How Military Training Reshapes Mouth Bacteria
Imagine moving into a new apartment with roommates. Within weeks, you notice curious changes: you start using the same expressions, develop similar habits, and even crave the same foods. Now, consider this same phenomenon occurring at a microscopic level—with the trillions of bacteria living in your mouth. This isn't science fiction, but a fascinating scientific discovery that emerged from an unlikely setting: a military training base at Fort Benning, Georgia.
The community of microorganisms inhabiting our mouths, including bacteria, fungi, and viruses.
When distinct microbial communities become more similar when sharing environments.
At this U.S. Army installation, researchers made a remarkable finding. When new recruits began their intensive 14-week training program, their oral microbiomes—the unique collection of bacteria inhabiting their mouths—underwent a startling transformation. Despite starting with distinct microbial communities, their mouth bacteria began to resemble each other's within just two weeks of training 1 2 . This temporary microbial convergence offers intriguing insights into how shared environments and experiences can reshape even the most personal aspects of our biology.
Military trainees represent an ideal population for studying microbiome dynamics, though perhaps not for reasons you might expect. The training environment creates what scientists call a "natural experiment"—conditions where multiple factors that can influence our microbiomes are standardized across a large group 1 8 .
Trainees experience unprecedented physical proximity in barracks and training facilities.
Identical diets, exercise regimens, and sleep schedules eliminate many variables that typically affect microbiome studies.
Both physiological and psychological stress can significantly impact microbial communities.
This combination of factors creates a perfect laboratory—one that would be both impractical and unethical to recreate artificially—for observing how human microbiomes respond to shared environmental pressures.
In 2016, a nationwide shortage of benzathine penicillin G (Bicillin) created an unexpected research opportunity at Fort Benning 2 . This antibiotic had routinely been administered to military trainees since the 1950s to prevent outbreaks of group A streptococcal infections 2 . The temporary suspension of this prophylaxis protocol in 2016 allowed researchers to conduct a compelling comparative study.
Received standard Bicillin inoculation at the start of training.
Did not receive antibiotic prophylaxis due to the shortage.
Between 2015 and 2016, scientists recruited U.S. Army Infantry trainees for a longitudinal observational study 1 . The design was straightforward yet powerful:
Both groups provided oropharyngeal swab samples at days 0, 7, 14, 28, 56, and 90 of training 2 .
This setup created a natural comparison group, allowing researchers to examine how the oral microbiome changes during training with and without mass antibiotic administration.
The most striking finding from the Fort Benning study emerged when researchers examined how the trainees' oral microbiomes changed over time. At the beginning of training, each recruit's oral microbiome was as unique as a fingerprint, showing considerable variation from person to person 1 8 . But something remarkable happened around day 14 of training.
| Day of Training | Samples Collected | Key Microbial Observations |
|---|---|---|
| Day 0 | Baseline | High individual variation in microbiome composition |
| Day 7 | Yes | Early signs of convergence |
| Day 14 | Yes | Peak microbiome convergence among trainees |
| Day 28 | Yes | Convergence pattern begins to weaken |
| Day 56 | Yes | Further divergence of microbiomes |
| Day 90 | Final assessment | Microbiomes distinct again |
The data revealed a temporary but significant convergence of the oral microbiomes—meaning that the bacterial communities in different trainees' mouths became more similar to each other than they had been at the start 1 8 . This phenomenon coincided with an increase in sequences associated with Streptococcus bacteria, though the convergence pattern extended beyond just this one bacterial group 2 .
Perhaps even more interesting was what happened after this period of convergence. As training continued, the microbiomes gradually became distinct again, suggesting that this harmonization effect is temporary when humans share an environment 1 .
Given the study design comparing groups with and without antibiotic prophylaxis, you might expect that the 2015 cohort (which received Bicillin) would show dramatically different results from the 2016 cohort. The actual findings, however, surprised the researchers.
Despite the theoretical expectation that penicillin would significantly reduce Streptococcus bacteria, the data told a different story. There was no significant difference in Streptococcus abundance over time between the 2015 and 2016 cohorts 1 2 . The statistical analysis yielded a P-value of .07, which falls short of the conventional threshold for statistical significance 1 8 .
| Cohort | Antibiotic Prophylaxis | Streptococcus Abundance | Statistical Significance |
|---|---|---|---|
| 2015 | Bicillin administered | Increased during convergence period | P = .07 (not significant) |
| 2016 | No prophylaxis | Increased during convergence period |
This unexpected result suggests that the convergence phenomenon may be driven more powerfully by shared environmental factors than by the presence or absence of antibiotic prophylaxis—at least at the overall Streptococcus population level.
Conducting comprehensive microbiome research requires specialized tools and techniques. Here's a breakdown of the key methods used in the Fort Benning study and similar investigations:
| Tool or Technique | Function | Application in This Study |
|---|---|---|
| Oropharyngeal swabs | Sample collection from back of throat | Collected microbial specimens from trainees |
| 16S rRNA sequencing | Identify bacterial types | Amplified V1-V3 regions to classify bacteria |
| Illumina MiSeq | High-throughput DNA sequencing | Generated ~50,000 reads per sample |
| QIIME 2 | Bioinformatics analysis | Processed sequencing data and diversity analysis |
| DADA2 | Algorithm for precise sequence variant identification | Distinguished fine-scale differences in bacterial strains |
| PicoGreen assay | Quantify DNA concentration | Ensured sufficient DNA for sequencing |
| Weighted UniFrac | Measure microbial community differences | Quantified microbiome convergence between trainees |
A technique that targets a specific gene region to identify and classify bacteria without needing to culture them.
Computational tools that process and analyze large sequencing datasets to extract meaningful biological insights.
These tools form the foundation of modern microbiome research, allowing scientists to move beyond what can be grown in petri dishes (since most mouth bacteria can't be cultured traditionally) and instead profile entire microbial communities through their genetic signatures 6 .
While the Fort Benning study focused on military trainees, its implications extend far beyond the training grounds. The observation that human microbiomes can temporarily converge under shared living conditions aligns with similar findings in other settings 1 . This dynamic response supports the concept that our personal microbiomes are not static, but constantly responding to our environments, behaviors, and interactions with others.
Family members sharing homes show similar microbial profiles.
Students in classrooms may experience microbial convergence.
Patients and healthcare workers in hospitals share microbes.
The research also highlights the incredible resilience of our microbial ecosystems. Even when challenged by factors like antibiotic exposure or extreme stress, the oral microbiome demonstrates a remarkable capacity to maintain its overall structure and function.
For the scientific community, military populations offer unique opportunities for controlled studies of the human microbiome under diverse conditions 1 . The standardized environment, detailed monitoring, and careful record-keeping create ideal conditions for investigating fundamental questions about how our microbial inhabitants shape and respond to our health.
As research continues, each study like the Fort Benning investigation brings us closer to understanding the complex dance between our bodies and the trillions of microbes we host—a relationship that turns out to be far more dynamic and interconnected than we ever imagined.