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Minimal attachment killing (MAK): a versatile method for susceptibility testing of attached biofilm-positive and
Johannes K-M Knobloch1, Heimke Von Osten, Matthias A Horstkotte
1Institut für Medizinische Mikrobiologie und Immunologie, Universitätsklinikum Hamburg-Eppendorf, Martinistr. 52, Germany. knobloch@uke.uni-hamburg.de
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.
Insights
A new method for testing antibiotic susceptibility of attached Staphylococcus epidermidis was developed. This minimal attachment killing concentration (MAK) test helps predict treatment outcomes for device-associated infections, even without biofilms.
Area of Science:
- Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Staphylococcus epidermidis is a major cause of chronic medical device-associated infections.
- Attached S. epidermidis exhibits reduced susceptibility to antibiotics, with poor correlation between standard tests and clinical outcomes.
- Current susceptibility testing methods do not adequately reflect the challenges of treating attached bacteria.
Purpose of the Study:
- To establish a versatile and easy method for antimicrobial susceptibility testing of attached S. epidermidis.
- To determine the minimal attachment killing concentration (MAK) for various antibiotics against biofilm-positive and biofilm-negative S. epidermidis.
- To investigate the role of biofilm formation in antibiotic persistence of attached S. epidermidis.
Main Methods:
- Developed a novel antimicrobial susceptibility testing method for attached S. epidermidis.
- Utilized readily available equipment for testing both biofilm-negative and biofilm-positive bacteria.
- Determined minimal attachment killing concentrations (MAK) for penicillin, oxacillin, vancomycin, and gentamicin against wild-type strains and their biofilm-negative mutants.
Main Results:
- Differentiated between heterogeneous (MAK(hetero)) and homogeneous (MAK(homo)) resistance, suggesting a persister cell model.
- Biofilm-negative mutants showed lower MAK(homo) than wild types for some antibiotics, likely due to inoculum differences.
- Comparable MAK(hetero) for mutants and wild types indicated biofilm is not essential for persistence under antibiotic treatment.
Conclusions:
- Biofilm formation is not a prerequisite for S. epidermidis persistence on surfaces during antibiotic treatment.
- This finding could explain treatment failures in foreign body-associated infections caused by biofilm-negative S. epidermidis.
- MAK determination offers a promising approach to predict therapeutic outcomes in S. epidermidis device-associated infections, regardless of biofilm status.
