Plant defence compounds inhibit the growth of human pathogenic bacteria

Berries are good sources of phenolic compounds, which often act as defence compounds in plants. In humans plant phenolics besides their other health-inducing effects have been found to possess antimicrobial activities. Our previous studies showed that phenolic berry extracts especially inhibited the growth of Gram-negative but not Gram-positive bacteria such as Lactobacillus. Especially cloudberry, raspberry and strawberry extracts proved to be strong inhibitors of the avirulent Salmonella (1). Materials and Methods Bilberry (Vaccinium myrtillus), lingonberry (Vaccinium vitis-idaea), cranberry (Vaccinium oxycoccus), raspberry (Rubus idaeus, var. Ottawa), cloudberry (Rubus chamaemorus), strawberry (Fragaria ananassa Senga Sengana), blackcurrant (Ribes nigrum, var. Öjeby) and sea buckthorn berry (Hippophae rhamnoides), were used in the study. For antimicrobial studies berries were freeze-dried and ground to fine powder. Phenolics were extracted from the berries by aqueous 70% acetone (2). Antimicrobial activity of freeze-dried grounded berries (2 or 10 mg/ml) and phenolic berry extracts (1 or 5 mg/ml) was analysed by determining the bacterial growth curves in liquid cultures (1). The bacterial strains used were Listeria monocytogenes VTT E-991205, L. innocua VTT E-981011, Salmonella enterica ser. Typhimurium VTT E-981151, S. enterica ser. Infantis VTT E-97738, Staphylococcus aureus VTT E-70045 and Lactobacillus rhamnosus VTT E-96666 (ATCC 53103). Results Salmonella strains were inhibited by all the berries. Cloudberry, raspberry, strawberry and bilberry extracts inhibited the growth during the first 12 h incubation. However, during the next 12 h the number of viable cells started to increase. Cloudberry extract was the best inhibitor showing strong antimicrobial effects. Staphylococcus strains were strongly inhibited by all the berries and berry extracts. The number of viable cells did not increase at the end of cultivation. The tested Listeria or Lactobacillus strains were not sensitive to berries or berry extracts, with the exception of cranberry, which showed inhibitory effects against Listeria bacteria. Conclusions The results showed that antimicrobial properties of the berry compounds aren't restricted to the avirulent Salmonella but inhibited also the virulent strains. The inhibition of the Staphylococcus aureus suggested that also the structure of Gram-positive bacteria is sensitive to the antimicrobial compounds of the berries. Bioactive berry compounds seem to inhibit the growth of pathogenic bacteria with several mechanisms. The difference in the antimicrobial properties of the berries and the berry extracts also suggested that other bioactive compounds are involved in the cell growth inhibition. In the future, the most interesting berry extracts will be fractionated with the aim of identifying the antimicrobially active components. In addition the mechanism of action against gastrointestinal bacteria, especially intestinal pathogens, will be extensively studied. The results will be utilised in functional food development and in pharmaceutical applications.