we know that resistant strains (MDR, XDR, PDR) have arisen in single incidents around the globe but why are these bacteria not common and not spreading quickly as humans keep using antibiotics and killing the non-resistant strains which these Amr strains have to compete for resources. One might say that they ARE spreading but my question is why is the increase in population so slow. compare it with malaria parasite 'plasmodium' which despite being a higher organism with slower reproduction cycle kills so many.
Natural selection is environment-dependent. A mutation that makes an individual more fit in one context, might make it less fit in a different context.
The mutations that make bacteria more fit in an environment where they're exposed to antibiotics generally make them less fit in a "natural" environment where they don't have to deal with antibiotics. That means that MDR bacteria are often out-competed by their less-resistant competitors, unless they're in the "right" environment for them, such as a hospital or an infected, antibiotic-treated patient.
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