The incidence of bacteriuria in the neonate is low, ranging from 0.1% to 1.9% in full-term infants and up to 10% in low-birth-weight newborns. In the neonatal period, a preponderance of infection occurs in male infants. In infants less than 2 months of age presenting with fever, urinary tract infection is found in 7.5% of patients. Many studies have reported an increased susceptibility to urinary tract infections in uncircumcised male infants. Other reports suggest that breast-feeding is associated with a lower incidence of infection.

Urinary tract infections can be acquired by hematogenous infection of the kidney in association with neonatal bacteremia or by the ascending route via the urethra. The short female urethra is thought to allow for ascending infection and explains the higher frequency of infection in girls older than 3 months. In the uncircumcised male infant, accumulation of bacteria in preputial folds with meatal contamination is likely. Specific fimbrial receptors on the foreskin and along the urethra may allow for ascending infection. Malformations of the urinary tract predispose to infection. Between 4% and 20% of infants presenting with urinary tract infection have an underlying malformation of the urinary tract.

Escherichia coli is the most frequent pathogen, causing 75% to 85% of infections. Other gram-negative organisms such as Klebsiella pneumoniae, Enterobacter species, Proteus vulgaris, and Pseudomonas aeruginosa are encountered less often. Gram-positive bacteria (including enterococci, group B streptococci, and Staphylococcus species) are uncommon pathogens in neonates. Candidal infections are seen as part of disseminated candidiasis, in the presence of an indwelling urinary catheter and in newborns requiring intensive care. A few patients have been reported with mixed bacterial infections.

In infants, 50% to 70% of E. coli strains causing urinary infection belong to one of the eight common pyelonephritogenic O serotypes found in older patients; data conflict concerning the frequency of specific polysaccharide K antigens on the surface of E. coli. Furthermore, E. coli can attach to specific receptors on uroepithelial cells. E. coli strains isolated from infants with urinary tract infection show a higher percentage of P and X fimbriation and more type 1 pili than found in matched control patients. Other recognized E. coli virulence factors include hemolysin production, colicin production, resistance to serum bactericidal activity, and the ability to acquire iron. The virulence factors may play a role not only in the localization of the infection (cystitis or pyelonephritis) but also in its severity.

Few specific symptoms or signs of urinary tract infection are recognizable in the newborn period. Conversely, clinical manifestations vary widely, and many infants are completely asymptomatic. When symptoms are present, they often consist of fever, irritability, decreased feeding, and lethargy. Some patients present with diarrhea, vomiting, or weight loss. Jaundice is seen in approximately 7% of cases and can be accompanied by hepatomegaly and splenomegaly. The genitalia should be carefully inspected and the abdomen palpated gently to detect malformations or enlargement of the kidneys and bladder. Occasionally, an alert caretaker notices crying on urination (i.e., dysuria) or an increased number of wet diapers (i.e., frequency).

The diagnosis of urinary tract infection is based on examination and culture of an appropriately collected urine specimen. A urine culture should be included in the sepsis workup of all infants older than 72 hours of age. Within the first 3 days of life, urinary tract infection occurs secondary to bacteremia; therefore, such infections can be identified by blood culture. The most reliable test is when urine is obtained by suprapubic bladder puncture. This technique is safe and easy; bleeding or perforation of the bowel occurs rarely. Dehydration, abdominal distention, and a bleeding diathesis are contraindications for suprapubic aspiration. To optimize the yield of a successful tap, the aspiration should be done 30 to 60 minutes after the infant has voided. Any bacterial growth in cultures obtained by a suprapubic puncture is considered significant. Catheterization of the bladder is a valuable and safe procedure when suprapubic aspiration is unsuccessful.

The simplest method, but the least reliable, of collecting a urine culture is by application of a sterile plastic bag after careful disinfection of the perineum; the bag is removed shortly after the child has voided. Results of urine cultures obtained by bagged specimens are helpful when they are sterile, but a positive result is not necessarily indicative of infection because of frequent contamination during the collection process. Therefore, this method of obtaining a urine culture should be considered a screening technique, and the diagnosis must always be confirmed by a better method for urine culture. False-positive rates of 33% and 15% have been reported after obtaining one and two bagged urine specimens, respectively. Before initiation of antibiotics in infants evaluated for possible sepsis, an appropriate urine specimen should be obtained for culture by suprapubic aspiration or catheterization.