PmrAB, a two-component regulatory system of Pseudomonas aeruginosa that modulates resistance to cationic antimicrobial peptides and addition of aminoarabinose to lipid A

Pharmacokinetics and pharmacodynamics of 'old' polymyxins: what is new?

'Old' colistin and polymyxin B are increasingly used as last-line therapy against multidrug-resistant Gram-negative bacteria Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. For intravenous administration, colistin is dosed as its inactive prodrug colistin methanesulfonate (sodium), while polymyxin B is used as its sulfate (active antibacterial). Over the last decade, significant progress has been made in understanding their chemistry, pharmacokinetics (PK), and pharmacodynamics (PD). The first scientifically based dosing suggestions are now available for colistin methanesulfonate to generate a desired target steady-state plasma concentration of formed colistin in various categories of critically ill patients. As simply increasing polymyxin dosage regimens is not an option for optimizing their PK/PD due to nephrotoxicity, combination therapy with other antibiotics has great potential to maximize the efficacy of polymyxins while minimizing emergence of resistance. We must pursue rational approaches to the use of polymyxins and other existing antibiotics through the application of PK/PD principles.

An orphan chemotaxis sensor regulates virulence and antibiotic tolerance in the human pathogen Pseudomonas aeruginosa

The synthesis of virulence factors by pathogenic bacteria is highly regulated and occurs in response to diverse environmental cues. An array of two component systems (TCSs) serves to link perception of different cues to specific changes in gene expression and/or bacterial behaviour. Those TCSs that regulate functions associated with virulence represent attractive targets for interference in anti-infective strategies for disease control. We have previously identified PA2572 as a putative response regulator required for full virulence of Pseudomonas aeruginosa, the opportunistic human pathogen, to Galleria mellonella (Wax moth) larvae. Here we have investigated the involvement of candidate sensors for signal transduction involving PA2572. Mutation of PA2573, encoding a probable methyl-accepting chemotaxis protein, gave rise to alterations in motility, virulence, and antibiotic resistance, functions which are also controlled by PA2572. Comparative transcriptome profiling of mutants revealed that PA2572 and PA2573 regulate expression of a common set of 49 genes that are involved in a range of biological functions including virulence and antibiotic resistance. Bacterial two-hybrid analysis indicated a REC-dependent interaction between PA2572 and PA2573 proteins. Finally expression of PA2572 in the PA2573 mutant background restored virulence to G. mellonella towards wild-type levels. The findings indicate a role for the orphan chemotaxis sensor PA2573 in the regulation of virulence and antibiotic tolerance in P. aeruginosa and indicate that these effects are exerted in part through signal transduction involving PA2572.

Bacterial resistance to cationic antimicrobial peptides

Naturally occurring cationic antimicrobial peptides (CAMPs) have been considered as promising candidates to treat infections caused by pathogenic bacteria to animals and humans. This assumption is based on their mechanism of action, which is mainly performed through electrostatic membrane interactions. Unfortunately, the rise in the reports that describe bacterial resistance to CAMPs has redefined their role as therapeutic agents. In this review, we describe the state of the art of the most common resistance mechanisms developed by bacteria to CAMPs, making special emphasis on resistance selection. Considering most of the resistance mechanisms here reviewed, the emergence of resistance is unlikely in the short term, however we also described evidences that show the evolution of resistance to CAMPs, reevaluating their use as good antibacterial agents. Finally, the knowledge related to the description of CAMP resistance mechanisms may provide useful information for improving strategies to control infections.

HOG-MAPK signaling regulates the adaptive responses of Aspergillus fumigatus to thermal stress and other related stress

Aspergillus fumigatus is naturally exposed to a highly variable environment and subjected to various kinds of stresses. High-osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway plays a crucial role in regulating cellular homeostasis in response to environmental changes. Here, we explored the contribution of HOG-MAPK pathway to the adaptive responses to thermal stress and other related stresses in A. fumigatus. We observed the phenotype features of wild-type strains and their derived mutants at 37 and 48 °C, and the results suggested that tcsB participates in response to high temperature. Furthermore, susceptibility test for antifungal drugs showed that SHO1 branch is probably involved in the susceptibility of A. fumigatus to itraconazole at high temperature. Although sakA expression at mRNA level appeared unchanged in wild-type AF293 subjected to thermal stress, phosphorylated SakAp level increased significantly in the strains exposed to cold stress, 250 mmol/L nystatin or 10 % dimethyl sulfoxide in a manner dependent on the SLN1 branch and independent on the SHO1 branch. Taken together, these results indicate that HOG-MAPK pathway, especially the SLN1 branch, plays an important role in the adaptations of A. fumigatus to thermal stress and other related stresses.

Cation concentration variability of four distinct Mueller-Hinton agar brands influences polymyxin B susceptibility results

Polymyxins have been the only alternative therapeutic option for the treatment of serious infections caused by multidrug-resistant Acinetobacter baumannii or Pseudomonas aeruginosa isolates. For this reason, it is of crucial importance that susceptibility tests provide accurate results when testing these drug-pathogen combinations. In this study, the effect of cation concentration variability found on different commercial brands of Mueller-Hinton agar (MHA) for testing polymyxin B susceptibility was evaluated. The polymyxin B susceptibilities determined using Etest and disk diffusion were compared to those determined by the CLSI reference broth microdilution method. In general, the polymyxin B MIC values were higher when determined by Etest than when determined by broth microdilution against both A. baumannii and P. aeruginosa isolates. A high very major error rate (10%) was observed, as well as a trend toward lower MICs, compared to those determined by broth microdilution when the Merck MHA was tested by Etest. Poor essential agreement rates (10 to 70%) were observed for P. aeruginosa when all MHA brands were tested by Etest. Although an excellent categorical agreement rate (100%) was seen between the disk diffusion and broth microdilution methods for P. aeruginosa, larger zones of inhibition were shown obtained using the Merck MHA. The high cation concentration variability found for the MHA brands tested correlated to the low accuracy, and discrepancies in the polymyxin B MICs were determined by Etest method, particularly for P. aeruginosa isolates.

Impact of two-component regulatory systems PhoP-PhoQ and PmrA-PmrB on colistin pharmacodynamics in Pseudomonas aeruginosa

The in vitro pharmacodynamics of colistin against Pseudomonas aeruginosa PAO1 wild-type and isogenic knockout strains of phoP and pmrA were evaluated. Colistin killing at subinhibitory concentrations was greater against the phoP and pmrA mutants than the wild type within the first 8 h: the concentration that results in 50% of maximal effect (EC(50)) of the pmrA mutant (0.413 mg/liter) was less than that of the wild type (0.718 mg/liter) (P < 0.05). An in vitro pharmacodynamic model simulating human colistin regimens displayed initial killing followed by regrowth in the phoP mutant and gradual regrowth in the pmrA mutant and wild type.

