-related bloodstream infection, haematopoietic stem cell transplant recipients, and patients receiving immunosuppressive agents. However, T6SS was not a prognostic factor for mortality. Our results suggest that the T6SS of A. baumannii is associated with virulence and contributes to infections in immunocompromised patients and those with implanted medical devices.

The Immune Response against Acinetobacter baumannii, an Emerging Pathogen in Nosocomial Infections

Acinetobacter baumannii is the etiologic agent of a wide range of nosocomial infections, including pneumonia, bacteremia, and skin infections. Over the last 45 years, an alarming increase in the antibiotic resistance of this opportunistic microorganism has been reported, a situation that hinders effective treatments. In order to develop effective therapies against A. baumannii it is crucial to understand the basis of host–bacterium interactions, especially those concerning the immune response of the host. Different innate immune cells such as monocytes, macrophages, dendritic cells, and natural killer cells have been identified as important effectors in the defense against A. baumannii; among them, neutrophils represent a key immune cell indispensable for the control of the infection. Several immune strategies to combat A. baumannii have been identified such as recognition of the bacteria by immune cells through pattern recognition receptors, specifically toll-like receptors, which trigger bactericidal mechanisms including oxidative burst and cytokine and chemokine production to amplify the immune response against the pathogen. However, a complete picture of the protective immune strategies activated by this bacteria and its potential therapeutic use remains to be determined and explored.

Current advances and challenges in the development of Acinetobacter vaccines

Acinetobacter baumannii is a major cause of healthcare-associated infections worldwide with high morbidity and mortality. The clinical treatment of A. baumannii infections has become increasingly difficult because of the rapid emerging of multidrug and extremely drug resistant strains. Thus, there is an urgent need for the development of novel intervention strategies to combat this multidrug-resistant pathogen. Vaccine is one of the most effective medical measures for infection control and is likely to overcome the development of multidrug resistance by A. baumannii. Here we discussed the recent advances and potential challenges in development of A. baumannii vaccines with a focus on the 3 most important steps in the preclinical vaccine development: antigen selection, immune correlates of protection, and animal models for efficacy evaluation.

Evaluate the frequency distribution of nonadhesive virulence factors in carbapenemase-producing Acinetobacter baumannii isolated from clinical samples in Kermanshah

Background:

Acinetobacter baumannii which is a Gram-negative bacterium can cause several different infections. The appearance of carbapenemase-producing A. baumannii in recent years has made the treatment process more difficult. The identification of virulence factors (VFs), such as nonadhesives in A. baumannii, helps to fight against related infections.

Materials and Methods:

A total of 104 samples from teaching hospitals in Kermanshah, Iran, were collected during a 24 months period (2011-2013). Sample identification was first carried out by biochemical tests, and then their susceptibility to carbapenems was determined using the Kirby–Bauer method. For confirmation of carbapenemase-producing A. baumannii, polymerase chain reaction (PCR) was done for carbapenemase-encoding genes. In addition, the frequency of nonadhesive VFs in carbapenemase-producing isolates was determined by PCR.

Results:

There were 50 isolates that were identified as carbapenemase-producing A. baumannii. The PCR results showed; 40 isolates (80%) for traT, 17 isolates (34%) for cvaC, and 8 isolates (16%) for iutA, and these encode serum resistance, colicin V and aerobactin, respectively. No significant correlation was observed between these three genes.

Conclusions:

The mechanism of A. baumannii virulence has always been in question. The role of VFs has also been recognized in other Gram-negative bacteria. According to the prevalence of traT, cvaC and iutA, as nonadhesive VFs, we can suggest that they could be the main mechanism of carbapenemase-producing A. baumannii pathogenesis.

Outer membrane Protein A plays a role in pathogenesis of Acinetobacter nosocomialis

Acinetobacter nosocomialis is an important nosocomial pathogen that causes a variety of human infections. However, the specific virulence factors of this microorganism have not yet been determined. We investigated the role of outer membrane protein A (OmpA) in the pathogenesis of A. nosocomialis. A ΔompA mutant of the A. nosocomialis ATCC 17903(T) strain was constructed using markerless gene deletion. The ΔompA mutant displayed reduced biofilm formation in polystyrene tubes and reduced adherence to A549 cells in comparison to the wild-type strain. These virulence traits of the ΔompA mutant strain were restored when the ompA gene was complemented. Cytotoxicity was not significantly different between the wild-type strain and the ΔompA mutant when A549 cells were infected with bacteria or treated with outer membrane vesicles (OMVs). However, OMVs from the wild-type strain induced cytotoxicity in HEp-2 cells, whereas OMVs from the ΔompA mutant did not induce cytotoxicity. Proteomic analysis of OMVs revealed that OmpA influenced the distribution of envelope and periplasmic proteins. Overall, this study is the first report that links OmpA to A. nosocomialis pathogenesis, and highlights OmpA as a putative target to develop anti-virulence agents or vaccines against A. nosocomialis infection.

