Colistin resistance in Acinetobacter baumannii is mediated by complete loss of lipopolysaccharide production

Lipopolysaccharide biogenesis and transport at the outer membrane of Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer containing a unique glycolipid, lipopolysaccharide (LPS) in its outer leaflet. LPS molecules confer to the OM peculiar permeability barrier properties enabling Gram-negative bacteria to exclude many toxic compounds, including clinically useful antibiotics, and to survive harsh environments. Transport of LPS poses several problems to the cells due to the amphipatic nature of this molecule. In this review we summarize the current knowledge on the LPS transport machinery, discuss the challenges associated with this process and present the solutions that bacterial cells have evolved to address the problem of LPS transport and assembly at the cell surface. Finally, we discuss how knowledge on LPS biogenesis can be translated for the development of novel antimicrobial therapies. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Polymyxin: Alternative Mechanisms of Action and Resistance

Antibiotic resistance among pathogenic bacteria is an ever-increasing issue worldwide. Unfortunately, very little has been achieved in the pharmaceutical industry to combat this problem. This has led researchers and the medical field to revisit past drugs that were deemed too toxic for clinical use. In particular, the cyclic cationic peptides polymyxin B and colistin, which are specific for Gram-negative bacteria, have been used as "last resort" antimicrobials. Before the 1980s, these drugs were known for their renal and neural toxicities; however, new clinical practices and possibly improved manufacturing have made them safer to use. Previously suggested to primarily attack the membranes of Gram-negative bacteria and to not easily select for resistant mutants, recent research exploring resistance and mechanisms of action has provided new perspectives. This review focuses primarily on the proposed alternative mechanisms of action, known resistance mechanisms, and how these support the alternative mechanisms of action.

A penicillin-binding protein inhibits selection of colistin-resistant, lipooligosaccharide-deficient Acinetobacter baumannii

The Gram-negative bacterial outer membrane fortifies the cell against environmental toxins including antibiotics. Unique glycolipids called lipopolysaccharide/lipooligosaccharide (LPS/LOS) are enriched in the cell-surface monolayer of the outer membrane and promote antimicrobial resistance. Colistin, which targets the lipid A domain of LPS/LOS to lyse the cell, is the last-line treatment for multidrug-resistant Gram-negative infections. Lipid A is essential for the survival of most Gram-negative bacteria, but colistin-resistant Acinetobacter baumannii lacking lipid A were isolated after colistin exposure. Previously, strain ATCC 19606 was the only A. baumannii strain demonstrated to subsist without lipid A. Here, we show that other A. baumannii strains can also survive without lipid A, but some cannot, affording a unique model to study endotoxin essentiality. We assessed the capacity of 15 clinical A. baumannii isolates including 9 recent clinical isolates to develop colistin resistance through inactivation of the lipid A biosynthetic pathway, the products of which assemble the LOS precursor. Our investigation determined that expression of the well-conserved penicillin-binding protein (PBP) 1A, prevented LOS-deficient colony isolation. The glycosyltransferase activity of PBP1A, which aids in the polymerization of the peptidoglycan cell wall, was lethal to LOS-deficient A. baumannii Global transcriptomic analysis of a PBP1A-deficient mutant and four LOS-deficient A. baumannii strains showed a concomitant increase in transcription of lipoproteins and their transporters. Examination of the LOS-deficient A. baumannii cell surface demonstrated that specific lipoproteins were overexpressed and decorated the cell surface, potentially compensating for LOS removal. This work expands our knowledge of lipid A essentiality and elucidates a drug resistance mechanism.

Toxic Accumulation of LPS Pathway Intermediates Underlies the Requirement of LpxH for Growth of Acinetobacter baumannii ATCC 19606

The lipid A moiety of lipopolysaccharide (LPS) is the main constituent of the outer leaflet of the Gram-negative bacterial outer membrane (OM) and is essential in many Gram-negative pathogens. An exception is Acinetobacter baumannii ATCC 19606, where mutants lacking enzymes occurring early in lipid A biosynthesis (LpxA, LpxC or LpxD), and correspondingly lacking LPS, can grow. In contrast, we show here that LpxH, an enzyme that occurs downstream of LpxD in the lipid A biosynthetic pathway, is essential for growth in this strain. Multiple attempts to disrupt lpxH on the genome were unsuccessful, and when LpxH expression was controlled by an isopropyl β-d-1-thiogalactopyranoside (IPTG) inducible promoter, cell growth under typical laboratory conditions required IPTG induction. Mass spectrometry analysis of cells shifted from LpxH-induced to uninduced (and whose growth was correspondingly slowing as LpxH was depleted) showed a large cellular accumulation of UDP-2,3-diacyl-GlcN (substrate of LpxH), a C14:0(3-OH) acyl variant of the LpxD substrate (UDP-3-O-[(R)-3-OH-C₁₄]-GlcN), and disaccharide 1-monophosphate (DSMP). Furthermore, the viable cell counts of the LpxH depleted cultures dropped modestly, and electron microscopy revealed clear defects at the cell (inner) membrane, suggesting lipid A intermediate accumulation was toxic. Consistent with this, blocking the synthesis of these intermediates by inhibition of the upstream LpxC enzyme using CHIR-090 abrogated the requirement for IPTG induction of LpxH. Taken together, these data indicate that LpxH is essential for growth in A. baumannii ATCC19606, because, unlike earlier pathway steps like LpxA or LpxC, blockage of LpxH causes accumulation of detergent-like pathway intermediates that prevents cell growth.

Molecular mechanisms of polymyxin resistance: knowns and unknowns

Colistin, also referred to as polymyxin E, is an effective antibiotic against most multidrug-resistant Gram-negative bacteria and is currently used as a last-line drug for treating severe bacterial infections. Colistin resistance has increased gradually for the last few years, and knowledge of its multifaceted mechanisms is expanding. This includes the newly discovered plasmid-mediated colistin resistance gene mcr-1, which has been detected in over 20 countries within 3 months of its first report. We previously reported all of the known mechanisms of polymyxin resistance in our first review in 2014, but an update seems necessary in 2016, considering the significant recent discoveries that have been made in this domain. This review provides an update about what is already known, what is new, and some unresolved questions with respect to colistin resistance.

Unstable tandem gene amplification generates heteroresistance (variation in resistance within a population) to colistin in Salmonella enterica

Heteroresistance, a phenomenon where subpopulations of a bacterial isolate exhibit different susceptibilities to an antibiotic, is a growing clinical problem where the underlying genetic mechanisms in most cases remain unknown. We isolated colistin resistant mutants in Escherichia coli and Salmonella enterica serovar Typhimurium at different concentrations of colistin. Genetic analysis showed that genetically stable pmrAB point mutations were responsible for colistin resistance during selection at high drug concentrations for both species and at low concentrations for E. coli. In contrast, for S. Typhimurium mutants selected at low colistin concentrations, amplification of different large chromosomal regions conferred a heteroresistant phenotype. All amplifications included the pmrD gene, which encodes a positive regulator that up-regulates proteins that modify lipid A, and as a result increase colistin resistance. Inactivation and over-expression of the pmrD gene prevented and conferred resistance, respectively, demonstrating that the PmrD protein is required and sufficient to confer resistance. The heteroresistance phenotype is explained by the variable gene dosage of pmrD in a population, where sub-populations with different copy number of the pmrD gene show different levels of colistin resistance. We propose that variability in gene copy number of resistance genes can explain the heteroresistance observed in clinically isolated pathogenic bacteria.

