Antimicrobial activity of essential oils in milk samples from mastitic sheep

Introduction: Some essential oils, extracted from many plants, in vitro show antimicrobial activity against various pathogenic bacteria and fungi. The purpose of the present study was to evaluate the antibacterial activity of the of the essential oils (EOs) of Citrus bergamia Risso & Poiteau, Melaleuca alternifolia (tea-tree oil) and Chenopodium botrys in vitro against strains of pathogens isolated commonly in milk samples from mastitic sheep. This activity was compared with the effect of some antimicrobial agents used in veterinary practice against micro-organisms frequently involved in ovine mastitis. Materials and methods: The research was carried out on 55 milk samples, taken from sheep, suffering from clinically diagnosed mastitis and kept in free stall housing in on Sicilian. The animals suffering from intramammary mastitic infection (IMI) showed: an udder that was increased in volume, reddened, hot and painful on palpation with decreased mammary secretion and contained milk with alterated chemical physical and organoleptic characteristics. General symptoms included slightly raised body temperature, anorexia and depression. Diagnosis was made on clinical signs and by means of bacteriological testing of the milk. Gram-negative and Gram-positive bacteria in milk samples were isolated on McConkey Agar and Chapman-Stone agar (Difco, Italy), respectively and identified biochemically using the API System (Bio Merieux Italy). The antimicrobial activity of EOs and of some antibiotics used in veterinary practice (ampicillin, amoxicillin, amikacin, thiamphenicol, erythromycin and tetracycline) was evaluated by the agar disk diffusion method (Kirby-Bauer) by measuring the diameter of the growth inhibition zone. Furthermore MIC and MBC values were determined. The data obtained were elaborated using Student’s t- test. Statistical significance was defined as P < 0.05. Results: The bacterial strains isolated from the milk samples were: Staphylococcus xylosus (no.11), Staphylococcus epidermidis (no.8), Streptococcus salivarius (no. 5), Enterococcus faecium (no.4), Streptococcus agalactiae (no 17) and Pseudomonas aeruginosa (no 10). Results indicated that EOs showed a higher antibacterial activity against all the micro-organism (with low MIC values) than the tested antimicrobial agents. A significant bacterial reduction in the milk samples was observed with low MIC values, compared with the tested antimicrobial agents. In particular, Melaleuca alternifolia showed the greater antimicrobial activity, followed by Citrus bergamia Risso and Chenopodium botrys 1-4 Discussion: The data obtained in the present study indicate that EOs are active in vitro against all the tested micro-organism, suggesting their potential role for the treatment of susceptible bacterial agents in ovine mastitis. Preliminary in vivo study showed a therapeutic efficacy of Melaleuca alternifolia in sheep affected of IMI caused by Gram positive bacteria. Further investigations will be carried out to appraise the therapeutic effectiveness of these EOs in the bacterial mastitis of sheep and cattle in vivo.