PmrB mutations promote polymyxin resistance of Pseudomonas aeruginosa isolated from colistin-treated cystic fibrosis patients

Pseudomonas aeruginosa can develop resistance to polymyxin and other cationic antimicrobial peptides. Previous work has shown that mutations in the PmrAB and PhoPQ regulatory systems can confer low to moderate levels of colistin (polymyxin E) resistance in laboratory strains and clinical isolates of this organism (MICs of 8 to 64 mg/liter). To explore the role of PmrAB in high-level clinical polymyxin resistance, P. aeruginosa isolates from chronically colistin-treated cystic fibrosis patients, most with colistin MICs of >512 mg/liter, were analyzed. These cystic fibrosis isolates contained probable gain-of-function pmrB alleles that conferred polymyxin resistance to strains with a wild-type or pmrAB deletion background. Double mutant pmrB alleles that contained mutations in both the periplasmic and dimerization-phosphotransferase domains markedly augmented polymyxin resistance. Expression of mutant pmrB alleles induced transcription from the promoter of the arnB operon and stimulated addition of 4-amino-l-arabinose to lipid A, consistent with the known role of this lipid A modification in polymyxin resistance. For some highly polymyxin-resistant clinical isolates, repeated passage without antibiotic selection pressure resulted in loss of resistance, suggesting that secondary suppressors occur at a relatively high frequency and account for the instability of this phenotype. These results indicate that pmrB gain-of-function mutations can contribute to high-level polymyxin resistance in clinical strains of P. aeruginosa.

Differential Role of Two-Component Regulatory Systems (phoPQ and pmrAB) in Polymyxin B Susceptibility of Pseudomonas aeruginosa

Polymyxins are often considered as a last resort to treat multidrug resistant P. aeruginosa but polymyxin resistance has been increasingly reported worldwide in clinical isolates. Polymyxin resistance in P. aeruginosa is known to be associated with alterations in either PhoQ or PmrB. In this study, mutant strains of P. aeruginosa carrying amino acid substitution, a single and/or dual inactivation of PhoQ and PmrB were constructed to further understand the roles of PhoQ and PmrB in polymyxin susceptibility. Polymyxin B resistance was caused by both inactivation and/or amino acid substitutions in PhoQ but by only amino acid substitutions of PmrB. Alterations of both PhoQ and PmrB resulted in higher levels of polymyxin B resistance than alteration of either PhoQ or PmrB alone. These results were confirmed by time-killing assays suggesting that high-level polymyxin resistance in P. aeruginosa is caused by alterations of both PhoQ and PmrB.

Surface-localized spermidine protects the Pseudomonas aeruginosa outer membrane from antibiotic treatment and oxidative stress

Extracellular DNA acts as a cation chelator and induces the expression of antibiotic resistance genes regulated by Mg(2+) levels. Here we report the characterization of novel DNA-induced genes in Pseudomonas aeruginosa that are annotated as homologs of the spermidine synthesis genes speD (PA4773) and speE (PA4774). The addition of sublethal concentrations of DNA and membrane-damaging antibiotics induced expression of the genes PA4773 to PA4775, as shown using transcriptional lux fusions and quantitative RT-PCR. Exogenous polyamine addition prevented DNA- and peptide-mediated gene induction. Mutation of PA4774 resulted in an increased outer membrane (OM) susceptibility phenotype upon polymyxin B, CP10A, and gentamicin treatment. When the membrane-localized fluorescent probe C(11)-BODIPY(581/591) was used as an indicator of peroxidation of membrane lipids, the PA4774::lux mutant demonstrated an increased susceptibility to oxidative membrane damage from H(2)O(2) treatment. Addition of exogenous polyamines protected the membranes of the PA4774::lux mutant from polymyxin B and H(2)O(2) treatment. Polyamines from the outer surface were isolated and shown to contain putrescine and spermidine by using high-performance liquid chromatography and mass spectrometry. The PA4774::lux mutant did not produce spermidine on the cell surface, but genetic complementation restored surface spermidine production as well as the antibiotic and oxidative stress resistance phenotypes of the membrane. We have identified new functions for spermidine on the cell surface and propose that polyamines are produced under Mg(2+)-limiting conditions as an organic polycation to bind lipopolysaccharide (LPS) and to stabilize and protect the outer membrane against antibiotic and oxidative damage.

PAMPs, PRRs and the genomics of gram negative bacterial recognition in fish

Understanding the mechanisms that underpin pathogen recognition and subsequent orchestration of the immune response in fish is an area of significant importance for both basic research and management of health in aquaculture. In recent years much attention has been given to the identification of pattern recognition receptors (PRRs) in fish, however, characterisation of interactions with specific pathogen-associated molecular patterns (PAMPs) is still incomplete. Microarray studies have significantly contributed to functional studies and early descriptions of PAMP-PRR driven activation of specific response cassettes in the genome have been obtained although much is left to be done. In this review we will address gram negative (G-negative) bacterial recognition in fish addressing contributing factors such as structure-function relationships between G-negative PAMPs, current knowledge of fish PRRs and the input achieved by microarray-based studies ranging from in vivo infection studies to directed in vitro PAMP-cell studies. Finally we revisit the endotoxic recognition paradigm in fish and suggest a series of future perspectives that could contribute toward the further elucidation of G-negative bacterial recognition across the highly diverse group of vertebrates that encompass the fishes.

Synthesis of Neoglycoconjugates Containing 4-Amino-4-deoxy-l-arabinose Epitopes Corresponding to the Inner Core of Burkholderia and Proteus Lipopolysaccharides

Disaccharides that contain 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) and d-glycero-d-talo-oct-2-ulosonic acid (Ko) substituted at the 8-position by 4-amino-4-deoxy-β-l-arabinopyranosyl (Ara4N) residues have been prepared. Coupling an N-phenyltrifluoroacetimidate-4-azido-4-deoxy-l-arabinosylglycosyl donor to acetyl-protected allyl glycosides of Kdo and Ko afforded anomeric mixtures of disaccharide products in 74 and 90 % yield, respectively, which were separated by chromatography. Further extension of an intermediate Ara4N-(1→8)-Kdo disaccharide acceptor, which capitalized on a regioselective glycosylation with a Kdo bromide donor under Helferich conditions, afforded the branched trisaccharide α-Kdo-(2→4)[β-l-Ara4N-(1→8)]-α-Kdo derivative. Deprotection of the protected di- and trisaccharide allyl glycosides was accomplished by TiCl₄-promoted benzyl ether cleavage followed by the removal of ester groups and reduction of the azido group with thiol or Staudinger reagents, respectively. The reaction of the anomeric allyl group with 1,3-propanedithiol under radical conditions afforded the thioether-bridged spacer glycosides, which were efficiently coupled to maleimide-activated bovine serum albumin. The neoglycoconjugates serve as immunoreagents with specificity for inner core epitopes of Burkholderia and Proteus lipopolysaccharides.

