Bactericidal activities against pathogenic bacteria by selected constituents of plant extracts in carrot broth

HPLC-DAD analysis provided evidence for the certain identification of some constituents of hydroalcoholic extracts from Lithospermum erythrorhizon, Rheum palmatum, Thymus vulgaris, Lippia citriodora, and a mixture of Rosmarinus officinalis, Salvia lavandulifolia and Thymus mastichina. Their inhibitory and bactericidal activities in vitro against Escherichia coli and Staphylococcus aureus were evaluated either in Luria–Bertani (LB) broth or a model food system, Tyndallised carrot broth (TCB). Shikonin exhibited high inhibitory activity against both microorganisms, but to a lesser extent than the basic unit naphthazarin (5,8-dihydroxy-1,4-naphthoquinone). Rhein and carnosic acid showed noticeable inhibitory effects on S. aureus. In inoculated TCB containing 15 mg/L naphthazarin or 47 mg/L shikonin there was about 6 log reductions in E. coli counts, while 20 mg/L naphthazarin, 26.4 mg/L rhein, 45 mg/L shikonin or 68.7 mg/L carnosic acid caused 1.6, 2.4, 3.1 and 3.1 log reductions in S. aureus inocula, respectively. Log reductions obtained for S. aureus in TCB compared unfavourably with those found in LB broth.