Antibacterial activity of yoghurt against viridans streptococci in vitro

Objective: Yoghurt consumption leads to a selective decrease in the oral level of mutans streptococci. It is not clear whether this decrease is due to the bactericidal activity of yoghurt or other mechanisms. The present study investigated the differences in susceptibility to yoghurt between several strains of viridans streptococci. Design: The sources of variation were minimised, at the expense of the external validity of the study, using culture collection strains. Each strain was tested separately on five occasions in planktonic form and logarithmic growth phase. Two strains of each of the following Streptococcus species were tested: mutans, sobrinus, gordonii, oralis, parasanguinis and sanguinis. One millilitre [108 colony-forming units (cfu)] of each strain was incubated (37 °C, 60 min) with 9 mL of fat-free plain yoghurt containing Streptococcus thermophilus and Lactobacillus bulgaricus (108 and 107 cfu/g, respectively) in gently vortexed tubes. Survival rates were calculated every 15 min by dividing the number of viable cells, obtained using conventional laboratory procedures, by the baseline number. Results: Survival rates were 8% (S. mutans 6519T), 12% (S. mutans 31738), 35% (S. oralis 25671) and >50% (all other species tested) after 15 min, and 0.01% (S. mutans) and >10% (all other species tested) after 30 min. Overall, S. parasanguinis and S. sobrinus were the most resistant species. When heat-treated yoghurt (<10 cfu/g bacteria and inactivated bacteriocins) was used, this antibacterial activity was not found. Conclusion: In vitro, yoghurt with live bacteria showed selective anti-mutans activity, suggesting that the overall decrease in mutans streptococci in vivo could be due to a bactericidal effect on S. mutans but not on S. sobrinus. © 2008 Elsevier Ltd. All rights reserved.