PhoQ mutations promote lipid A modification and polymyxin resistance of Pseudomonas aeruginosa found in colistin-treated cystic fibrosis patients

Pseudomonas aeruginosa can develop resistance to polymyxin and other cationic antimicrobial peptides. Previous work has shown that mutations in the PmrAB and PhoPQ regulatory systems can confer low to moderate levels of polymyxin resistance (MICs of 8 to 64 mg/liter) in laboratory and clinical strains of this organism. To explore the role of PhoPQ in high-level clinical polymyxin resistance, P. aeruginosa strains with colistin MICs > 512 mg/liter that had been isolated from cystic fibrosis patients treated with inhaled colistin (polymyxin E) were analyzed. Probable loss-of-function phoQ alleles found in these cystic fibrosis strains conferred resistance to polymyxin. Partial and complete suppressor mutations in phoP were identified in some cystic fibrosis strains with resistance-conferring phoQ mutations, suggesting that additional loci can be involved in polymyxin resistance in P. aeruginosa. Disruption of chromosomal phoQ in the presence of an intact phoP allele stimulated 4-amino-l-arabinose addition to lipid A and induced transcription from the promoter of the pmrH (arnB) operon, consistent with the known role of this lipid A modification in polymyxin resistance. These results indicate that phoQ loss-of-function mutations can contribute to high-level polymyxin resistance in clinical strains of P. aeruginosa.

Inhaled antibiotics for Gram-negative respiratory infections

Several disease states create conditions that lead to opportunistic Gram-negative respiratory infections. Inhalation is the most direct and, until recently, underutilized means of antimicrobial drug targeting for respiratory tract infections. All approved antimicrobial agents for administration by inhalation are indicated for Pseudomonas aeruginosa infections in patients with cystic fibrosis. These inhaled therapies have directly contributed to a significant reduction in exacerbations and hospitalizations in this patient population over the last few decades. The relentless adaptation of pathogenic organisms to current treatment options demands that the pharmaceutical industry continue designing next-generation antimicrobial agents over 70 years after they were first introduced. Recent technological advances in inhalation devices and drug formulation techniques have broadened the scope of antimicrobial structural classes that can be investigated by inhalation; however, there is an urgent need to discover novel compounds with improved resistance profiles relative to those drugs that are already marketed.

Effect of colistin exposure and growth phase on the surface properties of live Acinetobacter baumannii cells examined by atomic force microscopy

The diminishing antimicrobial development pipeline has forced the revival of colistin as a last line of defence against infections caused by multidrug-resistant Gram-negative 'superbugs' such as Acinetobacter baumannii. The complete loss of lipopolysaccharide (LPS) mediates colistin resistance in some A. baumannii strains. Atomic force microscopy was used to examine the surface properties of colistin-susceptible and -resistant A. baumannii strains at mid-logarithmic and stationary growth phases in liquid and in response to colistin treatment. The contribution of LPS to surface properties was investigated using A. baumannii strains constructed with and without the lpxA gene. Bacterial spring constant measurements revealed that colistin-susceptible cells were significantly stiffer than colistin-resistant cells at both growth phases (P<0.01), whilst colistin treatment at high concentrations (32 mg/L) resulted in more rigid surfaces for both phenotypes. Multiple, large adhesive peaks frequently noted in force curves captured on colistin-susceptible cells were not evident for colistin-resistant cells. Adhesion events were markedly reduced following colistin exposure. The cell membranes of strains of both phenotypes remained intact following colistin treatment, although fine topographical details were illustrated. These studies, conducted for the first time on live A. baumannii cells in liquid, have contributed to our understanding of the action of colistin in this problematic pathogen.

Pseudomonas aeruginosa: resistance to the max

Pseudomonas aeruginosa is intrinsically resistant to a variety of antimicrobials and can develop resistance during anti-pseudomonal chemotherapy both of which compromise treatment of infections caused by this organism. Resistance to multiple classes of antimicrobials (multidrug resistance) in particular is increasingly common in P. aeruginosa, with a number of reports of pan-resistant isolates treatable with a single agent, colistin. Acquired resistance in this organism is multifactorial and attributable to chromosomal mutations and the acquisition of resistance genes via horizontal gene transfer. Mutational changes impacting resistance include upregulation of multidrug efflux systems to promote antimicrobial expulsion, derepression of ampC, AmpC alterations that expand the enzyme's substrate specificity (i.e., extended-spectrum AmpC), alterations to outer membrane permeability to limit antimicrobial entry and alterations to antimicrobial targets. Acquired mechanisms contributing to resistance in P. aeruginosa include β-lactamases, notably the extended-spectrum β-lactamases and the carbapenemases that hydrolyze most β-lactams, aminoglycoside-modifying enzymes, and 16S rRNA methylases that provide high-level pan-aminoglycoside resistance. The organism's propensity to grow in vivo as antimicrobial-tolerant biofilms and the occurrence of hypermutator strains that yield antimicrobial resistant mutants at higher frequency also compromise anti-pseudomonal chemotherapy. With limited therapeutic options and increasing resistance will the untreatable P. aeruginosa infection soon be upon us?

Bile salts induce resistance to polymyxin in enterohemorrhagic Escherichia coli O157:H7

Many enteric bacteria use bile as an environmental cue to signal resistance and virulence gene expression. Microarray analysis of enterohemorrhagic Escherichia coli O157:H7 (EHEC) treated with bile salts revealed upregulation of genes for an efflux system (acrAB), a two-component signal transduction system (basRS/pmrAB), and lipid A modification (arnBCADTEF and ugd). Bile salt treatment of EHEC produced a basS- and arnT-dependent resistance to polymyxin.

A Mig-14-like protein (PA5003) affects antimicrobial peptide recognition in Pseudomonas aeruginosa

The evolution of antibiotic resistance in pathogenic bacteria is a growing global health problem which is gradually making the treatment of infectious diseases less efficient. Antimicrobial peptides are small charged molecules found in organisms from the complete phylogenetic spectrum. The peptides are attractive candidates for novel drug development due to their activity against bacteria that are resistant to conventional antibiotics, and reports of peptide resistance are rare in the clinical setting. Paradoxically, many clinically relevant bacteria have mechanisms that can recognize and respond to the presence of cationic antimicrobial peptides (CAMPs) in the environment by changing the properties of the microbial surface thereby increasing the tolerance of the microbes towards the peptides. In Pseudomonas aeruginosa an essential component of this inducible tolerance mechanism is the lipopolysaccharide modification operon arnBCADTEF-PA3559 which encodes enzymes required for LPS alterations leading to increased antimicrobial peptide tolerance. The expression of the operon is induced by the presence of CAMPs in the environment but the molecular mechanisms underlying the cellular recognition of the peptides are poorly elucidated. In this work, we investigate the factors influencing arnB expression by transposon mutagenesis and arnB promoter green fluorescent protein reporters. We have identified a novel gene encoding a Mig-14-like protein that is required for recognition of the CAMPs colistin and Novispirin G10 by P. aeruginosa. Moreover, we show that this gene is also required for the formation of CAMP-tolerant subpopulations in P. aeruginosa hydrodynamic flow chamber biofilms.

