I don't know about Bactrum but I used to get UTIs constantly until I started on a regimen of OTC supplements made by Uqora. It took about 8 months for the products to work but I now almost never get a UTI. They mail the products to you

My dear husband gets frequent UTI’s due needing a catheter & Bactrum is what he is given with the same dosage

Bactrim is a broad spectrum antibiotic, meaning it will kill a wide variety of bacteria. They will often do a blood culture which tells the doctor what bacteria are present and what antibiotics they are sensitive to to. Since UTIs are common in elderly, some doctors don’t do a culture. If you have a UTI which was treated but comes right back, a culture should be done. Sometimes you can have a primary infection and a secondary infection which doesn’t show up until the primary infection is treated. The secondary infection may need a different antibiotic to treat it.