Biofilm-associated infections with Staphylococcus aureus are difficult to treat even after administration of antibiotics that according to the standard susceptibility assays are effective. Currently, the assays used in the clinical laboratories to determine the sensitivity of S. aureus towards antibiotics are not representing the behaviour of biofilm-associated S. aureus, since these assays are performed on planktonic bacteria. In research settings, microcalorimetry has been used for antibiotic susceptibility studies. Therefore, in this study we investigated if we can use isothermal microcalorimetry to monitor the response of biofilm towards antibiotic treatment in real-time. We developed a reproducible method to generate biofilm in an isothermal microcalorimeter setup. Using this system, the sensitivity of 5 methicillin-sensitive S. aureus (MSSA) and 5 methicillin-resistant S. aureus (MRSA) strains from different genetic lineages were determined towards: flucloxacillin, cefuroxime, cefotaxime, gentamicin, rifampicin, vancomycin, levofloxacin, clindamycin, erythromycin, linezolid, fusidic acid, co-trimoxazole, and doxycycline. In contrast to conventional assays, our calorimetry-based biofilm susceptibility assay showed that S. aureus biofilms, regardless MSSA or MRSA, can survive the exposure to the maximum serum concentration of all tested antibiotics. The only treatment with a single antibiotic showing a significant reduction in biofilm survival was rifampicin, yet in 20% of the strains, emerging antibiotic resistance was observed. Furthermore, the combination of rifampicin with flucloxacillin, vancomycin or levofloxacin was able to prevent S. aureus biofilm from becoming resistant to rifampicin. Isothermal microcalorimetry allows real-time monitoring of the sensitivity of S. aureus biofilms towards antibiotics in a fast and reliable way.
Willem van Wamel, PhD is an Associate Professor, Department of Medical Microbiology and Infectious Diseases Erasmus University Medical Centre Rotterdam, The Netherlands. He earned his doctoral degree in microbiology from the Eijkman Winkler Institute for Microbiology, Infectious Diseases, and Inflammation at the University of Utrecht in Utrecht, The Netherlands; the topic of his doctoral thesis was “Regulation of Virulence Factors in Staphylococci.” In addition, he completed postdoctoral work and research there, as well as at the Laboratory of Bacterial Pathogenesis and Immunology at Rockefeller University in New York, New York, and Dartmouth Medical School in Hanover, New Hampshire. His current areas of research include the pathogenesis of Staphylococcus aureus (S. aureus) with a focus on difficult to treat chronic infection. In his group strategies were developed to monitor in vivo/ex vivo expression of virulence factors but also ways to monitor (real-time) the metabolic status of biofilms associated bacteria.
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