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

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.

Membrane-Active Small Molecules: Designs Inspired by Antimicrobial Peptides

Infectious diseases continue to be one of the major contributors to human morbidity. The rapid rate at which pathogenic microorganisms have developed resistance against frontline antimicrobials has compelled scientists to look for new alternatives. Given their vast antimicrobial repertoire, substantial research effort has been dedicated toward the development of antimicrobial peptides (AMPs) as alternative drugs. However, inherent limitations of AMPs have driven substantial efforts worldwide to develop synthetic mimics of AMPs. This review focuses on the progress that has been made toward the development of small molecules that emulate the properties of AMPs, both in terms of design and biological activity. Herein we provide an extensive discussion of the structural features of various designs and we examine biological properties that have been exploited. Furthermore, we raise a number of questions for which the field has yet to provide solutions and discuss possible future research directions that remain either unexploited or underexploited.

Bloodstream infections in intensive care unit patients: distribution and antibiotic resistance of bacteria

Bloodstream infections (BSIs) are among the leading infections in critically ill patients. The case-fatality rate associated with BSIs in patients admitted to intensive care units (ICUs) reaches 35%–50%. The emergence and diffusion of bacteria with resistance to antibiotics is a global health problem. Multidrug-resistant bacteria were detected in 50.7% of patients with BSIs in a recently published international observational study, with methicillin resistance detected in 48% of Staphylococcus aureus strains, carbapenem resistance detected in 69% of Acinetobacter spp., in 38% of Klebsiella pneumoniae, and in 37% of Pseudomonas spp. Prior hospitalization and antibiotic exposure have been identified as risk factors for infections caused by resistant bacteria in different studies. Patients with BSIs caused by resistant strains showed an increased risk of mortality, which may be explained by a higher incidence of inappropriate empirical therapy in different studies. The molecular genetic characterization of resistant bacteria allows the understanding of the most common mechanisms underlying their resistance and the adoption of surveillance measures. Knowledge of epidemiology, risk factors, mechanisms of resistance, and outcomes of BSIs caused by resistant bacteria may have a major influence on global management of ICU patients. The aim of this review is to provide the clinician an update on BSIs caused by resistant bacteria in ICU patients.

The Prevalence of IS Aba 1 and IS Aba 4 in Acinetobacter baumannii Species of Different International Clone Lineages Among Patients With Burning in Tehran, Iran

Background:

Multidrug resistant strains of Acinetobacter baumannii (MDR-AB) have emerged as alarming nosocomial pathogens among patients with burning.

Objectives:

The current study aimed to determine the susceptibility of A. baumannii species, carbapenems resistance patterns, and their association with IS_Aba1 and IS_Aba4 elements upstream of the bla_OXA-like genes, and the distribution of international clone (IC) of A. baumannii isolates among patients with burning in Tehran, Iran.

Materials and Methods:

In the current study, 62 A. baumannii species isolates from patients with burning in Tehran, Iran, in 2012 were evaluated for the antimicrobial susceptibility, genetic relationships, ICs, carbapenemase encoding genes, and insertion elements IS_Aba upstream of bla_OXA-like genes.

Results:

The highest rates of susceptibility were observed with colistin (88.7%) and tigecycline (82.2%). The extensively drug-resistance and pan drug-resistance were observed in 37.1% and 8.1% of the isolates, respectively. About 98.3% of 17 genotypes categorized into three distinct clusters. Thirty-six of the 62 isolates (58%) belonged to the IC II lineage. The most prevalent acquired OXA-type carbapenemase was bla_OXA-23-like (62.9%). IS_Aba1 and IS_Aba4 were detected upstream of bla_OXA-23-like genes in 45.1% and 12.9% of isolates, respectively. In 32.2% of all isolates, IS_Aba1 laid upstream of bla_OXA-51-like genes. The PCR results were negative for carbapenemase genes of Ambler class A and B, except bla_VIM-2. (1.6%).

Conclusions:

It was the first study that attempted to detect the insertion elements IS_Aba and IC lineages in MDR-AB species isolated from patients with burning in Iran.

In vitro activity of colistin in antimicrobial combination against carbapenem-resistant Acinetobacter baumannii isolated from patients with ventilator-associated pneumonia in Vietnam

Acinetobacter baumannii has become one of the major infection threats in intensive care units (ICUs) globally. Since 2008, A. baumannii has been the leading cause of ventilator-associated pneumonia (VAP) in our ICU at an infectious disease hospital in southern Vietnam. The emergence of this pathogen in our setting is consistent with the persistence of a specific clone exhibiting resistance to carbapenems. Antimicrobial combinations may be a strategy to treat infections caused by these carbapenem-resistant A. baumannii. Therefore, we assessed potential antimicrobial combinations against local carbapenem-resistant A. baumannii by measuring in vitro interactions of colistin with four antimicrobials that are locally certified for treating VAP. We first performed antimicrobial susceptibility testing and multilocus variable number tandem repeat analysis (MLVA) genotyping on 74 A. baumannii isolated from quantitative tracheal aspirates from patients with VAP over an 18-month period. These 74 isolates could be subdivided into 21 main clusters by MLVA and >80 % were resistant to carbapenems. We selected 56 representative isolates for in vitro combination synergy testing. Synergy was observed in four (7 %), seven (13 %), 20 (36 %) and 38 (68 %) isolates with combinations of colistin with ceftazidime, ceftriaxone, imipenem and meropenem, respectively. Notably, more carbapenem-resistant A. baumannii isolates (36/43; 84 %) exhibited synergistic activity with a combination of colistin and meropenem than carbapenem-susceptible A. baumannii isolates (2/13; 15 %) (P = 0.023; Fisher's exact test). Our findings suggest that combinations of colistin and meropenem should be considered when treating carbapenem-resistant A. baumannii infections in Vietnam, and we advocate clinical trials investigating combination therapy for VAP.

