For the following infectious disease patient, what is the definitive etiological diagnostic method? A 32-year-old female with persistent fever for 10 days, accompanied by headache and poor appetite, passing stools 3-4 times daily. Treated with chloramphenicol and SMZCO at a local clinic without fever resolution. Widal test shows 'O' 1:160, 'H' 1:320. Clinical diagnosis: typhoid fever.