Investigation of In-Vitro Adaptation toward Sodium Bituminosulfonate in Staphylococcus aureus

The global increase in antimicrobial resistance has revived the interest in "old" substances with antimicrobial activity such as sodium bituminosulfonate. However, for those "old" compounds, scientific studies are still sparse and the ones available do not mostly meet the current standards. Since this compound is used for topical applications, investigation of a potential increase in minimal inhibitory concentrations (MICs) is of particular importance. For selection of phenotypes with decreased susceptibility, a collection of 30 genetically diverse methicillin-susceptible (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) strains were cultured on bi-layered linear gradient agar plates containing sub-inhibitory concentrations of the active agents. The stability of phenotypes with increased MICs was determined by serial passage on agent-free medium. Within 10 passages, only slight and, in most cases, reversible increases in MSSA and MRSA MIC levels toward sodium bituminosulfonate were obtained. Fusidic acid, used as a control, showed exponential expansions in MIC based on mutations in the fusA gene (elongation factor G or EF-G) with no reduction during the recovery phase. The only marginal and largely reversible changes of S. aureus MICs after exposition to sodium bituminosulfonate indicate a low potential for resistance development.