Serious bacterial infections in febrile infants younger than 90 days of age: the importance of ampicillin-resistant pathogens.

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Intrapartum antibiotic prophylaxis against group B Streptococcus (GBS) has reduced the occurrence of serious bacterial infections (SBI) in young infants caused by GBS. Recommendations for initial antibiotic therapy for the febrile infant 1 to 90 days old were developed when infections with GBS were common and antibiotic resistance was rare. To document the pathogens responsible for SBI in recent years in febrile infants 1 to 90 days old and the antibiotic susceptibility of these organisms. The results of bacterial cultures from infants 1 to 90 days old evaluated for fever at Primary Children's Medical Center in Salt Lake City, Utah, between July 1999 and April 2002 were analyzed. Antibiotic susceptibility profiles were collected and patient records were reviewed to determine if initial antibiotic therapy was changed following the identification of the organism. Of 1298 febrile infants enrolled from the Primary Children's Medical Center emergency department, 105 (8%) had SBI. The mean age of the infants with SBI was 39 days (range 2-82 days) and 2 (2%) were <7 days. SBI included urinary tract infection (UTI; 67%), bacteremia (16%), bacteremia and UTI (6%), bacteremia and meningitis (5%), meningitis (2%), abscess (2%), meningitis and UTI (1%), and meningitis and gastroenteritis (1%). Eighty-three (79%) of 105 episodes of SBI were caused by Gram-negative bacteria, including 92% of UTI, 54% of bacteremia, and 44% of meningitis cases. The most common pathogen was Escherichia coli (61%). Other Gram-negative pathogens were responsible for 19% of SBI. Staphylococcus aureus was the most common Gram-positive pathogen, causing 8% of SBI. GBS accounted for 6% of SBI. Of the 105 pathogens, 56 (53%) were resistant to ampicillin. Of the pathogens causing meningitis, UTI, and bacteremia, 78%, 53%, and 50%, respectively, were resistant to ampicillin. Antibiotic therapy was changed in 54% of cases of SBI following identification of the organism. In Utah, ampicillin-resistant Gram-negative bacteria are the most common cause of SBI in febrile infants <90 days old. This finding impacts antibiotic selection, especially in cases of meningitis. Local surveillance of pathogens and antibiotic susceptibility patterns is critical to determine appropriate antibiotic therapy.