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It seems that a human bite can be very dangerous, because of the myriads of bacterial species found in saliva. This leads to several questions that, perhaps, may have the same single answer. But, I'll boil it down to a single question per the comment.

Is there a mechanism by which the body "tames" their own oral bacteria that could otherwise be dangerous pathogens, so that the bacteria don't cause pathology but also don't arouse a typical immune response or inflammation?

No doubt this question has been explored often, but I haven't been able to find a clear answer.

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  • $\begingroup$ This is a good questions, but it is difficult to answer in brief. To point toward an answer, consider that many members of the oral microbiome are considered pathobionts, meaning that they are symbiotic in their native niche but pathogenic in other contexts. $\endgroup$
    – acvill
    Feb 4, 2020 at 17:20
  • $\begingroup$ Upon reading the article to which you provided a link, it seems that the interactions between different species in the oral bacterial community may be important for keeping potentially pathogenic members of that community in check -- but the article doesn't indicate how that occurs. I know for example that bacteria can modify the pH of a medium and thereby control bacteria or yeasts that require a specific range of pH. But in a complex community like the mouth, a change that inhibits some pathogens might well benefit other pathogens. Are there lots of independent factors analogous to pH? $\endgroup$
    – S. McGrew
    Feb 4, 2020 at 17:42
  • $\begingroup$ Please limit yourself to one question per post. Take a close look at your linked reference to see if it answers any of the questions for you. That might help you narrow things down. $\endgroup$
    – De Novo
    Feb 5, 2020 at 4:02

2 Answers 2

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One way to identify mechanisms by which the body tames the microbes in the mouth is to observe what happens when those mechanisms are impaired:

Deviation from symbiosis among the bacterial community leads to “dysbiosis”, a state of community disturbance. Dysbiosis occurs due to many confounding factors that predispose a shift in the composition and relative abundance of microbial communities. (Dentistry Journal)

1) Immunity

Normal innate, cellular and humoral immunity helps to prevent oral flora from causing infections.

a) When the immunity is impaired due to a disease (HIV/AIDS, cancer...) or long-term treatment with antibiotics, steroids, immunosuppressants or chemotherapeutics, a part of normal flora, such as the yeast Candida, can overgrow and cause oral thrush.

Host defense against infections with Candida spp. depends on rapid activation of an acute inflammatory response by innate immunity, followed by an incremental stimulation of specific immune responses mediated by T-cells (cellular immunity) or B-cells (humoral immunity). (Immunology of oral candidiasis, J Pharm Bioallied Sci.)

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Picture 1: Oral candidiasis (source: Wikipedia, creative commons license)

b) Impaired immunity can result in osteomyelitis of the mandible, caused, for example, by the Actinomyces bacteria:

Systemic factors such as diabetes mellitus, agranulocytosis, leukemia, severe anemia, malnutrition, or alcohol abuse affect immune surveillance and lead to impairing the osteomyelitis

Actinomycotic druses and filaments were detected from the sequestrum of the fracture site (Rapidly Progressing Osteomyelitis of the Mandible, Case Reports in Dentistry)

2) Saliva

Saliva helps to prevent mouth infections by flushing microbes into the gut and by the antimicrobial substances.

Among such protective factors, the flushing effect of saliva flow is the most important one...(Antimicrobial function of human saliva--how important is it for oral health?, Acta Odontol Scand.)

Importantly, saliva is crucial for defense against microbial species, as it is rich in antimicrobial compounds such as hydrogen peroxide, lactoferrin, and lysozymes...disruptions in saliva secretion increase the frequency of oral conditions such as oral candidiasis, gum disease, and tooth decay (caries), as well as respiratory tract infections (The power of saliva: Antimicrobial and beyond, PLoS)

Impaired saliva excretion due to a disease (salivary stones, Sjögren's syndrome, untreated diabetes, radiation injury) or drugs (antihypertensives, antidepressants, tranquilizers, diuretics, antihistamines...) can result in dry mouth (xerostomia), which increases the risk of oral thrush and other mouth infections.

3) Lack of substrate for microbial overgrowth

Clean healthy mouth does not contain enough food for microbial overgrowth.