The pmrCAB operon mediates polymyxin resistance in Acinetobacter baumannii ATCC 17978 and clinical isolates through phosphoethanolamine modification of lipid A

The emergence of multidrug resistance among Acinetobacter baumannii is leading to an increasing dependence on the use of polymyxins as last-hope antibiotics. Here, we utilized genetic and biochemical methods to define the involvement of the pmrCAB operon in polymyxin resistance in this organism. Sequence analysis of 16 polymyxin B-resistant strains, including 6 spontaneous mutants derived from strain ATCC 17978 and 10 clinical isolates from diverse sources, revealed that they had independent mutations in the pmrB gene, encoding a sensor kinase, or in the response regulator PmrA. Knockout of the pmrB gene in two mutants and two clinical isolates led to a decrease in the polymyxin B susceptibility of these strains, which could be restored with the cloned pmrAB genes from the mutants but not from the wild type. Reverse transcription-quantitative PCR (RT-qPCR) analysis also showed a correlation between the expression of pmrC and polymyxin B resistance. Characterization of lipid A species from the mutant strains, by thin-layer chromatography and mass spectrometry, indicated that the addition of phosphoethanolamine to lipid A correlated with resistance. This addition is performed in Salmonella enterica serovar Typhimurium by the product of the pmrC gene, which is a homolog of the pmrC gene from Acinetobacter. Knockout of this gene in the mutant R2 [pmrB(T235I)] reversed resistance as well as phosphoethanolamine modification of lipid A. These results demonstrate that specific alterations in the sequence of the pmrCAB operon are responsible for resistance to polymyxins in A. baumannii.

Genetic and Functional Diversity of Pseudomonas aeruginosa Lipopolysaccharide

Lipopolysccharide (LPS) is an integral component of the Pseudomonas aeruginosa cell envelope, occupying the outer leaflet of the outer membrane in this Gram-negative opportunistic pathogen. It is important for bacterium-host interactions and has been shown to be a major virulence factor for this organism. Structurally, P. aeruginosa LPS is composed of three domains, namely, lipid A, core oligosaccharide, and the distal O antigen (O-Ag). Most P. aeruginosa strains produce two distinct forms of O-Ag, one a homopolymer of D-rhamnose that is a common polysaccharide antigen (CPA, formerly termed A band), and the other a heteropolymer of three to five distinct (and often unique dideoxy) sugars in its repeat units, known as O-specific antigen (OSA, formerly termed B band). Compositional differences in the O units among the OSA from different strains form the basis of the International Antigenic Typing Scheme for classification via serotyping of different strains of P. aeruginosa. The focus of this review is to provide state-of-the-art knowledge on the genetic and resultant functional diversity of LPS produced by P. aeruginosa. The underlying factors contributing to this diversity will be thoroughly discussed and presented in the context of its contributions to host-pathogen interactions and the control/prevention of infection.

Phosphoethanolamine modification of lipid A in colistin-resistant variants of Acinetobacter baumannii mediated by the pmrAB two-component regulatory system

Colistin resistance is rare in Acinetobacter baumannii, and little is known about its mechanism. We investigated the role of PmrCAB in this trait, using (i) resistant and susceptible clinical strains, (ii) laboratory-selected mutants of the type strain ATCC 19606 and of the clinical isolate ABRIM, and (iii) a susceptible/resistant pair of isogenic clinical isolates, Ab15/133 and Ab15/132, isolated from the same patient. pmrAB sequences in all the colistin-susceptible isolates were identical to reference sequences, whereas resistant clinical isolates harbored one or two amino acid replacements variously located in PmrB. Single substitutions in PmrB were also found in resistant mutants of strains ATCC 19606 and ABRIM and in the resistant clinical isolate Ab15/132. No mutations in PmrA or PmrC were found. Reverse transcriptase (RT)-PCR identified increased expression of pmrA (4- to 13-fold), pmrB (2- to 7-fold), and pmrC (1- to 3-fold) in resistant versus susceptible organisms. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry showed the addition of phosphoethanolamine to the hepta-acylated form of lipid A in the resistant variants and in strain ATCC 19606 grown under low-Mg(2+) induction conditions. pmrB gene knockout mutants of the colistin-resistant ATCC 19606 derivative showed >100-fold increased susceptibility to colistin and 5-fold decreased expression of pmrC; they also lacked the addition of phosphoethanolamine to lipid A. We conclude that the development of a moderate level of colistin resistance in A. baumannii requires distinct genetic events, including (i) at least one point mutation in pmrB, (ii) upregulation of pmrAB, and (iii) expression of pmrC, which lead to addition of phosphoethanolamine to lipid A.

Correlation between overexpression and amino acid substitution of the PmrAB locus and colistin resistance in Acinetobacter baumannii

Relationships between the PmrAB two-component system and colistin resistance were investigated in Acinetobacter baumannii. The sequences of pmrA, pmrB and pmrC in 26 colistin-susceptible (ColS) and 7 colistin-resistant (ColR) A. baumannii isolates were determined. In addition, 30 ColR mutants (colistin minimum inhibitory concentration >64 mg/L) were selected in vitro from 10 ColS strains and the pmrA, pmrB and pmrC sequences of the in-vitro-selected ColR mutants were also determined. Expression of pmrA and pmrB was compared between the ColR mutants and their parent ColS strains using a quantitative real-time polymerase chain reaction method. Elevated expression of pmrA and pmrB genes was evident both in wild-type and in in-vitro-selected ColR strains. However, no amino acid differences in the pmrA, pmrB and pmrC genes were found between wild-type ColR and ColS isolates. Although six kinds of amino acid alterations in pmrB were identified in in-vitro-selected ColR mutants, no changes were found in some of the mutants. These findings indicate that increased expression of the PmrAB system is essential for colistin resistance in A. baumannii but that amino acid alterations might be only partially responsible for resistance.

Pseudomonas aeruginosa: resistance to the max

Pseudomonas aeruginosa is intrinsically resistant to a variety of antimicrobials and can develop resistance during anti-pseudomonal chemotherapy both of which compromise treatment of infections caused by this organism. Resistance to multiple classes of antimicrobials (multidrug resistance) in particular is increasingly common in P. aeruginosa, with a number of reports of pan-resistant isolates treatable with a single agent, colistin. Acquired resistance in this organism is multifactorial and attributable to chromosomal mutations and the acquisition of resistance genes via horizontal gene transfer. Mutational changes impacting resistance include upregulation of multidrug efflux systems to promote antimicrobial expulsion, derepression of ampC, AmpC alterations that expand the enzyme's substrate specificity (i.e., extended-spectrum AmpC), alterations to outer membrane permeability to limit antimicrobial entry and alterations to antimicrobial targets. Acquired mechanisms contributing to resistance in P. aeruginosa include β-lactamases, notably the extended-spectrum β-lactamases and the carbapenemases that hydrolyze most β-lactams, aminoglycoside-modifying enzymes, and 16S rRNA methylases that provide high-level pan-aminoglycoside resistance. The organism's propensity to grow in vivo as antimicrobial-tolerant biofilms and the occurrence of hypermutator strains that yield antimicrobial resistant mutants at higher frequency also compromise anti-pseudomonal chemotherapy. With limited therapeutic options and increasing resistance will the untreatable P. aeruginosa infection soon be upon us?

