title: Standard susceptibility testing overlooks potent azithromycin activity and cationic peptide synergy against MDR Stenotrophomonas maltophilia.
abstract: OBJECTIVE
  The Gram-negative bacillus Stenotrophomonas maltophilia (SM) is an emerging MDR opportunistic pathogen. Recent studies identify a potentially relevant activity of azithromycin against Gram-negative bacteria overlooked in standard bacteriological testing. We investigated azithromycin activity against SM in testing conditions incorporating mammalian tissue culture medium and host defence factors.
  
  
  METHODS
  MIC testing, chequerboard assays, time-kill assays and fluorescence microscopy were performed for azithromycin, the cationic peptide antibiotic colistin and the human defence peptide cathelicidin LL-37 alone or in combination in cation-adjusted Mueller-Hinton broth or mammalian tissue culture media. Azithromycin sensitization of SM to host immune clearance was tested in a human neutrophil killing assay and a murine pneumonia model.
  
  
  RESULTS
  We observed potent bactericidal activity of azithromycin against SM in mammalian tissue culture medium absent in bacteriological medium. Colistin and LL-37 strongly potentiated azithromycin killing of SM by increasing drug entry. Additionally, azithromycin sensitized SM to neutrophil killing and increased SM clearance in the murine pneumonia model.
  
  
  CONCLUSIONS
  Despite lack of activity in standard MIC testing, azithromycin synergizes with cationic peptide antibiotics to kill SM in medium mimicking tissue fluid conditions. Azithromycin, alone or in combination with colistin, merits further exploration in therapy of drug-resistant SM infections.
authors.affiliation: Division of Infectious Diseases, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Monika
authors.initials: M
authors.lastname: Kumaraswamy
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Leo
authors.initials: L
authors.lastname: Lin
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Joshua
authors.initials: J
authors.lastname: Olson
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA School of Medicine, China Medical University, Taichung 40402, Taiwan.
authors.forename: Ching-Fang
authors.initials: CF
authors.lastname: Sun
authors.affiliation: Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Poochit
authors.initials: P
authors.lastname: Nonejuie
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Ross
authors.initials: R
authors.lastname: Corriden
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Simon
authors.initials: S
authors.lastname: Döhrmann
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Syed Raza
authors.initials: SR
authors.lastname: Ali
authors.affiliation: Department of Pathology, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Deirdre
authors.initials: D
authors.lastname: Amaro
authors.affiliation: Central Facility for Microscopy, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Rohde
authors.affiliation: Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Joe
authors.initials: J
authors.lastname: Pogliano
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: George
authors.initials: G
authors.lastname: Sakoulas
authors.affiliation: Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093, USA Rady Children's Hospital, San Diego, CA 92123, USA vnizet@ucsd.edu.
authors.forename: Victor
authors.initials: V
authors.lastname: Nizet
mesh_terms: D000818:Animals; D000890:Anti-Infective Agents / Q000494:pharmacology*; D023181:Antimicrobial Cationic Peptides / Q000494:pharmacology*; D017963:Azithromycin / Q000494:pharmacology*; D003091:Colistin / Q000494:pharmacology; D004195:Disease Models, Animal; D004357:Drug Synergism; D016905:Gram-Negative Bacterial Infections / Q000188:drug therapy / Q000382:microbiology; D006801:Humans; D051379:Mice; D008826:Microbial Sensitivity Tests; D009504:Neutrophils / Q000276:immunology; D018410:Pneumonia, Bacterial / Q000188:drug therapy / Q000382:microbiology; D020615:Stenotrophomonas maltophilia / Q000187:drug effects*; D016896:Treatment Outcome; D054804:Cathelicidins
chemical_list: D000890:Anti-Infective Agents; D023181:Antimicrobial Cationic Peptides; D017963:Azithromycin; D003091:Colistin; D054804:Cathelicidins
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