title: Minimal attachment killing (MAK): a versatile method for susceptibility testing of attached biofilm-positive and -negative Staphylococcus epidermidis.
abstract: Due to its ability to attach to polymeric surfaces Staphylococcus epidermidis is a common pathogen in chronic, medical device-associated infections. Attached S. epidermidis displays reduced susceptibility against a variety of antimicrobial substances, and little correlation between standard susceptibility test results and clinical outcome of antibiotic treatment is observed. In this study we established a new, versatile, and easy method of antimicrobial susceptibility testing for attached Staphylococcus epidermidis, suitable for both biofilm-negative and biofilm-positive attached bacteria using readily available equipment. For three biofilm-positive wild-type strains and their biofilm-negative mutants minimal attachment killing concentrations (MAK) of penicillin, oxacillin, vancomycin, and gentamicin were determined. Depending on strain and investigated antibiotics, a heterogeneous MAK (MAK(hetero)) could be differentiated from a homogeneous resistance (MAK(homo)), favoring a model of few persisters within attached cells under antibiotic treatment. For the biofilm-negative mutants, a lower MAK(homo) was detected than for the corresponding wild types for some of the tested antibiotics, which probably resulted from higher bacterial inocula of wild-type strains, whereas the MAK(hetero) were comparable for mutants and wild types for most of the tested antibiotics and strains. These data indicate that biofilm formation is not a necessary prerequisite for persistence of attached S. epidermidis cells under antibiotic treatment, which could explain therapeutic failure in foreign body-associated infections due to biofilm-negative S. epidermidis isolates. The highly individual resistance phenotypes of the investigated strains with different antibiotics suggests that MAK determination could help to predict the therapeutic outcome of foreign body-associated infections with both biofilm-positive and biofilm-negative S. epidermidis.
authors.affiliation: Institut für Medizinische Mikrobiologie und Immunologie, Universitätsklinikum Hamburg-Eppendorf, Martinistr. 52, Germany. knobloch@uke.uni-hamburg.de
authors.forename: Johannes K-M
authors.initials: JK
authors.lastname: Knobloch
authors.forename: Heimke
authors.initials: H
authors.lastname: Von Osten
authors.forename: Matthias A
authors.initials: MA
authors.lastname: Horstkotte
authors.forename: Holger
authors.initials: H
authors.lastname: Rohde
authors.forename: Dietrich
authors.initials: D
authors.lastname: Mack
mesh_terms: D000900:Anti-Bacterial Agents / Q000145:classification / Q000276:immunology; D001422:Bacterial Adhesion; D018441:Biofilms / Q000187:drug effects*; D004352:Drug Resistance, Microbial; D008826:Microbial Sensitivity Tests / Q000379:methods*; D008954:Models, Biological; D013212:Staphylococcus epidermidis / Q000187:drug effects* / Q000235:genetics / Q000502:physiology
chemical_list: D000900:Anti-Bacterial Agents