Canine being treated for 5 weeks with imipenem /Cil

I have a 9 year old male boxer who had a TPLO surgery 3 months ago. He developed a staff infection that is resistant to all anti biotics except primaxine (imipenem cilastatin).

He has been on this drug for 5 weeks.When the vet asked me to reduce the dosage to twice daily and 1 ml...the bacteria came back like gangbusters and the amount of puss coming from the leg was by the cupful. We went back to 3 IM injections daily each 1.5ml.There is no sign of puss.

How do I know that after 14 days the bacteria is dead and not hiding in the screws or plate in his back leg? This is a people drug made by Merck and even my vets have no data base as far as results with canine usage. THey have talked to Tufts and Cornell and they don't have an info either.

Please give me some advice as I am out here on my own with this one.

Sincerely,

Lori Fogler