Inhibition of LpxC Increases Antibiotic Susceptibility in Acinetobacter baumannii

LpxC inhibitors have generally shown poor in vitro activity against Acinetobacter baumannii We show that the LpxC inhibitor PF-5081090 inhibits lipid A biosynthesis, as determined by silver staining and measurements of endotoxin levels, and significantly increases cell permeability. The presence of PF-5081090 at 32 mg/liter increased susceptibility to rifampin, vancomycin, azithromycin, imipenem, and amikacin but had no effect on susceptibility to ciprofloxacin and tigecycline. Potentiating existing antibiotics with LpxC inhibitors may represent an alternative treatment strategy for multidrug-resistant A. baumannii.

Modulation of virulence in Acinetobacter baumannii cells surviving photodynamic treatment with toluidine blue

INTRODUCTION

Widespread resistance to antimicrobial agents has led to a dearth of therapeutic choices in treating Acinetobacter baumannii infections, leading to new strategies for treatment being needed. We evaluated the effects of photodynamic therapy (PDT) as an alternative antimicrobial modality on the virulence features of cell-surviving PDT.

MATERIALS AND METHODS

To determine the sublethal PDT (sPDT), a colistin-resistant, extensively drug-resistant A. baumannii (CR-XDR-AB) clinical isolate and A. baumannii and ATCC 19606 strains, photosensitized with toluidine blue O (TBO), were irradiated with light emitting diodes, following bacterial viability measurements. The biofilm formation ability, outer membrane (OM) integrity, and antimicrobial susceptibility profiles were assessed for cell-surviving PDT. The effects of sPDT on the expression of virulent genes were evaluated by real-time quantitative reverse transcription PCR (qRT-PCR).

RESULTS

sPDT resulted in the reduction of the biofilm formation capacity, and its metabolic activity in strains. The OM permeability and efflux pump inhibition of the sPDT-treated CR-XDR-AB cells were increased; however, there was no significant change in OM integrity in ATCC 19606 strain after sPDT. sPDT reduced the minimum inhibitory concentrations of the most tested antimicrobials by ≥2-fold in CR-XDR-AB. lpsB, blsA, and dnaK were upregulated after the strains were treated with sPDT; however, a reduction in the expression of csuE, epsA, and abaI was observed in the treated strains after sPDT.

CONCLUSION

The susceptibility of CR-XDR-AB to a range of antibiotics was enhanced following sPDT. The virulence of strains is reduced in cells surviving PDT with TBO, and this may have potential implications of PDT for the treatment of A. baumannii infections.

An "Unlikely" Pair: The Antimicrobial Synergy of Polymyxin B in Combination with the Cystic Fibrosis Transmembrane Conductance Regulator Drugs KALYDECO and ORKAMBI

Novel combination therapies are desperately needed for combating lung infections caused by bacterial "superbugs". This study aimed to investigate the synergistic antibacterial activity of polymyxin B in combination with the cystic fibrosis (CF) drugs KALYDECO (ivacaftor) and ORKAMBI (ivacaftor + lumacaftor) against Gram-negative pathogens that commonly colonize the CF lung, in particular, the problematic Pseudomonas aeruginosa. The in vitro synergistic activity of polymyxin B combined with ivacaftor or lumacaftor was assessed using checkerboard and static time-kill assays against a panel of polymyxin-susceptible and polymyxin-resistant P. aeruginosa isolates from the lungs of CF patients. Polymyxin B, ivacaftor, and lumacaftor were ineffective when used individually against polymyxin-resistant (MIC ≥ 4 mg/L) isolates. However, when used together, the combination of clinically relevant concentrations of polymyxin B (2 mg/L) combined with ivacaftor (8 mg/L) or ivacaftor (8 mg/L) + lumacaftor (8 mg/L) displayed synergistic killing activity against polymyxin-resistant P. aeruginosa isolates as demonstrated by a 100-fold decrease in the bacterial count (CFU/mL) even after 24 h. The combinations also displayed excellent antibacterial activity against P. aeruginosa under CF relevant conditions in a sputum medium assay. The combination of lumacaftor (alone) with polymyxin B showed additivity against P. aeruginosa. The potential antimicrobial mode of action of the combinations against P. aeruginosa was investigated using different methods. Treatment with the combinations induced cytosolic GFP release from P. aeruginosa cells and showed permeabilizing activity in the nitrocefin assay, indicating damage to both the outer and inner Gram-negative cell membranes. Moreover, scanning and transmission electron micrographs revealed that the combinations produce outer membrane damage to P. aeruginosa cells that is distinct from the effect of each compound per se. Ivacaftor was also shown to be a weak inhibitor of the bacterial DNA gyrase and topoisomerase IV with no effect on either human type I or type IIα topoisomerases. Lumacaftor displayed the ability to increase the cellular production of damaging reactive oxygen species. In summary, the combination of polymyxin B with KALYDECO or ORKAMBI exhibited synergistic activity against highly polymyxin-resistant P. aeruginosa CF isolates and can be potentially useful for otherwise untreatable CF lung infections.

Lipopolysaccharide transport to the cell surface: periplasmic transport and assembly into the outer membrane

Gram-negative bacteria possess an outer membrane (OM) containing lipopolysaccharide (LPS). Proper assembly of the OM not only prevents certain antibiotics from entering the cell, but also allows others to be pumped out. To assemble this barrier, the seven-protein lipopolysaccharide transport (Lpt) system extracts LPS from the outer leaflet of the inner membrane (IM), transports it across the periplasm and inserts it selectively into the outer leaflet of the OM. As LPS is important, if not essential, in most Gram-negative bacteria, the LPS biosynthesis and biogenesis pathways are attractive targets in the development of new classes of antibiotics. The accompanying paper (Simpson BW, May JM, Sherman DJ, Kahne D, Ruiz N. 2015 Phil. Trans. R. Soc. B 370, 20150029. (doi:10.1098/rstb.2015.0029)) reviewed the biosynthesis of LPS and its extraction from the IM. This paper will trace its journey across the periplasm and insertion into the OM.

Antibiotic combinations for controlling colistin-resistant Enterobacter cloacae

Enterobacter cloacae is a Gram-negative bacterium associated with high morbidity and mortality in intensive care patients due to its resistance to multiple antibiotics. Currently, therapy against multi-resistant bacteria consists of using colistin, in spite of its toxic effects at higher concentrations. In this context, colistin-resistant E. cloacae strains were challenged with lower levels of colistin combined with other antibiotics to reduce colistin-associated side effects. Colistin-resistant E. cloacae (ATCC 49141) strains were generated by serial propagation in subinhibitory colistin concentrations. After this, three colistin-resistant and three nonresistant replicates were isolated. The identity of all the strains was confirmed by MALDI-TOF MS, VITEK 2 and MicroScan analysis. Furthermore, cross-resistance to other antibiotics was checked by disk diffusion and automated systems. The synergistic effects of the combined use of colistin and chloramphenicol were observed via the broth microdilution checkerboard method. First, data here reported showed that all strains presented intrinsic resistance to penicillin, cephalosporin (except fourth generation), monobactam, and some associations of penicillin and β-lactamase inhibitors. Moreover, a chloramphenicol and colistin combination was capable of inhibiting the induced colistin-resistant strains as well as two colistin-resistant clinical strains. Furthermore, no cytotoxic effect was observed by using such concentrations. In summary, the data reported here showed for the first time the possible therapeutic use of colistin-chloramphenicol for infections caused by colistin-resistant E. cloacae.

