Born in the wild: the ecological roles of key Enterobacteriaceae virulence factors

Many Enterobacteriaceae family pathogens are not truly human adapted but are instead opportunists able to thrive in various different niches. Survival in environments outside human hosts provides a means for transmission and also represents an important selective pressure shaping the evolution of these bacterial species. Pathogens such as Escherichia coli and Klebsiella pneumoniae benefit from highly diverse genomes and horizontally acquired virulence or host fitness factors that collectively determine infection type and severity. These same “virulence” factors can also drive environmental survival. Here we will review current knowledge of the environmental roles of canonical host fitness factors of opportunistic pathogens, including capsule, siderophores, curli, and type VI secretion systems. These systems are primarily known for their roles in protection, nutrient acquisition, and intermicrobial competition within the host but can also make dramatic contributions to environmental survival. Investigating these roles can give us new perspectives on how these factors may function within the host and help us to understand the evolutionary forces shaping bacterial pathogens.