Ventilator-associated infection: the role for inhaled antibiotics

PURPOSE OF REVIEW

Despite multiple protocols for the prevention of ventilator-associated pneumonia (VAP), respiratory infections have not been eliminated in the ICU. The profound disruption in both airway integrity and mucociliary clearance caused by the endotracheal tube makes it unlikely there will ever be a zero rate of respiratory infection in critically ill ventilated patients or a 100% cure rate when infection is present. In fact, options for treatment are diminishing as bacteria resistant to most, or in some hospitals all, systemic antibiotics increase in prevalence from our liberal use of systemic antibiotics. Inhaled therapy with proper delivery will result in the high concentrations of antibiotics needed in the treatment of increasingly resistant organisms.

RECENT FINDINGS

Data from many recent investigations have focused on inhaled antibiotics as: adjunctive therapy to systemic antibiotic for VAP, monotherapy for VAP, and as monotherapy for ventilator-associated tracheobronchitis. The clinical outcomes of these studies will be reviewed as well as their effect on multidrug-resistant organisms.

SUMMARY

The present review will focus on the rationale for inhaled therapy, the current studies examining the delivery and clinical efficacy of inhaled antibiotics, and the potential role for this mode of delivery actually decreasing antibiotic resistance in the respiratory tract.

In Vivo Selection of Pan-Drug Resistant Acinetobacter baumannii during Antibiotic Treatment

PURPOSE

Colistin resistance in Acinetobacter baumannii (A. baumannii) is mediated by a complete loss of lipopolysaccharide production via mutations in lpxA, lpxC, and lpxD gene or lipid A modifications via mutations in the pmrA and pmrB genes. However, the exact mechanism of therapy-induced colistin resistance in A. baumannii is not well understood.

MATERIALS AND METHODS

We investigated the genotypic and phenotypic changes that underlie pan-drug resistance mechanisms by determining differences between the alterations in extensively drug-resistant (XDR) A. baumannii (AB001 and AB002) isolates and a pan-drug resistant (PDR) counterpart (AB003) recovered from one patient before and after antibiotic treatment, respectively.

RESULTS

All three clinical isolates shared an identical sequence type (ST138), belonging to the global epidemic clone, clonal complex 92, and all produced OXA-23 carbapenemase. The PDR AB003 showed two genetic differences, acquisition of armA gene and an amino acid substitution (Glu229Asp) in pmrB gene, relative to XDR isolates. No mutations were detected in the pmrA, pmrC, lpxA, lpxC, or lpxD genes in all three isolates. In matrix-assisted laser desorption ionization-time of flight analysis, the three isolates commonly showed two major peaks at 1728 m/z and 1912 m/z, but peaks at 2034 m/z, 2157 m/z, 2261 m/z, and 2384 m/z were detected only in the PDR A. baumannii AB003 isolate.

CONCLUSION

Our results show that changes in lipid A structure via a mutation in the pmrB gene and acquisition of armA gene might confer resistance to colistin and aminoglycosides to XDR A. baumannii strains, resulting in appearance of a PDR A. baumannii strain of ST138.

Antimicrobial heteroresistance: an emerging field in need of clarity

"Heteroresistance" describes a phenomenon where subpopulations of seemingly isogenic bacteria exhibit a range of susceptibilities to a particular antibiotic. Unfortunately, a lack of standard methods to determine heteroresistance has led to inappropriate use of this term. Heteroresistance has been recognized since at least 1947 and occurs in Gram-positive and Gram-negative bacteria. Its clinical relevance may be considerable, since more resistant subpopulations may be selected during antimicrobial therapy. However, the use of nonstandard methods to define heteroresistance, which are costly and involve considerable labor and resources, precludes evaluating the clinical magnitude and severity of this phenomenon. We review the available literature on antibiotic heteroresistance and propose recommendations for definitions and determination criteria for heteroresistant bacteria. This will help in assessing the global clinical impact of heteroresistance and developing uniform guidelines for improved therapeutic outcomes.

Treatment options for carbapenem-resistant and extensively drug-resistant Acinetobacter baumannii infections

Acinetobacter baumannii is a leading cause of healthcare-associated infections worldwide. Because of various intrinsic and acquired mechanisms of resistance, most β-lactam agents are not effective against many strains, and carbapenems have played an important role in therapy. Recent trends show many infections are caused by carbapenem-resistant or even extensively drug-resistant (XDR) strains, for which effective therapy is not well established. Evidence to date suggests that colistin constitutes the backbone of therapy, but the unique pharmacokinetic properties of colistin have led many to suggest the use of combination antimicrobial therapy. However, the combination of agents and dosing regimens that delivers the best clinical efficacy while minimizing toxicity is yet to be defined. Carbapenems, sulbactam, rifampin and tigecycline have been the most studied in the context of combination therapy. Most data regarding therapy for invasive, resistant A. baumannii infections come from uncontrolled case series and retrospective analyses, though some clinical trials have been completed and others are underway. Early institution of appropriate antimicrobial therapy is shown to consistently improve survival of patients with carbapenem-resistant and XDR A. baumannii infection, but the choice of empiric therapy in these infections remains an open question. This review summarizes the most current knowledge regarding the epidemiology, mechanisms of resistance, and treatment considerations of carbapenem-resistant and XDR A. baumannii.

Reinforcing Lipid A Acylation on the Cell Surface of Acinetobacter baumannii Promotes Cationic Antimicrobial Peptide Resistance and Desiccation Survival

Acinetobacter baumannii is an emerging Gram-negative pathogen found in hospitals and intensive care units. In order to persist in hospital environments, A. baumannii withstands desiccative conditions and can rapidly develop multidrug resistance to conventional antibiotics. Cationic antimicrobial peptides (CAMPs) have served as therapeutic alternatives because they target the conserved lipid A component of the Gram-negative outer membrane to lyse the bacterial cell. However, many Gram-negative pathogenic bacteria, including A. baumannii, fortify their outer membrane with hepta-acylated lipid A to protect the cell from CAMP-dependent cell lysis. Whereas in Escherichia coli and Salmonella, increased production of the outer membrane acyltransferase PagP results in formation of protective hepta-acylated lipid A, which reinforces the lipopolysaccharide portion of the outer membrane barrier, A. baumannii does not carry a gene that encodes a PagP homolog. Instead, A. baumannii has evolved a PagP-independent mechanism to synthesize protective hepta-acylated lipid A. Taking advantage of a recently adapted A. baumannii genetic recombineering system, we characterized two putative acyltransferases in A. baumannii designated LpxLAb (A. baumannii LpxL) and LpxMAb (A. baumannii LpxM), which transfer one and two lauroyl (C12:0) acyl chains, respectively, during lipid A biosynthesis. Hepta-acylation of A. baumannii lipid A promoted resistance to vertebrate and polymyxin CAMPs, which are prescribed as last-resort treatment options. Intriguingly, our analysis also showed that LpxMAb-dependent acylation of lipid A is essential for A. baumannii desiccation survival, a key resistance mechanism for survival in hospital environments. Compounds that inhibit LpxMAb-dependent hepta-acylation of lipid A could act synergistically with CAMPs to provide innovative transmission prevention strategies and treat multidrug-resistant infections.

