Is streptococcus agalactiae uti serious if left untreated? leukocyte esterase is also present in urine. i have no symptoms. thank you?

Follow your doctor's advice about treatment. At your age, any and all UTIs should be treated, regardless of symptoms or not. Not treating will leave you at risk for potentially serious complications, like spread of infection to one or both kidneys (pyelonephritis), requiring hospitalization and IV antibiotics. The particular bacteria causing the infection makes no difference. Good luck.

If there is a significant amount of bacteria growing in the urine in a young person, plus the positive leukocyte esterase, antibiotic treatment is definitely recommended, even without symptoms, because urine is normally sterile.