Temperature-dependent changes in susceptibility of Stenotrophomonas maltophilia to the essential oils of sweet basil (Ocimum basilicum) and black pepper (Piper nigrum)

The aims of this study was to determine the activities of essential oils from sweet basil and black pepper against Stenotrophomonas maltophilia and, as temperature is known to affect the in vitro susceptibility of Sten. maltophilia to aminoglycosides and quinolones, to identify temperature-dependent susceptibility. Minimum inhibitory concentrations (MICs) of the oils were determined using broth microdilution at 30 degrees C and 37 degrees C. Linalool was further studied using an agar-incorporation method and gradient plates using tryptone soya agar containing lecithin and Tween 80 (TSALT agar) at 30 degrees C and 37 degrees C. Linalool was more active (MIC 0.125% v/v) than black pepper oil (MIC 8% v/v) at 37 degrees C using broth microdilution. MICs for linalool were two- to fourfold higher in the agar-incorporation assay compared with the broth microdilution. At 30 degrees C, MICs for linalool and black pepper oil were twofold higher than those obtained at 37 degrees C. Temperature-dependent susceptibility to linalool was demonstrated using TSALT gradient plates. In conclusions, linalool is more active than black pepper oil against Sten. maltophilia . Susceptibility to both oils manifests temperature dependence. Susceptibility to linalool is affected by the nature of the assay (i.e., agar vs broth techniques). These results highlight the need for standardization of methods used to assess susceptibility of microorganisms to plant essential oils.