IMPORTANCE

Acinetobacter baumannii infections can be life threatening, and disease can progress in a variety of host tissues. Current antibiotic regimen and disinfectant strategies have failed to limit nosocomial A. baumannii infections. Instead, the rate of A. baumannii infection among health care communities has skyrocketed due to the bacterium's adaptability. Its aptitude for survival over extended periods on inanimate objects, such as catheters, respirators, and surfaces in intensive care units, or on the hands of health care workers and its ability to rapidly develop antibiotic resistance make A. baumannii a threat to health care communities. Emergence of multidrug- and extremely drug-resistant A. baumannii illustrates the ineffectiveness of current prevention and treatment options. Our analysis to understand how A. baumannii resists cationic antimicrobial peptide (CAMP)-mediated and desiccative killing revealed two lipid A acyltransferases that produce protective hepta-acylated lipid A. Our work suggests that inhibiting lipid A biosynthesis by targeting the acyltransferase LpxMAb (A. baumannii LpxM) could provide a novel target to combat this pathogen.

Retention of virulence following adaptation to colistin in Acinetobacter baumannii reflects the mechanism of resistance

OBJECTIVES

Colistin resistance in Acinetobacter baumannii has been associated with loss of virulence and a negative impact on isolate selection. In this study, exposure of clinical isolates to suboptimal concentrations of colistin was used to explore the capacity to develop resistance by diverse mechanisms, and whether acquired resistance always reduces fitness and virulence.

METHODS

Twelve colistin-susceptible clinical A. baumannii isolates were exposed to a sub-MIC concentration of colistin over 6 weeks with weekly increases in concentration. Stable resistance was then phenotypically investigated with respect to antibiotic/biocide resistance, virulence in Galleria mellonella and growth rate. Putative mechanisms of resistance were identified by targeted sequencing of known resistance loci.

RESULTS

Eight A. baumannii isolates acquired resistance to colistin within 1 week with MICs ranging from 2 to >512 mg/L. By 6 weeks 11 isolates were resistant to colistin; this was linked to the development of mutations in pmr or lpx genes. Strains that developed mutations in lpxACD showed a loss of virulence and increased susceptibility to several antibiotics/disinfectants tested. Two of the colistin-resistant strains with mutations in pmrB retained similar virulence levels to their respective parental strains in G. mellonella.

CONCLUSIONS

Acquisition of colistin resistance does not always lead to a loss of virulence, especially when this is linked to mutations in pmrB. This underlines the importance of understanding the mechanism of colistin resistance as well as the phenotype.

A distinct alleles and genetic recombination of pmrCAB operon in species of Acinetobacter baumannii complex isolates

To investigate pmrCAB sequence divergence in 5 species of Acinetobacter baumannii complex, a total of 80 isolates from a Korean hospital were explored. We evaluated nucleotide and amino acid polymorphisms of pmrCAB operon, and phylogenetic trees were constructed for each gene of prmCAB operon. Colistin and polymyxin B susceptibility was determined for all isolates, and multilocus sequence typing was also performed for A. baumannii isolates. Our results showed that each species of A. baumannii complex has divergent pmrCAB operon sequences. We identified a distinct pmrCAB allele allied with Acinetobacter nosocomialis in gene trees. Different grouping in each gene tree suggests sporadic recombination or emergence of pmrCAB genes among Acinetobacter species. Sequence polymorphisms among Acinetobacter species might not be associated with colistin resistance. We revealed that a distinct pmrCAB allele may be widespread across the continents such as North America and Asia and that sporadic genetic recombination or emergence of pmrCAB genes might occur.

Kibdelomycin is a bactericidal broad-spectrum aerobic antibacterial agent

Bacterial resistance to antibiotics continues to grow and pose serious challenges, while the discovery rate for new antibiotics declines. Kibdelomycin is a recently discovered natural-product antibiotic that inhibits bacterial growth by inhibiting the bacterial DNA replication enzymes DNA gyrase and topoisomerase IV. It was reported to be a broad-spectrum aerobic Gram-positive agent with selective inhibition of the anaerobic bacterium Clostridium difficile. We have extended the profiling of kibdelomycin by using over 196 strains of Gram-positive and Gram-negative aerobic pathogens recovered from worldwide patient populations. We report the MIC50s, MIC90s, and bactericidal activities of kibdelomycin. We confirm the Gram-positive spectrum and report for the first time that kibdelomycin shows strong activity (MIC90, 0.125 μg/ml) against clinical strains of the Gram-negative nonfermenter Acinetobacter baumannii but only weak activity against Pseudomonas aeruginosa. We confirm that well-characterized resistant strains of Staphylococcus aureus and Streptococcus pneumoniae show no cross-resistance to kibdelomycin and quinolones and coumarin antibiotics. We also show that kibdelomycin is not subject to efflux in Pseudomonas, though it is in Escherichia coli, and it is generally affected by the outer membrane permeability entry barrier in the nonfermenters P. aeruginosa and A. baumannii, which may be addressable by structure-based chemical modification.