Resurgence of colistin: a review of resistance, toxicity, pharmacodynamics, and dosing

Colistin is a polymyxin antibiotic that was discovered in the late 1940s for the treatment of gram-negative infections. After several years of clinical use, its popularity diminished because of reports of significant nephrotoxicity and neurotoxicity. Recently, the antibiotic has resurfaced as a last-line treatment option for multidrug-resistant organisms such as Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. The need for antibiotics with coverage of these gram-negative pathogens is critical because of their high morbidity and mortality, making colistin a very important treatment option. Unfortunately, however, resistance to colistin has been documented among all three of these organisms in case reports. Although the exact mechanism causing colistin resistance has not been defined, it is hypothesized that the PmrA-PmrB and PhoP-PhoQ genetic regulatory systems may play a role. Colistin dosages must be optimized, as colistin is a last-line treatment option; in addition, suboptimal doses have been linked to the development of resistance. The lack of pharmacokinetic and pharmacodynamic studies and no universal harmonization of dose units, however, have made it difficult to derive optimal dosing regimens and specific dosing guidelines for colistin. In critically ill patients who may have multiorgan failure, renal insufficiency may alter colistin pharmacokinetics. Therefore, dosage alterations in this patient population are imperative to achieve maximal efficacy and minimal toxicity. With regard to colistin toxicity, most studies show that nephrotoxicity is reversible and less frequent than once thought, and neurotoxicity is rare. Further research is needed to fully understand the impact that the two regulatory systems have on resistance, as well as the dosages of colistin needed to inhibit and overcome these developing patterns.

Pseudomonas aeruginosa biofilms in cystic fibrosis

The persistence of chronic Pseudomonas aeruginosa lung infections in cystic fibrosis (CF) patients is due to biofilm-growing mucoid (alginate-producing) strains. A biofilm is a structured consortium of bacteria, embedded in a self-produced polymer matrix consisting of polysaccharide, protein and DNA. In CF lungs, the polysaccharide alginate is the major part of the P. aeruginosa biofilm matrix. Bacterial biofilms cause chronic infections because they show increased tolerance to antibiotics and resist phagocytosis, as well as other components of the innate and the adaptive immune system. As a consequence, a pronounced antibody response develops, leading to immune complex-mediated chronic inflammation, dominated by polymorphonuclear leukocytes. The chronic inflammation is the major cause of the lung tissue damage in CF. Biofilm growth in CF lungs is associated with an increased frequency of mutations, slow growth and adaptation of the bacteria to the conditions in the lungs, and to antibiotic therapy. Low bacterial metabolic activity and increase of doubling times of the bacterial cells in CF lungs are responsible for some of the tolerance to antibiotics. Conventional resistance mechanisms, such as chromosomal β-lactamase, upregulated efflux pumps, and mutations of antibiotic target molecules in the bacteria, also contribute to the survival of P. aeruginosa biofilms. Biofilms can be prevented by early aggressive antibiotic prophylaxis or therapy, and they can be treated by chronic suppressive therapy.

Identification of nonclonal Pseudomonas aeruginosa isolates with reduced colistin susceptibility in Korea

The in vitro activity of colistin was evaluated against 215 nonduplicated Pseudomonas aeruginosa isolates, including 53 multidrug-resistant isolates, which were collected between 2006 and 2007 from nine tertiary care hospitals in Korea. Colistin-nonsusceptible P. aeruginosa (CNPA) isolates were genotyped using multilocus sequence typing. Sixteen (7.4%) CNPA isolates (minimum inhibitory concentration [MIC], >2 mg/l) were identified, including three resistant isolates. All but one of the MDR P. aeruginosa isolates was susceptible to colistin. Multilocus sequence typing analysis identified 12 sequence types (STs) among 16 CNPA isolates, indicating that colistin nonsusceptibility might arise independently. However, ST244 and ST292, which may be international clones, were found in multiple CNPA isolates. Our data indicate an increase of P. aeruginosa isolates with reduced colistin susceptibility, suggesting the need for continuous surveillance of P. aeruginosa.

Creeping baselines and adaptive resistance to antibiotics

The introduction of antimicrobial drugs in medicine gave hope for a future in which all infectious diseases could be controlled. Decades later it appears certain this will not be the case, because antibiotic resistance is growing relentlessly. Bacteria possess an extraordinary ability to adapt to environmental challenges like antimicrobials by both genetic and phenotypic means, which contributes to their evolutionary success. It is becoming increasingly appreciated that adaptation is a major mechanism behind the acquisition and evolution of antibiotic resistance. Adaptive resistance is a specific class of non-mutational resistance that is characterized by its transient nature. It occurs in response to certain environmental conditions or due to epigenetic phenomena like persistence. We propose that this type of resistance could be the key to understanding the failure of some antibiotic therapy programs, although adaptive resistance mechanisms are still somewhat unexplored. Similarly, hard wiring of some of the changes involved in adaptive resistance might explain the phenomenon of "baseline creep" whereby the average minimal inhibitory concentration (MIC) of a given medically important bacterial species increases steadily but inexorably over time, making the likelihood of breakthrough resistance greater. This review summarizes the available information on adaptive resistance.

Impaired virulence and in vivo fitness of colistin-resistant Acinetobacter baumannii

Acinetobacter baumannii (American Type Culture Collection strain 19606) acquires mutations in the pmrB gene during the in vitro development of resistance to colistin. The colistin-resistant strain has lower affinity for colistin, reduced in vivo fitness (competition index, .016), and decreased virulence, both in terms of mortality (0% lethal dose, 6.9 vs 4.9 log colony-forming units) and survival in a mouse model of peritoneal sepsis. These results may explain the low incidence and dissemination of colistin resistance in A. baumannii in clinical settings.

A two-component regulatory system interconnects resistance to polymyxins, aminoglycosides, fluoroquinolones, and β-lactams in Pseudomonas aeruginosa

Constitutive overexpression of the active efflux system MexXY/OprM is a major cause of resistance to aminoglycosides, fluoroquinolones, and cefepime in clinical strains of Pseudomonas aeruginosa. Upregulation of this pump often results from mutations occurring in mexZ, the local repressor gene of the mexXY operon. In this study, analysis of MexXY-overproducing mutants selected in vitro from reference strain PAO1Bes on amikacin (at a concentration 1.5-fold higher than the MIC) led to identification of a new class of mutants harboring an intact mexZ gene and exhibiting increased resistance to colistin and imipenem in addition to aminoglycosides, fluoroquinolones, and cefepime. Reverse transcription-quantitative PCR (RT-qPCR) experiments on a selected clone named PAOW2 demonstrated that mexXY overexpression was independent of mexZ and the PA5471 gene, which is required for drug-dependent induction of mexXY. Furthermore, the transcript levels of the oprD gene, which encodes the carbapenem-selective porin OprD, were found to be reduced drastically in PAOW2. Whole-genome sequencing revealed a single mutation resulting in an M59I substitution in the ParR protein, the response regulator of the ParRS two-component regulatory system (with ParS being the sensor kinase), which is required for adaptive resistance of P. aeruginosa to polycationic peptides such as colistin. The multidrug resistance phenotype was suppressed in PAOW2 by deletion of the parS and parRS genes and conferred to PAO1Bes by chromosomal insertion of the mutated parRS locus from PAOW2. As shown by transcriptomic analysis, only a very small number of genes were expressed differentially between PAOW2 and PAO1Bes, including the lipopolysaccharide (LPS) modification operon arnBCADTEF-ugd, responsible for resistance to polycationic agents. Exposure of wild-type PAO1Bes to different polycationic peptides, including colistin, was shown to result in increased mexY and repressed oprD expression via ParRS, independent of PA5471. In agreement with these results, colistin antagonized activity of the MexXY/OprM substrates in PAO1Bes but not in a ΔparRS derivative. Finally, screening of clinical strains exhibiting the PAOW2 resistance phenotype allowed the identification of additional alterations in ParRS. Collectively, our data indicate that ParRS may promote either induced or constitutive multidrug resistance to four different classes of antibiotics through the activation of three distinct mechanisms (efflux, porin loss, and LPS modification).

