A 3-month old infant develops fever, vomiting, and watery diarrhea. Several family members also have symptoms of mild diarrhea. On examination the child appears to be lethargic, fontanelle is slightly depressed, hea rate is markedly increased, and there is hyperpnea. Examination of stool with methylene blue detects mucus and sheets of white cells. Which of the following diagnostic tests should be performed?