effect of fruit juices and pomace extracts on the growth of Gram-positive and Gram-negative bacteria

Extracts and juices of cultivated and wild fruits belonging to the families Rosaceae, Grossulariaceae, Moraceae, Berberidaceae, Polygonaceae, Caprifoliaceae and Cornaceae were examined for their growth reducing activity on four bacteria (Bacillus subtilis, B. cereus var. mycoides, Escherichia coli and Serratia marcescens). In vitro antibacterial activities were evaluated by microdilution plate assays. Black currant (Ribes nigrum), cornelian cherry (Cornus mas) and European rowan (Sorbus aucuparia) had the highest growth inhibition capacity. Acta Biol Szeged 52(2):267-270 (2008) Key WordS fruit juice antibacterial effect pomace extract Accepted July 28, 2008 *Corresponding author. E-mail: [email protected]. 267 Fruits are rich sources of vitamins, minerals and fibres, therefore, their consumption have positive role in the maintenance of human health. Phenolic compounds present in plants (phenolic acids, flavonoids, stilbenes, lignans and complex phenolic polymers) have antioxidant activity due to their redox properties (Kahkönen et al. 1999). They can also act as metal chelators and can have antimicrobial properties. The main target of antibacterial action is usually the cell membrane where destabilization and/or permeabilisation can occur. Phenolics can also inhibit extracellular enzymes or the multidrug resistance pumps of certain bacteria. Some berry extracts exhibit bacterial antiadhesion activity, so that bacteria cannot adhere to mucosal surfaces, which is an important prerequisite for colonisation and infection (Puupponen-Pimia et al. 2004). There has been a growing interest in plant-derived active compounds among food technologists and pharmacologists. Food industry is searching for natural preservatives and antioxidants; and medicine, for new antimicrobial agents without rapidly developing resistance. In this study, the antibacterial activity of 21 wild and cultivated fruits was investigated. Materials and Methods Bacteria and culture conditions Bacillus subtilis ssp. subtilis BD 170, B. cereus var. mycoides ATCC 9634, Escherichia coli SZMC 0582, and Serratia marcescens SZMC 0567 were grown on T1 medium (10g glucose, 4g beef extract, 4g peptone, 1g yeast extract, 1L H 2 O). Fruits and extraction methods The fruits tested are listed in Table 1. Fresh fruits were purchased on a local market (Szeged) or were harvested in the neighbourhood of Szeged or in the mountains of North-East Hungary. Fruit juices were freshly pressed and stored at -20°C. The remaining pomace was dried at 60°C in an oven for 12 h and then ground to powder. One gram of each powdered pomace was extracted 3 times with 10 ml of distilled water or methanol per cycle. The extracts were combined and evaporated to dryness at 100°C in an oven (water extracts) or at 35-40°C in a water bath (methanol extracts). The dry material was redissolved in 4 ml distilled water (water extracts) or 10% methanol-water solution (methanol extracts), and frozen in 1 ml aliquots. One sample from each extracts was dried again and weighed for dry matter content calculation. Juices and extracts were diluted in the appropriate media for the tests. Broth microdilution method Inocula from each bacterium (10 cells/ml) were prepared in LB medium (10g triptone, 10g NaCl, 2g yeast extract, 1L H 2 O). 100 μl of fivefold diluted and sterile filtered juice was mixed with 100 μl cell suspension in triplicates, incubated at 37°C for 48 h, and then absorbance was measured at 620 nm. For determination of growth curves absorbance was measured repeatedly in a time span of 48-72 hours.