CipA of Acinetobacter baumannii Is a Novel Plasminogen Binding and Complement Inhibitory Protein

Acinetobacter baumannii is an emerging opportunistic pathogen, responsible for up to 10% of gram-negative, nosocomial infections. The global increase of multidrug-resistant and pan-resistant Acinetobacter isolates presents clinicians with formidable challenges. To establish a persistent infection,A. baumannii must overcome the detrimental effects of complement as the first line of defense against invading microorganisms. However, the immune evasion principles underlying serum resistance inA. baumannii remain elusive. Here, we identified a novel plasminogen-binding protein, termed CipA. Bound plasminogen, upon conversion to active plasmin, degraded fibrinogen and complement C3b and contributed to serum resistance. Furthermore, CipA directly inhibited the alternative pathway of complement in vitro, irrespective of its ability to bind plasminogen. A CipA-deficient mutant was efficiently killed by human serum and showed a defect in the penetration of endothelial monolayers, demonstrating that CipA is a novel multifunctional protein that contributes to the pathogenesis ofA. baumannii.

Phylogenetic and genomic diversity in isolates from the globally distributed Acinetobacter baumannii ST25 lineage

Acinetobacter baumannii is a globally distributed nosocomial pathogen that has gained interest due to its resistance to most currently used antimicrobials. Whole genome sequencing (WGS) and phylogenetics has begun to reveal the global genetic diversity of this pathogen. The evolution of A. baumannii has largely been defined by recombination, punctuated by the emergence and proliferation of defined clonal lineages. In this study we sequenced seven genomes from the sequence type (ST)25 lineage and compared them to 12 ST25 genomes deposited in public databases. A recombination analysis identified multiple genomic regions that are homoplasious in the ST25 phylogeny, indicating active or historical recombination. Genes associated with antimicrobial resistance were differentially distributed between ST25 genomes, which matched our laboratory-based antimicrobial susceptibility typing. Differences were also observed in biofilm formation between ST25 isolates, which were demonstrated to produce significantly more extensive biofilm than an isolate from the ST1 clonal lineage. These results demonstrate that within A. baumannii, even a fairly recently derived monophyletic lineage can still exhibit significant genotypic and phenotypic diversity. These results have implications for associating outbreaks with sequence typing as well as understanding mechanisms behind the global propagation of successful A. baumannii lineages.

Translation Elongation Factor Tuf of Acinetobacter baumannii Is a Plasminogen-Binding Protein

Acinetobacter baumannii is an important nosocomial pathogen, causing a variety of opportunistic infections of the skin, soft tissues and wounds, urinary tract infections, secondary meningitis, pneumonia and bacteremia. Over 63% of A. baumannii infections occurring in the United States are caused by multidrug resistant isolates, and pan-resistant isolates have begun to emerge that are resistant to all clinically relevant antibiotics. The complement system represents the first line of defense against invading pathogens. However, many A. baumannii isolates, especially those causing severe bacteremia are resistant to complement-mediated killing, though the underlying mechanisms remain poorly understood. Here we show for the first time that A. baumannii binds host-derived plasminogen and we identify the translation elongation factor Tuf as a moonlighting plasminogen-binding protein that is exposed on the outer surface of A. baumannii. Binding of plasminogen to Tuf is at least partly dependent on lysine residues and ionic interactions. Plasminogen, once bound to Tuf can be converted to active plasmin and proteolytically degrade fibrinogen as well as the key complement component C3b. Thus, Tuf acts as a multifunctional protein that may contribute to virulence of A. baumannii by aiding in dissemination and evasion of the complement system.

