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?

This clinical picture is highly suggestive of Salmonella infection, gastroenteritis caused by an invasive process. Stool examination is likely to confirm Salmonella enteritidis. However, Campylobacter should also be considered as it is not infrequent among infants and young children. In the history of patients with Campylobacter infection, there is often a new puppy in the home instead of simultaneous infection of several persons, as in this case. Bacteremia is a possibility and, therefore, blood culture is also recommended. The dehydration appears to exceed 10%. Febrile infant less than 3 months of age with suspected Salmonella infection represents an indication for admission. Ref: Ray C.G., Ryan K.J. (2010). Chapter 33. Enterobacteriaceae. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e. b