A pinboard by
Daniel Pletzer

I'm a microbiologist having expertise with hard-to-treat pathogenic bacteria (esp. with WHO priority pathogens categorized "critical") involved in multi-drug resistance and biofilm infections. I have a broad knowledge of microbiology, biochemistry, molecular biology / genetics, and animal models as well as experience with bioinformatical data interpretation and analysis. Additionally, I gained a lot of experience as a teaching and research assistant that enabled me to help mentor and train students.

My long-term career goal is to become a project leader of my own research group and eventually attain a professorship at a research university. My personal motivation in continuing a scientific carrier is connected to the fact that antibiotic resistant bacteria (such as Pseudomonas aeruginosa) are emerging all over the world and current therapeutic approaches are rapidly losing their effectiveness. Addressing antibiotic resistance has captivated my interest over the past few years and I am eager to learn more about life-threatening bacteria in order to help to fight against the shrinking arsenal of antibiotics.


Fighting against our shrinking arsenal of antibiotics

The current therapy for infectious diseases is facing twin threats. On the one hand antibiotic and antiviral resistance is rising rapidly; on the other there are relatively few novel compounds or strategies under development or entering the clinic. One promising set of compounds are cationic host defence (antimicrobial) peptides that have both antimicrobial and immunomodulatory activities. These peptides are produced as a major component of the innate defense against infections by virtually all complex organisms ranging from plants and insects to humans.

Biofilm infections are especially recalcitrant to conventional antibiotic treatment and are a huge problem in trauma patients with major injuries, individuals with an implanted medical device, and in cystic fibrosis (CF) patients. Microbial biofilms are surface-associated, structured bacterial communities that grow in a protective polymeric matrix. The biofilm-mode of growth is a major lifestyle for bacteria in natural, industrial and clinical settings; indeed they are associated with 65-80% of all clinical infections. Bacterial growth as biofilms can result in as much as 1000-fold increase in resistance to most antimicrobial agents, due to (i) differentiation of bacteria within the biofilm, (ii) poor antibiotic penetration into the biofilm, and (iii) the stationary phase growth of bacteria underlying the surface layer. This type of transient resistance, which depends on the growth state of the organism, is termed adaptive resistance and has been proposed to play a major role in reducing therapeutic effectiveness of antibiotics. Currently, we have identified a number of novel anti-biofilm peptides that kill multiple species of bacteria in biofilms, including essentially all major clinically relevant bacteria, especially the so-called ESKAPE pathogens.

Recent studies have implicated a stress response termed the stringent response in the mechanism of action of these peptides. This response to amino acid starvation or other stresses involves the synthesis, through enzymes RelA and SpoT, of guanosine tetra-phosphate (ppGpp), which acts to direct bacterial RNA polymerase to modulate expression of many genes. However, the regulon still remains poorly defined. The downstream impacts of inhibition of synthesis of ppGpp by these peptides includes the inhibition of the biofilm development program, bacterial cell death (proposed to occur through the modulation of bacterial cell wall biosynthesis) and dispersal.


New Mouse Model for Chronic Infections by Gram-Negative Bacteria Enabling the Study of Anti-Infective Efficacy and Host-Microbe Interactions.

Abstract: Only a few, relatively cumbersome animal models enable long-term Gram-negative bacterial infections that mimic human situations, where untreated infections can last for weeks. Here, we describe a simple murine cutaneous abscess model that enables chronic or progressive infections, depending on the subcutaneously injected bacterial strain. In this model, Pseudomonas aeruginosa cystic fibrosis epidemic isolate LESB58 caused localized high-density skin and soft tissue infections and necrotic skin lesions for up to 10 days but did not disseminate in either CD-1 or C57BL/6 mice. The model was adapted for use with four major Gram-negative nosocomial pathogens, Acinetobacter baumannii, Klebsiella pneumoniae, Enterobacter cloacae, and Escherichia coli This model enabled noninvasive imaging and tracking of lux-tagged bacteria, the influx of activated neutrophils, and production of reactive oxygen-nitrogen species at the infection site. Screening antimicrobials against high-density infections showed that local but not intravenous administration of gentamicin, ciprofloxacin, and meropenem significantly but incompletely reduced bacterial counts and superficial tissue dermonecrosis. Bacterial RNA isolated from the abscess tissue revealed that Pseudomonas genes involved in iron uptake, toxin production, surface lipopolysaccharide regulation, adherence, and lipase production were highly upregulated whereas phenazine production and expression of global activator gacA were downregulated. The model was validated for studying virulence using mutants of more-virulent P. aeruginosa strain PA14. Thus, mutants defective in flagella or motility, type III secretion, or siderophore biosynthesis were noninvasive and suppressed dermal necrosis in mice, while a strain with a mutation in the bfiS gene encoding a sensor kinase showed enhanced invasiveness and mortality in mice compared to controls infected with wild-type P. aeruginosa PA14.IMPORTANCE More than two-thirds of hospital infections are chronic or high-density biofilm infections and difficult to treat due to adaptive, multidrug resistance. Unfortunately, current models of chronic infection are technically challenging and difficult to track without sacrificing animals. Here we describe a model of chronic subcutaneous infection and abscess formation by medically important nosocomial Gram-negative pathogens that is simple and can be used for tracking infections by imaging, examining pathology and immune responses, testing antimicrobial treatments suitable for high-density bacterial infections, and studying virulence. We propose that this mouse model can be a game changer for modeling hard-to-treat Gram-negative bacterial chronic and skin infections.

