Introduction: Mouse prolactin-inducible protein (mPIP) is secreted in mouse saliva and has been found to bind oral bacteria, showing the highest affinity for streptococci. Comparisons between the oral flora of mPIP knockout mice and their wild-type controls showed differences in the genera colonizing the two groups of mice. These findings suggested a role for mPIP in the colonization of the mouse oral cavity, possibly modulating the oral flora. In this in vitro study, we focused on the contribution of this protein to aggregation of oral bacteria, a process thought to promote the clearance of bacteria from the oral cavity, and one that could influence the composition of the oral bacterial community.
Methods: The aggregation of selected human and mouse oral streptococci was measured spectrophotometrically. The aggregation of oral bacteria by saliva from mPIP knockout mice, which lack mPIP, was compared with that of saliva from wild-type mice.
Results: Both wild-type saliva and mPIP knockout mouse saliva induced aggregation of human strain Streptococcus gordonii SK120 and mouse streptococci strains M105/6 and M106/2. Bacterial aggregation induced by the saliva of wild-type mice was significantly higher than the aggregation induced by saliva from mPIP knockout mice for all the bacterial strains.
Conclusion: In this study it was confirmed that mPIP plays a role in the aggregation of oral bacteria. The salivary components promoting aggregation of oral bacteria are considered to be part of the oral defense mechanisms so these findings provide insight into a possible function of mPIP in host defense by promoting aggregation of oral bacteria.