Lipopolysaccharide transport to the cell surface: biosynthesis and extraction from the inner membrane

The cell surface of most Gram-negative bacteria is covered with lipopolysaccharide (LPS). The network of charges and sugars provided by the dense packing of LPS molecules in the outer leaflet of the outer membrane interferes with the entry of hydrophobic compounds into the cell, including many antibiotics. In addition, LPS can be recognized by the immune system and plays a crucial role in many interactions between bacteria and their animal hosts. LPS is synthesized in the inner membrane of Gram-negative bacteria, so it must be transported across their cell envelope to assemble at the cell surface. Over the past two decades, much of the research on LPS biogenesis has focused on the discovery and understanding of Lpt, a multi-protein complex that spans the cell envelope and functions to transport LPS from the inner membrane to the outer membrane. This paper focuses on the early steps of the transport of LPS by the Lpt machinery: the extraction of LPS from the inner membrane. The accompanying paper (May JM, Sherman DJ, Simpson BW, Ruiz N, Kahne D. 2015 Phil. Trans. R. Soc. B 370, 20150027. (doi:10.1098/rstb.2015.0027)) describes the subsequent steps as LPS travels through the periplasm and the outer membrane to its final destination at the cell surface.

Colistin and tigecycline resistance in carbapenemase-producing Gram-negative bacteria: emerging resistance mechanisms and detection methods

A literature review was undertaken to ascertain the molecular basis for tigecycline and colistin resistance mechanisms and the experimental basis for the detection and delineation of this resistance particularly in carbapenemase-producing Gram-negative bacteria. Pubmed, Google Scholar and Science Direct were searched with the keywords colistin, tigecycline, resistance mechanisms and detection methods. Trans-complementation and comparative MIC studies, mass spectrometry, chromatography, spectrofluorometry, PCR, qRT-PCR and whole genome sequencing (WGS) were commonly used to determine tigecycline and colistin resistance mechanisms, specifically modifications in the structural and regulatory efflux (acrAB, OqxAB, kpgABC adeABC-FGH-IJK, mexAB-XY-oprJM and soxS, rarA robA, ramRAB marRABC, adeLRS, mexRZ and nfxb) and lipid A (pmrHFIJFKLM, lpxA, lpxC lpxD and mgrB, pmrAB, phoPQ,) genes respectively. Mutations in the ribosomal 16S rRNA operon rrnBC, also yielded resistance to tigecycline through target site modifications. The mcr-1 gene conferring resistance to colistin was identified via WGS, trans-complementation and a murine thigh infection model studies. Common detection methods are mainly antibiotic sensitivity testing with broth microdilution while molecular identification tools are mostly PCR and WGS. Spectrofluorometry, MALDI-TOF MS, micro-array and real-time multiplex PCR hold much promise for the future as new detection tools.

Antibacterial Drug Discovery Targeting the Lipopolysaccharide Biosynthetic Enzyme LpxC

The enzyme LpxC (UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase) is broadly conserved across Gram-negative bacteria and is essential for synthesis of lipid A, the membrane anchor of the lipopolysaccharides (LPSs), which are a major component of the outer membrane in nearly all Gram-negative bacteria. LpxC has been the focus of target-directed antibiotic discovery projects in numerous pharmaceutical and academic groups for more than 20 years. Despite intense effort, no LpxC inhibitor has been approved for therapeutic use, and only one has yet reached human studies. This article will summarize the history of LpxC as a drug target and the parallel history of research on LpxC biology. Both academic and industrial researchers have used LpxC inhibitors as tool compounds, leading to increased understanding of the differing mechanisms for regulation of LPS synthesis in Escherichia coli and Pseudomonas aeruginosa.

The Power of Asymmetry: Architecture and Assembly of the Gram-Negative Outer Membrane Lipid Bilayer

Determining the chemical composition of biological materials is paramount to the study of natural phenomena. Here, we describe the composition of model gram-negative outer membranes, focusing on the predominant assembly, an asymmetrical bilayer of lipid molecules. We also give an overview of lipid biosynthetic pathways and molecular mechanisms that organize this material into the outer membrane bilayer. An emphasis is placed on the potential of these pathways as targets for antibiotic development. We discuss deviations in composition, through bacterial cell surface remodeling, and alternative modalities to the asymmetric lipid bilayer. Outer membrane lipid alterations of current microbiological interest, such as lipid structures found in commensal bacteria, are emphasized. Additionally, outer membrane components could potentially be engineered to develop vaccine platforms. Observations related to composition and assembly of gram-negative outer membranes will continue to generate novel discoveries, broaden biotechnologies, and reveal profound mysteries to compel future research.

The induction and identification of novel Colistin resistance mutations in Acinetobacter baumannii and their implications

Acinetobacter baumannii is a significant cause of opportunistic hospital acquired infection and has been identified as an important emerging infection due to its high levels of antimicrobial resistance. Multidrug resistant A. baumannii has risen rapidly in Vietnam, where colistin is becoming the drug of last resort for many infections. In this study we generated spontaneous colistin resistant progeny (up to >256 μg/μl) from four colistin susceptible Vietnamese isolates and one susceptible reference strain (MIC <1.5 μg/μl). Whole genome sequencing was used to identify single nucleotide mutations that could be attributed to the reduced colistin susceptibility. We identified six lpxACD and three pmrB mutations, the majority of which were novel. In addition, we identified further mutations in six A. baumannii genes (vacJ, pldA, ttg2C, pheS and conserved hypothetical protein) that we hypothesise have a role in reduced colistin susceptibility. This study has identified additional mutations that may be associated with colistin resistance through novel resistance mechanisms. Our work further demonstrates how rapidly A. baumannii can generate resistance to a last resort antimicrobial and highlights the need for improved surveillance to identified A. baumannii with an extensive drug resistance profile.

Synergy between Colistin and the Signal Peptidase Inhibitor MD3 Is Dependent on the Mechanism of Colistin Resistance in Acinetobacter baumannii

Synergy between colistin and the signal peptidase inhibitor MD3 was tested against isogenic mutants and clinical pairs of Acinetobacter baumannii isolates. Checkerboard assays and growth curves showed synergy against both colistin-susceptible strains (fractional inhibitory concentration index [FICindex] = 0.13 to 0.24) and colistin-resistant strains with mutations in pmrB and phosphoethanolamine modification of lipid A (FICindex = 0.14 to 0.25) but not against colistin-resistant Δlpx strains with loss of lipopolysaccharide (FICindex = 0.75 to 1). A colistin/MD3 combination would need to be targeted to strains with specific colistin resistance mechanisms.

Colistin and Polymyxin B Dosage Regimens against Acinetobacter baumannii: Differences in Activity and the Emergence of Resistance

Infections caused by multidrug-resistant Acinetobacter baumannii are a major public health problem, and polymyxins are often the last line of therapy for recalcitrant infections by such isolates. The pharmacokinetics of the two clinically used polymyxins, polymyxin B and colistin, differ considerably, since colistin is administered as an inactive prodrug that undergoes slow conversion to colistin. However, the impact of these substantial pharmacokinetic differences on bacterial killing and resistance emergence is poorly understood. We assessed clinically relevant polymyxin B and colistin dosage regimens against one reference and three clinical A. baumannii strains in a dynamic one-compartment in vitro model. A new mechanism-based pharmacodynamic model was developed to describe and predict the drug concentrations and viable counts of the total and resistant populations. Rapid attainment of target concentrations was shown to be critical for polymyxin-induced bacterial killing. All polymyxin B regimens achieved peak concentrations of at least 1 mg/liter within 1 h and caused ≥4 log10 killing at 1 h. In contrast, the slow rise of colistin concentrations to 3 mg/liter over 48 h resulted in markedly reduced bacterial killing. A significant (4 to 6 log10 CFU/ml) amplification of resistant bacterial populations was common to all dosage regimens. The developed mechanism-based model explained the observed bacterial killing, regrowth, and resistance. The model also implicated adaptive polymyxin resistance as a key driver of bacterial regrowth and predicted the amplification of preexisting, highly polymyxin-resistant bacterial populations following polymyxin treatment. Antibiotic combination therapies seem the most promising option for minimizing the emergence of polymyxin resistance.