Design, synthesis, and evaluation of a new fluorescent probe for measuring polymyxin-lipopolysaccharide binding interactions

Fluorescence assays employing semisynthetic or commercial dansyl-polymyxin B have been widely employed to assess the affinity of polycations, including polymyxins, for bacterial cells and lipopolysaccharide (LPS). The five primary γ-amines on diaminobutyric acid residues of polymyxin B are potentially derivatized with dansyl-chloride. Mass spectrometric analysis of the commercial product revealed a complex mixture of di- or tetra-dansyl-substituted polymyxin B. We synthesized a mono-substituted fluorescent derivative, dansyl[Lys]¹polymyxin B₃. The affinity of polymyxin for purified gram-negative LPS and whole bacterial cells was investigated. The affinity of dansyl[Lys]¹polymyxin B₃ for LPS was comparable to polymyxin B and colistin, and considerably greater (K(d)<1 μM) than for whole cells (K(d)∼6-12μM). Isothermal titration calorimetric studies demonstrated exothermic enthalpically driven binding between both polymyxin B and dansyl[Lys]¹polymyxin B₃ to LPS, attributed to electrostatic interactions. The hydrophobic dansyl moiety imparted a greater entropic contribution to the dansyl[Lys]¹polymyxin B₃-LPS reaction. Molecular modeling revealed a loss of electrostatic contact within the dansyl[Lys]¹polymyxin B₃-LPS complex due to steric hindrance from the dansyl[Lys]¹ fluorophore; this corresponded with diminished antibacterial activity (MIC≥16μg/mL). Dansyl[Lys]¹polymyxin B₃ may prove useful as a screening tool for drug development.

Polymyxins and their novel derivatives

The emerging very multiresistant Gram-negative bacteria cause remarkable therapeutic challenges. There are no novel classes of agents in clinical development for the treatment of Gram-negative infections. Polymyxins (polymyxin B and colistin) were abandoned in the seventies but are now back in the therapy as the last resort. Their nephrotoxicity may complicate the therapy or even necessitate its discontinuation. Less toxic polymyxin derivatives would be highly welcome. Novel derivatives lack in strategic positions two of the five cationic charges of polymyxins, differ from polymyxins in their renal handling and affinity to kidney brush-border membrane, and are in preclinical studies. Less characterized other recent derivatives, also reviewed here, have increased the collective knowledge on the structure-function relationships in polymyxins. Acquired resistance to polymyxins has been encountered. However, the resistance mechanism compromises the function of the bacterial outer membrane as a permeability barrier to other noxious agents.

Salmonella-regulated lipopolysaccharide modifications

Salmonella enterica are Gram-negative enteric pathogens that cause typhoid fever and gastroenteritis in humans. Many bacteria, including Salmonella, use signal transduction cascades such as two-component regulatory systems to detect and respond to stimuli in the local microenvironment. During infection, environmental sensing allows bacteria to regulate gene expression to evade host immune defenses and thrive in vivo. Activation of the Salmonella two-component regulatory systems PhoP-PhoQ and PmrA-PmrB and the RcsC-RcsD-RcsB phosphorylay by specific environmental signals in the intestine and within host cells leads to several lipopolysaccharide modifications that promote bacterial survival, cationic antimicrobial peptide resistance and virulence. Many pathogens encode orthologs to Salmonella two-component regulatory systems and also modify the lipopolysaccharide to escape killing by the host immune response. However, these organisms often regulate their virulence genes, including those responsible for lipopolysaccharide modification, in ways that differ from Salmonella. Further examination of bacterial virulence gene regulation and lipopolysaccharide modifications may lead to improved antimicrobial therapies and vaccines.

Efficient Synthesis of 4-Amino-4-deoxy-L-arabinose and Spacer-equipped 4-Amino-4-deoxy-L-arabinopyranosides by Transglycosylation Reactions

Methyl 4-azido-4-deoxy-β-L-arabinopyranoside has been synthesized in five steps starting from methyl β-D-xylopyranoside in a multigram scale without chromatographic purification in 78% overall yield. The transformation relied on selective tosylation/nosylation at O-4 followed by acylation, S(N)2 displacement with sodium azide and subsequent deprotection. The methyl 4-azido-4-deoxy-arabinoside was then converted into allyl, propenyl, ω-bromohexyl and chlorethoxyethyl spacer glycosides by transglycosylation with the respective alcohols in good yields and fair anomeric selectivity. Reduction of the azido group and further transformations of the aglycon afforded ω-thiol-containing spacer derivatives. Coupling to maleimide-activated BSA provided a potent immunogen which was used to generate murine and rabbit polyclonal sera binding to LPS-core epitopes containing 4-amino-4-deoxy-arabinose residues.

Molecular characterization of the PhoPQ-PmrD-PmrAB mediated pathway regulating polymyxin B resistance in Klebsiella pneumoniae CG43

Background

The cationic peptide antibiotic polymyxin has recently been reevaluated in the treatment of severe infections caused by gram negative bacteria.

Methods

In this study, the genetic determinants for capsular polysaccharide level and lipopolysaccharide modification involved in polymyxin B resistance of the opportunistic pathogen Klebsiella pneumoniae were characterized. The expressional control of the genes responsible for the resistance was assessed by a LacZ reporter system. The PmrD connector-mediated regulation for the expression of pmr genes involved in polymyxin B resistance was also demonstrated by DNA EMSA, two-hybrid analysis and in vitro phosphor-transfer assay.

Results

Deletion of the rcsB, which encoded an activator for the production of capsular polysaccharide, had a minor effect on K. pneumoniae resistance to polymyxin B. On the other hand, deletion of ugd or pmrF gene resulted in a drastic reduction of the resistance. The polymyxin B resistance was shown to be regulated by the two-component response regulators PhoP and PmrA at low magnesium and high iron, respectively. Similar to the control identified in Salmonella, expression of pmrD in K. pneumoniae was dependent on PhoP, the activated PmrD would then bind to PmrA to prolong the phosphorylation state of the PmrA, and eventually turn on the expression of pmr for the resistance to polymyxin B.

Conclusions

The study reports a role of the capsular polysaccharide level and the pmr genes for K. pneumoniae resistance to polymyxin B. The PmrD connector-mediated pathway in governing the regulation of pmr expression was demonstrated. In comparison to the pmr regulation in Salmonella, PhoP in K. pneumoniae plays a major regulatory role in polymyxin B resistance.

Colistin susceptibility testing: evaluation of reliability for cystic fibrosis isolates of Pseudomonas aeruginosa and Stenotrophomonas maltophilia

OBJECTIVES

Antibiotic susceptibility methods that are commonly used to test bacterial isolates from patients with cystic fibrosis are of uncertain reliability for the polymyxins. To assess the reliability of four standard testing methods, this pilot study used a challenge set that included polymyxin-resistant isolates of Pseudomonas aeruginosa and Stenotrophomonas maltophilia.