On the other hand, in untreated diabetes mellitus, the abundance of glucose in the saliva promotes the overgrowth of the yeast Candida (oral thrush):

A dry mouth coupled with a higher amount of glucose in the saliva can also make for favourable conditions for thrush. (Diabetes.co.uk)

Mouth full of food debris due to poor hygiene can promote bacterial overgrowth:

Periodontitis is caused by certain bacteria (known as periodontal bacteria) and by the local inflammation triggered by those bacteria. Although these periodontal bacteria are naturally present in the mouth, they are only harmful when the conditions are right for them to increase dramatically in numbers. This happens when a layer of bacteria and food debris, known as plaque, builds up and is left undisturbed on the teeth...(What is periodontitis, European Federation of Periodontology)

The bacteria from normal oral flora often involved in periodontitis: Porphyromonas and Streptococcus (Lumenlearning.com).

4) pH

The normal pH in the mouth is 6.2-7.6 (Journal of Indian Society of Periodontology). When it falls under certain value known as critical pH (~5.5), acids derived from bacterial fermentation of dietary sugars start to dissolve tooth enamel and cause caries. The bacterium Streptococcus mutans is a part of normal oral flora, but is commonly involved in caries (Lumenlearning.com).

5) Low susceptibility of mouth mucosa for certain microbial toxins

Certain microbes, such as Staphyloccocus aureus, which are a part of normal mouth flora do not likely cause infections in the mouth, even when they appear in the mouth in much greater amounts, for example, in food poisoning and even if these microbes can cause severe diarrhea (Centers of Disease Control and Prevention). This is because the mouth mucosa is not susceptible for the toxins secreted by these bacteria, while the stomach or intestinal mucosa is.

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  • $\begingroup$ This answer is definitely useful, but not quite satisfying. #1(a) indicates that the immune system is active in manipulating the ratios of oral flora without necessarily eliminating any particular species. #2 points out that saliva flow constrains the ratios according to the relative abilities of species to find safe niches to avoid being washed out. #3 indicates that the ratios of different available nutrients affect the growth ratios of different species that depend on specific nutrients. #4 mentions pH, which can be considered analogous to temperature and will affect species ratios $\endgroup$
    – S. McGrew
    Feb 10, 2020 at 14:48
  • $\begingroup$ @S.McGrew What did you expect? There are surely more mechanisms and more specifics, but I don't think I need to cover just about everything. My intention was to show there are certain mechanisms that allow the oral flora to exist without being harmful. $\endgroup$
    – Jan
    Feb 10, 2020 at 14:57
  • $\begingroup$ according to species' preferred pH range. #5 points out that different bacterial species have different inherent ability to break down specific tissues in the body. All of this supports the idea that oral flora form an ecosystem in which the various member species regulate each others' growth via the usual competition, cooperation, and other such interactions, and that the body "simply" imposes constraints that tend to shape the overall microbial ecosystem. $\endgroup$
    – S. McGrew
    Feb 10, 2020 at 14:59
  • $\begingroup$ I would like a better understanding of how and why those constraints work, and how the microflora interact, and what the modes and conditions of stability are. Links to research on that topic would be welcome! $\endgroup$
    – S. McGrew
    Feb 10, 2020 at 14:59
  • $\begingroup$ I personally find it impossible to say why something works when it works normally. Because it just works. For me, the only way to say how it works is to observe mechanism when they are disrupted and conclude from that how they normally work. To go into more specifics, I would need to know what goal, beyond just knowing some facts, are you trying to reach. There were interesting sub-questions in your original question, but that appeared to be broad. You may want to open some of those as separate questions. $\endgroup$
    – Jan
    Feb 10, 2020 at 15:11
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It's an interesting question. More interestingly there are a number of articles and a lot of studies are going on to prove that by manipulating oral microbiota, Caries or Periodontal diseases can be prevented. At this I want to point out as the OP has asked about the body's way of 'taming' the flora, I will share the references of the studies in which the oral flora was manipulated not by body's own mechanism but externally. And this resulted in prevention of oral diseases commonly seen. This answer is an add on knowledge which I will share, I know not the exact answer since how the oral cavity by itself provides immunity has been satisfactorily answered by @Jan.

Probiotic therapy or - bacteriotherapy’ (Rajendhran and Gunasek- aran, 2010) has the potential to naturally cure and prevent disease at its early stages by incorporating beneficial bacteria that can reestablish an ecological balance or enhance the biodiversity of a microflora. For example, research showed that individuals with high amounts of Capnocytophaga ochracea had lower amounts of P. gingivalis and displayed no periodontal disease progression. Individuals with low C. ochracea did exhibit disease progression. In probiotic treatment, a patient found to have elevated levels of P. gingivalis could be given C. ochracea probiotics to reestablish a healthy equilibrium before any periodontal disease generates.

Here is the article

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