Paradoxical Effect of Polymyxin B: High Drug Exposure Amplifies Resistance in Acinetobacter baumannii

Administering polymyxin antibiotics in a traditional fashion may be ineffective against Gram-negative ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens. Here, we explored increasing the dose intensity of polymyxin B against two strains of Acinetobacter baumannii in the hollow-fiber infection model. The following dosage regimens were simulated for polymyxin B (t1/2 = 8 h): non-loading dose (1.43 mg/kg of body weight every 12 h [q12h]), loading dose (2.22 mg/kg q12h for 1 dose and then 1.43 mg/kg q12h), front-loading dose (3.33 mg/kg q12h for 1 dose followed by 1.43 mg/kg q12h), burst (5.53 mg/kg for 1 dose), and supraburst (18.4 mg/kg for 1 dose). Against both A. baumannii isolates, a rapid initial decline in the total population was observed within the first 6 h of polymyxin exposure, whereby greater polymyxin B exposure resulted in greater maximal killing of -1.25, -1.43, -2.84, -2.84, and -3.40 log10 CFU/ml within the first 6 h. Unexpectedly, we observed a paradoxical effect whereby higher polymyxin B exposures dramatically increased resistant subpopulations that grew on agar containing up to 10 mg/liter of polymyxin B over 336 h. High drug exposure also proliferated polymyxin-dependent growth. A cost-benefit pharmacokinetic/pharmacodynamic relationship between 24-h killing and 336-h resistance was explored. The intersecting point, where the benefit of bacterial killing was equal to the cost of resistance, was an fAUC0-24 (area under the concentration-time curve from 0 to 24 h for the free, unbound fraction of drug) of 38.5 mg · h/liter for polymyxin B. Increasing the dose intensity of polymyxin B resulted in amplification of resistance, highlighting the need to utilize polymyxins as part of a combination against high-bacterial-density A. baumannii infections.

Proteomic characterization of Nα- and Nε-acetylation in Acinetobacter baumannii

Nα- and Nε-acetylation represent a pivotal post-translational modification used by both eukaryotes and prokaryotes to modulate diverse biological processes. Acinetobacter baumannii has been described as an important nosocomial pathogen for the past 30 years, frequently involved in ventilator-associated pneumonia, bloodstream and urinary tract infections. Many aspects of the biology of A. baumannii remain elusive, in particular the extent and function of N-acetylation. We investigated here N-acetylation in A. baumannii strain ATCC 17978 by proteomic analysis, and we showed the usefulness of using different analytical approaches. Overall, we identified 525 N-acetylated proteins in which, 145 were Nα-acetylated and 411 were Nε-acetylated. Among them, 41 proteins carried both types of N-acetylation. We found that N-acetylation may play a role in biofilm formation, bacterial virulence (e.g. in several iron acquisition pathways), as well as a number of phenotypes, such as, stress adaptation and drug resistance.

BIOLOGICAL SIGNIFICANCE

This study is the first to perform the N-acetylome of A. baumannii using different analytical approaches. Each analytical tool permitted to characterize distinctive modified peptides. The combination of all these methods allowed us to identify 145 and 411 Nα- and Nε-acetylated proteins. Besides the fact that acetylation was involved in central metabolism as previously described in other bacteria, some N-acetylated proteins showed interesting role in bacterial virulence (iron acquisition), biofilm formation, stress adaptation and drug resistance of A. baumannii.

Polymyxin Resistance in Acinetobacter baumannii: Genetic Mutations and Transcriptomic Changes in Response to Clinically Relevant Dosage Regimens

Polymyxins are often last-line therapeutic agents used to treat infections caused by multidrug-resistant A. baumannii. Recent reports of polymyxin-resistant A. baumannii highlight the urgent need for research into mechanisms of polymyxin resistance. This study employed genomic and transcriptomic analyses to investigate the mechanisms of polymyxin resistance in A. baumannii AB307-0294 using an in vitro dynamic model to mimic four different clinically relevant dosage regimens of polymyxin B and colistin over 96 h. Polymyxin B dosage regimens that achieved peak concentrations above 1 mg/L within 1 h caused significant bacterial killing (~5 log₁₀CFU/mL), while the gradual accumulation of colistin resulted in no bacterial killing. Polymyxin resistance was observed across all dosage regimens; partial reversion to susceptibility was observed in 6 of 8 bacterial samples during drug-free passaging. Stable polymyxin-resistant samples contained a mutation in pmrB. The transcriptomes of stable and non-stable polymyxin-resistant samples were not substantially different and featured altered expression of genes associated with outer membrane structure and biogenesis. These findings were further supported via integrated analysis of previously published transcriptomics data from strain ATCC19606. Our results provide a foundation for understanding the mechanisms of polymyxin resistance following exposure to polymyxins and the need to explore effective combination therapies.

Colistin: Revival of an Old Polymyxin Antibiotic

Colistin (polymyxin E) is a positively charged deca-peptide antibiotic that disrupts the integrity of the outer membrane of the cell wall of gram-negative bacteria by binding to the lipid A moiety of lipopolysaccharides, resulting in cell death. The endotoxic activity of lipopolysaccharides is simultaneously inhibited. Colistin is increasingly being prescribed as rescue treatment for infections with multidrug-resistant bacilli. Nephrotoxicity and, to a lesser degree, neurotoxicity occur often during systemic colistin therapy, and have severely limited its application in the past. However, these side effects are largely reversible and can be managed through close monitoring. The prodrug colistimethate sodium (CMS) is less toxic and is, therefore, the preferred formulation for parenteral administration. Importantly, resistance to colistin seems to emerge often unless it is combined with another antibiotic, but further studies into this phenomenon are necessary. Pharmacokinetic and pharmacodynamic properties have received little attention, partly because of the physicochemical peculiarities of polymyxin antibiotics, especially their propensity to stick to other molecules and surfaces. The ratio between the area under the curve of free colistin and the pathogen's Minimal Inhibitory Concentration (MIC) best predicts microbiological and clinical responses, but more studies are needed in this area. Likewise, further standardization is needed in production and labeling of colistin formulations, and in the way the susceptibility of bacteria to colistin is determined.

Evolved resistance to colistin and its loss due to genetic reversion in Pseudomonas aeruginosa

The increased reliance on colistin for treating multidrug-resistant Gram-negative bacterial infections has resulted in the emergence of colistin-resistant Pseudomonas aeruginosa. We attempted to identify genetic contributors to colistin resistance in vitro evolved isogenic colistin-resistant and -susceptible strains of two P. aeruginosa lineages (P5 and P155). Their evolutionary paths to acquisition and loss of colistin resistance were also tracked. Comparative genomic analysis revealed 13 and five colistin resistance determinants in the P5 and P155 lineages, respectively. Lipid A in colistin-resistant mutants was modified through the addition of 4-amino-L-arabinose; this modification was absent in colistin-susceptible revertant strains. Many amino acid substitutions that emerged during the acquisition of colistin resistance were reversed in colistin-susceptible revertants. We demonstrated that evolved colistin resistance in P. aeruginosa was mediated by a complicated regulatory network that likely emerges through diverse genetic alterations. Colistin-resistant P. aeruginosa became susceptible to the colistin upon its withdrawal because of genetic reversion. The mechanisms through which P. aeruginosa acquires and loses colistin resistance have implications on the treatment options that can be applied against P. aeruginosa infections, with respect to improving bactericidal efficacy and preventing further resistance to antibiotics.

