The most useful choice in this design is the mouth. Sampling the oral microbiome alongside the vaginal one gives every participant her own comparison site [2][3], which is how a four-person study earns any inferential right at all. Had both sites moved together, the likely explanations would have been dull and shared: sampling and handling artefacts, or anything about the day that touched the whole body at once. A divergence between two sites in the same woman is harder to explain that way, and divergence is what the authors report, with the vaginal community changing more [3][11].
What the design cannot pull apart is gravity from the stress it provokes. Cortisol was up after the flights [5], and the team describes the exposure as inducing acute physiological stress and altering host-microbiome interactions [17]. A parabola serves microgravity and hypergravity together [2], and nobody in this study received the stress without the altered gravity, so the two candidate causes named in the paper's own framing stay bundled [4].
Then the denominator. Four participants [2] means one woman carries a quarter of the sample [16], and the published account describes no group that stayed on the ground [15]. The word "significantly" is there [4]; the arithmetic behind it is not, since this account reports no effect sizes, diversity statistics or named taxa [14]. Direction is on offer. Magnitude is still in the tables.
The site was worth the trouble for reasons Mathyk states plainly: the vaginal microbiome has become a noninvasive biomarker for gynecological disease detection, cancer, pregnancy and health monitoring [9], and stress-driven disruption of its balance has been linked to inflammation, greater infection potential and altered immune responses [8], with dysbiosis in endometriosis associated with chronic inflammation and disease progression [18]. Those links are why a shift is worth measuring. They are not evidence that a shift measured around a parabolic flight does anything to a crew member on month four of a transit.
Nobody had examined this site under space travel conditions before [10], and the group that did it also ran the first gynecologic ultrasound studies in microgravity [13], working with partners from the National Research Council Canada and the International Institute of Astronautical Sciences [12]. Feasibility plus a two-site divergence is a reasonable basis for a larger cohort, and Mathyk has already specified what that cohort has to settle: whether the change persists, and whether it carries clinical weight over a longer mission [6].