Colistin, mechanisms and prevalence of resistance

BACKGROUND

Infections caused by multi-drug-resistant Gram-negative bacteria, particularly Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae, that cause nosocomial infections, represent a growing problem worldwide. The rapid increase in the prevalence of Gram-negative pathogens that are resistant to fluoroquinolones and aminoglycosides as well as all β-lactams, including carbapenems, monobactam, cephalosporins and broad-spectrum penicillins, has prompted the reconsideration of colistin as a valid therapeutic option. Colistin is an old class of cationic, which act by disrupting the bacterial membranes resulting in cellular death. Although there has been a significant recent increase in the data gathered on colistin, focusing on its chemistry, antibacterial activity, mechanism of action and resistance, pharmacokinetics, pharmacodynamics and new clinical application, the prevalence of colistin resistance has been very little reported in the literature. This review concentrates on recent literature aimed at optimizing the clinical use of this important antibiotic.

METHODS

The available evidence from various studies (microbiological and clinical studies, retrieved from the PubMed, and Scopus databases) regarding the mechanisms and prevalence of resistance was evaluated.

RESULTS

Increasing use of colistin for treatment of infections caused by these bacteria has led to the emergence of colistin resistance in several countries worldwide. Although resistance to polymyxins is generally less than 10%, it is higher in the Mediterranean and South-East Asia (Korea and Singapore), where colistin resistance rates are continually increasing.

CONCLUSION

There is a critical need for effective infection prevention and control measures and strict use of antibiotics in the world to control the rise and spread of colistin resistance.

Emergence of colistin resistance without loss of fitness and virulence after prolonged colistin administration in a patient with extensively drug-resistant Acinetobacter baumannii

The spread of extensively drug-resistant (XDR) gram-negative bacteria has boosted colistin use, with a resultant selection of colistin-resistant, often pandrug-resistant strains. Whether acquisition of further resistance mechanisms translates into a reduced virulence is the subject of active research. In this report, we describe clinical features of an immunocompromised patient who developed infection due to colistin-resistant Acinetobacter baumannii while on long-term colistin therapy. We analyzed phenotypic and genotypic characteristics, molecular mechanisms of colistin resistance, and in vitro and in vivo fitness of sequential colistin-sensitive and colistin-resistant strains isolated from the patient. Both colistin-sensitive and colistin-resistant strains were XDR and showed identical ST78 genotype. At variance with prior reports on colistin-resistant strains of A. baumannii, resistance to colistin due to P233S mutation in PmrB sensor kinase did not associate with any measurable reduction in strain fitness, growth characteristics, and virulence.

Emerging broad-spectrum resistance in Pseudomonas aeruginosa and Acinetobacter baumannii: Mechanisms and epidemiology

Multidrug resistance is quite common among non-fermenting Gram-negative rods, in particular among clinically relevant species including Pseudomonas aeruginosa and Acinetobacter baumannii. These bacterial species, which are mainly nosocomial pathogens, possess a diversity of resistance mechanisms that may lead to multidrug or even pandrug resistance. Extended-spectrum β-lactamases (ESBLs) conferring resistance to broad-spectrum cephalosporins, carbapenemases conferring resistance to carbapenems, and 16S rRNA methylases conferring resistance to all clinically relevant aminoglycosides are the most important causes of concern. Concomitant resistance to fluoroquinolones, polymyxins (colistin) and tigecycline may lead to pandrug resistance. The most important mechanisms of resistance in P. aeruginosa and A. baumannii and their most recent dissemination worldwide are detailed here.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

The structure of apo ArnA features an unexpected central binding pocket and provides an explanation for enzymatic cooperativity

The bacterial protein ArnA is an essential enzyme in the pathway leading to the modification of lipid A with the pentose sugar 4-amino-4-deoxy-L-arabinose. This modification confers resistance to polymyxins, which are antibiotics that are used as a last resort to treat infections with multiple drug-resistant Gram-negative bacteria. ArnA contains two domains with distinct catalytic functions: a dehydrogenase domain and a transformylase domain. The protein forms homohexamers organized as a dimer of trimers. Here, the crystal structure of apo ArnA is presented and compared with its ATP- and UDP-glucuronic acid-bound counterparts. The comparison reveals major structural rearrangements in the dehydrogenase domain that lead to the formation of a previously unobserved binding pocket at the centre of each ArnA trimer in its apo state. In the crystal structure, this pocket is occupied by a DTT molecule. It is shown that formation of the pocket is linked to a cascade of structural rearrangements that emerge from the NAD(+)-binding site. Based on these findings, a small effector molecule is postulated that binds to the central pocket and modulates the catalytic properties of ArnA. Furthermore, the discovered conformational changes provide a mechanistic explanation for the strong cooperative effect recently reported for the ArnA dehydrogenase function.

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.

Colistin-resistant Acinetobacter baumannii: beyond carbapenem resistance

BACKGROUND

With an increase in the use of colistin methansulfonate (CMS) to treat carbapenem-resistant Acinetobacter baumannii infections, colistin resistance is emerging.

METHODS

Patients with infection or colonization due to colistin-resistant A. baumannii were identified at a hospital system in Pennsylvania. Clinical data were collected from electronic medical records. Susceptibility testing, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing (MLST) were performed. To investigate the mechanism of colistin resistance, lipid A was subjected to matrix-assisted laser desorption/ionization mass spectrometry.

RESULTS

Twenty patients with colistin-resistant A. baumannii were identified. Ventilator-associated pneumonia was the most common type of infection. Nineteen patients had received intravenous and/or inhaled CMS for treatment of carbapenem-resistant, colistin-susceptible A. baumannii infection prior to identification of colistin-resistant isolates. The 30-day all-cause mortality rate was 30%. The treatment regimen for colistin-resistant A. baumannii infection associated with the lowest mortality rate was a combination of CMS, a carbapenem, and ampicillin-sulbactam. The colistin-susceptible and -resistant isolates from the same patients were highly related by PFGE, but isolates from different patients were not, suggesting evolution of resistance during CMS therapy. By MLST, all isolates belonged to the international clone II, the lineage that is epidemic worldwide. Phosphoethanolamine modification of lipid A was present in all colistin-resistant A. baumannii isolates.

CONCLUSIONS

Colistin-resistant A. baumannii occurred almost exclusively among patients who had received CMS for treatment of carbapenem-resistant, colistin-susceptible A. baumannii infection. Lipid A modification by the addition of phosphoethanolamine accounted for colistin resistance. Susceptibility testing for colistin should be considered for A. baumannii identified from CMS-experienced patients.

