Insights into the Mechanistic Basis of Plasmid-Mediated Colistin Resistance from Crystal Structures of the Catalytic Domain of MCR-1

The polymixin colistin is a "last line" antibiotic against extensively-resistant Gram-negative bacteria. Recently, the mcr-1 gene was identified as a plasmid-mediated resistance mechanism in human and animal Enterobacteriaceae, with a wide geographical distribution and many producer strains resistant to multiple other antibiotics. mcr-1 encodes a membrane-bound enzyme catalysing phosphoethanolamine transfer onto bacterial lipid A. Here we present crystal structures revealing the MCR-1 periplasmic, catalytic domain to be a zinc metalloprotein with an alkaline phosphatase/sulphatase fold containing three disulphide bonds. One structure captures a phosphorylated form representing the first intermediate in the transfer reaction. Mutation of residues implicated in zinc or phosphoethanolamine binding, or catalytic activity, restores colistin susceptibility of recombinant E. coli. Zinc deprivation reduces colistin MICs in MCR-1-producing laboratory, environmental, animal and human E. coli. Conversely, over-expression of the disulphide isomerase DsbA increases the colistin MIC of laboratory E. coli. Preliminary density functional theory calculations on cluster models suggest a single zinc ion may be sufficient to support phosphoethanolamine transfer. These data demonstrate the importance of zinc and disulphide bonds to MCR-1 activity, suggest that assays under zinc-limiting conditions represent a route to phenotypic identification of MCR-1 producing E. coli, and identify key features of the likely catalytic mechanism.

Current status of selected oral peptide technologies in advanced preclinical development and in clinical trials

The development of oral dosage forms that allows absorption of therapeutic peptides to the systemic circulation is one of the greatest challenges for the pharmaceutical industry. Currently, a number of technologies including either mixtures of penetration enhancers or protease inhibitors and/or nanotechnology-based products are under clinical development. Typically, these formulations are presented in the form of enteric-coated tablets or capsules. Systems undergoing preclinical investigation include further advances in nanotechnology, including intestinal microneedle patches, as well as their combination with regional delivery to the colon. This review critically examines four selected promising oral peptide technologies at preclinical stage and the twelve that have progressed to clinical trials, as indicated in www.clinicaltrials.gov. We examined these technologies under the criteria of peptide selection, formulation design, system components and excipients, intestinal mechanism of action, efficacy in man, and safety issues. The conclusion is that most of the technologies in clinical trials are incremental rather than paradigm-shifting and that even the more clinically advanced oral peptide drugs examples of oral bioavailability appear to yield oral bioavailability values of only 1-2% and are, therefore, only currently suitable for a limited range of peptides.

Colistin in Pig Production: Chemistry, Mechanism of Antibacterial Action, Microbial Resistance Emergence, and One Health Perspectives

Colistin (Polymyxin E) is one of the few cationic antimicrobial peptides commercialized in both human and veterinary medicine. For several years now, colistin has been considered the last line of defense against infections caused by multidrug-resistant Gram-negative such as Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Colistin has been extensively used orally since the 1960s in food animals and particularly in swine for the control of Enterobacteriaceae infections. However, with the recent discovery of plasmid-mediated colistin resistance encoded by the mcr-1 gene and the higher prevalence of samples harboring this gene in animal isolates compared to other origins, livestock has been singled out as the principal reservoir for colistin resistance amplification and spread. Co-localization of the mcr-1 gene and Extended-Spectrum-β-Lactamase genes on a unique plasmid has been also identified in many isolates from animal origin. The use of colistin in pigs as a growth promoter and for prophylaxis purposes should be banned, and the implantation of sustainable measures in pig farms for microbial infection prevention should be actively encouraged and financed. The scientific research should be encouraged in swine medicine to generate data helping to reduce the exacerbation of colistin resistance in pigs and in manure. The establishment of guidelines ensuring a judicious therapeutic use of colistin in pigs, in countries where this drug is approved, is of crucial importance. The implementation of a microbiological withdrawal period that could reduce the potential contamination of consumers with colistin resistant bacteria of porcine origin should be encouraged. Moreover, the management of colistin resistance at the human-pig-environment interface requires the urgent use of the One Health approach for effective control and prevention. This approach needs the collaborative effort of multiple disciplines and close cooperation between physicians, veterinarians, and other scientific health and environmental professionals. This review is an update on the chemistry of colistin, its applications and antibacterial mechanism of action, and on Enterobacteriaceae resistance to colistin in pigs. We also detail and discuss the One Health approach and propose guidelines for colistin resistance management.

Invitro Activities of Polymyxins and Rifampicin against Carbapenem Resistant Acinetobacter baumannii at a Tertiary Care Hospital from South India

INTRODUCTION

Acinetobacter baumannii (A.baumannii) is rapidly emerging as a potent organism causing a multitude of nosocomial infections. The organism also carries various resistance mechanisms to antibiotics, making treatment more difficult. Very few choices are left, as A.baumannii strains have begun to develop resistance against cephalosporins, aminoglycosides and even carbapenems.

AIM

To examine the sensitivity pattern of three older antibiotics namely colistin, polymyxin B and rifampicin against carbapenem resistant A.baumannii by disk diffusion method and the sensitivity of colistin alone by Minimum Inhibitory Concentration (MIC) determination by VITEK automated system.

MATERIALS AND METHODS

Hundred clinical isolates of carbapenem resistant A. baumannii were tested for sensitivity to colistin, polymyxin B and rifampicin by Kirby-Bauer disk diffusion method. They were also tested for sensitivity to colistin by VITEK 2C (biomérieux) automated microbial identification system. The zone diameters and Minimum Inhibitory Concentration values for the above two methods, respectively were observed and analysed. All the Antibiotic Susceptibility Tests were done according to the CLSI guidelines.

RESULTS

By Kirby-Bauer disk diffusion method, 78% of the carbapenem resistant strains were found to be sensitive, 12% intermediate sensitive and 10% resistant to colistin. All the isolates were sensitive to polymyxin B and 80% were resistant to rifampicin. By the VITEK automated system, 99% of the isolates were sensitive to colistin (more in number than by disk diffusion method).

CONCLUSION

Polymyxins (colistin - polymyxin E and polymyxin B) are the next choice for multidrug resistant serious nosocomial infections like those of A. baumannii, till newer antibiotics are discovered to treat such infections. Rifampicin resistance was found to be very high and hence, is not advised for monotherapy.

