Key to the development of disease in many bacterial infections is expression of a bacterial toxin. Toxins come in many shapes and forms but all have a pretty similar goal, to directly induce damage to the cells of the host.
Toxins seem to conflict with the idea that pathogens are simply commensal bacteria that have lost the ability to tone down an attack once established in a host. What good is the host as a food source if you kill the host with your suite of toxins? But toxins are often integral to the normal lifecycle of the bacterium.
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