Influence of extraction method on antibacterial activity of ethanolic extracts of Ocimum gratissimum L.

The aim of this study was to define whether novel extraction methods such as microwave and ultrasound could obtain the most effective ethanolic extracts of Ocimum gratissimum as antibacterial agents. These extracts were compared with respect to extractive yield, eugenol content, antibacterial activity and brine-shrimp (Artemia salina) toxicity with extracts obtained by the classical procedures of maceration and Soxhlet. Significant differences among the extracts were observed in all analyses. Soxhlet extraction gave the highest yield (19.5%). Maceration and microwave extracts yielded the highest eugenol contents (11.6 and 11.8%, respectively). The bactericidal activity of the extracts was correlated with eugenol content (rs = 0.894). Maceration gave the extract with the broadest spectrum of activity. Ultrasound methods yielded an efficient extract for use as a topical antiseptic (minimum inhibitory concentration (MIC) = 0.66 to 1.32 mg/ml for Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus (MRSA)). The most active extracts to treat vancomycin-resistant enterococci (VRE) infections were obtained by Soxhlet  and microwave (MIC = 5.28 mg/ml). The extract obtained by maceration was the most toxic for brine shrimp, followed by the extracts obtained by ultrasonic horn, ultrasonic cleaning bath and Soxhlet /microwave. In conclusion, the antibacterial results showed that the extractive methodology can be chosen according to the intended use.   Key words: Ocimum gratissimum, microwave-assisted extraction (MAE), ultrasonic cleaning bath (UCB), ultrasonic horn (UH), antibacterial activity, toxicity, Artemia salina.