Ten years with colistin: a retrospective case series

OBJECTIVE

At the Shaare Zedek Medical Center, we have been using colistimethate sodium (CMS) for empiric as well as pathogen-directed treatment. We present our 10-year experience.

METHODS

We conducted a retrospective case-series analysis of patients admitted from 1 January 2004 through 1 May 2014 who received at least one dose of CMS. Patient characteristics analysed for all admission for which patients received CMS, included: age, number of re-admissions, admission ward, renal function, disposition and microbiology results. Overall trend in defined daily dose (DDD) for CMS and resistant isolates was analysed.

RESULTS

A total of 5603 admissions met inclusion criteria. Patients' mean (±SD) age was 80 ± 14 years, 1162 (48%) of the admissions were from a healthcare facility and 4367 (78%) of the admissions were to general Internal Medicine wards. The median number of hospital admissions per patient was 5, median admission and discharge creatinine (mg/dl) were 1.05 and 1.01, respectively; 2.3% of admissions required first-time dialysis. The discharge rate from the hospital was 58.4%. Excluding intrinsically CMS-resistant gram-negative organisms, bloodstream and urine isolates were 98% and 100% susceptible, respectively. CMS use (DDDs) increased during the study (p for trend = 0.04) without significant increase in incidence of multidrug-resistant organisms.

CONCLUSIONS

Colistimethate sodium use at our institution has increased during this 10-year period. Nevertheless, there is no increasing trend in CMS-resistant organisms, 58% of the patients were discharged alive, and we did not observe significant nephrotoxicity in patients prescribed CMS. CMS should be reserved for microbiologically confirmed extensively drug-resistant gram-negative infections.

The Lantibiotic NAI-107 Efficiently Rescues Drosophila melanogaster from Infection with Methicillin-Resistant Staphylococcus aureus USA300

We used the fruit fly Drosophila melanogaster as a cost-effective in vivo model to evaluate the efficacy of novel antibacterial peptides and peptoids for treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections. A panel of peptides with known antibacterial activity in vitro and/or in vivo was tested in Drosophila Although most peptides and peptoids that were effective in vitro failed to rescue lethal effects of S. aureus infections in vivo, we found that two lantibiotics, nisin and NAI-107, rescued adult flies from fatal infections. Furthermore, NAI-107 rescued mortality of infection with the MRSA strain USA300 with an efficacy equivalent to that of vancomycin, a widely applied antibiotic for the treatment of serious MRSA infections. These results establish Drosophila as a useful model for in vivo drug evaluation of antibacterial peptides.

Mutant prevention concentration of colistin alone and in combination with rifampicin for multidrug-resistant Acinetobacter baumannii

Colistin-susceptible isolates of Acinetobacter baumannii often contain subpopulations that are resistant to colistin. Monotherapy with colistin can lead to selective growth of these subpopulations and emergence of colistin-resistant strains. Our objectives were to explore the susceptibility pattern of colistin-resistant subpopulations and investigate if combining colistin with a second antibiotic could prevent their selective growth. Four colistin-susceptible clinical isolates of A. baumannii and one reference isolate were used. The mutant prevention concentration (MPC) of colistin, i.e. the concentration required to block growth of all single-step-mutant subpopulations, was determined by plating an inoculum of 10⁹ CFU on Mueller Hinton agar (MHA)-plates containing 2-fold dilutions of colistin (0.125–128 mg/L). Susceptibility testing of colistin-resistant subpopulations, obtained in the MPC assay, was performed with Etest. The MPC of colistin, in combination with rifampicin, was determined by plating an inoculum of 10⁹ CFU on MHA-plates containing colistin (0.125–128 mg/L) and fixed concentrations of rifampicin (1.1 mg/L or 4.4 mg/L). The colistin-resistant subpopulations demonstrated increased susceptibility to a number of agents compared to their main populations. These subpopulations were even susceptible to agents that normally are inactive against gram-negative bacteria and all had rifampicin MICs of < 0.002 mg/L. The combination of colistin and rifampicin completely inhibited the growth of all colistin-resistant subpopulations and significantly lowered the MPC of colistin for A. baumannii. Combining colistin with rifampicin could be a way to prevent selective growth of colistin-resistant subpopulations of A. baumannii and possibly the emergence of colistin-resistant strains.

Comparison of clinical characteristics of patients with acute kidney injury after intravenous versus inhaled colistin therapy

Background

The aim of this study was to investigate the incidence and clinical characteristics of intravenous (IV) or inhaled (IH) colistin-associated acute kidney injury (AKI) using the Risk, Injury, Failure, Loss, End-stage Renal Disease criteria.

Methods

From 2010 to 2014, 160 patients were treated with IV or IH colistin. Of these, we included 126 patients who received colistin for > 72 hours for the treatment of pneumonia and compared the incidence and clinical characteristics of patients in the IV (n = 107) and IH (n = 19) groups.

Results

The patients included 104 men and 22 women, with a mean age of 69 years (range, 24–91 years). The mortality rate was 45%, and AKI occurred in 75 (60%) patients. At the end of therapy, the bacteriologic cure rate was 66%. There were no differences in the clinical characteristics between the IV and IH groups except for age. In comparison with patients in the IV group, the patients in the IH group were older (74 ± 8 vs. 68 ± 12 years, P = 0.026). The incidence of AKI was not different between the 2 groups (62 vs. 47%, P = not significant), and there was no difference in the severity of AKI according to the Risk, Injury, Failure, Loss, End-stage Renal Disease criteria. Of the 83 patients with AKI, 6 and 1 patients underwent renal replacement therapy, respectively.

Conclusion

The incidence of AKI in patients with colistin therapy is 60% in our center. It seems that IH colistin therapy could not be better in safety than IV colistin therapy.

Evaluation of Risk Factors for Intravenous Colistin Use-related Nephrotoxicity

OBJECTIVES

We investigated the incidence of and risk factors for nephrotoxicity in patients using intravenous colistin.

METHODS

This retrospective, observational study was conducted at Karadeniz Technical University, Faculty of Medicine, clinics and intensive care unit between 1 January 2009 and 1 January 2013. Intravenous colistin was administered to 133 patients at a dose of 2.5-5.0 mg/kg/day.

