Explore the all part of the following question

Question- Kat has had repeated UTIs and her doctor has prescribed several courses of a cephalosporin antibiotic, cephalexin, to clear up her infection. Her UTI symptoms have cleared up but now she is having another problem-diarrhea.

Part 1- What bacterium is the likely cause of her diarrhea and how is her diarrhea related to her antibiotic usage?

Part 2- Cephalosporin's are broad-spectrum antibiotics. What characterizes an antibiotic as broad spectrum rather than narrow spectrum? Why don't doctors always treat infections with broad-spectrum antibiotics?

I need help to explain the characteristics of antibiotic.