For generations, the approach researchers took when studying microbes provided limited understanding, because they focused on individual species rather than the complex natural communities in which they really exist. In the last 10 to 20 years, advances in biomolecular sequencing and microscopic imaging, among others, have enabled the study of microbial communities more comprehensively. The most recent findings indicate that collaboration is a key driver of the microbiome. The individual organisms have evolved to share energy, genetic information, and metabolic activities, making members of a biofilm interdependent for surviving and thriving.
Earlier theories long held that microbes mostly compete with one another for nutrients, but the importance of collaboration and teamwork has become apparent. When researchers turned their attention to the microbiome as a whole, they tried to stitch together the information gleaned from the single species research approach. It did not work; major gaps in understanding occurred. For example, metabolic activity of single species in the laboratory rarely matched observations from real-world environments. The observations from the real world were always different than the laboratory observations of metabolic activity; sometimes more, sometimes less, but always different.
Even though the bacterial cells compete for the same resources within a biofilm, recent research suggests that evolution has promoted specialization and collaboration. How did these collaborations start in the first place? Physical proximity within a dense aggregation of microbes appears to be an important factor. Natural selection favors genes that make shared resources when different generations of bacteria are close to one another.
A natural assumption from reading the content above would be that this all refers to the oral cavity. It does not. The information comes from an article in Scientific
American in November 2018 on microbial communities half a mile below the surface of the ocean, in which a study of methane consumption by bacterial collaboration was described.1 It is difficult to conceive how many bacteria are all around us and how small they are. There are teeming microbial masses between grains of sand. A single drop of water from the tropical oceans contains about a million microbes . . . one drop of water. They exist on rocks deep below the ocean and on dust particles high in the atmosphere, and nearly every place in between. They also exist in massive numbers in the oral cavity.1
Can the information revealed by the researchers on deep sea microbiomes be applied to oral microbes? Some of the same properties have been identified in the mouth. The bacteria live in close proximity, they share genetic information, and they are interdependent. For example, periodontal pathogens such as Porphyromonas gingivalis (P. gingivalis) cannot survive in acidic conditions. The early colonizers on a new biofilm occupy space in the gingival sulcus, and their metabolic activities create a less acidic environment, which later enables colonization by periodontal pathogens that require neutral pH conditions. Even the sequence of arrival on a biofilm requires collaboration. If the periodontal pathogens arrive first, they could not tolerate the acidic conditions. They have to wait until the gingival sulcus is pH neutralized.
This interdependence among bacteria in a biofilm has a potential downside for the bacteria and a potential upside for the health of the oral cavity. If one member of the bacterial community is damaged in some way, the mutually dependent microbes could be negatively affected, potentially leading to the collapse of the entire community.1 This may be an avenue worth pursuing as a therapeutic approach.
In the oral cavity and in the rest of the world, the biosphere is very resilient and tenacious. We see this every day in the high incidence of gingivitis and periodontitis.
In the dental profession, we are so focused on what is going on in the oral cavity, and rightly so. It is helpful, however, to look beyond and observe consistencies with microbiomes in other settings. There is still much to learn about microbiomes, including metabolic partnerships, and the rapid pace of evolution from gene transfer among bacteria. The answer to the riddle of periodontal disease likely resides in the initial bacterial cause. Adding to the total body of knowledge from any source may provide some answers.
1. Marlow J, Braakman R. Team players. Sci Am. 2018;319(5):32-39. doi:10.1038/scientificamerican1118-32.