RESULTS

The patients mean age was 54.3±19.1 years and the mean duration of treatment was 13.5±3.6 days. Nephrotoxicity developed in 5.0±2.8 days in 38 (28.6%) patients. Based on RIFLE (risk, injury, failure, loss of kidney function, and end-stage kidney disease) criteria, 15 (39.5%) patients were class 1, 17 (44.7%) were class 2, six (15.8%) were class 3, and none were class 4. The mean duration of development of nephrotoxicity was 5.0±2.8 days. Hemodialysis requirement was observed in two (5.2%) of the 38 patients who developed nephrotoxicity. In these cases, colistin therapy was not discontinued. Nephrotoxicity was correlated with advanced age, high pretreatment serum creatinine levels, diabetes mellitus, and chronic obstructive pulmonary disease.

CONCLUSIONS

The use of colistin is relatively safe for patients that have normal renal functions. However, better standardization of the definition of nephrotoxicity in those patients with the use of scoring systems and close monitoring are necessary.

Real-time quantitative PCR assay with Taqman(®) probe for rapid detection of MCR-1 plasmid-mediated colistin resistance

Here we report the development of two rapid real-time quantitative PCR assays with TaqMan^® probes to detect the MCR-1 plasmid-mediated colistin resistance gene from bacterial isolates and faecal samples from chickens. Specificity and sensitivity of the assay were 100% on bacterial isolates including 18 colistin-resistant isolates carrying the mcr-1 gene (six Klebsiella pneumoniae and 12 Escherichia coli) with a calibration curve that was linear from 10¹ to 10⁸ DNA copies. Five out of 833 faecal samples from chickens from Algeria were positive, from which three E. coli strains were isolated and confirmed to harbour the mcr-1 gene by standard PCR and sequencing.

Activity of Colistin in Combination with Meropenem, Tigecycline, Fosfomycin, Fusidic Acid, Rifampin or Sulbactam against Extensively Drug-Resistant Acinetobacter baumannii in a Murine Thigh-Infection Model

Few effective therapeutic options are available for treating severe infections caused by extensively drug-resistant Acinetobacter baumannii (XDR-AB). Using a murine thigh-infection model, we examined the in vivo efficacy of colistin in combination with meropenem, tigecycline, fosfomycin, fusidic acid, rifampin, or sulbactam against 12 XDR-AB strains. Colistin, tigecycline, rifampin, and sulbactam monotherapy significantly decreased bacterial counts in murine thigh infections compared with those observed in control mice receiving no treatment. Colistin was the most effective agent tested, displaying bactericidal activity against 91.7% of strains at 48 h post-treatment. With strains showing a relatively low minimum inhibitory concentration (MIC) for meropenem (MIC ≤ 32 mg/L), combination therapy with colistin plus meropenem caused synergistic inhibition at both 24 h and 48 h post-treatment. However, when the meropenem MIC was ≥64 mg/L, meropenem did not significantly alter the efficacy of colistin. The addition of rifampin and fusidic acid significantly improved the efficacy of colistin, showing a synergistic effect in 100% and 58.3% of strains after 24 h of treatment, respectively, while the addition of tigecycline, fosfomycin, or sulbactam did not show obvious synergistic activity. No clear differences in activities were observed between colistin-rifampin and colistin-fusidic acid combination therapy with most strains. Overall, our in vivo study showed that administering colistin in combination with rifampin or fusidic acid is more efficacious in treating XDR-AB infections than other combinations. The colistin-meropenem combination may be another appropriate option if the MIC is ≤32 mg/L. Further clinical studies are urgently needed to confirm the relevance of these findings.

Genotyping of Methicillin Resistant Staphylococcus aureus from Tertiary Care Hospitals in Coimbatore, South India

Background:

Globally, methicillin resistant Staphylococcus aureus (MRSA) is one of the most common pathogens that causes hospital- and community-acquired infections. The use of molecular typing methods is essential for determining the origin of the isolates, their clonal relations, and also epidemiological investigations.

Objective:

The purpose of this study was to determine the prevalence of antibiotic-resistant MRSA investigate the accessory gene regulator (agr) and staphylococcal cassette chromosome mec (SCCmec) types and perform multilocus sequence typing (MLST). Furthermore, the minimum inhibitory concentration of MRSA isolates was determined for vancomycin and daptomycin.

Materials and Methods:

Two hundred and fifty-nine MRSA isolates were collected from Tertiary Care Hospitals in Coimbatore. Disk diffusion method was employed to assess the sensitivity of MRSA isolates to selected antibiotics and genetic analysis was performed using SCCmec, agr, and MLST typing by multiplex-polymerase chain reaction strategy. Minimal inhibitory concentration (MIC) was determined using Ezy MIC (vancomycin) and Biomerieux (daptomycin) E-test strip.

Results:

Of 259 MRSA isolates, 209 (80.7%) were confirmed as methicillin resistant. Antibiotic susceptibility pattern revealed that all the MRSA isolates were 100% sensitive to linezolid, rifampicin, teicoplanin, and vancomycin. MIC results showed that of 209 MRSA isolates, 10 were found to be vancomycin intermediate S. aureus and 100% of the MRSA isolates were daptomycin-susceptible. The agr group I and SCCmec Type III were the major type among MRSA isolates. In addition to these MLST typing revealed the prevalence of sequence type (ST) 239 (SLV of ST8) among the MRSA isolates.

Conclusion:

This study confirms that ST239 (Brazilian clone) of MRSA is predominant in this region which is responsible for the hospital-acquired MRSA infections. Thus, the study also suggests that vancomycin and daptomycin can still be used as an alternative drug for treating severe MRSA infections.

Insight into membrane selectivity of linear and branched polyethylenimines and their potential as biocides for advanced wound dressings

We report here structure-property relationship between linear and branched polyethylene imines by examining their antimicrobial activities against wide range of pathogens. Both the polymers target the cytoplasmic membrane of bacteria and yeasts, eliciting rapid microbicidal properties. Using multiscale molecular dynamic simulations, we showed that, in both fully or partially protonated forms LPEI discriminates between mammalian and bacterial model membranes whereas BPEI lacks selectivity for both the model membranes. Simulation results suggest that LPEI forms weak complex with the zwitterionic lipids whereas the side chain amino groups of BPEI sequester the zwitterionic lipids by forming tight complex. Consistent with these observations, label-free cell impedance measurements, cell viability assays and high content analysis indicate that BPEI is cytotoxic to human epithelial and fibroblasts cells. Crosslinking of BPEI onto electrospun gelatin mats attenuate the cytotoxicity for fibroblasts while retaining the antimicrobial activity against Gram-positive and yeasts strains. PEI crosslinked gelatin mats elicit bactericidal activity by contact-mediated killing and durable to leaching for 7days. The potent antimicrobial activity combined with enhanced selectivity of the crosslinked ES gelatin mats would expand the arsenel of biocides in the management of superficial skin infections. The contact-mediated microbicidal properties may avert antimicrobial resistance and expand the diversity of applications to prevent microbial contamination.

