Adult humans host potentially over 10 trillion (1013) microorganisms in and on their body, a human cell to bacteria ratio of approximately 1:1. The bulk of this microbiota is in the gastrointestinal tract, primarily the colon, with around 1012 microorganisms on the skin, and less elsewhere. Typically, a third of the colonic microbes are excreted when defaecating.1

The microbiota comprises bacteria, fungi, protozoa, archaea (single-cell organisms lacking a nucleus) and viruses, mainly as bacteriophages (viruses that infect bacteria). Bacteroidetes and Firmicutes normally account for over 90% of intestinal bacteria species, with Actinobacteria, Proteobacteria, Verrucomicrobia, and Fusobacteria being other common species.23,4,5

Due to acid, bile and pancreatic secretion, bacterial levels are much lower in the stomach and duodenum (estimated at under 1,000 cells per gramme of contents), mainly Lactobacillus and Streptococci species.3

Dysbiosis occurs when the microbiota is knocked out of a ‘healthy’ equilibrium, with a change in the function, diversity or relative quantities of microbes. Dysbiosis-associated gastric conditions include inflammatory bowel diseases, diarrhoea, and constipation; Clostridium difficile infection and rotavirus are associated with lower microbiota diversity.6,7

Antibiotics are well known to cause gut dysbiosis, but they are not the only drugs to do so. Researchers found that of more than 1,000 drugs screened against 40 representative gut bacteria, 24% inhibited the growth of at least one strain.8