Urinary tract infection, primarily Escherichia coli, is the most common cause of bacterial infection in well appearing children less than 90 days of age with fever and no source one exam.
Around 10% of well appearing febrile infants less than 90 days of age will have a serious bacterial infection. Most of these will be urinary tract infections with 1-2% being bacteremic and less than 0.5% having meningitis.
The most common pathogen is Escherichia coli followed by Group B Streptococcus(GBS). Other gram negatives include Klebsiella spp, Salmonella spp. Streptococcus pneumoniae, Enterococcus spp and Staphylococcus aureus. Listeria is a classically taught pathogen that is not seen in more recent studies.
Regimens vary by center and cover the common pathogens including:
A third generation cephalosporin alone regimen does not cover enterococcus or Listeria and ampicillin is the preferred agent for GBS.
While rare herpes simplex virus infection is also a consideration especially in neonates with vesicular skin lesions, seizures, or CSF pleocytosis.
References:
Biondi EA, Byington CL. Evaluation and Management of Febrile, Well-appearing Young Infants. Infect Dis Clin North Am. 2015 Sep;29(3):575-85. PMID: 26188607