STATEMENT OF SIGNIFICANCE

Current commercially available advanced wound dressings are either impregnated with metallic silver or silver salts which have side effects or may not avert antimicrobial resistance. In this article, we have used multidisciplinary approach comprising of computational, chemical and biological methods to understand the antimicrobial properties and biocompatibility of linear (LPEI) and branched (BPEI) polyethylenimines. We then applied this knowledge to develop dual purpose wound dressings containing these polymers, which encourages healing while maintain antimicrobial activity. In addition, the approach can be expanded to rationalize the antimicrobial vs. cytotoxicity of other cationic polymers and the method of crosslinking would enhance their potentials as biocides for advanced materials.

Amino Acid Substitutions of CrrB Responsible for Resistance to Colistin through CrrC in Klebsiella pneumoniae

Colistin is a last-resort antibiotic for treatment of carbapenem-resistant Klebsiella pneumoniae A recent study indicated that missense mutations in the CrrB protein contribute to colistin resistance. In our previous study, mechanisms of colistin resistance were defined in 17 of 26 colistin-resistant K. pneumoniae clinical isolates. Of the remaining nine strains, eight were highly resistant to colistin. In the present study, crrAB sequences were determined for these eight strains. Six separate amino acid substitutions in CrrB (Q10L, Y31H, W140R, N141I, P151S, and S195N) were detected. Site-directed mutagenesis was used to generate crrB loci harboring individual missense mutations; introduction of the mutated genes into a susceptible strain, A4528, resulted in 64- to 1,024-fold increases in colistin MICs. These crrB mutants showed increased accumulation of H239_3062, H239_3059, pmrA, pmrC, and pmrH transcripts by quantitative reverse transcription (qRT)-PCR. Deletion of H239_3062 (but not that of H239_3059) in the A4528 crrB(N141I) strain attenuated resistance to colistin, and H239_3062 was accordingly named crrC Similarly, accumulation of pmrA, pmrC, and pmrH transcripts induced by crrB(N141I) was significantly attenuated upon deletion of crrC Complementation of crrC restored resistance to colistin and accumulation of pmrA, pmrC, and pmrH transcripts in a crrB(N141I) ΔcrrC strain. In conclusion, novel individual CrrB amino acid substitutions (Y31H, W140R, N141I, P151S, and S195N) were shown to be responsible for colistin resistance. We hypothesize that CrrB mutations induce CrrC expression, thereby inducing elevated expression of the pmrHFIJKLM operon and pmrC (an effect mediated via the PmrAB two-component system) and yielding increased colistin resistance.

In vitro and in vivo efficacy, toxicity, bio-distribution and resistance selection of a novel antibacterial drug candidate

A synthetic antimicrobial peptide was identified as a possible candidate for the development of a new antibacterial drug. The peptide, SET-M33L, showed a MIC₉₀ below 1.5 μM and 3 μM for Pseudomonas aeruginosa and Klebsiella pneumoniae, respectively. In in vivo models of P. aeruginosa infections, the peptide and its pegylated form (SET-M33L-PEG) enabled a survival percentage of 60–80% in sepsis and lung infections when injected twice i.v. at 5 mg/Kg, and completely healed skin infections when administered topically. Plasma clearance showed different kinetics for SET-M33L and SET-M33L-PEG, the latter having greater persistence two hours after injection. Bio-distribution in organs did not show significant differences in uptake of the two peptides. Unlike colistin, SET-M33L did not select resistant mutants in bacterial cultures and also proved non genotoxic and to have much lower in vivo toxicity than antimicrobial peptides already used in clinical practice. The characterizations reported here are part of a preclinical development plan that should bring the molecule to clinical trial in the next few years.

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

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

Resistance to colistin: what is the fate for this antibiotic in pig production?

Colistin, a cationic polypeptide antibiotic, has reappeared in human medicine as a last-line treatment option for multidrug-resistant Gram-negative bacteria (MDR-GNB). Colistin is widely used in veterinary medicine for the treatment of gastrointestinal infections caused by Enterobacteriaceae. GNB resistant to colistin owing to chromosomal mutations have already been reported both in human and veterinary medicine, however several recent studies have just identified a plasmid-mediated mcr-1 gene encoding for colistin resistance in Escherichia coli colistin resistance. The discovery of a non-chromosomal mechanism of colistin resistance in E. coli has led to strong reactions in the scientific community and to concern among physicians and veterinarians. Colistin use in food animals and particularly in pig production has been singled out as responsible for the emergence of colistin resistance. The present review will focus mainly on the possible link between colistin use in pigs and the spread of colistin resistance in Enterobacteriaceae. First we demonstrate a possible link between Enterobacteriaceae resistance emergence and oral colistin pharmacokinetics/pharmacodynamics and its administration modalities in pigs. We then discuss the potential impact of colistin use in pigs on public health with respect to resistance. We believe that colistin use in pig production should be re-evaluated and its dosing and usage optimised. Moreover, the search for competitive alternatives to using colistin with swine is of paramount importance to preserve the effectiveness of this antibiotic for the treatment of MDR-GNB infections in human medicine.

Therapeutic drug monitoring of anti-infective agents in critically ill patients

Initial adequate anti-infective therapy is associated with significantly improved clinical outcomes for patients with severe infections. However, in critically ill patients, several pathophysiological and/or iatrogenic factors may affect the pharmacokinetics of anti-infective agents leading to suboptimal drug exposure, in particular during the early phase of therapy. Therapeutic drug monitoring (TDM) may assist to overcome this problem. We discuss the available evidence on the use of TDM in critically ill patient populations for a number of anti-infective agents, including aminoglycosides, β-lactams, glycopeptides, antifungals and antivirals. Also, we present the available evidence on the practices of anti-infective TDM and describe the potential utility of TDM to improve treatment outcome in critically ill patients with severe infections. For aminoglycosides, glycopeptides and voriconazole, beneficial effects of TDM have been established on both drug effectiveness and potential side effects. However, for other drugs, therapeutic ranges need to be further defined to optimize treatment prescription in this setting.

