Fig. 4

Stress may increase the human health risk posed by bacteria, but exposure to a diverse microbiome likely confers benefits. a Many stresses can induce horizontal gene transfer in bacteria, potentially increasing the spread of antibiotic resistance even when those genes do not confer a direct benefit. However, horizontal gene transfer can only occur until the intensity of the stress becomes high enough to inhibit bacterial survival. b A previously unexposed human (i.e., neonate) benefits rapidly from a high diversity of microbial exposures, as it allows acquired immunity to develop. Regular exposure to a relatively high-bacterial alpha-diversity helps maintain the acquired immunity over time. However, continued exposure to a high diversity of bacteria increases the chance of exposure to pathogens