Urinary excretion of pneumococcal cell wall polysaccharide in children

The urinary excretion of the cell wall polysaccharide of Streptococcus pneumoniae was studied in 92 children with the NOW test. Cell wall polysaccharide was detected in 65% of pneumococcal carriers and in 10% of noncarriers. Excretion rates were similar in healthy children and in children with acute otitis media. The high rate of antigen excretion among nonill carriers suggests that colonization is a major source of urinary antigen in children. Nasopharyngeal colonization with S. pneumoniae is common in childhood. It occurs with increasing frequency during the first several years of life. 1–5 At 6 months of age ∼25% of children in the United States can be colonized with one or more serotypes. 4 By 2 years of age, between 50 and 90% of the population have been colonized at least once. 3–5S. pneumoniae is the leading cause of otitis media and sinusitis as well as more serious disorders such as bacteremia, pneumonia and meningitis. 6 S. pneumoniae contains a number of immunologically important antigens such as capsular polysaccharide, cell wall polysaccharide, surface protein A, surface adhesin and pneumolysin. The cell wall polysaccharide is highly conserved and is present in all strains. 7 It is immunogenic and antibody is prevalent among young children. 8 The present study was designed to detect the frequency of urinary excretion of cell wall polysaccharide. The results suggest that antigen excretion is extremely common among young carriers of S. pneumoniae. Materials and methods. Population. Children younger than the age of 6 years were enrolled at three office sites. The enrollees had to be either healthy or ill with acute otitis media (AOM). The children were enrolled without regard to sex or race. Children were excluded from the study if they had been treated with antibiotics within the past month. Nasopharyngeal cultures. Nasopharyngeal samples were obtained with Mini-tip Culturettes (Becton Dickinson, Sparks, MD). The swabs were passed through the nasal canal until they touched the pharyngeal wall. The specimens were stored at 4°C and cultured within 12 h of collection. The specimens were cultured on sheep blood agar and chocolate agar. The plates were incubated at 36°C in 5% CO2 for 18 to 24 h. S. pneumoniae was identified by colonial morphology, Gram stain characteristics, optochin sensitivity and bile solubility. Urine collection and processing. Urine was collected in urine cups from older children and in urine bags or from cotton balls placed in diapers from younger children. The urine was kept refrigerated and processed within 12 h of collection. The NOW test (Binax Inc., Portland, ME) was used to detect pneumococcal cell wall polysaccharide. The test was performed according to the package insert. Each test was completed in 15 min. Results. Population. Ninety-two children were enrolled in the study. They ranged in age from 4 to 168 months with a median age of 25 months. There were 49 (53.3%) boys and 43 (47.7%) girls. Thirty-six (39%) children were classified as healthy and 56 (61%) were diagnosed with AOM. Thirty-one (33.7%) children were carriers of S. pneumoniae, and 61 (66.3%) were noncarriers. Antigen detection. Pneumococcal cell wall polysaccharide was detected in 26 of 92 (28.3%) urines. It was detected in 20 of 31 (64.5%) carriers of S. pneumoniae and in 6 of 61 (8.8%) noncarriers. Detection rates were similar for healthy carriers (66.7%) and children with AOM and positive nasopharyngeal cultures (64%, not significant). The rate of detection in noncarriers was 3.3% for healthy children and 16.1% for children with AOM. Discussion. This study demonstrated relatively frequent excretion of pneumococcal cell wall polysaccharide in the urine of children with positive nasopharyngeal cultures. Approximately 34% of the population carried the organism, and ∼65% excreted the antigen. This rate of detection was similar, albeit slightly higher, than rates reported by Dowell et al. 9 and Adegbola et al. 10 Whereas all the children in the study by Adegbola et al. 10 were healthy and 40% of the children in our study were healthy, all children with positive nasopharyngeal cultures were ill in the study of Dowell et al., 30% with pneumonia and 70% with nonrespiratory illness. 9 Excretion rates of cell wall polysaccharide did not appear to depend on the clinical disease but rather on the presence of colonization. Excretion rates did appear to be higher (80 to 90%) among individuals with blood culture-positive pneumococcal pneumonia. 11, 12 Thus the NOW test does not appear to be a good urinary test to establish the cause of pneumonia because antigen is excreted at a high rate during colonization. However, the test holds potential as a rapid antigen detection test when used on samples taken directly from the site where the organism resides such as the nasopharynx or middle ear. 13, 14 Urinary capsular antigens tests have been used in the past to detect the presence of S. pneumoniae infection. These assays include countercurrent immunoelectrophoresis, latex agglutination, coagglutination and enzyme-linked immunosorbent assay. Even during proven pneumococcal infections, the excretion rates of pneumococcal capsular antigen ranged from 0 to 76% with most below 50%. 15–17 Most if not all of these previous studies neglected to examine the contribution of nasopharyngeal colonization to the excretion of the antigen in the urine by failing to study healthy carriers. The large number of pneumococcal capsular types was a major limiting factor in establishing the utility of these tests. On the other hand the NOW test uses a ubiquitous, relatively conserved cell wall polysaccharide as the target antigen. Pneumococcal cell wall polysaccharide excretion was also detected among a small percentages of noncarriers in this report as in others. 9, 10 This could have been the result of undetectable, low grade colonization, cross-reaction to other bacteria or inadequate sampling technique. According to the manufacturer of the NOW test, one organism, Streptococcus mitis, of 144 tested, cross-reacted with S. pneumoniae. Overall the NOW test proved to be simple and quick in detecting pneumococcal cell wall polysaccharide excretion.