Objectives: To determine the effects of harvesting by centrifugation on the properties of single cells and single/dual-species biofilms of Streptococcus oralis and Actinomyces naeslundii.
Methods: Single/dual-species biofilms of S.oralis and A.naeslundii were grown for forty-eight hours in a parallel-plate-flow-chamber on glass slides coated with salivary proteins. Stress relaxation behavior was measured with a low-load-compression-tester. A generalized Maxwell model was fitted to the relaxation curve using three relaxation times. Separately, changes in zeta potential distributions and auto/co-aggregation of the bacteria were measured with fixed bacterial concentrations.
Results: Centrifugation of S.oralis significantly (ANOVA, Sidak, p<0.05, n=3) influenced two relaxation time ranges and total percent relaxation in single-species biofilms, while centrifugation of A.naeslundii had no significant (p>0.05) influence on its single-species biofilm relaxation. Auto-aggregation of S.oralis (0.199±0.115, 20 min. absorbance unit reduction, AU, n=5) only occurred for centrifugation speeds harvesting roughly 3x1011 bacteria or less. Changes in the bacterial zeta potential distribution were seen upon centrifugation when harvesting more than 3x1011 bacteria, concurrent with the loss of auto-aggregation properties (0.030±0.005 AU). Auto-aggregation of A.naeslundii cultures (0.009±0.003 AU) was not affected by centrifugation. Co-aggregation between both species widely varied with increasing centrifugation speeds of either strain. Auto-aggregation of first generation bacteria after high-speed centrifugation remained absent in S.oralis after subsequent culturing and low-speed centrifugation. Thus effects of high-speed centrifugation of S.oralis occurring in first generation bacteria may be transferred to subsequent cultures, suggesting changes in DNA-expression.
Conclusion: Centrifugation of oral bacteria may yield cell surface changes that impact their auto/co-aggregation as well as their zeta potentials in suspension. Moreover, effects of centrifugation remained observable after growth of bacteria into a biofilm from altered stress-relaxation, suggesting effects of centrifugation may be transferred to next generation bacteria. This was confirmed by auto-aggregation behavior of S.oralis after high-speed centrifugation which could not be restored after subsequent culturing.