Delving into Drool's Droves of Data: Quantifying the Microbial Load in Human Saliva
As anyone who has woken up to a blast of morning breath can attest, the human mouth plays host to a vibrant and diverse blend of microbes. Along with its obvious olfactory output, the oral microbiome impacts the digestive tract, immune system, and nearly every other facet of human health.
The mix of bacteria and fungi that comprise the oral microbiome has been studied extensively, yet the actual quantity has been largely untested. How many microbes are at work in the mouth throughout the day? And what are the potential health ramifications of both understanding and managing the microbial load?
Researchers from the Center for Microbiome Innovation at UC San Diego, led by Clarisse Marotz and corresponding author Karsten Zengler, designed a study to measure not just the composition of the oral microbiome, but the microbial load as well, and how it’s affected by daily activities such as eating and brushing one’s teeth. The results, entitled “Quantifying live microbial load in human saliva samples over time reveals stable composition and dynamic load,” were published in mSystems on February 16, 2021.
"While the proportion of bacteria can be the same, the overall numbers can vary from a few to trillions, showcasing the extreme dynamics of our microbial ecosystems,” according to Marotz. “We wanted to quantify how much microbial load can fluctuate in a healthy, host-associated microbiome over time."
The study was performed by collecting salivary samples from participants at specific intervals throughout the day and cross-referenced against perturbations such as meals and dental hygiene. The samples were measured for flow rate, tested for both microbial load and composition, and treated with propidium monoazide, which enabled tests to assess the mix and mass of active microbes.
By removing relic DNA from dead or inactive cells, the study provided a much clearer picture of how the quantity of living microbes ebbs and flows. “These results highlight the importance of removing dead cell signal in microbiome experiments, especially in longitudinal studies where variations over time can be masked by DNA from dead microbes of previous timepoints,” Marotz remarked when discussing the impact of the findings.
|Daily dynamics of the human saliva microbiome. The percentage of live microbial cells collected in unstimulated saliva (for 5 minutes) normalized to the first timepoint; samples were collected throughout the course of a day starting from right after waking to before going to bed.|
Based on these results, the team is embarking on further studies to determine how these methods can be adapted to other sample sites and microbiomes within the human body. If current testing methods based on the relative abundance of dead bacteria can be refined to focus on live microbes, researchers hope that this will allow for a deeper understanding of how variations in microbial activity are indicators of both health conditions and the progression of diseases.
Additional co-authors include Perris Navarro, Joanna Coker, Pedro Belda-Ferre, and Rob Knight, all at UC San Diego, as well as James T. Morton at the Simons Foundation.
This piece was written by CMI’s contributing editor Cassidy Symons
UC San Diego Center for Microbiome Innovation