Antibacterial mechanisms of polymyxin and bacterial resistance

Multidrug resistance in pathogens is an increasingly significant threat for human health. Indeed, some strains are resistant to almost all currently available antibiotics, leaving very limited choices for antimicrobial clinical therapy. In many such cases, polymyxins are the last option available, although their use increases the risk of developing resistant strains. This review mainly aims to discuss advances in unraveling the mechanisms of antibacterial activity of polymyxins and bacterial tolerance together with the description of polymyxin structure, synthesis, and structural modification. These are expected to help researchers not only develop a series of new polymyxin derivatives necessary for future medical care, but also optimize the clinical use of polymyxins with minimal resistance development.

The transcriptomic response of Acinetobacter baumannii to colistin and doripenem alone and in combination in an in vitro pharmacokinetics/pharmacodynamics model

OBJECTIVES

Colistin remains a last-line treatment for MDR Acinetobacter baumannii and combined use of colistin and carbapenems has shown synergistic effects against MDR strains. In order to understand the bacterial responses to these antibiotics, we analysed the transcriptome of A. baumannii following exposure to each.

METHODS

RNA sequencing was employed to determine changes in the transcriptome following treatment with colistin and doripenem, both alone and in combination, using an in vitro pharmacokinetics (PK)/pharmacodynamics model to mimic the PK of both antibiotics in patients.

RESULTS

After treatment with colistin (continuous infusion at 2 mg/L), >400 differentially regulated genes were identified, including many associated with outer membrane biogenesis, fatty acid metabolism and phospholipid trafficking. No genes were differentially expressed following treatment with doripenem (Cmax 25 mg/L, t1/2 1.5 h) for 15 min, but 45 genes were identified as differentially expressed after 1 h of growth under this condition. Treatment of A. baumannii with both colistin and doripenem together for 1 h resulted in >450 genes being identified as differentially expressed. More than 70% of these gene expression changes were also observed following colistin treatment alone.

CONCLUSIONS

These data suggest that colistin causes gross damage to the outer membrane, facilitates lipid exchange between the inner and outer membrane and alters the normal asymmetric outer membrane composition. The transcriptional response to colistin was highly similar to that observed for an LPS-deficient strain, indicating that many of the observed changes are responses to outer membrane instability resulting from LPS loss.

Whole-genome sequence of Chryseobacterium oranimense, a colistin-resistant bacterium isolated from a cystic fibrosis patient in France

For the first time, we report the whole-genome sequence analysis of Chryseobacterium oranimense G311, a multidrug-resistant bacterium, from a cystic fibrosis patient in France, including resistance to colistin. Whole-genome sequencing of C. oranimense G311 was performed using Ion Torrent PGM, and RAST, the EMBL-EBI server, and the Antibiotic Resistance Gene-ANNOTation (ARG-ANNOT) database were used for annotation of all genes, including antibiotic resistance (AR) genes. General features of the C. oranimense G311 draft genome were compared to the other available genomes of Chryseobacterium gleum and Chryseobacterium sp. strain CF314. C. oranimense G311 was found to be resistant to all β-lactams, including imipenem, and to colistin. The genome size of C. oranimense G311 is 4,457,049 bp in length, with 37.70% GC content. We found 27 AR genes in the genome, including β-lactamase genes which showed little similarity to the known β-lactamase genes and could likely be novel. We found the type I polyketide synthase operon followed by a zeaxanthin glycosyltransferase gene in the genome, which could impart the yellow pigmentation of the isolate. We located the O-antigen biosynthesis cluster, and we also discovered a novel capsular polysaccharide biosynthesis cluster. We also found known mutations in the orthologs of the pmrA (E8D), pmrB (L208F and P360Q), and lpxA (G68D) genes. We speculate that the presence of the capsular cluster and mutations in these genes could explain the resistance of this bacterium to colistin. We demonstrate that whole-genome sequencing was successfully applied to decipher the resistome of a multidrug resistance bacterium associated with cystic fibrosis patients.

Effect of carbonyl cyanide 3-chlorophenylhydrazone (CCCP) on killing Acinetobacter baumannii by colistin

We investigated the effect of cyanide 3-chlorophenylhydrazone (CCCP) and other efflux pump inhibitors (EPIs) on the colistin susceptibility in Acinetobacter baumannii. While minimum inhibitory concentrations (MICs) of colistin in all colistin-resistant strains decreased significantly with 25 μM of CCCP and 2,4-dinitrophenol (DNP), phenyl-arginine-β-naphthylamide (PAβN), and reserpine did not decrease the colistin MICs. However, CCCP and DNP as well as PAβN and reserpine did not have a significant effect on the MICs of the other agents. Efflux pump gene expressions in colistin-resistant strains were not increased compared with those in colistin-susceptible strains. When only 5X MIC of colistin (5 mg/L) was provided to a colistin-susceptible A. baumannii strain, the bacterial cell number was reduced by 9 h after exposure to colistin, but regrowth was observed. When CCCP was added to colistin, bacterial cells were completely killed after 24 to 48 h of incubation, which was not due to the toxicity of CCCP itself. Colistin resistance in A. baumannii may not be due to efflux pumps. Our present study suggests that bacterial cells with reduced metabolic activity by CCCP are more susceptible to colistin in A. baumannii. It may show the possibility that combined therapy with colistin and other antimicrobial agents could effective against A. baumannii infections.

Antimicrobial resistance in Acinetobacter baumannii: From bench to bedside

Acinetobacter baumannii (A. baumannii) is undoubtedly one of the most successful pathogens in the modern healthcare system. With invasive procedures, antibiotic use and immunocompromised hosts increasing in recent years, A. baumannii has become endemic in hospitals due to its versatile genetic machinery, which allows it to quickly evolve resistance factors, and to its remarkable ability to tolerate harsh environments. Infections and outbreaks caused by multidrug-resistant A. baumannii (MDRAB) are prevalent and have been reported worldwide over the past twenty or more years. To address this problem effectively, knowledge of species identification, typing methods, clinical manifestations, risk factors, and virulence factors is essential. The global epidemiology of MDRAB is monitored by persistent surveillance programs. Because few effective antibiotics are available, clinicians often face serious challenges when treating patients with MDRAB. Therefore, a deep understanding of the resistance mechanisms used by MDRAB can shed light on two possible strategies to combat the dissemination of antimicrobial resistance: stringent infection control and antibiotic treatments, of which colistin-based combination therapy is the mainstream strategy. However, due to the current unsatisfying therapeutic outcomes, there is a great need to develop and evaluate the efficacy of new antibiotics and to understand the role of other potential alternatives, such as antimicrobial peptides, in the treatment of MDRAB infections.

