Again, Septra is another name for Co-trimoxazole, so definitely the right drug for Steno.
Stenos are almost always resistant to all beta-lactams, including third genaration cephalosporins like Ceftazidime and the Carbapenems like Imipenem.
Beta-lactam antibiotics have been produced by fungi for millions of years in the environment to try an kill competitor bacteria, so many environmental bacteria, like Steno, have evolved to become resistant.
My research team has spent the last 10 years working out the mechanism of beta-lactam resistance in Steno, and hope to use what we know to design "resistance inhibitors" that will allow beta-lactams to be used to treat Steno infections.