Acinetobacter baumannii: evolution of antimicrobial resistance-treatment options

The first decade of the 20th century witnessed a surge in the incidence of infections due to several highly antimicrobial-resistant bacteria in hospitals worldwide. Acinetobacter baumannii is one such organism that turned from an occasional respiratory pathogen into a major nosocomial pathogen. An increasing number of A. baumannii genome sequences have broadened our understanding of the genetic makeup of these bacteria and highlighted the extent of horizontal transfer of DNA. Animal models of disease combined with bacterial mutagenesis have provided some valuable insights into mechanisms of A. baumannii pathogenesis. Bacterial factors known to be important for disease include outer membrane porins, surface structures including capsule and lipopolysaccharide, enzymes such as phospholipase D, iron acquisition systems, and regulatory proteins. A. baumannii has a propensity to accumulate resistance to various groups of antimicrobial agents. In particular, carbapenem resistance has become commonplace, accounting for the majority of A. baumannii strains in many hospitals today. Carbapenem-resistant strains are often resistant to all other routinely tested agents. Treatment of carbapenem-resistant A. baumannii infection therefore involves the use of combinations of last resort agents such as colistin and tigecycline, but the efficacy and safety of these approaches are yet to be defined. Antimicrobial-resistant A. baumannii has high potential to spread among ill patients in intensive care units. Early recognition and timely implementation of appropriate infection control measures is crucial in preventing outbreaks.

A mouse model of Acinetobacter baumannii-associated pneumonia using a clinically isolated hypervirulent strain

Acinetobacter baumannii is an important emerging pathogen in health care-acquired infections and is responsible for severe nosocomial and community-acquired pneumonia. Currently available mouse models of A. baumannii pneumonia show poor colonization with little to no extrapulmonary dissemination. Here, we describe a mouse model of A. baumannii pneumonia using a clinical isolate (LAC-4 strain) that reliably reproduces the most relevant features of human pulmonary A. baumannii infection and pathology. Using this model, we have shown that LAC-4 infection induced rapid bacterial replication in the lungs, significant extrapulmonary dissemination, and severe bacteremia by 24 h postintranasal inoculation. Infected mice showed severe bronchopneumonia and dilatation and inflammatory cell infiltration in the perivascular space. More significantly, 100% of C57BL/6 and BALB/c mice succumbed to 10(8) CFU of LAC-4 inoculation within 48 h. When this model was used to assess the efficacy of antimicrobials, all mice treated with imipenem and tigecycline survived a lethal intranasal challenge, with minimal clinical signs and body weight loss. Moreover, intranasal immunization of mice with formalin-fixed LAC-4 protected 40% of mice from a lethal (100× 100% lethal dose) intraperitoneal challenge. Thus, this model offers a reproducible acute course of A. baumannii pneumonia without requiring additional manipulation of host immune status, which will facilitate the development of therapeutic agents and vaccines against A. baumannii pneumonia in humans.

Biofilm formation avoids complement immunity and phagocytosis of Streptococcus pneumoniae

Streptococcus pneumoniae is a frequent member of the microbiota of the human nasopharynx. Colonization of the nasopharyngeal tract is a first and necessary step in the infectious process and often involves the formation of sessile microbial communities by this human pathogen. The ability to grow and persist as biofilms is an advantage for many microorganisms, because biofilm-grown bacteria show reduced susceptibility to antimicrobial agents and hinder recognition by the immune system. The extent of host protection against biofilm-related pneumococcal disease has not been determined yet. Using pneumococcal strains growing as planktonic cultures or as biofilms, we have investigated the recognition of S. pneumoniae by the complement system and its interactions with human neutrophils. Deposition of C3b, the key complement component, was impaired on S. pneumoniae biofilms. In addition, binding of C-reactive protein and the complement component C1q to the pneumococcal surface was reduced in biofilm bacteria, demonstrating that pneumococcal biofilms avoid the activation of the classical complement pathway. In addition, recruitment of factor H, the downregulator of the alternative pathway, was enhanced by S. pneumoniae growing as biofilms. Our results also show that biofilm formation diverts the alternative complement pathway activation by a PspC-mediated mechanism. Furthermore, phagocytosis of pneumococcal biofilms was also impaired. The present study confirms that biofilm formation in S. pneumoniae is an efficient means of evading both the classical and the PspC-dependent alternative complement pathways the host immune system.