Immunization with lipopolysaccharide-deficient whole cells provides protective immunity in an experimental mouse model of Acinetobacter baumannii infection

The increasing clinical importance of infections caused by multidrug resistant Acinetobacter baumannii warrants the development of novel approaches for prevention and treatment. In this context, vaccination of certain patient populations may contribute to reducing the morbidity and mortality caused by this pathogen. Vaccines against Gram-negative bacteria based on inactivated bacterial cells are highly immunogenic and have been shown to produce protective immunity against a number of bacterial species. However, the high endotoxin levels present in these vaccines due to the presence of lipopolysaccharide complicates their use in human vaccination. In the present study, we used a laboratory-derived strain of A. baumannii that completely lacks lipopolysaccharide due to a mutation in the lpxD gene (IB010), one of the genes involved in the first steps of lipopolysaccharide biosynthesis, for vaccination. We demonstrate that IB010 has greatly reduced endotoxin content (<1.0 endotoxin unit/106 cells) compared to wild type cells. Immunization with formalin inactivated IB010 produced a robust antibody response consisting of both IgG1 and IgG2c subtypes. Mice immunized with IB010 had significantly lower post-infection tissue bacterial loads and significantly lower serum levels of the pro-inflammatory cytokines IL-1β, TNF-α and IL-6 compared to control mice in a mouse model of disseminated A. baumannii infection. Importantly, immunized mice were protected from infection with the ATCC 19606 strain and an A. baumannii clinical isolate. These data suggest that immunization with inactivated A. baumannii whole cells deficient in lipopolysaccharide could serve as the basis for a vaccine for the prevention of infection caused by A. baumannii.

Mechanisms of polymyxin resistance: acquired and intrinsic resistance in bacteria

Polymyxins are polycationic antimicrobial peptides that are currently the last-resort antibiotics for the treatment of multidrug-resistant, Gram-negative bacterial infections. The reintroduction of polymyxins for antimicrobial therapy has been followed by an increase in reports of resistance among Gram-negative bacteria. Some bacteria, such as Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii, develop resistance to polymyxins in a process referred to as acquired resistance, whereas other bacteria, such as Proteus spp., Serratia spp., and Burkholderia spp., are naturally resistant to these drugs. Reports of polymyxin resistance in clinical isolates have recently increased, including acquired and intrinsically resistant pathogens. This increase is considered a serious issue, prompting concern due to the low number of currently available effective antibiotics. This review summarizes current knowledge concerning the different strategies bacteria employ to resist the activities of polymyxins. Gram-negative bacteria employ several strategies to protect themselves from polymyxin antibiotics (polymyxin B and colistin), including a variety of lipopolysaccharide (LPS) modifications, such as modifications of lipid A with phosphoethanolamine and 4-amino-4-deoxy-L-arabinose, in addition to the use of efflux pumps, the formation of capsules and overexpression of the outer membrane protein OprH, which are all effectively regulated at the molecular level. The increased understanding of these mechanisms is extremely vital and timely to facilitate studies of antimicrobial peptides and find new potential drugs targeting clinically relevant Gram-negative bacteria.

Tigecycline resistance in Acinetobacter baumannii mediated by frameshift mutation in plsC, encoding 1-acyl-sn-glycerol-3-phosphate acyltransferase

Acinetobacter baumannii is an important pathogen of healthcare-associated infections and shows multidrug resistance. With the increasing application of tigecycline, isolates resistant to this antibiotic are of growing concern clinically. However, the definitive mechanism of tigecycline resistance remains unclear. To explore the mechanism of tigecycline resistance in A. baumannii, a tigecycline-resistant strain was obtained by increasing the concentration of the antimicrobial in liquid culture. Three mutations were identified by the whole genome comparison, including one synonymous substitution in a hypothetical protein and a frameshift mutation in plsC and omp38. The plsC gene was confirmed to cause decreased susceptibility to tigecycline by a complementation experiment and cellular membrane change was detected by flow cytometry. By measuring the relative growth rate, the fitness cost of plsC was estimated to be approximately 8 %. In conclusion, plsC was found to play an important role in tigecycline resistance in A. baumannii. The minor fitness cost of plsC indicates a high risk of the emergence and development of tigecycline resistance in A. baumannii.

LpxC inhibitors as new antibacterial agents and tools for studying regulation of lipid A biosynthesis in Gram-negative pathogens

The problem of multidrug resistance in serious Gram-negative bacterial pathogens has escalated so severely that new cellular targets and pathways need to be exploited to avoid many of the preexisting antibiotic resistance mechanisms that are rapidly disseminating to new strains. The discovery of small-molecule inhibitors of LpxC, the enzyme responsible for the first committed step in the biosynthesis of lipid A, represents a clinically unprecedented strategy to specifically act against Gram-negative organisms such as Pseudomonas aeruginosa and members of the Enterobacteriaceae. In this report, we describe the microbiological characterization of LpxC-4, a recently disclosed inhibitor of this bacterial target, and demonstrate that its spectrum of activity extends to several of the pathogenic species that are most threatening to human health today. We also show that spontaneous generation of LpxC-4 resistance occurs at frequencies comparable to those seen with marketed antibiotics, and we provide an in-depth analysis of the mechanisms of resistance utilized by target pathogens. Interestingly, these isolates also served as tools to further our understanding of the regulation of lipid A biosynthesis and enabled the discovery that this process occurs very distinctly between P. aeruginosa and members of the Enterobacteriaceae. Finally, we demonstrate that LpxC-4 is efficacious in vivo against multiple strains in different models of bacterial infection and that the major first-step resistance mechanisms employed by the intended target organisms can still be effectively treated with this new inhibitor.

