Effects of melanized bacteria and soluble melanin on the intestinal homeostasis and microbiome in vivo Original paper

What was studied?

This brief in vivo safety study investigated whether melanized E. coli Nissle and soluble fungal allomelanin could be safely established in the gut without disrupting intestinal homeostasis or selected microbiome markers—an essential prerequisite for using melanin-based countermeasures to mitigate radiation-induced intestinal injury. Mice received either a single oral bolus of soluble allomelanin (derived from Inonotus obliquus and modified to enhance paramagnetic centers while remaining water-soluble) or a single oral gavage of a genetically engineered probiotic E. coli Nissle strain expressing a Bacillus megaterium tyrosinase gene to generate melanin (melanized E. coli Nissle). Outcomes emphasized short-term tolerability (body weight, organ weight, intestinal length), tissue inflammation (H&E histology), and targeted fecal microbial abundance/colonization readouts by culture and qPCR over hours to 48 hours.

Who was studied?

Two cohorts of CD-1 mice (both sexes) were used, with institutional animal ethics approval. For soluble allomelanin, male and female CD-1 mice were treated at 17 weeks of age after an overnight fast, with fecal collection immediately after gavage and again at 2, 12, and 24 hours. For the melanized probiotic experiment, 12-week-old male and female CD-1 mice received 1 × 10^7 CFU of either standard E. coli Nissle or melanized E. coli Nissle, with fecal sampling at 0, 3, 12, and 24 hours and necropsy-style physiology measures and histology assessed up to 48 hours. Group sizes were reported as n = 6 per group in the figure legends.

Most important findings

Across both interventions, no clinically apparent toxicity signal was detected over the acute observation window. Soluble allomelanin (2.5 mg/mouse) did not change body weight and did not measurably alter targeted fecal markers for Escherichia coli or Lactobacillus at 12 or 24 hours compared with controls, suggesting minimal short-term perturbation of these common gut taxa. In the engineered probiotic arm, melanized E. coli Nissle rapidly established detectable intestinal presence: viable melanized colonies and E. coli 16S rRNA qPCR signals were detectable by 3 hours and persisted through 24 hours after a single gavage, indicating fast colonization and short-term maintenance. Importantly for a microbiome signatures database, the authors reported no significant changes in selected bacterial groups (Salmonella, Lactobacillus, and Bacteroides fragilis) after melanized E. coli Nissle treatment, and no histologic inflammation or morphologic changes in the ileum or colon on H&E.

Key implications

Clinically, these data position melanized probiotic delivery and soluble melanin as plausibly “microbiome-safe” short-term candidates for radioprotection strategies, because they did not trigger weight loss, organ or gut-length changes, or histologic intestinal inflammation while allowing rapid, trackable gut colonization by the engineered strain. However, the microbiome readouts were intentionally limited (targeted taxa rather than community-wide sequencing), and only acute effects were assessed; for translation, longer-duration exposure and comprehensive microbiome profiling (e.g., 16S/shotgun metagenomics plus metabolomics) are necessary to detect subtle dysbiosis, functional shifts, or host immune effects relevant to radiation therapy patients.

Citation

Zhang YG, Malo ME, Tschirhart T, Xia Y, Wang Z, Dadachova E, Sun J. Effects of melanized bacteria and soluble melanin on the intestinal homeostasis and microbiome in vivo. Toxics. 2023;11(1):13. doi:10.3390/toxics11010013