Cryo-electron tomography analysis of membrane vesicles from Acinetobacter baumannii ATCC19606 T

Acinetobacter baumannii is an important nosocomial pathogen responsible for colonization and infection of critically ill patients. Its virulence attributes together with the condition of the host determine the pathogenicity of A. baumannii. These virulence factors may be delivered to host cells by membrane vesicles. The aim of this study was to characterize the formation and morphology of membrane vesicles (MVs) from A. baumannii ATCC19606(T) using cryo-electron microscopy. Cryo-electron microscopy imaging of A. baumannii in broth cultures revealed the formation of small (≈ 30 nm) outer membrane vesicles at distal ends of early log-phase bacteria and larger (200-500 nm) membrane vesicles at septa of dividing bacteria. In the stationary phase vesicles comprising both inner and outer membranes were observed. In addition, we noted the presence of highly branched membrane structures originating from bacterial remnants forming large numbers of vesicles that were covered with proteins. Exposure of A. baumannii to sub-inhibitory concentrations of the antibiotic ceftazidime resulted in an increase in formation of MVs. Together, our results revealed multiple ways of vesicle formation leading to morphologically different MVs in the various stages of in vitro bacterial cultures.

Biofilm formed by a hypervirulent (hypermucoviscous) variant of Klebsiella pneumoniae does not enhance serum resistance or survival in an in vivo abscess model

A new hypervirulent (hypermucoviscous) clinical variant of Klebsiella pneumoniae (hvKP) has emerged over the last decade. Our goal is to identify new mechanisms, which increase the virulence hvKP. It has been shown that hvKP strains produce more biofilm than "classical" stains of K. pneumoniae, therefore we hypothesized that biofilm formation may contribute to the pathogenesis of systemic infection. To test this hypothesis, transposon mutants of the model pathogen hvKP1 were generated and screened for decreased production of biofilm. Three mutant constructs with disruptions in glnA [putatively encodes glutamine synthetase, hvKP1 glnA:: EZ::TN < KAN-2 > (glnA::Tn)], sucD [putatively encodes succinyl-CoA synthase α subunit, hvKP1 sucD:: EZ::TN < KAN-2 > (sucD::Tn)], and tag [putatively encodes transcriptional antiterminator of glycerol uptake operon, hvKP1 tag:: EZ::TN < KAN-2 > (tag::Tn)] were chosen for further characterization and use in biologic studies. Quantitative assays performed in rich laboratory medium and human ascites confirmed the phenotype and a hypermucoviscosity assay established that capsule production was not affected. However, compared with its wild-type parent, neither planktonic cells nor biofilms of glnA::Tn, sucD::Tn and tag::Tn displayed a change to the bactericidal activity of 90% human serum. Likewise, when assessed in a rat subcutaneous abscess model, the growth and survival of glnA::Tn, sucD::Tn and tag::Tn in abscess fluid was similar to hvKP1. In this report we identify three new genes that contribute to biofilm formation in hvKP1. However, decreased biofilm production due to disruption of these genes does not affect the sensitivity of these mutant constructs to 90% human serum when in planktonic form or within a biofilm. Further, their virulence in an in vivo abscess model was unaffected.

Differences in Acinetobacter baumannii strains and host innate immune response determine morbidity and mortality in experimental pneumonia

Despite many reports documenting its epidemicity, little is known on the interaction of Acinetobacter baumannii with its host. To deepen our insight into this relationship, we studied persistence of and host response to different A. baumannii strains including representatives of the European (EU) clones I–III in a mouse pneumonia model. Neutropenic mice were inoculated intratracheally with five A. baumannii strains and an A. junii strain and at several days morbidity, mortality, bacterial counts, airway inflammation, and chemo- and cytokine production in lungs and blood were determined. A. baumannii RUH875 and RUH134 (EU clone I and II, respectively) and sporadic strain LUH8326 resulted in high morbidity/mortality, whereas A. baumannii LUH5875 (EU clone III, which is less widespread than clone I and II) caused less symptoms. A. baumannii type strain RUH3023^T and A. junii LUH5851 did not cause disease. All strains, except A. baumannii RUH3023^T and A. junii LUH5851, survived and multiplied in the lungs for several days. Morbidity and mortality were associated with the severity of lung pathology and a specific immune response characterized by low levels of anti-inflammatory (IL-10) and specific pro-inflammatory (IL-12p40 and IL-23) cytokines at the first day of infection. Altogether, a striking difference in behaviour among the A. baumannii strains was observed with the clone I and II strains being most virulent, whereas the A. baumannii type strain, which is frequently used in virulence studies appeared harmless.