Journal of Applied Microbiology 1 998, 84, 1 52–1 58The effect of essential oils of basil on the growth ofAeromonas hydrophila and Pseudomonas fluorescensJ. Wan, A. Wilcock and M.J. CoventryAustralian Food Industry Science Centre, Werribee, Victoria, Australia601 5/1 2/96: received 6 December 1 996 and accepted 20 March 1 997J. WAN, A. WILCOCK AND M.J. COVENTRY. 1 998. Basil essential oils, including basil sweetlinalool (BSL) and basil methyl chavicol (BMC), were screened for antimicrobial activityagainst a range ofGram-positive and Gram-negative bacteria, yeasts and moulds usingan agar well diffusion method. Both essential oils showed antimicrobial activityagainst most ofthe micro-organisms examined except Clostridium sporogenes, Flavimonasoryzihabitans, and three species ofPseudomonas. The minimum inhibitoryconcentration (MIC) ofBMC against Aeromonas hydrophila and Pseudomonas fluorescensin TSYE broth (as determined using an indirect impedance method) was 0·125and 2% (v/v), respectively; the former was not greatly affected by the increase ofchallenge inoculum from 103to 106cfu ml−1. Results with resting cellsdemonstrated that BMC was bactericidal to both Aer. hydrophila and Ps. fluorescens.The growth ofAer. hydrophila in filter-sterilized lettuce extract was completelyinhibited by 0·1% (v/v) BMC whereas that ofPs. fluorescens was not significantlyaffected by 1% (v/v) BMC. In addition, the effectiveness ofwashing fresh lettucewith 0·1 or 1% (v/v) BMC on survival ofnatural microbial flora was comparable withthat effected by 125 ppm chlorine.INTRODUCTIONalso an increasing trend to the development of ready-to-serve salad products and the inclusion of herbs and unusualvarieties in MPF vegetable products (Anon 1993; Anon 1996).Sweet basil (Ocimum basilicum L.) is a popular culinary herband has been used widely as a food ingredient for flavouringconfectionary and baked goods, condiments (tomato pastes,sauces, pickles and vinegars) and meat products such as saus-ages and canned meats (Dziezak 1989). In addition, basilessential oils have been reported to be antimicrobial againsta variety ofGram-positive, Gram-negative bacteria and yeastsand moulds (Reuveni etal. 1984; Deans and Ritchie 1987;Dube etal. 1989; Gangrade etal. 1989; Meena and Sethi1994). It has also been demonstrated that the antimicrobialactivities ofessential oils ofdifferent species ofOcimum werepredominantly associated with the main constituents of lin-alool and methyl chavicol (Sinha and Gulati 1990). In thecurrent study, the effects of commercially available basilessential oils were screened against a variety ofbacteria, yeastsand moulds occurring in foods to verify the spectrum ofactivity. Antimicrobial activity against Aer. hydrophila andPs. fluorescens was further characterized with resting cells andgrowing cells in broth medium and filter-sterilized lettuceextract. In addition, the effect of washing with basil oil sus-pensions on survival ofthe natural microflora offresh lettuceMinimally processed fresh (MPF) vegetable products pack-aged in modified atmospheres are a growing market segmentforhorticultural produce (Ahvenainen 1996). However, effec-tive refrigerated temperature control during manufacture,distribution and retailing is required for maintaining themicrobiological quality and safety of these products. Psy-chrotrophic pathogens (such as Aeromonas, Listeria, Yersiniaand some types ofClostridium botulinum) and spoilage bacteria(such as Pseudomonas and Enterobacteriaceae) are the majormicrobiological concerns with MPF products (Lund 1992;Nguyen-the and Carlin 1994; Beuchat 1996). In particular,Aer. hydrophila poses a potential risk as it has been reportedto occur at high microbial loads in these products (up to 104–106cfu g−1, Marchetti etal. 1992) and cytotoxicity occurs ina large proportion ofAeromonas isolates from various sources(Callister and Agger 1987; Fricker and Tompsett 1989).Preservation systems using food-grade antimicrobials offerthe potential ofadditional hurdles for more effective controlof the microbiological quality of MPF vegetable productsthroughout the processing and distribution chains. There isCorrespondence to: M.J. Coventry, Australian Food Industry ScienceCentre, Private Bag 16, Werribee, Victoria 3030, Australia.© 1 998 The Society for Applied Microbiology
