The emergence of new infectious diseases, the resurgence of several infections that appeared to have been controlled and the increase in bacterial resistance have created the necessity for studies directed towards the development of new antimicrobials. Considering the failure to acquire new molecules with antimicrobial properties from microorganisms, the optimization for screening methods used for the identification of antimicrobials from other natural sources is of great importance. The objective of this study was to evaluate technical variants used in screening methods to determine antibacterial activity of natural products. Thus, a varied range of natural products of plant, fungi and lichen origin were tested against two bacterial species, Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922, by two variants of the agar diffusion method (well and disc), two variants of the bioautographic method (direct and indirect) and by microdilution assay. We concluded that the well-variant of the diffusion method was more sensitive than the disc-variant, whilst the direct-variant of the bioautographic method exhibited a greater sensitivity if compared to indirectvariant. Bioautographic and diffusion techniques were found to have similar sensitivity; however the latter technique provided more suitable conditions for the microbial growth. In this study, we also discussed the best conditions for the determination of minimal inhibitory concentration.

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IntroductionThere is an increasing interest in phytochemicals as new sources of natural antioxidant and antimicrobial agents. The use of synthetic antioxidants in the food industry is severely restricted as to both application and level (TAWAHA et al., 2007;PENG et al., 2005). Currently, there is a strong debate about the safety aspects of chemical preservatives, since they are considered responsible for many carcinogenic and teratogenic attributes, as well as residual toxicity (MOREIRA et al., 2005).Plant-derived polyphenols receive considerable attention because of their potential antioxidant and antimicrobial properties (MOREIRA et al., 2005;PROESTOS;SERELI;KOMAITIS, 2006). Phenolic compounds exhibit a considerable free-radical scavenging (antioxidant) activity, which is determined by their reactivity as hydrogen-or electron-donating agents, the stability of the resulting antioxidant-derived radical, their reactivity with other antioxidants and, finally, their metal chelating properties (RICE-EVANS; MILLER; PAGANGA, 1997).Rosemary (Rosmarinus officinalis L.) is a spice and medicinal herb widely used around the world. Of the natural antioxidants, rosemary has been widely accepted as one of the spices with the highest antioxidant activity (PENG et al., 2005). Rosemary essential oil is also used as an antibacterial, antifungal (OLUWATUYI; KAATZ;GIBBONS, 2004;FERNÁNDEZ-LÓPEZ et al., 2005;KABOUCHE et al., 2005;REZZOUG;BOUTEKEDJIRET;ALLAF, 2005) and anticancer agent (LEAL et al., 2003). Ouattara et al. (1997) investigated the antibacterial activity of selected essential oils against some food spoilage organisms. They concluded that the essential oils of cinnamon, clove and rosemary were the most active. Similar results were obtained by Valero and Salmeron (2003) for the antibacterial activity of rosemary essential oil against Bacillus cereus strains grown in carrot broth.Many compounds have been isolated from rosemary, including flavones, diterpenes, steroids, and triterpenes. Of these, ResumoExtratos das partes aéreas de alecrim foram obtidos utilizando fluidos supercríticos (FSC) e extrações Sohxlet. Sua composição foi analisada com GC-MS. Os extratos foram analisados com base nos compostos repostados na literatura como tendo atividade antimicrobiana e antioxidante. Os extratos de alecrim foram testados quanto à atividade antioxidante (radicais DPPH e conteúdo total de fenólicos -reagente

The antibacterial activity of coumarin per se and other 45 coumarin derivatives was tested against strains of Bacillus cereus MIP 96016, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 25923. The inhibitory effects of coumarins were affected by their substitution patterns. Osthenol (44) showed the most effective antibacterial activity against Gram-positive bacteria with MIC values ranging between 125 and 62.5 µg/ml. These results suggested that the prenyl chain of 44 at position 8 and the presence of OH at position 7 of the benzenic ring are required for the antibacterial activity against these strains.