Although effective, current ATT carries a significant risk of gastrointestinal and neurological side effects[11]. Another potential important side effect of ATT could be disruption to the host microbiome or dysbiosis[28]. We profiled the gut microbiota of LTBI patients initiating TB disease prophylaxis with 3HP or 4R. We found that the rifamycins induced a modest decrease in alpha diversity but a significant shift in beta diversity correlated with the treatment duration and dose. In our healthy volunteer group unexposed to the rifamycins, we observed a modest decrease in alpha diversity but no significant shift in beta diversity. These findings are consistent with prior reports that ATT induces immediate and significant gut microbial dysbiosis[12, 13]. In our study, patients with LTBI received either 600 mg of RIF daily for four months or 900mg of RPT and 600mg of INH weekly for three months. Standard therapy for TB disease is much longer and uses a combination of at least four antimicrobials daily during the first two months of the 6-month minimum therapy[11]. These differences in dosage and duration of the anti-TB drugs may account for the modest effect of the LTBI therapy on the gut microbiota we observed here, in comparison to other studies done in TB patients treated with combination therapy for a longer period of time[13, 14, 16, 29].
The gut microbiota plays a crucial role in health[30]. Cross-talk between a healthy gut and the lung microbiota via the gut-lung axis is hypothesized to support the host in fending off challenges by respiratory pathogens, such as Mtb[31]. We observed that LTBI patients differed from healthy volunteers on the relative abundance of Tenericutes. However, there was no significant difference in Shannon diversity between the two groups. This observation contrasts with a limited number of prospective studies showing that Mtb infection itself results in gut dysbiosis, manifested as a decrease in bacterial diversity[14, 16]. However, because substantial inter-individual variation in gut microbiota is expected, a comparison of the gut microbial diversity of LTBI versus healthy volunteers is unlikely to tell us much about the effect of Mtb infection on gut dysbiosis[32]. None of our LTBI participants were treated because of recent exposure to an infectious TB case (i.e., recent contact); assuming an effect of Mtb infection, the gut microbiota composition we observed might better be described as a stabilized version of the "pre-Mtb" gut microbiota.
We observed that while TB prophylaxis with the rifamycins resulted in a modest decrease in the mean Shannon diversity index during treatment, alpha diversity increased two months posttreatment completion compared to pretreatment. In particular, the bacterial families Lachnospiraceae (Genus Coprococcus), Ruminococcaceae, and the genus Parasutterella increased in relative abundance during treatment. In contrast, the relative abundance of Roseburia (Family Lachnospiraceae) decreased during treatment. Our observations suggest that treatment is reverting the alpha diversity to a pre-Mtb state. Indeed, treatment naïve TB patients have previously been shown to differ from healthy controls on the relative abundance of Ruminococcaceae and Lachnospiraceae, which are characterized by their anti-inflammatory capacity as well as their ability to utilize carbohydrates in simple and polymeric forms to produce short-chain fatty acids (SCFAs) essential in the maintenance of health[28]. Although our numbers are too small to test this formally, prior reports suggest that a low level of Ruminococcaceae at baseline is associated with antibiotic-induced diarrhea[33]. Two LTBI patients who reported gastrointestinal discomfort upon treatment initiation saw a resolution of their symptoms once treatment was completed. However, it is important to note that one patient reported the use of probiotics to alleviate these GI symptoms. We were able to observe a partial recovery of the gut microbiota to baseline two months after rifamycin-based LTBI therapy was completed in some patients. This finding of an incomplete recovery of the gut microbiota following antimicrobial treatment is consistent with prior reports with longer follow-up[34, 35].
This study provides initial findings on the effect of TB prophylaxis with RIF or RPT on the gut microbiota. However, our results should be evaluated with caution, considering that our study sample is small. Also, we followed our study participants through the Thanksgiving and Christmas holidays and the first few weeks of stay-at-home orders to control COVID-19 spread. Participants reported eating and drinking alcohol more heavily during these times. LTBI patients were advised not to consume alcohol during therapy to limit liver injury[36]. One LTBI patient on 4R reported they also eliminated dairy during treatment to mitigate gastrointestinal side effects. However, volunteers were not instructed to modify their diet or lifestyle. Two healthy volunteers reported taking other drugs at the posttreatment time point; one took Zyrtec® for allergies while the other took amoxicillin. Similarly, one LTBI patient took amoxicillin during the two-month posttreatment follow-up period. Alcohol and other drastic diet changes have been shown to alter the composition and diversity of the gut microbiota[37, 38]. A similar effect in this study may have contributed to the control gut microbiota resembling more the gut microbiota of the treated LTBI patients, thus resulting in a smaller overall impact of the rifamycin exposure[32, 39]. In addition, exposure to amoxicillin during the two months posttreatment follow-up may have further confounded our estimates of the gut microbial reversal to baseline for one LTBI participant. Finally, we discontinued follow-up two months after LTBI treatment was completed, which likely was too short of a follow-up time to document complete reversal to baseline for all LTBI patients[35]. In particular, three out of six LTBI patients had trace concentrations of the parent rifamycins or their partially active metabolites in the stool samples collected two months after taking their last treatment dose. The rifamycins are primarily eliminated in the stool and to a lesser extent in urine[11]. In these patients, the two-month stool may have represented delayed clearance of the rifamycins and confounded our estimate of posttreatment recovery.
In conclusion, important literature on the synergistic interplay between the human gut and lung microbiome on the prevention and control of TB is slowly emerging, but questions remain. Our study adds to this literature by investigating the effect of TB prophylaxis with 3HP or 4R on the diversity and composition of the gut microbiota. We showed a small decrease in alpha diversity following exposure to the rifamycins that was not significantly different from gut dysbiosis observed in a shadow control population sampled in parallel. We also documented a reversal of the gut microbial community to baseline two months posttreatment completion, albeit this recovery was incomplete. These findings need to be confirmed in a larger study where patients are followed for longer than two months posttreatment completion.