Potential mechanisms of increased disease in humans from antimicrobial resistance in food animals.

There are at least 5 potential mechanisms by which antimicrobial resistance can have adverse effects on human health. The first, called the "attributable fraction," relates to individuals who become infected only because they are taking an antimicrobial agent (for unrelated reasons) to which the pathogen is resistant: the antimicrobial agent, by suppressing their normal microbiota, renders them more vulnerable to infection. A second mechanism involves the linkage of virulence traits to resistance traits so that resistant organisms may be more virulent than susceptible organisms. A third mechanism is that treatment may be rendered ineffective by the choice of a drug to which the pathogens are resistant or may be complicated by the need to use an agent with less desirable attributes than would otherwise be the case. A fourth mechanism is the animal equivalent of the attributable fraction: resistant pathogens acquired by this mechanism in food animals may then be transmitted through the food chain to humans. Last, resistance traits can be acquired by the commensal flora of animals; from this reservoir, resistance traits could find their way through the food chain to commensals and pathogens of humans.