Mechanisms of Antimicrobial Peptide Resistance in Gram-Negative Bacteria

Cationic antimicrobial peptides (CAMPs) are important innate immune defenses that inhibit colonization by pathogens and contribute to clearance of infections. Gram-negative bacterial pathogens are a major target, yet many of them have evolved mechanisms to resist these antimicrobials. These resistance mechanisms can be critical contributors to bacterial virulence and are often crucial for survival within the host. Here, we summarize methods used by Gram-negative bacteria to resist CAMPs. Understanding these mechanisms may lead to new therapeutic strategies against pathogens with extensive CAMP resistance.

Occurrence and ecological risk assessment of fluoroquinolone antibiotics in hospital waste of Lahore, Pakistan

In the present study, wastewater and sludge samples of two major hospitals of Lahore, Pakistan were analyzed by developing an HPLC-UV method for the possible occurrence of five frequently used fluoroquinolone antibiotics i.e. ofloxacin, ciprofloxacin, sparfloxacin, moxifloxacin and gemifloxacin. The highest detected concentration was for moxifloxacin in both wastewater (224 μg/L) and sludge samples (219 μg/kg. The highest concentration of ofloxacin, ciprofloxacin, sparfloxacin and gemifloxacin were found to be 66, 18, 58 and 0.2 μg/L respectively. Risk quotient (RQ) was also calculated based on maximum measured concentrations and the RQ values were very high particularly for ofloxacin and ciprofloxacin. The maximum RQ values for ofloxacin against Vibrio fisheri, Pseudomonas putida, fish, Daphnia, Green algae and Pseudokirchneriella subcapitata were 3300, 66,000, 124, 46, 3300 and 6000, respectively. In case of ciprofloxacin, RQ values were found to be 1750 and 3500 against green algae and Microcystis aeruginosa, respectively.

Emergence of multidrug-resistant Acinetobacter baumannii producing OXA-23 Carbapenemase in Qatar

The objective of our study was to describe the molecular support of carbapenem resistance from randomly selected clinical isolates of multidrug-resistant (MDR) Acinetobacter baumannii as a pilot study from the Hamad Medical Corporation (HMC), Qatar. Results of our report will be used to study carbapenemases using molecular techniques in all isolated MDR A. baumannii. Forty-eight MDR A. baumannii were randomly selected from isolates preserved at HMC. Identification of all isolates was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Antibiotic resistance was tested phenotypically by Phoenix and confirmed by Etest. The molecular support of carbapenemases (bla OXA-23, bla OXA-24, bla OXA-58, bla NDM) was investigated by real-time PCR. The epidemiologic relatedness of the isolates was verified by phylogenetic analysis based on partial sequences of CsuE and bla OXA-51 genes. All 48 isolates were identified as A. baumannii and were confirmed to be resistant to most antibiotics, especially meropenem, imipenems, ciprofloxacin, levofloxacin, amikacin, gentamicin and most of the β-lactams; they were sensitive to colistin. All the isolates were positive for bla OXA-23 and negative for the other tested carbapenemase genes. Clonality analysis demonstrated that different lineages were actually circulating in Qatar; and we suggest that an outbreak occurred in the medical intensive care unit of HMC between 2011 and 2012. Here we report the emergence of MDR A. baumannii producing the carbapenemase OXA-23 in Qatar.

Activity and Predicted Nephrotoxicity of Synthetic Antibiotics Based on Polymyxin B

The polymyxin lipodecapeptides colistin and polymyxin B have become last resort therapies for infections caused by highly drug-resistant Gram-negative bacteria. Unfortunately, their utility is compromised by significant nephrotoxicity and polymyxin-resistant bacterial strains. We have conducted a systematic activity–toxicity investigation by varying eight of the nine polymyxin amino acid free side chains, preparing over 30 analogues using a novel solid-phase synthetic route. Compounds were tested against a panel of Gram-negative bacteria and counter-screened for in vitro cell toxicity. Promising compounds underwent additional testing against primary kidney cells isolated from human kidneys to better predict their nephrotoxic potential. Many of the new compounds possessed equal or better antimicrobial potency compared to polymyxin B, and some were less toxic than polymyxin B and colistin against mammalian HepG2 cells and human primary kidney cells. These initial structure–activity and structure–toxicity studies set the stage for further improvements to the polymyxin class of antibiotics.

Management of a hospital outbreak of extensively drug-resistant Acinetobacter baumannii using a multimodal intervention including daily chlorhexidine baths

BACKGROUND

Extensively drug-resistant Acinetobacter baumannii (XDR-Ab) is an increasingly important cause of healthcare-associated infection. Uncertainties remain concerning optimal control measures for healthcare-associated outbreaks.

AIM

To describe the epidemiology and control of an XDR-Ab outbreak that involved multiple units of a large hospital from March 2012 to January 2014.

METHODS

Case-finding included screening of rectum, groin, throat, nose, wounds, iatrogenic portals of entry, and catheterized sites. Antimicrobial susceptibility was evaluated by disc diffusion and E-test. Resistance genes were detected by polymerase chain reaction. Clonality was assessed by pulsed-field gel electrophoresis. Charts of cases were reviewed to identify risk factors for invasive infection. Control measures included isolation and cohorting of cases, hand hygiene reinforcement, environmental decontamination, and source control with daily baths using wipes pre-impregnated with chlorhexidine gluconate.

FINDINGS

A single clonal strain of XDR-Ab colonized or infected 29 patients. Five patients died of XDR-Ab bacteraemia. Transmission occurred primarily on two wards. Colonization was detected at all anatomical screening sites; only 57% (16/28) of cases were rectal carriers. Advanced malignancy was a risk factor for bacteraemia (relative risk: 5.8; 95% confidence interval: 1.2-27.0). Transmission ended following implementation of the multimodal control strategy. No additional nosocomial cases occurred during the following 20 months.

CONCLUSION

Our study highlights the need to screen multiple anatomic sites to diagnose carriage and identifies risk factors for XDR-Ab bacteraemia. A multimodal intervention that included daily chlorhexidine baths for cases was rapidly followed by the termination of the outbreak. Hospitals should consider similar interventions when managing future XDR-Ab outbreaks.

