Comparative evaluation of small molecules reported to be inhibitors of Staphylococcus aureus biofilm formation

Abstract

Biofilm formation is an important characteristic of many Staphylococcus aureus infections because it limits the efficacy of host defenses and conventional antibiotic therapy. There are many literature reports that describe small molecule inhibitors of S. aureus biofilm formation, but the lack of direct comparisons and differences in the methods used to assess inhibition make it impossible to assess the efficacy of these compounds relative to each other. To address this, we compared 19 compounds reported in the literature to be inhibitors of S. aureus biofilm formation. We used the methicillin-susceptible clinical osteomyelitis isolate UAMS-1 in a microtiter plate biofilm assay shown to maximize biofilm formation and provide results consistent with those observed in vivo . Under these conditions, telithromycin (Ketek) showed the greatest inhibitory activity, limiting biofilm formation to a degree comparable to a UAMS-1 sarA mutant at a concentration of 0.49 µM (0.40 µg/mL). Similar results were obtained for the methicillin-resistant USA300 strain LAC. Telithromycin is a bacteriostatic ketolide antibiotic that is not currently approved in the United States for use as an antibiotic due to its concerning safety profile. However, the concentration required to limit growth in vitro was higher than the concentration required to limit biofilm formation, suggesting that low-dose telithromycin might be useful to limit biofilm formation and increase the efficacy of other antibiotics in biofilm-associated infections. Irrespective of whether telithromycin is ultimately proven clinically useful in this regard, these results emphasize the need for widespread use of a standardized in vitro approach to evaluate prospective inhibitors. IMPORTANCE Because biofilm formation is such a problematic feature of Staphylococcus aureus infections, much effort has been put into identifying biofilm inhibitors. However, the results observed with these compounds are often reported in isolation, and the methods used to assess biofilm formation vary between labs, making it impossible to assess relative efficacy and prioritize among these putative inhibitors for further study. The studies we report address this issue by directly comparing putative biofilm inhibitors using a consistent in vitro assay. This assay was previously shown to maximize biofilm formation, and the results observed with this assay have been proven to be relevant in vivo . Of the 19 compounds compared using this method, many had no impact on biofilm formation under these conditions. Indeed, only one proved effective at limiting biofilm formation without also inhibiting growth.

Increased production of aureolysin and staphopain A is a primary determinant of the reduced virulence of Staphylococcus aureus sarA mutants in osteomyelitis

“…Since mutation of sarA has long been known to inhibit biofilm formation, cytotoxicity and virulence in osteomyelitis ( 4 , 6 , 8 , 15 , 25 , 26 ), several reports have already described the identification of such inhibitors with a primary focus on limiting biofilm formation ( 27 – 32 ). However, we recently examined a number of these putative inhibitors using the same biofilm assay used in this report, and we failed to identify any that effectively inhibited biofilm formation to the same degree as mutation of sarA in LAC and UAMS-1 ( 33 ).…”