Study of Streptococcus mutans and lactobacilli on dentin caries formation

Objectives:

To investigate biofilm formation of monoculture or co-culture of Streptococcus mutans (MS) and lactobacilli (LB) on dentin and their ability to induce dentin caries in vitro.

Methods:

Biofilm formation and dentin caries induction were studied in 3 culture conditions: S. mutans UA159, lactobacilli rhamnosus 7469, and co-culture of the two bacteria. Midcoronal dentin blocks were gamma irradiated and rehydrated in 2 ml of Brain Heart Infusion broth with 1% sucrose. Mono-cultures of S. mutans, L.  rhamnosus, or mixed cultures of the two bacteria in individual cell culture plates containing dentin blocks were incubated anaerobically for 7 days with half of the medium changed at day 3. The supernatants of the culture medium were collected at day 3 and day 7 for final pH measurements, bacteria counts by selective culture, cell density measurement by microplate spectrometer, and calcium evaluation using atomic absorption spectroscopy. The biofilm on dentin blocks were removed by sonication and measured by optical density and selective culture at Day 7. All assays were done in duplicate or quadruplicate and were repeated once.

Results:

Compared to mono-culture of S. mutans or co-culture of S. mutans and Lactobacilli, mono-culture of Lactobacilli had significantly less biofilm formation, less pH reduction, and less calcium release from dentin blocks (ANOVA, P<0.05). There was no statistically significant difference in biofilm formation or demineralization of the dentin blocks between mono-culture of S. mutans and the co-culture (ANOVA, P>0.05). Interestingly, Lactobacilli served as the dominant bacteria in the biofilm of co-culture.

Groups	Biofilm OD Mean(SE)	Day 3 Calcium (ppm) Mean(SE)	Day 7 Calcium (ppm) Mean(SE)
MS	0.30(0.03)	168(7)	180(13)
LB	0.00(0.00)	30(13)	79(33)
MS+LB	0.26(0.04)	153(15)	154(10)

Conclusions:

Mono-culture of Lactobacilli forms poor biofilm on dentin. Mixed culture of Lactobacilli with S. mutans enhances its colonization on dentin surfaces and promotes caries formation on dentin.