Impaired growth under iron-limiting conditions associated with the acquisition of colistin resistance in Acinetobacter baumannii

Acquisition of colistin resistance in Acinetobacter baumannii has been associated with reduced bacterial fitness and virulence, although the mechanisms underlying this fitness loss have not been well characterised. In this study, the role played by environmental iron levels on the growth and survival of colistin-resistant strains of A. baumannii was assessed. Growth assays with the colistin-susceptible ATCC 19606 strain and its colistin-resistant derivative RC64 [colistin minimum inhibitory concentration (MIC) of 64 mg/L] demonstrated that the strains grew similarly in rich laboratory medium (Mueller-Hinton broth), whereas RC64 demonstrated impaired growth compared with ATCC 19606 in human serum (>100-fold at 24 h). Compared with RC64, ATCC 19606 grew in the presence of higher concentrations of the iron-specific chelator 2,2'-bipyridine and grew more readily under iron-limiting conditions in solid and liquid media. In addition, iron supplementation of human serum increased the growth of RC64 compared with unsupplemented human serum to a greater extent than ATCC 19606. The ability of 11 colistin-resistant clinical isolates with mutations in the pmrB gene to grow in iron-replete and iron-limiting conditions was assessed, demonstrating that eight of the strains showed reduced growth under iron limitation. Individual mutations in the pmrB gene did not directly correlate with a decreased capacity for growth under iron limitation, suggesting that mutations in pmrB may not directly produce this phenotype. Together these results indicate that acquisition of colistin resistance in A. baumannii can be associated with a decreased ability to grow in low-iron environments.

Acinetobacter infections prevalence and frequency of the antibiotics resistance: comparative study of intensive care units versus other hospital units

INTRODUCTION

This study aims to determine the Acinetobacter sp clinical isolates frequency and its antibiotic susceptibility pattern by comparing results obtained from the Intensive Care Units (ICUs) to that of other units at the Mohammed V Military Teaching Hospital in Rabat.

METHODS

This is a retrospective study over a 2-years period where we collected all clinical isolates of Acinetobacter sp obtained from samples for infection diagnosis performed on hospitalized patients between 2012 to 2014.

RESULTS

During the study period, 441 clinical and non-repetitive isolates of Acinetobacter sp were collected representing 6.94% of all bacterial clinical isolates (n = 6352) and 9.6% of Gram negative rods (n = 4569). More than a half of the isolates were from the ICUs and were obtained from 293 infected patients of which 65, 2% (191 cases) were males (sex ratio = 1.9) and the median age was 56 years (interquartile range: 42-68 years). Acinetobacter clinical isolates were obtained from respiratory samples (44.67%) followed by blood cultures (14.51%). The resistance to ciprofloxacin, ceftazidime, piperacillin / tazobactam, imipenem, amikacin, tobramycin, netilmicin, rifampicin and colistin was respectively 87%, 86%, 79%, 76%; 52%, 43%, 33% 32% and 1.7%. The difference in resistance between the ICUs and the other units was statistically significant (p <0.05) except for colistin, tetracycline and rifampicin.

CONCLUSION

This paper shows that solving the problem of prevalence and high rate of multidrug resistant Acinetobacter infection which represents a therapeutic impasse, requires the control of the hospital environment and optimizing hands hygiene and antibiotics use in the hospital.

Functional Genome Screening to Elucidate the Colistin Resistance Mechanism

Antibiogram profile of 1590 clinical bacterial isolates based on thirteen different antimicrobial compounds showed that 1.6% of the bacterial isolates are multidrug resistant. Distribution pattern based on 16S rRNA sequence analysis showed that Pseudomonas aeruginosa constituted the largest group (83.6%) followed by Burkholderia pseudomallei sp. A191 (5.17%), Staphylococcus sp. A261 (3.45%). Among the various antibiotics used, colistin appeared to be the most effective against the Gram negative bacteria. Burkholderia pseudomallei sp. A191 and Pseudomonas aeruginosa sp. A111 showed resistance to 1500 μg/ml and 750 μg/ml of colistin respectively which constitutes 7.7% of the bacterial population. A functional genomics strategy was employed to discover the molecular support for colistin resistance in Burkholderia pseudomallei sp. A191. A pUC plasmid-based genomic expression library was constructed with an estimated library size of 2.1 × 10⁷bp. Five colistin resistant clones were obtained after functional screening of the library. Analysis of DNA sequence of five colistin resistant clones showed homology to two component regularity systems (TCRS) encoding for a histidine kinase (mrgS) and its regulatory component (mrgR). Cross complementation assay showed that mutations in mrgS were sufficient enough to confer colistin resistant phenotype in a sensitive strain.

Genome dynamics of multidrug-resistant Acinetobacter baumannii during infection and treatment

BACKGROUND

Limited treatment options are available for patients infected with multidrug (MDR)- or pan-drug (PDR)-resistant bacterial pathogens, resulting in infections that can persist for weeks or months. In order to better understand transmission and evolutionary dynamics of MDR Acinetobacter baumannii (Ab) during long-term infection, we analyzed genomes from a series of isolates from individual patients at isolate-specific, patient-specific, and population levels.

METHODS

Whole genome analysis of longitudinal isolates (range 2-10 isolates per patient spanning 0-829 days) from 40 patients included detection of single-nucleotide variants (SNVs), insertion sequence (IS) mapping, and gene content changes.

RESULTS

Phylogenetic analysis revealed that a significant fraction of apparently persistent infections are in fact due to re-infection with new strains. SNVs primarily resulted in protein coding changes, and IS events primarily interrupted genes or were in an orientation such that the adjacent gene would be over-expressed. Mutations acquired during infection were over-represented in transcriptional regulators, notably pmrAB and adeRS, which can mediate resistance to the last line therapies colistin and tigecycline, respectively, as well as transporters, surface structures, and iron acquisition genes.

CONCLUSIONS

Most SNVs and IS events were isolate-specific indicating these mutations did not become fixed on the time scale investigated, yet over-representation of independent mutations in some genes or functional categories suggests that they are under selective pressure. Genome analysis at the population-level suggests that gene transfer including recombination also contributes to Ab evolutionary dynamics. These findings provide important insight into the transmission dynamics of Ab and the identification of patients with repeat infections has implications for infection control programs targeted to this pathogen.

Global metabolic analyses identify key differences in metabolite levels between polymyxin-susceptible and polymyxin-resistant Acinetobacter baumannii

Multidrug-resistant Acinetobacter baumannii presents a global medical crisis and polymyxins are used as the last-line therapy. This study aimed to identify metabolic differences between polymyxin-susceptible and polymyxin-resistant A. baumannii using untargeted metabolomics. The metabolome of each A. baumannii strain was measured using liquid chromatography-mass spectrometry. Multivariate and univariate statistics and pathway analyses were employed to elucidate metabolic differences between the polymyxin-susceptible and -resistant A. baumannii strains. Significant differences were identified between the metabolic profiles of the polymyxin-susceptible and -resistant A. baumannii strains. The lipopolysaccharide (LPS) deficient, polymyxin-resistant 19606R showed perturbation in specific amino acid and carbohydrate metabolites, particularly pentose phosphate pathway (PPP) and tricarboxylic acid (TCA) cycle intermediates. Levels of nucleotides were lower in the LPS-deficient 19606R. Furthermore, 19606R exhibited a shift in its glycerophospholipid profile towards increased abundance of short-chain lipids compared to the parent polymyxin-susceptible ATCC 19606. In contrast, in a pair of clinical isolates 03-149.1 (polymyxin-susceptible) and 03-149.2 (polymyxin-resistant, due to modification of lipid A), minor metabolic differences were identified. Notably, peptidoglycan biosynthesis metabolites were significantly depleted in both of the aforementioned polymyxin-resistant strains. This is the first comparative untargeted metabolomics study to show substantial differences in the metabolic profiles of the polymyxin-susceptible and -resistant A. baumannii.