Quantitation of Polymyxin-Lipopolysaccharide Interactions Using an Image-Based Fluorescent Probe

The frequency of polymyxin-resistant pathogenic Gram-negative bacteria appearing in the clinic is increasing, and the consequences are largely mediated by modification of lipopolysaccharide (LPS) in the outer membrane. As polymyxins exert their antibacterial effect by binding to LPS, understanding their mode of binding will prove highly valuable for new antibiotic discovery. In this study, we assess the potential of MIPS-9451, a fluorescent polymyxin analogue designed for imaging studies, as a fluorescent reporter molecule, titrating it against 17 different Gram-negative species and/or strains of LPS. MIPS-9451 bound to the various species and/or strains of LPS with a dissociation constant ranging between 0.14 ± 0.01 μM (Escherichia coli) and 0.90 ± 0.42 μM (Porphyromonas gingivalis; mean ± standard error). Furthermore, we assessed the applicability of MIPS-9451 to rank affinities of polymyxin B to different LPS species in a displacement assay which yielded inhibition constants of 6.2 μM ± 33%, 7.2 μM ± 30%, and 0.95 μM ± 13% for Klebsiella pneumoniae, Pseudomonas aeruginosa, and Salmonella enterica, respectively (mean ± coefficient of variation). The results from this study are concordant with those observed with similarly structured polymyxin probes, confirming the potential of MIPS-9451 for quantitation of polymyxin-LPS affinities in discovery programs of novel polymyxin antibiotics.

New Structural Templates for Clinically Validated and Novel Targets in Antimicrobial Drug Research and Development

The development of bacterial resistance against current antibiotic drugs necessitates a continuous renewal of the arsenal of efficacious drugs. This imperative has not been met by the output of antibiotic research and development of the past decades for various reasons, including the declining efforts of large pharma companies in this area. Moreover, the majority of novel antibiotics are chemical derivatives of existing structures that represent mostly step innovations, implying that the available chemical space may be exhausted. This review negates this impression by showcasing recent achievements in lead finding and optimization of antibiotics that have novel or unexplored chemical structures. Not surprisingly, many of the novel structural templates like teixobactins, lysocin, griselimycin, or the albicidin/cystobactamid pair were discovered from natural sources. Additional compounds were obtained from the screening of synthetic libraries and chemical synthesis, including the gyrase-inhibiting NTBI's and spiropyrimidinetrione, the tarocin and targocil inhibitors of wall teichoic acid synthesis, or the boronates and diazabicyclo[3.2.1]octane as novel β-lactamase inhibitors. A motif that is common to most clinically validated antibiotics is that they address hotspots in complex biosynthetic machineries, whose functioning is essential for the bacterial cell. Therefore, an introduction to the biological targets-cell wall synthesis, topoisomerases, the DNA sliding clamp, and membrane-bound electron transport-is given for each of the leads presented here.

Impairment of Pseudomonas aeruginosa Biofilm Resistance to Antibiotics by Combining the Drugs with a New Quorum-Sensing Inhibitor

Pseudomonas aeruginosa plays an important role in chronic lung infections among patients with cystic fibrosis (CF) through its ability to form antibiotic-resistant biofilms. In P. aeruginosa, biofilm development and the production of several virulence factors are mainly regulated by the rhl and las quorum-sensing (QS) systems, which are controlled by two N-acyl-homoserine lactone signal molecules. In a previous study, we discovered an original QS inhibitor, N-(2-pyrimidyl)butanamide, called C11, based on the structure of C4-homoserine lactone, and found that it is able to significantly inhibit P. aeruginosa biofilm formation. However, recent data indicate that P. aeruginosa grows under anaerobic conditions and forms biofilms in the lungs of CF patients that are denser and more robust than those formed under aerobic conditions. Our confocal microscopy observations of P. aeruginosa biofilms developed under aerobic and anaerobic conditions confirmed that the biofilms formed under these two conditions have radically different architectures. C11 showed significant dose-dependent antibiofilm activity on biofilms grown under both aerobic and anaerobic conditions, with a greater inhibitory effect being seen under conditions of anaerobiosis. Gene expression analyses performed by quantitative reverse transcriptase PCR showed that C11 led to the significant downregulation of rhl QS regulatory genes but also to the downregulation of both las QS regulatory genes and QS system-regulated virulence genes, rhlA and lasB. Furthermore, the activity of C11 in combination with antibiotics against P. aeruginosa biofilms was tested, and synergistic antibiofilm activity between C11 and ciprofloxacin, tobramycin, and colistin was obtained under both aerobic and anaerobic conditions. This study demonstrates that C11 may increase the efficacy of treatments for P. aeruginosa infections by increasing the susceptibility of biofilms to antibiotics and by attenuating the pathogenicity of the bacterium.

Correlation of β-Lactamase Production and Colistin Resistance among Enterobacteriaceae Isolates from a Global Surveillance Program

The increasing use of carbapenems for treating multidrug-resistant (MDR) Gram-negative bacterial infections has contributed to the global dissemination of carbapenem-resistant Enterobacteriaceae (CRE). Serine and metallo-β-lactamases (MBLs) that hydrolyze carbapenems have become prevalent and endemic in some countries, necessitating the use of older classes of agents, such as colistin. A total of 19,719 isolates of Enterobacteriaceae (excluding Proteeae and Serratia spp., which have innate resistance to colistin) were collected from infected patients during 2012 and 2013 in a global surveillance program and tested for antimicrobial susceptibility using CLSI methods. Isolates of CRE were characterized for carbapenemases and extended-spectrum β-lactamases (ESBLs) by PCR and sequencing. Using EUCAST breakpoints, the rate of colistin susceptibility was 98.4% overall, but it was reduced to 88.0% among 482 carbapenemase-positive isolates. Colistin susceptibility was higher among MBL-positive isolates (92.6%) than those positive for a KPC (87.9%) or OXA-48 (84.2%). Of the agents tested, only tigecycline (MIC90, 2 to 4 μg/ml) and aztreonam-avibactam (MIC90, 0.5 to 1 μg/ml) consistently tested with low MIC values against colistin-resistant, ESBL-positive, and carbapenemase-positive isolates. Among the 309 (1.6%) colistin-resistant isolates from 10 species collected in 38 countries, 58 carried a carbapenemase that included KPCs (38 isolates), MBLs (6 isolates), and OXA-48 (12 isolates). These isolates were distributed globally (16 countries), and 95% were Klebsiella pneumoniae. Thirty-nine (67.2%) isolates carried additional ESBL variants of CTX-M, SHV, and VEB. This sample of Enterobacteriaceae demonstrated a low prevalence of colistin resistance overall. However, the wide geographic dispersion of colistin resistance within diverse genus and species groups and the higher incidence observed among carbapenemase-producing MDR pathogens are concerning.

