Oral Lactobacilli and Dental Caries: A Model for Niche Adaptation in Humans

Abstract

Lactobacilli have been associated with dental caries for over a century. Here, we review the pertinent literature along with findings from our own study to formulate a working hypothesis about the natural history and role of lactobacilli. Unlike most indigenous microbes that stably colonize a host, lactobacilli appear to be planktonic, opportunistic settlers that can gather and multiply only in certain restrictive niches of the host, at least within the oral cavity. We postulate that the following essential requirements are necessary for sustained colonization of lactobacilli in humans: 1) a stagnant, retentive niche that is mostly anaerobic; 2) a low pH milieu; and 3) ready access to carbohydrates. Three sites on the human body meet these specifications: caries lesions, the stomach, and the vagina. Only a handful of Lactobacillus species is found in caries lesions, but they are largely absent in caries-free children. Lactobacilli present in caries lesions represent both a major contributor to caries progression and a major reservoir to the gastrointestinal (GI) tract. We extend the assertion from other investigators that lactobacilli found in the GI tract originate in the oral cavity by proposing that lactobacilli in the oral cavity arise from caries lesions. This, in turn, leads us to reflect on the health implications of the lactobacilli in the mouth and downstream GI and to ponder whether these or any of the Lactobacillus species are truly indigenous to the human GI tract or the oral cavity.

Keywords: Lactobacillus; colonization; gastrointestinal tract; mouth; mutans streptococci; natural history.

Figure 1.

Prevalence of the different Lactobacillus species isolated from children with severe early childhood caries (S-ECC) (n = 35) and their mothers (n = 15); for description of study cohort, see the Appendix (Li et al. 2015).

Figure 2.

Proposed model showing caries lesions as the major or only source of Lactobacillus (LB) from the oral cavity to the GI tract. Left panel: Primary teeth (molar and incisor) in a caries-free child in the absence of LB. Most children harbor mutans streptococci (MS) as commensal bacteria of the oral cavity. Right Panel: 1. An early caries lesion with MS in the presence of simple carbohydrates (CHO; e.g., sucrose). Some teeth of children with severe early childhood caries (S-ECC) have retentive niches in the form of enamel hypoplasia (~EHP) as well as pits and fissures found on the occlusal and buccal surfaces of the molars. 2. MS and other acidogenic microbes coalesce and form a retentive niche characterized by low pH and anaerobic conditions. Select species of LB from food or other humans accumulate in the retentive, low pH niche. If early caries continues to dentin, LB may be able to bind and/or degrade exposed dentinal collagen. 3. Caries has progressed due to lower pH contributed by both MS and LB. LB can dominate lesions to the exclusion of MS. 4. LB from caries lesions spill over into the saliva and are swallowed, seeding the downstream gastrointestinal (GI) tract, including retentive sites in the low pH stomach. We hypothesize that in the absence of dental caries, the oral cavity does not harbor LB, and as a result, the downstream GI tract no longer has a source of LB except those present in food.