Comparative Genomics of Two ST 195 Carbapenem-Resistant Acinetobacter baumannii with Different Susceptibility to Polymyxin Revealed Underlying Resistance Mechanism

Acinetobacter baumannii is a Gram-negative nosocomial pathogen of importance due to its uncanny ability to acquire resistance to most antimicrobials. These include carbapenems, which are the drugs of choice for treating A. baumannii infections, and polymyxins, the drugs of last resort. Whole genome sequencing was performed on two clinical carbapenem-resistant A. baumannii AC29 and AC30 strains which had an indistinguishable ApaI pulsotype but different susceptibilities to polymyxin. Both genomes consisted of an approximately 3.8 Mbp circular chromosome each and several plasmids. AC29 (susceptible to polymyxin) and AC30 (resistant to polymyxin) belonged to the ST195 lineage and are phylogenetically clustered under the International Clone II (IC-II) group. An AbaR4-type resistance island (RI) interrupted the comM gene in the chromosomes of both strains and contained the bla OXA-23 carbapenemase gene and determinants for tetracycline and streptomycin resistance. AC29 harbored another copy of bla OXA-23 in a large (~74 kb) conjugative plasmid, pAC29b, but this gene was absent in a similar plasmid (pAC30c) found in AC30. A 7 kb Tn1548::armA RI which encodes determinants for aminoglycoside and macrolide resistance, is chromosomally-located in AC29 but found in a 16 kb plasmid in AC30, pAC30b. Analysis of known determinants for polymyxin resistance in AC30 showed mutations in the pmrA gene encoding the response regulator of the two-component pmrAB signal transduction system as well as in the lpxD, lpxC, and lpsB genes that encode enzymes involved in the biosynthesis of lipopolysaccharide (LPS). Experimental evidence indicated that impairment of LPS along with overexpression of pmrAB may have contributed to the development of polymyxin resistance in AC30. Cloning of a novel variant of the bla AmpC gene from AC29 and AC30, and its subsequent expression in E. coli also indicated its likely function as an extended-spectrum cephalosporinase.

Highly potent antimicrobial peptide derivatives of bovine cateslytin

The antimicrobial activity of bovine cateslytin was optimized in a series of sequential modifications leading to synlytin, a highly potent non-haemolytic antimicrobial peptide.

Antibacterial Low Molecular Weight Cationic Polymers: Dissecting the Contribution of Hydrophobicity, Chain Length and Charge to Activity

The balance of cationicity and hydrophobicity can profoundly affect the performance of antimicrobial polymers. To this end a library of 24 cationic polymers with uniquely low degrees of polymerization was synthesized via Cu(0)-mediated polymerization, using three different cationic monomers and two initiators: providing two different hydrocarbon chain tail lengths (C2 and C12). The polymers exhibited structure-dependent antibacterial activity when tested against a selection of bacteria, viz, Staphylococcus aureus ATCC 29213, Klebsiella pneumoniae ATCC 13883, Acinetobacter baumannii ATCC 19606, and Pseudomonas aeruginosa ATCC 27853 as a representative palette of Gram-positive and Gram-negative ESKAPE pathogens. The five best-performing polymers were identified for additional testing against the polymyxin-resistant A. baumannii ATCC 19606R strain. Polymers having the lowest DP and a C12 hydrophobic tail were shown to provide the broadest antimicrobial activity against the bacteria panel studied as evidenced by lower minimum inhibitory concentrations (MICs). An optimal polymer composition was identified, and its mechanism of action investigated via membrane permeability testing against Escherichia coli. Membrane disruption was identified as the most probable mechanism for bacteria cell killing.

Tackling Threats and Future Problems of Multidrug-Resistant Bacteria

With the advent of the antibiotic era, the overuse and inappropriate consumption and application of antibiotics have driven the rapid emergence of multidrug-resistant pathogens. Antimicrobial resistance increases the morbidity, mortality, length of hospitalization and healthcare costs. Among Gram-positive bacteria, Staphylococcus aureus (MRSA) and multidrug-resistant (MDR) Mycobacterium tuberculosis, and among the Gram-negative bacteria, extended-spectrum beta-lactamase (ESBLs)-producing bacteria have become a major global healthcare problem in the 21st century. The pressure to use antibiotics guarantees that the spread and prevalence of these as well as of future emerging multidrug-resistant pathogens will be a persistent phenomenon. The unfeasibility of reversing antimicrobial resistance back towards susceptibility and the critical need to treat bacterial infection in modern medicine have burdened researchers and pharmaceutical companies to develop new antimicrobials effective against these difficult-to-treat multidrug-resistant pathogens. However, it can be anticipated that antibiotic resistance will continue to develop more rapidly than new agents to treat these infections become available and a better understanding of the molecular, evolutionary and ecological mechanisms governing the spread of antibiotic resistance is needed. The only way to curb the current crisis of antimicrobial resistance will be to develop entirely novel strategies to fight these pathogens such as combining antimicrobial drugs with other agents that counteract and obstruct the antibiotic resistant mechanisms expressed by the pathogen. Furthermore, as many antibiotics are often inappropriately prescribed, a more personalized approach based on precise diagnosis tools will ensure that proper treatments can be promptly applied leading to more targeted and effective therapies. However, in more general terms, also the overall use and release of antibiotics in the environment needs to be better controlled.

Triple-targeting Gram-negative selective antimicrobial peptides capable of disrupting the cell membrane and lipid A biosynthesis

A potent Gram-negative selective triple-targeting antimicrobial peptide was developed that attacks the membrane and inhibits two enzymes involved in early lipid A biosynthesis.

Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

The genus Acinetobacter encompasses multiple nosocomial opportunistic pathogens that are of increasing worldwide relevance because of their ability to survive exposure to various antimicrobial and sterilization agents. Among these, Acinetobacter baumannii, Acinetobacter nosocomialis, and Acinetobacter pittii are the most frequently isolated in hospitals around the world. Despite the growing incidence of multidrug-resistant Acinetobacter spp., little is known about the factors that contribute to pathogenesis. New strategies for treating and managing infections caused by multidrug-resistant Acinetobacter strains are urgently needed, and this requires a detailed understanding of the pathobiology of these organisms. In recent years, some virulence factors important for Acinetobacter colonization have started to emerge. In this review, we focus on several recently described virulence factors that act at the bacterial surface level, such as the capsule, O-linked protein glycosylation, and adhesins. Furthermore, we describe the current knowledge regarding the type II and type VI secretion systems present in these strains.

