Here’s some follow-up thoughts on my Salmonella-related moment of fame that I reposted yesterday
So while on vacation, I was mentioned in a NY Times article about diseases that can be caught from your fish tank. The moral of the story is when cleaning your fish tank, pretend it’s toilet water, clean everything including the sink that comes in contact with the aquarium water, and you’ll be fine. (Here’s the Emerging and Infectious Disease article that led to the NY Times story).
I’m happy with the coverage: the issue of antibiotic resistance needs it. But one important message-that antibiotic use in agriculture or aquaculture has influenced the evolution of a bacterial pathogen-didn’t really make it into the story. From my perspective, that, and not drinking water that your fish have crapped in, is the important part.
The reason I was contacted for the story is that the disease outbreak was caused by a multi-drug resistant (resistant to multiple antibiotics) strain of Salmonella paratyphi found in fish tanks (~20 cases in Australia isn’t exactly a pandemic, but knowing that your fish tank water is not ‘clean’ is useful public health information). One of the antibiotics that this strain of S. paratyphi is resistant to is chloramphenicol. That is not that unusual: chloramphenicol resistance is fairly common. Chloramphenicol is also not very widely used any more in medicine or agriculture, except in some developing countries to treat typhoid fever (including S. paratyphi), so chloramphenicol resistance isn’t exactly a public health disaster either.
What set off warning bells is that the chloramphenicol resistance in S. paratyphi is due to a gene called floR. floR does result in bacterial resistance to chloramphenicol, but it also confers resistance to florfenicol, an antibiotic only used in agriculture and aquaculture (fish farming). In S. paratyphi, this florfenicol resistance gene (floR) is linked to other specific resistance genes in what is called a gene cassette. This cassette has been found world-wide for several years in isolates from this particular strain of S. paratyphi, so multi-drug resistance isn’t a one time, freak evolutionary event that happened in someone’s tropical fish store. For this to have happened as a result of antibiotic use by tropical fish culturists, the identical genes would have wind up in the exactly same strain of Salmonella, in the exact same order, multiple times, which isn’t very likely (or even probable).
But the antibiotic resistance punchline is this: floR was first observed in agricultural and aquacultural isolates, so this is pretty good evidence that antibiotic use in agriculture led to chloramphenicol (and florfenicol) resistance in S. paratyphi. Even now, in E. coli and Salmonella, most floR genes are found in agricultural isolates, not clinical ones.
While it could jump to another strain (and in the reporter’s defense, I did say that), the observation that these genes are in S. paraptyphi, a known pathogen, and probably were selected by agricultural antibiotic use is the real story. Whether these linked genes evolved in another bacterium, such as E. coli and transferred into S. paratyphi as a unit or were assembled de novo in S. paratyphi is unclear. But the presence of floR clearly indicates that agricultural use of florfenicol is having a definite effect of the evolution of a known pathogen.