Bacterial Fitness and Antimicrobial Resistance

I just posted about how little we understand the rise and fall of MRSA. As a follow-on, read this recently published work from Denmark—which investigates the relative “fitness” of different S. aureus isolates within clonal complex 8 (using older terminology, phage type 83A). As might be expected, the more resistant strains were relatively less “fit”, and in the absence of antimicrobials were "out-competed" by the more susceptible strains. The authors suggest that one explanation for the waning of Denmark’s MRSA epidemic was the bacterial fitness cost of maintaining resistance in the absence of antimicrobial pressure.

We often discuss the transmission and spread of resistant bacteria on this blog, but we rarely discuss the downside, from the bacteria’s standpoint, of maintaining multiple drug-resistance mechanisms. This concept is quite old, and fairly intuitive. Wearing 30 pounds of body armor and lugging an 80-pound rucksack might favor the survival of a soldier in combat, but if I wore this gear for my daily walk to work (through non-hostile territory), I might not make it to my office. See this Nature Blog post from 2010 for a great general discussion of this topic.

Illustration by Don Smith