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

Background

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

Methods

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

Results

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

Conclusions

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

Polymyxin B versus colistin: an update

Polymyxin B and colistin (polymyxin E) are polypeptide antibiotics that were developed in the 1940s, but fell into disfavor due to their high toxicity rates. These two antibiotics were previously regarded to be largely equivalent, due to similarities in their chemical structure and spectrum of activity. In recent years, several pertinent differences, especially in terms of potency and disposition, have been revealed between polymyxin B and colistin. These differences are mainly attributed to the fact that polymyxin B is administered parenterally in its active form, while colistin is administered parenterally as an inactive pro-drug, colistimethate. In this review, we summarize the similarities and differences between polymyxin B and colistin. We also discuss the potential clinical implications of these findings, and provide our perspectives on how polymyxins should be employed to preserve their utility in this era of multi-drug resistance.

Cationic Peptides Facilitate Iron-induced Mutagenesis in Bacteria

Pseudomonas aeruginosa is the causative agent of chronic respiratory infections and is an important pathogen of cystic fibrosis patients. Adaptive mutations play an essential role for antimicrobial resistance and persistence. The factors that contribute to bacterial mutagenesis in this environment are not clear. Recently it has been proposed that cationic antimicrobial peptides such as LL-37 could act as mutagens in P. aeruginosa. Here we provide experimental evidence that mutagenesis is the product of a joint action of LL-37 and free iron. By estimating mutation rate, mutant frequencies and assessing mutational spectra in P. aeruginosa treated either with LL-37, iron or a combination of both we demonstrate that mutation rate and mutant frequency were increased only when free iron and LL-37 were present simultaneously. Colistin had the same effect. The addition of an iron chelator completely abolished this mutagenic effect, suggesting that LL-37 enables iron to enter the cells resulting in DNA damage by Fenton reactions. This was also supported by the observation that the mutational spectrum of the bacteria under LL-37-iron regime showed one of the characteristic Fenton reaction fingerprints: C to T transitions. Free iron concentration in nature and within hosts is kept at a very low level, but the situation in infected lungs of cystic fibrosis patients is different. Intermittent bleeding and damage to the epithelial cells in lungs may contribute to the release of free iron that in turn leads to generation of reactive oxygen species and deterioration of the respiratory tract, making it more susceptible to the infection.

The role of vancomycin in addition with colistin and meropenem against colistin-sensitive multidrug resistant Acinetobacter baumannii causing severe infections in a Paediatric Intensive Care Unit

Background

Acinetobacter baumannii has been associated with high morbidity and mortality rates, even in pediatric patients. Therapeutic options are limited, especially when the strain is multidrug resistant.

Methods

Clinical and microbiological analyses of 4 cases of systemic infections caused by multi drug resistant A. baumannii treated with colistin/vancomycin combination at a Pediatric Intensive Care Unit were performed in order to explore the potential synergistic activity of colistin plus vancomycin. All the patients were treated with colistin, meropenem and vancomycin.

Results

Four severe infections due to MDR A. baumannii were observed. All patients treated with colistin/vancomycin combination had a positive outcome with no infection relapses. Most importantly, no significant adverse events related to the simultaneous administration of COL plus VAN were observed. In our in-vitro experiments, the synergistic effect of the combination COL plus VAN showed an early bactericidal activity even at VAN concentration of 16 mg/L, which reflects the serum trough concentrations obtained in patients.

Discussion

An antimicrobial strategy based on the activity of colistin plus vancomycin was in-vitro and in-vivo effective in life-threatening infections caused by multidrug-resistant A. baumannii in a Pediatric Intensive Care Unit, in the absence of adverse effects. Colistin plus vancomycin were highly synergic and bactericidal against carbapenem-resistant, colistin sensitive A. baumannii whereas the addition of meropenem did not enhance the in-vitro activity of colistin plus vancomycin.

Conclusions

Our results confirm existing data on the potential synergistic activity of a therapeutic strategy including colistin plus vancomycin and provide important new clinical information for its potential use as a therapeutic option against MDR A. baumannii infections, especially in the pediatric population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-1133-3) contains supplementary material, which is available to authorized users.

A 30-years review on pharmacokinetics of antibiotics: is the right time for pharmacogenetics?

Drug bioavailability may vary greatly amongst individuals, affecting both efficacy and toxicity: in humans, genetic variations account for a relevant proportion of such variability. In the last decade the use of pharmacogenetics in clinical practice, as a tool to individualize treatment, has shown a different degree of diffusion in various clinical fields.

In the field of infectious diseases, several studies identified a great number of associations between host genetic polymor-phisms and responses to antiretroviral therapy. For example, in patients treated with abacavir the screening for HLA-B*5701 before starting treatment is routine clinical practice and standard of care for all patients; efavirenz plasma levels are influenced by single nucleotide polymorphism (SNP) CYP2B6-516G> T (rs3745274). Regarding antibiotics, many studies investigated drug transporters involved in antibiotic bioavailability, especially for fluoroquinolones, cephalosporins, and antituberculars. To date, few data are available about pharmacogenetics of recently developed antibiotics such as tigecycline, daptomycin or linezolid. Considering the effect of SNPs in gene coding for proteins involved in antibiotics bioavailability, few data have been published.

Increasing knowledge in the field of antibiotic pharmacogenetics could be useful to explain the high drug inter-patients variability and to individualize therapy. In this paper we reported an overview of pharmacokinetics, pharmacodynamics, and pharmacogenetics of antibiotics to underline the importance of an integrated approach in choosing the right dosage in clinical practice.

N-Lipidated Peptide Dimers: Effective Antibacterial Agents against Gram-Negative Pathogens through Lipopolysaccharide Permeabilization

Treating infections caused by multidrug-resistant Gram-negative pathogens is challenging, and there is concern regarding the toxicity of the most effective antimicrobials for Gram-negative pathogens. We hypothesized that conjugating a fatty acid moiety onto a peptide dimer could maximize the interaction with lipopolysaccharide (LPS) and facilitate the permeabilization of the LPS barrier, thereby improving potency against Gram-negative pathogens. We systematically designed a series of N-lipidated peptide dimers that are active against Gram-negative bacteria, including carbapenem-resistant Enterobacteriaceae (CRE). The optimized lipid length was 6-10 carbons. At these lipid lengths, the N-lipidated peptide dimers exhibited strong LPS permeabilization. Compound 23 exhibited synergy with select antibiotics in most of the combinations tested. 23 and 32 also displayed rapid bactericidal activity. Importantly, 23 and 32 were nonhemolytic at 10 mg/mL, with no cellular or in vivo toxicity. These characteristics suggest that these compounds can overcome the limitations of current Gram-negative-targeted antimicrobials such as polymyxin B.

