How long does a strain of bacteria remain in a cow?

[spirochete]

I've been reading about methods for preventing E. coli O157:H7 carriage in cattle, and found that cows inoculated with O157:H7 carry the strain for about a month before it's no longer detected in the feces.

My question is: is this normal or unusual? Do some strains take up permanent residence or is everything transitory?

[canalon]

It depends. I don't know for O157:H7 or for cows specifically, but there are a few papers on the evolution of the gut content of mammals (I remebre dogs and human at least) that suggest that the gut contains transient and persistant clones of bacteria in different and variable quantities over time.
And I have found at the herd level (in pigs) that the same is true, and that after more than one year some clones are stil present, while many individuals went from piglets to bacon.

So you cannot rule out that O157 can persist in the gut in cattle. So I would not consider that unuasual. And remeber that O157:H7 carrying cattle is perfecly healthy.

[spirochete]

In experimental models animals animals infected with O157:H7 invariably seem to stop carrying it after a certain time period, maximum about 40 days. I would speculate it's because O157:H7 is outcompeted by non pathogenic E. coli which aren't wasting energy maintaining virulence factors and the extra DNA which does for them.