How sad for this company.
That is a LOT of recalls for a company that produces a relatively small amount of food.
I used to like Merrick and look what happened to them. Now I wouldn't let my dog touch their products with a ten foot pole.
Unfortunately for the pups, salmonella can cause very similar symptoms to what humans get from it. Most cases probably go undiagnosed because the money is just not there to test all cases of GI upset. Instead they get the Metro and the bland diet talk. And for immunocompromised dogs, it can potentially kill them.
It would be interesting to find out what brands of kibble test positive for it. I see way fewer recalls in general with the larger companies. They put more money into quality control. That's the main reason why if I could go grab any kibble for my dog, it would very likely be Science Diet (gasp).