New antibiotics are needed for the effective treatment of serious infections caused by Gram-negative pathogens, and the responsibility of identifying new drug candidates rests squarely on the shoulders of the infectious disease community. The limited number of validated cellular targets and approaches, along with the increasing amount of antibiotic resistance that is spreading throughout the clinical environment, has prompted us to explore the utility of inhibitors of novel targets and pathways in these resistant organisms, since preexisting target-based resistance should be negligible. Lipid A biosynthesis is an essential process for the formation of lipopolysaccharide, which is a critical component of the Gram-negative outer membrane. In this report, we describe the in vitro and in vivo characterization of novel inhibitors of LpxC, an enzyme whose activity is required for proper lipid A biosynthesis, and demonstrate that our lead compound has the requisite attributes to warrant further consideration as a novel antibiotic.

Distribution of bla(OXA-23), ISAba , Aminoglycosides resistant genes among burned & ICU patients in Tehran and Sari, Iran

BACKGROUND

Multidrug resistant strains of Acinetobacter baumannii (MDR-AB) have emerged as alarming nosocomial pathogens among patients admitted to Intensive Care Unit and burned patients. The aim of this study was to determine the susceptibility of A. baumannii isolates, the carbapenems resistance patterns bla(OXA-23) and also ISAba elements of A. baumannii isolates among burned and ICU patients in Tehran and Sari, Iran.

METHODS

In this study, 100 A. baumannii isolates from burned and ICU patients in Tehran and Sari (Iran) during 2013 were tested for determination of antimicrobials susceptibility by the disc-diffusion method on Mueller Hinton agar recommended by the guidelines of Clinical and Laboratory Standards Institute (CLSI), and frequency bla(OXA-23) carbapenemase genes, and insertion elements ISAba genes were studied by PCR method.

RESULTS

The highest rates of susceptibility were observed with Colistin (88.7%), Tigecycline (82.2%), Imipenem (67%) and ISAba (32.2%). The extensively drug-resistance and pan drug-resistance were observed in 37.1% and 8.1% isolates, respectively. Results indicated among isolates resistant to Aminoglycoside and Carbapenem, the highest resistance was observed to Streptomycin (90%) ' and the most sensitivity was to Imipenem (67%).

CONCLUSIONS

This is the most study that attempted to detect Acinetobacter baumanii the insertion elements ISAba , bla(OXA-23) and aminoglycosides resistance in MDR-AB isolates from burned and ICU patients in Iran. In a timely manner, antimicrobial resistance surveillance and strict infection control strategies are still lacking in burn ward and ICU in Iran, despite the alarming emergence of MDR-AB strains, particularly among those isolates that are not susceptible to Colistin. The results of this study are consistent with a recent report in which a number of combinations exhibited potent activity against Multidrug resistant strains of A. baumannii (MDR-AB).

Variation in the OC locus of Acinetobacter baumannii genomes predicts extensive structural diversity in the lipooligosaccharide

Lipooligosaccharide (LOS) is a complex surface structure that is linked to many pathogenic properties of Acinetobacter baumannii. In A. baumannii, the genes responsible for the synthesis of the outer core (OC) component of the LOS are located between ilvE and aspS. The content of the OC locus is usually variable within a species, and examination of 6 complete and 227 draft A. baumannii genome sequences available in GenBank non-redundant and Whole Genome Shotgun databases revealed nine distinct new types, OCL4-OCL12, in addition to the three known ones. The twelve gene clusters fell into two distinct groups, designated Group A and Group B, based on similarities in the genes present. OCL6 (Group B) was unique in that it included genes for the synthesis of L-Rhamnosep. Genetic exchange of the different configurations between strains has occurred as some OC forms were found in several different sequence types (STs). OCL1 (Group A) was the most widely distributed being present in 18 STs, and OCL6 was found in 16 STs. Variation within clones was also observed, with more than one OC locus type found in the two globally disseminated clones, GC1 and GC2, that include the majority of multiply antibiotic resistant isolates. OCL1 was the most abundant gene cluster in both GC1 and GC2 genomes but GC1 isolates also carried OCL2, OCL3 or OCL5, and OCL3 was also present in GC2. As replacement of the OC locus in the major global clones indicates the presence of sub-lineages, a PCR typing scheme was developed to rapidly distinguish Group A and Group B types, and to distinguish the specific forms found in GC1 and GC2 isolates.

Current and future treatment options for infections caused by multidrug-resistant Gram-negative pathogens

ABSTRACT 

The spread of multidrug-resistant, extensively drug-resistant and pan-drug-resistant pathogens is causing an unprecedented public health crisis. The limited current therapeutic options led to the revival of two ‘old’ antibiotics – colistin and fosfomycin – for which a better understanding of their pharmacokinetics in the critically ill patient and in specific body compartments is required. Tigecycline's use in clinical practice for nonapproved indication based on its in vitro activity against problematic pathogens requires caution and probably higher doses. Furthermore, all three antibiotics should be used as part of combination regimens in order to prevent resistance and optimize outcomes. The development of new antibacterials in the near future, namely combinations of avibactam, ceftolozane/tazobactam and plazomicin, seems promising; however, they will only partially address current mechanisms of resistance.

Update on the Epidemiology, Treatment, and Outcomes of Carbapenem-resistant Acinetobacter infections

Carbapenem-resistant Acinetobacter species are increasingly recognized as major nosocomial pathogens, especially in patients with critical illnesses or in intensive care. The ability of these organisms to accumulate diverse mechanisms of resistance limits the available therapeutic agents, makes the infection difficult to treat, and is associated with a greater risk of death. In this review, we provide an update on the epidemiology, resistance mechanisms, infection control measures, treatment, and outcomes of carbapenem-resistant Acinetobacter infections.

In vitro synergism of combinations of colistin with selected antibiotics against colistin-resistant Acinetobacter baumannii

Aim: The in vitro activity of colistin in combination with sulbactam, netilmicin, and vancomycin against colistin-resistant A. baumannii strains was investigated. Furthermore, the clonal relationship of the strains was analyzed.

Methods: Clonal relationship was investigated using rep-PCR. To screen for synergysm, the fractional inhibitory concentration index (FICI) was calculated using checkerboard assay. The killing kinetics of the combination of colistin with vancomycin was assessed using time-kill assay.

