Figure 4. Contextual pathogenicity.

a, Microbes exhibit contextual pathogenicity along a spectrum. Host factors such as barrier breaches and immunosuppression bias microbes towards pathogenic behaviour, whereas homeostatic conditions bias them towards mutualistic behaviour. On the microbial side, virulence gene expression and microbe–microbe interactions can also push microbial behaviour to be mutualistic or pathogenic. In a mutualistic host–microbe relationship, the host provides nutrients, while the microbe promotes epithelial and immune homeostasis as well as pathogen resistance through microbial products and occupation of metabolic niches. In a pathogenic relationship, the microbe invades past the epithelium, causing inflammation, and sometimes also benefiting from a host inflammatory response. b, Both S. epidermidis and S. aureus are examples of contextual pathogenicity; S. epidermidis is biased towards mutualistic behaviour whereas S. aureus displays more pathogenic character.