METHODS

Twenty-five P. aeruginosa and 12 S. maltophilia isolates were tested for susceptibility to colistin (polymyxin E). Repeatability (concordance of replicates performed concurrently), reproducibility (concordance of replicates performed over time) and comparability (concordance of different methods) of agar dilution, broth microdilution, Etest and disc diffusion were assessed through the use of descriptive statistics and scatterplot analyses.

RESULTS

All four methods displayed excellent repeatability (overall concordance rate of 99%). However, analysis of reproducibility revealed substantially lower rates of concordance (74% for agar dilution, 84% for broth microdilution and Etest, and 91% for disc diffusion). In addition, comparability to agar dilution of the three other methods was generally poor, with overall rates of very major error ranging from 12% for broth microdilution to 18% for Etest and disc diffusion.

CONCLUSIONS

Compared with agar dilution, other susceptibility testing methods give high rates of apparent false polymyxin susceptibility for cystic fibrosis isolates of P. aeruginosa and S. maltophilia. Prospective study of the correlation between in vitro susceptibility and clinical response is needed to clarify whether these discrepancies reflect oversensitivity of the agar dilution method or insensitivity of the other methods.

Biological 'glue' and 'Velcro': molecular tools for adhesion and biofilm formation in the hairy and gluey bug Pseudomonas aeruginosa

Pseudomonas aeruginosa contains an extraordinarily large number of loci encoding systems facilitating a communal lifestyle and binding to supports of various natures. These P. aeruginosa systems are reviewed here and may be categorized as classical or non-classical systems. They highlight the panoply of strategies that this hairy and gluey bacterium has developed for dealing with the diverse environments with which it is faced during various types of infection, involving complex regulatory networks that have not yet been fully elucidated but several aspects of which are discussed here.

Comprehensive structure characterization of lipid A extracted from Yersinia pestis for determination of its phosphorylation configuration

We report on comprehensive structure characterization of lipid A extracted from Yersinia pestis (Yp) for determination of its phosphorylation configuration that was achieved by combining the methods of molecular biology with high-resolution tandem mass spectrometry. The phosphorylation pattern of diphosphorylated lipid A extracted from Yp has recently been found to be a heterogeneous mixture of C-1 and C-4' bisphosphate, C-1 pyrophosphate, and C-4' pyrophosphate (Proc. Natl. Acad. Sci. 2008, 105, 12742). To reduce the inherent phosphate heterogeneity of diphosphorylated lipid A extracted from Yp, we incorporated specific C-1 and C-4' position phosphatases into wild type KIM6+ Yp grown at 37 degrees C. Comprehensive high-resolution tandem mass spectrometric analyses of lipid A extracted from Yp modified with either the C-1 or C-4' phosphatase allowed for unambiguous structure assignment of monophosphorylated and diphosphorylated lipid A and distinction of isomeric bisphosphate and pyrophosphate forms. The prevalent aminoarabinose modification was determined to be exclusively attached to the lipid A disaccharide via a phospho-diester linkage at either or both the C-1 and C-4' positions.

Attenuation of colistin bactericidal activity by high inoculum of Pseudomonas aeruginosa characterized by a new mechanism-based population pharmacodynamic model

Colistin is increasingly being utilized against Gram-negative pathogens, including Pseudomonas aeruginosa, resistant to all other antibiotics. Since limited data exist regarding killing by colistin at different initial inocula (CFUo), we evaluated killing of Pseudomonas aeruginosa by colistin at several CFUo and developed a mechanism-based mathematical model accommodating a range of CFUo. In vitro time-kill experiments were performed using >or=8 concentrations up to 64 x the MIC of colistin against P. aeruginosa PAO1 and two clinical P. aeruginosa isolates at CFUo of 10(6), 10(8), and 10(9) CFU/ml. Serial samples up to 24 h were simultaneously modeled in the NONMEM VI (results shown) and S-ADAPT software programs. The mathematical model was prospectively "validated" by additional time-kill studies assessing the effect of Ca(2+) and Mg(2+) on killing of PAO1 by colistin. Against PAO1, killing of the susceptible population was 23-fold slower at the 10(9) CFUo and 6-fold slower at the 10(8) CFUo than at the 10(6) CFUo. The model comprised three populations with different second-order killing rate constants (5.72, 0.369, and 0.00210 liters/h/mg). Bacteria were assumed to release signal molecules stimulating a phenotypic change that inhibits killing. The proposed mechanism-based model had a good predictive performance, could describe killing by colistin for all three studied strains and for two literature studies, and performed well in a prospective validation with various concentrations of Ca(2+) and Mg(2+). The extent and rate of killing of P. aeruginosa by colistin were markedly decreased at high CFUo compared to those at low CFUo. This was well described by a mechanism-based mathematical model, which should be further validated using dynamic in vitro models.

Substitution of the Bordetella pertussis lipid A phosphate groups with glucosamine is required for robust NF-kappaB activation and release of proinflammatory cytokines in cells expressing human but not murine Toll-like receptor 4-MD-2-CD14

Bordetella pertussis endotoxin is a key modulator of the host immune response, mainly due to the role of its lipid A moiety in Toll-like receptor 4 (TLR4)-mediated signaling. We have previously demonstrated that the lipid A phosphate groups of B. pertussis BP338 can be substituted with glucosamine in a BvgAS-regulated manner. Here we examined the effect of this lipid A modification on the biological activity of B. pertussis endotoxin. We compared purified endotoxin and heat-killed B. pertussis BP338 whole cells that have modified lipid A phosphate groups to an isogenic mutant lacking this modification with respect to their capacities to induce the release of inflammatory cytokines by human and murine macrophages and to participate in the TLR4-mediated activation of NF-kappaB in transfected HEK-293 cells. We found inactivated B. pertussis cells to be stronger inducers of proinflammatory cytokines in THP-1-derived macrophages when lipid A was modified. Most notably, lack of lipid A modification abolished the ability of purified B. pertussis endotoxin to induce the release of inflammatory cytokines by human THP-1-derived macrophages but led to only slightly reduced inflammatory cytokine levels when stimulating murine (RAW 264.7) macrophages. Accordingly, upon stimulation of HEK-293 cells with inactivated bacteria and purified endotoxin, lack of lipid A modification led to impaired NF-kappaB activation only when human, and not when murine, TLR4-MD-2-CD14 was expressed. We speculate that in B. pertussis, lipid A modification has evolved to benefit the bacteria during human infection by modulating immune defenses rather than to evade innate immune recognition.