Results: Three different clones were found among 10 clinical isolates of colistin-resistant A. baumannii strains. Thereof, 8 strains were susceptible to netilmicin. Synergistic interaction was detected in 1 strain with the combination of colistin-netilmicin, in 5 strains with colistin-sulbactam, and in 9 strains with colistin-vancomycin. None of combinations had antagonistic activity. Colistin-vancomycin combination resulted in rapid bactericidal activity.

Conclusion: These results show a distinct in vitro synergism between colistin and vancomycin, which might be useful to treat infection with multiple-resistant strains, prevent emergence of resistant strains, and to lower doses for both antibiotics to be used.

Efficacy of the small molecule inhibitor of Lipid II BAS00127538 against Acinetobacter baumannii

Objective

To test the activity of a small molecule compound that targets Lipid II against Acinetobacter baumannii.

Methods

Susceptibility to small molecule Lipid II inhibitor BAS00127538 was assessed using carbapenem- and colistin-resistant clinical isolates of A. baumannii. In addition, synergy between colisitin and this compound was assessed.

Results

Small molecule Lipid II inhibitor BAS00127538 potently acts against A. baumannii and acts synergistically with colistin.

Conclusion

For the first time, a compound that targets Lipid II is described that acts against multi-drug resistant isolates of A. baumannii. The synergy with colistin warrants further lead development of BAS00127538.

Endotoxin structures in the psychrophiles Psychromonas marina and Psychrobacter cryohalolentis contain distinctive acyl features

Lipid A is the essential component of endotoxin (Gram-negative lipopolysaccharide), a potent immunostimulatory compound. As the outer surface of the outer membrane, the details of lipid A structure are crucial not only to bacterial pathogenesis but also to membrane integrity. This work characterizes the structure of lipid A in two psychrophiles, Psychromonas marina and Psychrobacter cryohalolentis, and also two mesophiles to which they are related using MALDI-TOF MS and fatty acid methyl ester (FAME) GC-MS. P. marina lipid A is strikingly similar to that of Escherichia coli in organization and total acyl size, but incorporates an unusual doubly unsaturated tetradecadienoyl acyl residue. P. cryohalolentis also shows structural organization similar to a closely related mesophile, Acinetobacter baumannii, however it has generally shorter acyl constituents and shows many acyl variants differing by single methylene (-CH2-) units, a characteristic it shares with the one previously reported psychrotolerant lipid A structure. This work is the first detailed structural characterization of lipid A from an obligate psychrophile and the second from a psychrotolerant species. It reveals distinctive structural features of psychrophilic lipid A in comparison to that of related mesophiles which suggest constitutive adaptations to maintain outer membrane fluidity in cold environments.

The Influence of Ziegler-Natta and Metallocene Catalysts on Polyolefin Structure, Properties, and Processing Ability

50 years ago, Karl Ziegler and Giulio Natta were awarded the Nobel Prize for their discovery of the catalytic polymerization of ethylene and propylene using titanium compounds and aluminum-alkyls as co-catalysts. Polyolefins have grown to become one of the biggest of all produced polymers. New metallocene/methylaluminoxane (MAO) catalysts open the possibility to synthesize polymers with highly defined microstructure, tacticity, and steroregularity, as well as long-chain branched, or blocky copolymers with excellent properties. This improvement in polymerization is possible due to the single active sites available on the metallocene catalysts in contrast to their traditional counterparts. Moreover, these catalysts, half titanocenes/MAO, zirconocenes, and other single site catalysts can control various important parameters, such as co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, and branching of the polymer. However, in most cases research in this area has reduced academia as olefin polymerization has seen significant advancements in the industries. Therefore, this paper aims to further motivate interest in polyolefin research in academia by highlighting promising and open areas for the future.

Practical agar-based disk potentiation test for detection of fosfomycin-nonsusceptible Escherichia coli clinical isolates producing glutathione S-transferases

The number of reports concerning Escherichia coli clinical isolates that produce glutathione S-transferases responsible for fosfomycin resistance (FR-GSTs) has been increasing. We have developed a disk-based potentiation test in which FR-GST producers expand the growth inhibition zone around a Kirby-Bauer disk containing fosfomycin in combination with sodium phosphonoformate (PPF). PPF, an analog of fosfomycin, is a transition-state inhibitor of FosA(PA), a type of FR-GST from Pseudomonas aeruginosa. Considering its mechanism of action, PPF was expected to inhibit a variety of FR-GSTs. In the presence of PPF, zone enlargement around the disk containing fosfomycin was observed for FosA3-, FosA4-, and FosC2-producing E. coli clinical isolates. Moreover, the growth inhibition zone was remarkably enlarged when the Mueller-Hinton (MH) agar plate contained 25 μg/ml glucose-6-phosphate (G6P). When we retrospectively tested 12 fosfomycin-resistant (MIC, ≥256 μg/ml) E. coli clinical isolates from our hospital with the potentiation test, 6 FR-GST producers were positive phenotypically by potentiation disk and were positive for FR-GST genes: 5 harbored fosA3 and 1 harbored fosA4. To identify the production of FR-GSTs, we set the provisional cutoff value, 5-mm enlargement, by adding PPF to a fosfomycin disk on the MH agar plates containing G6P. Our disk-based potentiation test reliably identifies FR-GST producers and can be performed easily; therefore, it will be advantageous in epidemiological surveys and infection control of fosfomycin-resistant bacteria in clinical settings.

Combination therapy in severe Acinetobacter baumannii infections: an update on the evidence to date

ABSTRACT: 

Acinetobacter baumannii is a drug-resistant Gram-negative pathogen increasingly causing hospital-acquired infections in critically ill patients. In this review, we summarize the current mechanisms of antimicrobial resistance in A. baumannii and describe in detail recent in vitro and in vivo experimental data on the activity of antimicrobial combinations against this microorganism. We then introduce the rationale for the use of combination antibiotic therapy in resistant A. baumannii infections. Finally, we present and critically discuss both uncontrolled clinical studies and the few randomized clinical trials of combination antimicrobial therapy for these infections, with a special focus on ongoing multinational trials and optimal approach to future research in this field.