Polymyxin E Induces Rapid Paenibacillus polymyxa Death by Damaging Cell Membrane while Ca2+ Can Protect Cells from Damage

Polymyxin E, produced by Paenibacillus polymyxa, is an important antibiotic normally against Gram-negative pathogens. In this study, we found that polymyxin E can kill its producer P. polymyxa, a Gram-positive bacterium, by disrupting its cell membrane. Membrane damage was clearly revealed by detecting the leakage of intracellular molecules. The observation using scanning electron microscopy also supported that polymyxin E can destroy the cell membrane and cause an extensive cell surface alteration. On the other hand, divalent cations can give protection against polymyxin E. Compared with Mg²⁺, Ca²⁺ can more effectively alleviate polymyxin E-induced damage to the cell membrane, thus remarkably increasing the P. polymyxa survival. Our findings would shed light on a not yet described bactericidal mechanism of polymyxin E against Gram-positive bacteria and more importantly the nature of limited fermentation output of polymyxin E from P. polymyxa.

Antimicrobial Peptide Novicidin Synergizes with Rifampin, Ceftriaxone, and Ceftazidime against Antibiotic-Resistant Enterobacteriaceae In Vitro

The spread of antibiotic resistance among Gram-negative bacteria is a serious clinical threat, and infections with these organisms are a leading cause of mortality worldwide. Traditional novel drug development inevitably leads to the emergence of new resistant strains, rendering the new drugs ineffective. Therefore, reviving the therapeutic potentials of existing antibiotics represents an attractive novel strategy. Novicidin, a novel cationic antimicrobial peptide, is effective against Gram-negative bacteria. Here, we investigated novicidin as a possible antibiotic enhancer. The actions of novicidin in combination with rifampin, ceftriaxone, or ceftazidime were investigated against 94 antibiotic-resistant clinical Gram-negative isolates and 7 strains expressing New Delhi metallo-β-lactamase-1. Using the checkerboard method, novicidin combined with rifampin showed synergy with >70% of the strains, reducing the MICs significantly. The combination of novicidin with ceftriaxone or ceftazidime was synergistic against 89.7% of the ceftriaxone-resistant strains and 94.1% of the ceftazidime-resistant strains. Synergistic interactions were confirmed using time-kill studies with multiple strains. Furthermore, novicidin increased the postantibiotic effect when combined with rifampin or ceftriaxone. Membrane depolarization assays revealed that novicidin alters the cytoplasmic membrane potential of Gram-negative bacteria. In vitro toxicology tests showed novicidin to have low hemolytic activity and no detrimental effect on cell cultures. We demonstrated that novicidin strongly rejuvenates the therapeutic potencies of ceftriaxone or ceftazidime against resistant Gram-negative bacteria in vitro. In addition, novicidin boosted the activity of rifampin. This strategy can have major clinical implications in our fight against antibiotic-resistant bacterial infections.

Identification of synergists that potentiate the action of polymyxin B against Burkholderia cenocepacia

Burkholderia cenocepacia and other members of the Burkholderia cepacia complex (BCC) are highly multidrug-resistant bacteria that cause severe pulmonary infections in patients with cystic fibrosis. A screen of 2686 compounds derived from marine organisms identified molecules that could synergise with polymyxin B (PMB) to inhibit the growth of B. cenocepacia. At 1 μg/mL, five compounds synergised with PMB and inhibited the growth of B. cenocepacia by ≥70% compared with growth in PMB alone. Follow-up testing revealed that one compound from the screen, the aminocoumarin antibiotic novobiocin, synergised with PMB and colistin against tobramycin-resistant clinical isolates of B. cenocepacia and Burkholderia multivorans. In parallel, we show that novobiocin sensitivity is common among BCC species and that these bacteria are even more susceptible to an alternative aminocoumarin, clorobiocin, which also had an additive effect with PMB against B. cenocepacia. These studies support using aminocoumarin antibiotics to treat BCC infections and show that synergisers can be found to increase the efficacy of antimicrobial peptides and polymyxins against BCC bacteria.

Mucin Binding Reduces Colistin Antimicrobial Activity

Colistin has found increasing use in treating drug-resistant bacterial lung infections, but potential interactions with pulmonary biomolecules have not been investigated. We postulated that colistin, like aminoglycoside antibiotics, may bind to secretory mucin in sputum or epithelial mucin that lines airways, reducing free drug levels. To test this hypothesis, we measured binding of colistin and other antibiotics to porcine mucin, a family of densely glycosylated proteins used as a surrogate for human sputum and airway mucin. Antibiotics were incubated in dialysis tubing with or without mucin, and concentrations of unbound antibiotics able to penetrate the dialysis tubing were measured over time using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The percentage of antibiotic measured in the dialysate after 4 h in the presence of mucin, relative to the amount without mucin, was 15% for colistin, 16% for polymyxin B, 19% for tobramycin, 52% for ciprofloxacin, and 78% for daptomycin. Antibiotics with the strongest mucin binding had an overall polybasic positive charge, whereas those with comparatively little binding were less basic. When comparing MICs measured with or without added mucin, colistin and polymyxin B showed >100-fold increases in MICs for multiple Gram-negative bacteria. Preclinical evaluation of mucin binding should become a standard procedure when considering the potential pulmonary use of new or existing antibiotics, particularly those with a polybasic overall charge. In the airways, mucin binding may reduce the antibacterial efficacy of inhaled or intravenously administered colistin, and the presence of sub-MIC effective antibiotic concentrations could result in the development of antibiotic resistance.

Preliminary method for direct quantification of colistin methanesulfonate by attenuated total reflectance Fourier transform infrared spectroscopy

Colistin use has increased in response to the advent of infections caused by multidrug-resistant organisms. It is administered parenterally as an inactive prodrug, colistin methanesulfonate (CMS). Various formulations of CMS and labeling conventions can lead to confusion about colistin dosing, and questions remain about the pharmacokinetics of CMS. Since CMS does not have strong UV absorbance, current methods employ a laborious process of chemical conversion to colistin followed by precolumn derivatization to detect formed colistin by high-performance liquid chromatography. Here, we report a method for direct quantification of colistin methanesulfonate by attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR).