Perturbation of the intestinal microbiome is seen consistently as an effect of microplastic exposure. Bacteria such as Firmicutes and Bacteroides species can live on and colonize microplastic particles, but their numbers may not be increased in the intestinal tract. In a study using colonic bacterial samples from two people in an artificial colon with three compartments to mimic the ascending, transverse, and descending colon, significant perturbations in bacterial prevalence were observed upon dosage with high concentrations of MP. The bacterial species most affected by MP depended upon the colonic compartment, though the effect of MP on each bacterial species was consistent throughout the artificial colon. Chlamydia, Desulfobacterota, Cloacabacillus, Synergisteles, and Bilophila species increased. α-Proteobacteria increased in the human colon model and other species , but in other species, their levels were reduced. Similarly, Firmicutes species levels increased in the human colon model and decreased in other organisms. β-Proteobacteria and Parabacteroides decreased in the human model, and Bacteroides levels decreased both in the human colon model and in zebrafish and mice. In mice, Coprococcus and Anaeroplasma increased while many other bacterial levels decreased in the presence of MP; the expression of metabolic and amino acid genes was substantially altered from their expression in the absence of microplastics. Given the importance of the microbiome in human health, microplastics may thus be able to alter the metabolism of many organisms, including humans. One active question from microplastic exposure is whether microplastics can concentrate and/or transport toxic compounds, including additives in plastic manufacture such as phthalates, unreacted monomers, and flame retardants such as polybrominated diaryl ethers (PBDE), and persistent organic pollutants (POP) such as DDT, pharmaceutical agents, or metals released into the environment. In algae, microplastics and copper exerted cooperative toxicity; in daphnids, microfibers and phenanthrene also showed cooperative toxicity. In mussels, the combination of 2-6 m polystyrene particles and fluoranthene exhibited significant toxicity, with GI inflammation, hemocyte impairment, and increased oxidative stress enzymes. Though fluoranthene partitioned selectively into the microparticles over water or algae, it was not transported out of the mussels' GI tracts. When pyrene, polystyrene and polyethylene were administered to mussels, pyrene was found in the GI tract and gills, while DNA strand breaks were found in hemocytes. In lugworms, polychlorinated biphenyl (PCB)-contaminated sediment and polystyrene microplastics were co-administered; the microplastics reduced feeding and increased PCB concentrations in lugworms. Lugworms treated with 5% PVC microplastics in sand in combination with nonylphenol, Triclosan, or PBDE-47 showed reduced feeding or phagocytosis and increased oxidative stress than in the absence of the particles; the particles increased concentrations of the added substances, and with Triclosan, 55% of the lugworms died. Goby fish given 1-5 m polystyrene microparticles with pyrene showed an increase in bile metabolites, decreased lipid metabolism, and inhibition of AChE. Interestingly, the goby fish showed delayed pyrene-induced mortality. In European sea bass, combinations of microplastics and mercury were evaluated; while low concentrations of microplastics did little, high concentrations of microplastics with or without mercury altered lipid metabolism in muscle, and both high microplastic concentrations (without mercury) and low concentrations of microplastics with mercury inhibited AChE. Another potential hazard microplastic particles may present is the ability to carry bacteria and viruses. Plastics are known to harbor bacterial biofilms; an earlier document showed that small plastic pieces immersed in seawater accreted biofilms. Amaral-Zettler et al. characterized the bacteria which colonized microplastics and found that they differed significantly from the bacteria in the waters where the plastics were found; the microplastic colonies were less diverse than free colonies but were less dominated by single species than were free colonies. UV exposure and aging of plastics increased the accretion of bacteria, likely by altering the surface potential and charge. Dinoflagellates, photosynthetic bacteria, and Vibrio species (though V. pseudohaemolytica and not V. cholerae) were found on microplastics. A recent article found that the Escherichia coli phage T4 adhered to polystyrene microplastic particles; as with bacteria, the adherence depended on surface functionalization and potential and increased with aging. T4 phages on polystyrene microplastics survived better than in culture and retained their infectivity. While viruses have not been shown to be carried by microplastics in the environment, and the corresponding survival and infectivity of pathogenic viruses are not known, the article suggests that viruses may also be carried on microplastics. In addition, the permeability of the intestinal tract to microplastics suggests that if intestinal viruses and bacteria colonize microplastics, they may provide a vector for intestinal diseases and may enhance their severity.