Anthelmintic closantel enhances bacterial killing of polymyxin B against multidrug-resistant Acinetobacter baumannii

Polymyxins, an old class of antibiotics, are currently used as the last resort for the treatment of multidrug-resistant (MDR) Acinetobacter baumannii. However, recent pharmacokinetic and pharmacodynamic data indicate that monotherapy can lead to the development of resistance. Novel approaches are urgently needed to preserve and improve the efficacy of this last-line class of antibiotics. This study examined the antimicrobial activity of novel combination of polymyxin B with anthelmintic closantel against A. baumannii. Closantel monotherapy (16 mg l(-1)) was ineffective against most tested A. baumannii isolates. However, closantel at 4-16 mg l(-1) with a clinically achievable concentration of polymyxin B (2 mg l(-1)) successfully inhibited the development of polymyxin resistance in polymyxin-susceptible isolates, and provided synergistic killing against polymyxin-resistant isolates (MIC ⩾4 mg l(-1)). Our findings suggest that the combination of polymyxin B with closantel could be potentially useful for the treatment of MDR, including polymyxin-resistant, A. baumannii infections. The repositioning of non-antibiotic drugs to treat bacterial infections may significantly expedite discovery of new treatment options for bacterial 'superbugs'.

Characterization of an Acinetobacter baumannii lptD Deletion Strain: Permeability Defects and Response to Inhibition of Lipopolysaccharide and Fatty Acid Biosynthesis

Lipid A on the Gram-negative outer membrane (OM) is synthesized in the cytoplasm by the Lpx pathway and translocated to the OM by the Lpt pathway. Some Acinetobacter baumannii strains can tolerate the complete loss of lipopolysaccharide (LPS) resulting from the inactivation of early LPS pathway genes such as lpxC. Here, we characterized a mutant deleted for lptD, which encodes an OM protein that mediates the final translocation of fully synthesized LPS to the OM. Cells lacking lptD had a growth defect comparable to that of an lpxC deletion mutant under the growth conditions tested but were more sensitive to hydrophobic antibiotics, revealing a more significant impact on cell permeability from impaired LPS translocation than from the loss of LPS synthesis. Consistent with this, ATP leakage and N-phenyl-1-naphthylamine (NPN) fluorescence assays indicated a more severe impact of lptD deletion than of lpxC deletion on inner and outer membrane permeability, respectively. Targeted liquid chromatography-mass spectrometry (LCMS) analysis of LPS intermediates from UDP-3-O-R-3-hydroxylauroyl-N-acetyl-α-d-glucosamine through lipid IV(A) showed that the loss of LptD caused an accumulation of lipid IV(A). This suggested that pathway intermediate accumulation or mislocalization caused by the blockage of later LPS pathway steps impacts envelope integrity. Supporting this notion, chemical inhibition of lipid A precursor enzymes, including LpxC and FabB/F, in the lptD deletion strain partially rescued growth and permeability defects.

IMPORTANCE

New antibiotics to treat Gram-negative bacterial infections are urgently needed. Inhibition of LPS biosynthesis is attractive because this would impact viability and cell permeability. Therefore, a better understanding of this pathway is important, especially in strains such as A. baumannii ATCC 19606, where LPS biosynthesis is not essential in vitro. We show that ATCC 19606 also survives the loss of the final translocation of LPS into the OM (lptD deletion). Intriguingly, this impaired cell envelope integrity more than the loss of LPS biosynthesis (lpxC deletion), presumably due to the accumulation of toxic intermediates. Supporting this, chemical inhibition of LPS biosynthesis partially reversed this permeability defect. This extends our understanding of the LPS machinery and provides insights into potential interrelationships of the target steps along this important pathway.

Rapid dissemination of colistin and carbapenem resistant Acinetobacter baumannii in Central Greece: mechanisms of resistance, molecular identification and epidemiological data

Background

Colistin-resistant/carbapenem-resistant Acinetobacter baumannii is a significant challenge for antibiotic treatment and infection control policies. Since 2012, in Central Greece an increase of colistin/pan- resistant A. baumannii has occurred, indicating the need for further analysis.

Methods

A total of 86 colistin-resistant/carbapenem-resistant out of 1228 A. baumannii clinical isolates, consecutively collected between 2012 and 2014 in a tertiary Greek hospital of Central Greece, as well as one environmental isolate from surveillance cultures were studied. Molecular typing and mechanisms of resistance to colistin and to carbapenems were assessed, whereas, epidemiological and clinical data of the patients were reviewed.

Results

During the study period, the rate of colistin resistance gradually increased and reached 21.1 % in 2014. All colistin-resistant/carbapenem-resistant A. baumannii belonged to 3LST ST101 clone that corresponds to the international clonal lineage II. Carbapenem resistance was associated with the presence of bla_oxa-23-like, while resistance to colistin probably correlated with G54E and R109H amino acid substitutions in PmrA and PmrC, respectively.

Conclusions

Epidemiological data of the patients indicated that the first detection of colistin-resistant/carbapenem-resistant ST101 clone in the University Hospital of Larissa (UHL) was associated with a patient who previously had received colistin, while, the movement of the infected patients into the hospital probably resulted to its spread.

An Amphipathic Undecapeptide with All d-Amino Acids Shows Promising Activity against Colistin-Resistant Strains of Acinetobacter baumannii and a Dual Mode of Action

Multiple strains of Acinetobacter baumannii have developed multidrug resistance (MDR), leaving colistin as the only effective treatment. The cecropin-α-melittin hybrid BP100 (KKLFKKILKYL-NH2) and its analogs have previously shown activity against a wide array of plant and human pathogens. In this study, we investigated the in vitro antibacterial activities of 18 BP100 analogs (four known and 14 new) against the MDR A. baumannii strain ATCC BAA-1605, as well as against a number of other clinically relevant human pathogens. Selected peptides were further evaluated against strains of A. baumannii that acquired resistance to colistin due to mutations of the lpxC, lpxD, pmrA, and pmrB genes. The novel analogue BP214 showed antimicrobial activity at 1 to 2 μM and a hemolytic 50% effective concentration (EC50) of >150 μM. The lower activity of its enantiomer suggests a dual, specific and nonspecific mode of action. Interestingly, colistin behaved antagonistically to BP214 when pmrAB and lpxC mutants were challenged.

Phoenix dactylifera (date palm) pit aqueous extract mediated novel route for synthesis high stable silver nanoparticles with high antifungal and antibacterial activity

The biological synthesis of silver nanoparticles (AgNPs) was conducted using date palm pit aqueous extract. The first visible sign of the synthesis of AgNPs was the change in colour of reaction mixtures from yellowish to reddish brown. The resulting synthesised AgNPs were characterised using UV-visible spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The UV-visible spectra gave surface plasmon resonance at 428 nm. XRD confirmed that the silver particles formed in our experiments were in the form of nanocrystals. TEM images revealed the formation of AgNPs with spherical shape and sizes in the range between 1-40 nm. DLS showed nanoparticles with an average size of 27 nm. Fourier transform infrared spectroscopy indicated the role of different possible functional groups (carboxyl, amine, aromatic and hydroxyl) in the formation of AgNPs. AgNPs were stable at 28°C in vitro for over a year without any precipitation or decreased production of antimicrobial effect. Then, the antifungal and antibacterial activities of synthesised AgNPs were investigated. The synthesised AgNPs showed significant inhibitory effects on Rhizoctonia solani (AG2_2) cultures, so that the concentration of 25 µg/ml prevented approximately 83% of the mycelium growth of the fungus. Then, the broth macro-dilution method was used for examining antibacterial effect of AgNPs. The minimum inhibitory concentration and minimum bactericide concentration against Klebsiella pneumonia (PCI 602) and Acinetobacter baumannii (ATCC 19606) were recorded as 1.56 and 3.12 µg/ml AgNPs, respectively.

