How bacterial pathogens use type III and type IV secretion systems to facilitate their transmission

Work on type III or type IV secretion systems (T3SSs or T4SSs) is often focused on elucidating how these sophisticated bacterial virulence factors manipulate host cell physiology to cause disease. But to fully understand their role in pathogen biology, it is important to consider whether the morbidity or mortality they trigger is somehow linked to enhancing communicability of the microbe. Recent work on Salmonella enterica and Brucella abortus provide captivating examples of how manipulation of host cells with T3SSs or T4SSs instigates distant downstream consequences that promote spread of the pathogen. It is clear from these examples that T3SSs and T4SSs are ultimately transmission factors placed under selection by an incredibly complex series of events unfolding during host pathogen interaction.