"That finding, right there — the migration...

2010-07-22T06:47:19.359-05:00
"That finding, right there — the migration of resistance to a human-only drug into an organism carried by an animal — signals one of the insoluble problems of overuse of antibiotics."

This is not a comment about the paper, per se (which is not a journal we subscribe to); I just wanted to reflect on your comment above. I'm aware that many reports such as these readily supply the observed resistance phenotypes of strains, but commentary on the molecular epidemiology of such resistance cannot be made without addressing the actual determinants and their mechanisms of action (btw, did they do this for those isolates with linezolid resistance?)

Resistance to oxazolidinones, amongst other things has, in the clinic, been seen to be conferred by the determinant cfr, in S. aureus (when not conferred by a chromosomal point mutation). However, this does not mean that there aren't resistance determinates in the environment - entirely independent of clinical or agricultural prophylaxis - that are similarly capable conferring resistance; it's just that they are not the ones seen in the lab/clinic.

If the reference paper 'Sampling the antibiotic resistome' (of soil-dwelling bacteria) is to be built upon, we can anticipate that numerous uncharacterised mechanisms exist for conferring resistance to a/biotics, beyond those that we most commonly see in the clinic. The concern was (and is) that it is only a matter of time before such determinants find their way into the clinic. Although, I should add that I'm also aware there is literature to support a correlation between resistance profile of isolates collected from farms and the drugs used for prophylaxis at the same sites.

However, it cannot be wholly stated that the presence of resistance to 'recent' a/biotics in animals is a direct product of the overuse of such a/biotics without knowing the mechanism by which resistance is conferred. Furthermore, the horizontal transfer of such determinants from soil->animals, or animals->humans may not necessarily result from a/biotic selective pressure, but alternatively via selection for another determinant/virulence factor that is genetically linked with the a/biotic resistance determinant.

I think the persistence of antibiotic resistance in farm isolates, despite discontinued prophylaxis (and thus apparent selection) raises interesting questions. I look at the fitness costs that are imposed on bacteria as a result of the a/biotic determinants they encode. Frequently in the lab we can raise highly resistant strains, but they aren't very 'fit', so in the absence of selection they revert to their ancestral, sensitive forms. Occasionally slightly less resistant forms arise, after multiple generations, which retain their fitness.

Thus, bacteria evolve strategies to mitigate the fitness costs imparted by their a/biotic resistance determinants; indeed, it is the epidemic strains of MRSA that can be said to have overcome such costs.

Comments on Superbug: Hospitals want patients to eat antibiotic-free mea...

"That finding, right there — the migration...

Plan B: Switch over to veggie/myco-based meat subs...