Pub.: 02 Mar '17, Pinned: 16 Aug '17

Mechanisms of intrinsic resistance and acquired susceptibility of Pseudomonas aeruginosa isolated from cystic fibrosis patients to temocillin, a revived antibiotic.

Abstract: The β-lactam antibiotic temocillin (6-α-methoxy-ticarcillin) shows stability to most extended spectrum β-lactamases, but is considered inactive against Pseudomonas aeruginosa. Mutations in the MexAB-OprM efflux system, naturally occurring in cystic fibrosis (CF) isolates, have been previously shown to reverse this intrinsic resistance. In the present study, we measured temocillin activity in a large collection (n = 333) of P. aeruginosa CF isolates. 29% of the isolates had MICs ≤ 16 mg/L (proposed clinical breakpoint for temocillin). Mutations were observed in mexA or mexB in isolates for which temocillin MIC was ≤512 mg/L (nucleotide insertions or deletions, premature termination, tandem repeat, nonstop, and missense mutations). A correlation was observed between temocillin MICs and efflux rate of N-phenyl-1-naphthylamine (MexAB-OprM fluorescent substrate) and extracellular exopolysaccharide abundance (contributing to a mucoid phenotype). OpdK or OpdF anion-specific porins expression decreased temocillin MIC by ~1 two-fold dilution only. Contrarily to the common assumption that temocillin is inactive on P. aeruginosa, we show here clinically-exploitable MICs on a non-negligible proportion of CF isolates, explained by a wide diversity of mutations in mexA and/or mexB. In a broader context, this work contributes to increase our understanding of MexAB-OprM functionality and help delineating how antibiotics interact with MexA and MexB.

Pub.: 17 Jan '17, Pinned: 16 Aug '17

Structure and function of the PiuA and PirA siderophore-drug receptors from Pseudomonas aeruginosa and Acinetobacter baumannii.

Abstract: The outer membrane of Gram-negative bacteria presents an efficient barrier to the permeation of antimicrobial molecules. One strategy pursued to circumvent this obstacle is to hijack transport systems for essential nutrients such as iron. BAL30072 and MC-1 are two monobactams conjugated to a dihydroxypyridone siderophore that are active against Pseudomonas aeruginosa and Acinetobacter baumannii Here, we investigated the mechanism of action of these molecules in A. baumannii We identified two novel TonB-dependent receptors, termed Ab-PiuA and Ab-PirA that are required for antimicrobial activity of both agents. Deletion of either piuA or pirA in A. baumannii resulted in 4 to 8 fold decreased susceptibility, while their overexpression in the heterologous host P. aeruginosa increased susceptibility to the two siderophore-drug conjugates by 4 to 32 fold. Crystal structures of PiuA and PirA from A. baumannii and their orthologues from P. aeruginosa were determined. The structures revealed a similar architecture, however structural differences between PirA and PiuA point to potential differences between their cognate siderophore ligands. Spontaneous mutants, selected upon exposure to BAL30072, harbored frame-shift mutations in either the ExbD3 or the TonB3 proteins of A. baumannii, forming the cytoplasmic membrane complex providing the energy for the siderophore translocation process. The results of this study provide insight for the rational design of novel siderophore-drug conjugates against problematic Gram-negative pathogens.

Pub.: 01 Feb '17, Pinned: 16 Aug '17