Minimal attachment killing (MAK): a versatile method for susceptibility testing of attached biofilm-positive and -negative Staphylococcus epidermidis.

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.