Characterization of UDP-glucose dehydrogenase and UDP-glucose pyrophosphorylase mutants of Proteus mirabilis: defectiveness in polymyxin B resistance, swarming, and virulence

Proteus mirabilis is known to be highly resistant to the action of polymyxin B (PB). However, the mechanism underlying PB resistance is not clear. In this study, we used Tn5 transposon mutagenesis to identify genes that may affect PB resistance in P. mirabilis. Two genes, ugd and galU, which may encode UDP-glucose dehydrogenase (Ugd) and UDP-glucose pyrophosphorylase (GalU), respectively, were identified. Knockout mutants of ugd and galU were found to be extremely sensitive to PB, presumably because of alterations in lipopolysaccharide (LPS) structure and cell surface architecture in these mutants. These mutants were defective in swarming, expressed lower levels of virulence factor hemolysin, and had lower cell invasion ability. Complementation of the ugd or galU mutant with the full-length ugd or galU gene, respectively, led to the restoration of wild-type phenotypic traits. Interestingly, we found that the expression of Ugd and GalU was induced by PB through RppA, a putative response regulator of the bacterial two-component system that we identified previously. Mutation in either ugd or galU led to activation of RpoE, an extracytoplasmic function sigma factor that has been shown to be activated by protein misfolding and alterations in cell surface structure in other bacteria. Activation of RpoE or RpoE overexpression was found to cause inhibition of FlhDC and hemolysin expression. To our knowledge, this is the first report describing the roles and regulation of Ugd and GalU in P. mirabilis.

Proteus mirabilis pmrI, an RppA-regulated gene necessary for polymyxin B resistance, biofilm formation, and urothelial cell invasion

Proteus mirabilis is naturally resistant to polymyxin B (PB). To investigate the underlying mechanisms, Tn5 mutagenesis was performed, and a mutant exhibiting increased PB susceptibility was isolated. The mutant was found to have Tn5 inserted into the PpmrI (Proteus pmrI) gene, a gene which may encode a UDP-glucuronic acid decarboxylase. In other bacteria, pmrI belongs to the seven-gene pmrF operon, which is involved in lipopolysaccharide (LPS) modification. While the PpmrI knockout mutant had a wild-type LPS profile and produced amounts of LPS similar to those produced by the wild type, LPS of the knockout mutant had higher PB-binding activity than that of the wild type. PB could induce alterations of LPS in the wild type but not in the PpmrI knockout mutant. Moreover, the PpmrI knockout mutant exhibited decreased abilities in biofilm formation and urothelial cell invasion. Complementation of the PpmrI mutant with the full-length PpmrI gene led to restoration of the wild-type phenotypic traits. Previously we identified RppA, a response regulator of the bacterial two-component system, as a regulator of PB susceptibility and virulence factor expression in P. mirabilis. Here we showed that RppA could mediate the induction of PpmrI expression by PB. An electrophoretic mobility shift assay further demonstrated that RppA could bind directly to the putative PpmrI promoter. Together, these results provide a new insight into the regulatory mechanism underlying PB resistance and virulence expression in P. mirabilis.

Colistin in the 21st century

PURPOSE OF REVIEW

Colistin is a 50-year-old antibiotic that is being used increasingly as a 'last-line' therapy to treat infections caused by multidrug-resistant Gram-negative bacteria, when essentially no other options are available. Despite its age, or because of its age, there has been a dearth of knowledge on its pharmacological and microbiological properties. This review focuses on recent studies aimed at optimizing the clinical use of this old antibiotic.

RECENT FINDINGS

A number of factors, including the diversity in the pharmaceutical products available, have hindered the optimal use of colistin. Recent advances in understanding of the pharmacokinetics and pharmacodynamics of colistin, and the emerging knowledge on the relationship between the pharmacokinetics and pharmacodynamics, provide a solid base for optimization of dosage regimens. The potential for nephrotoxicity has been a lingering concern, but recent studies provide useful new information on the incidence, severity and reversibility of this adverse effect. Recent approaches to the use of other antibiotics in combination with colistin hold promise for increased antibacterial efficacy with less potential for emergence of resistance.

SUMMARY

Because few, if any, new antibiotics with activity against multidrug-resistant Gram-negative bacteria will be available within the next several years, it is essential that colistin is used in ways that maximize its antibacterial efficacy and minimize toxicity and development of resistance. Recent developments have improved use of colistin in the 21st century.

The role of Pseudomonas lipopolysaccharide in cystic fibrosis airway infection

Pseudomonas aeruginosa (PA) is a ubiquitous environmental Gram-negative bacterium found in soil and water. This opportunistic pathogen can cause infections in individuals with impaired phagocytic function, such as those with burns, exposure to chemotherapy, or cystic fibrosis (CF). PA infects the lungs of most individuals with CF, and is associated with severe progressive pulmonary disease that is the major cause of premature death in this disorder. The specific adaptations of PA to the CF airway responsible for bacterial persistence and antibiotic tolerance are not completely understood but may include increased alginate production (i.e., mucoid phenotype), biofilm formation, and specific lipid A modifications. During adaptation to the CF airway, PA synthesizes a variety of lipid A structures that alter host innate immune responses and promote bacterial persistence and chronic infection. The synthesis of specific lipid A structures is attributable to bacterial enzymes that: (1) remove the 3OH-C10:0 acyl chain from the 3-position (PagL); (2) add a C16:0 acyl chain to the 3OH-C10:0 chain at the 3'-position (PagP); (3) add C12:0 and 2OH-C12:0 acyl chains to the 3OH-C12:0 chains at the 2- and 2'-positions (HtrB and LpxO); and (4) add aminoarabinose to phosphate groups at the 1- and 4'-positions (PmrH, PmrF, PmrI, PmrJ, PmrK, and PmrE). These lipid A modifications represent an essential aspect of PA adaptation to the CF airway.

Alterations in two-component regulatory systems of phoPQ and pmrAB are associated with polymyxin B resistance in clinical isolates of Pseudomonas aeruginosa

Polymyxins are often the only option to treat acquired multidrug-resistant Pseudomonas aeruginosa. Polymyxin susceptibility in P. aeruginosa PAO1 is associated with the lipopolysaccharide structure that is determined by arnBCADTEF and modulated by phoPQ and pmrAB. We examined five clonally unrelated clinical isolates of polymyxin B-resistant P. aeruginosa to investigate the molecular basis of polymyxin resistance. All isolates grew with 4 microg/ml polymyxin B (MIC, 8 microg/ml), whereas P. aeruginosa PAO1 grew with 0.25 mug/ml polymyxin B (MIC, 0.5 microg/ml). The resistant isolates were converted to susceptible ones (the MICs fell from 8 to 0.5 microg/ml) following the introduction of phoPQ (four isolates) and pmrAB (one isolate), which had been cloned from strain PAO1. DNA sequence analysis revealed that a single-nucleotide substitution in three isolates replaced a single amino acid of PhoQ, the deletion of 17 nucleotides in one isolate truncated the protein of PhoQ, and two nucleotide substitutions in one isolate replaced two amino acids of PmrB. The involvement of these amino acid substitutions or the truncated protein of PhoQ and PmrB in polymyxin B resistance was confirmed using strain PAO1 lacking phoPQ or pmrAB that was transformed by phoPQ or pmrAB containing the amino acid substitutions or the truncated protein. The resistant clinical isolates were sensitized by the inactivation of arnBCADTEF (the MICs fell from 8 to 0.5 microg/ml). These results suggest that polymyxin B resistance among clinical isolates of P. aeruginosa is associated with alterations in two-component regulatory systems of phoPQ or pmrAB.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.