The ingestion of microplastics has become a frequent occurrence. This is because plastic' s inability to biodegrade leads it to break down into smaller pieces that make their way into the water and food that humans consume. As humans ingest microplastics, the trillions of bacteria living in the gut are exposed to these particles. The gut bacteria are vital for human health due to their role in the immune system and in the digestive process; thus, it is important to understand the effects of microplastics on bacteria in the gut microbiome. This study exposed four human gut bacteria - E. coli and E. aerogenes, which are Proteobacteria, and S. epidermidis and L. acidophilus, which are Firmicutes - to two different types of microplastics - polyethylene microbeads and polyester microfibers. This study sought to examine the effects of microplastics on the growth of bacteria in the gut microbiome. To simulate human digestion, an in vitro digestion was performed on the microplastics. The differences between the effects of undigested and digested microplastics were also evaluated to determine whether microplastics undergo any property changes during digestion. The bacteria were exposed to the microplastics for six hours and the growth of the bacteria was measured with a spectrophotometer.

This study found that both the digested and undigested microplastics had a significant effect on the growth of the Firmicutes, but no effect on the Proteobacteria; however, each type of plastic had a varying degree to how it affected each bacteria. The undigested polyethylene and polyester made S. epidermidis grow 40.8% and 53.2% more, respectively, while the digested polyethylene made it grow 11.7% more. The undigested polyethylene made the L. acidophilus grow 28.4% less, while all other experimental groups had no significant effects on the growth of this bacteria. Furthermore, for the Firmicutes, a significant difference between the effects of undigested and digested microplastics was found. For S. epidermidis, the digested polyethylene and polyester had 71.3% and 99.4% less of an effect on the growth of bacteria respectively than the undigested polyethylene. For L. acidophilus, the digested polyethylene had 95.8% less of an effect on the growth of the bacteria than the undigested polyethylene. These differences suggest that the microplastics undergo property changes when digested. Understanding the effects of microplastics on the growth of bacteria in the gut microbiome is important because of the myriad of roles the gut microbiome plays in human health. The alteration of the Firmicute populations by the microplastics has implications in immune health because this phylum of bacteria plays an important role in the immune system. Furthermore, because there is competition for space and resources among the different bacteria in the gut, the alteration of the Firmicute population will likely lead to gut dysbiosis, throwing all of the bacteria populations off balance. It is important that future studies look into the health effects that the gut dysbiosis leads to, as well as perform in vivo studies to expand the knowledge of the effects of ingesting microplastics.