Impact of a Cross-Kingdom Signaling Molecule of Candida albicans on Acinetobacter baumannii Physiology

Multidrug-resistant (MDR) Acinetobacter baumannii is an opportunistic human pathogen that has become highly problematic in the clinical environment. Novel therapies are desperately required. To assist in identifying new therapeutic targets, the antagonistic interactions between A. baumannii and the most common human fungal pathogen, Candida albicans, were studied. We have observed that the C. albicans quorum-sensing molecule, farnesol, has cross-kingdom interactions, affecting the viability of A. baumannii. To gain an understanding of its mechanism, the transcriptional profile of A. baumannii exposed to farnesol was examined. Farnesol caused dysregulation of a large number of genes involved in cell membrane biogenesis, multidrug efflux pumps (AcrAB-like and AdeIJK-like), and A. baumannii virulence traits such as biofilm formation (csuA, csuB, and ompA) and motility (pilZ and pilH). We also observed a strong induction in genes involved in cell division (minD, minE, ftsK, ftsB, and ftsL). These transcriptional data were supported by functional assays showing that farnesol disrupts A. baumannii cell membrane integrity, alters cell morphology, and impairs virulence characteristics such as biofilm formation and twitching motility. Moreover, we showed that A. baumannii uses efflux pumps as a defense mechanism against this eukaryotic signaling molecule. Owing to its effects on membrane integrity, farnesol was tested to see if it potentiated the activity of the membrane-acting polymyxin antibiotic colistin. When coadministered, farnesol increased sensitivity to colistin for otherwise resistant strains. These data provide mechanistic understanding of the antagonistic interactions between diverse pathogens and may provide important insights into novel therapeutic strategies.

Multiple Genetic Mutations Associated with Polymyxin Resistance in Acinetobacter baumannii

We studied polymyxin B resistance in 10 pairs of clinical Acinetobacter baumannii isolates, two of which had developed polymyxin B resistance in vivo. All polymyxin B-resistant isolates had lower growth rates than and substitution mutations in the lpx or pmrB gene compared to their parent isolates. There were significant differences in terms of antibiotic susceptibility and genetic determinants of resistance in A. baumannii isolates that had developed polymyxin B resistance in vivo compared to isolates that had developed polymyxin B resistance in vitro.

Gene expression analysis in human polymorphonuclear leukocytes stimulated by LPSs from nosocomial opportunistic pathogens

Innate immunity coordinates LPS detection via TLR4 on polymorphonuclear leukocytes (PMNs) to elicit responses to many Gram-negative bacteria. In this study, we describe the effects of five subtypes of LPS [isolated from Escherichia coli B4, Pseudomonas aeruginosa PAO1, multidrug-resistant P. aeruginosa (MDRP), Acinetobacter baumannii and multidrug-resistant A. baumannii (MDRA)] on gene expression in PMNs. LPS isolated from B4, PAO1, and A. baumannii did not significantly alter TLR2 expression. However, LPS from MDRP and MDRA caused a 0.6-fold decrease and 2.7-fold increase, respectively, in TLR2 expression. Similarly, TLR4 expression was not significantly altered by LPS isolated from B4, PAO1 and A. baumannii but was down-regulated by LPS isolated from MDRP and MDRA by 0.1- and 0.6-fold, respectively. All LPS subtypes, excluding PAO1, down-regulated CD14 expression in PMNs. However, all five LPS subtypes up-regulated TNFA, IL1B, IL6, IL10 and TREM1 expression in a concentration-dependent manner, with the most substantial responses observed following exposure to LPS from MDRP and MDRA. These different effects on the gene expression in PMNs may depend on variation in LPS structural modifications related to acquired drug resistance, such as acylation and/or glycosylation.

Translational deficiencies in antibacterial discovery and new screening paradigms

An impending disaster is currently developing in the infectious disease community: the combination of rapidly emerging multidrug-resistance among clinically relevant bacterial pathogens, together with an unprecedented withdrawal from industrial dedication to this disease area, is jeopardizing human health on a societal level. For those who remain focused and dedicated to identifying solutions to this growing problem, additional challenges await when in vitro activity does not correlate with in vivo efficacy. Thus the development of more effective translational assays will greatly improve and streamline the process of identifying novel antibacterial agents that can stand the test of preclinical and clinical development. Here we describe recent examples of research that justify the need for such assays.

Lipopolysaccharide loss produces partial colistin dependence and collateral sensitivity to azithromycin, rifampicin and vancomycin in Acinetobacter baumannii

Treatment options for multidrug-resistant (MDR) strains of Acinetobacter baumannii that acquire resistance to colistin are limited. Acinetobacter baumannii can become highly resistant to colistin through complete loss of lipopolysaccharide (LPS) owing to mutations in the genes encoding the first three enzymes involved in lipid A biosynthesis (lpxA, lpxC and lpxD). The objective of this study was to characterise the susceptibility to 15 clinically relevant antibiotics and 6 antimicrobial peptides (AMPs) of MDR A. baumannii clinical isolates that acquired colistin resistance due to mutations in lpxA, lpxC and lpxD as well as their colistin-susceptible counterparts. A dramatic increase in antibiotic susceptibility (≥16-fold increase) was observed upon LPS loss for azithromycin, rifampicin and vancomycin, whereas a moderate increase in susceptibility was seen for amikacin, ceftazidime, imipenem, cefepime and meropenem. Importantly, concentrations ranging from 8 mg/L to 32 mg/L of the six AMPs were able to reduce bacterial viability by ≥3 log10 in growth curve assays. We also demonstrate that colistin resistance results in partial colistin dependence for growth in LPS-deficient strains containing mutations in lpxA, lpxC and lpxD, but not when colistin resistance occurs via LPS modification due to mutations in the PmrA/B two-component system. The results of this study indicate that loss of LPS expression results in collateral sensitivity to azithromycin, rifampicin and vancomycin, and that the six AMPs tested retain activity against LPS-deficient strains, indicating that these antibiotics may be viable treatment options for infections caused by these strains.

Genetic basis of antimicrobial resistance and clonal dynamics of carbapenem-resistant Acinetobacter baumannii sequence type 191 in a Korean hospital

This study investigated the genetic basis of antimicrobial resistance and the epidemiological characteristics of 125 carbapenem-resistant Acinetobacter baumannii (CRAB) isolates collected from 2011 to 2012 in a Korean hospital. All CRAB isolates showed an extensively drug-resistant phenotype, but were susceptible to tigecycline. The blaOXA-23 and armA genes were mainly responsible for resistance to carbapenems and aminoglycosides, respectively. Four colistin-resistant CRAB isolates with different pulsotypes were identified. All four colistin-resistant isolates had a deletion at nucleotide 776 in lpxA, while one also had an insertion at nucleotide 732 in lpxA. All CRAB isolates belonged to three sequence types (STs): ST191 (n=118), ST208 (n=6), and ST436 (n=1), but were classified into 33 arbitrary pulsotypes. Of the CRAB ST191 isolates, two main arbitrary pulsotypes 5 (n=20) and 18 (n=17) emerged sequentially, but were not clonally related to CRAB isolates collected from 2009 to 2010 in the same hospital. Furthermore, of the two main pulsotypes identified among CRAB ST191 isolates from 2009 to 2010, one was clonally related to sporadic CRAB ST191 isolates from 2011 to 2012, but the other was not related to any CRAB isolate from 2011 to 2012. In conclusion, this study shows the clonal dynamics of CRAB ST191 isolates in a Korean hospital during the last four years.