Molecular Analysis by Pulsed-Field Gel Electrophoresis and Antibiogram of Streptococcus pneumoniae Serotype 6B Isolates from Selected Areas within the United States

Fifty-eight clinical isolates of Streptococcus pneumoniae serotype 6B, including 16 from Alaska, 14 from Arizona, 11 from Washington, and 17 from seven additional states, were analyzed. The antibiograms of these isolates were assigned to 10 antibiotic profiles based on their susceptibilities to penicillin, erythromycin, tetracycline, and trimethoprim-sulfamethoxazole. Thirty-two (55%) of these isolates were penicillin nonsusceptible, while 21 (36%) were intermediate or resistant to three or more antibiotics. The restriction endonucleases ApaI and SmaI were used to digest intact chromosomes, and the fragments were resolved by pulsed-field gel electrophoresis (PFGE). The ApaI and SmaI PFGE patterns were combined, and 13 of the 16 Alaskan isolates showed indistinguishable PFGE patterns. One other isolate exhibited highly related ApaI and SmaI PFGE patterns, differing by only one band after restriction with ApaI. Among the 14 isolates from Arizona, 1 was indistinguishable from the predominant ApaI and SmaI PFGE patterns seen in the Alaskan isolates; 5 others were highly related (61 band after cutting with either enzyme) to the Alaskan isolates, suggesting a common ancestral origin. Of the remaining eight isolates, six additional ApaI plus SmaI PFGE patterns were observed. The 28 isolates from the various contiguous states had 22 ApaI plus SmaI PFGE patterns. No correlations were found between specific PFGE patterns, antibiograms, dates of isolation, or geography. The serotype 6B isolates across the contiguous United States were genetically diverse, while the 6B isolates from Alaska appeared to be much less diverse. Streptococcus pneumoniae remains a leading cause of bacterial pneumonia, otitis media, sepsis, meningitis, and bacteremia worldwide, resulting in significant morbidity and mortality. In the United States it is estimated that S. pneumoniae is responsible for at least one-fourth of all community-acquired pneumonia (6). Over the past decade in the United States, there has been an increase in the number of reports of pneumococcal isolates that are either moderately or completely resistant to penicillin, erythromycin, and trimethoprim-sulfamethoxazole (TMP-SMZ) (28). Isolates resistant to these antibiotics appear to belong to a few selected serogroups and/or serotypes (6B, 14, 19F, 23F, 9V) (1, 11, 14). These serotypes are commonly associated with invasive disease and are most frequently isolated from children with serious infections (4). In Alaska, the highest reported rate of invasive pneumococcal disease exists among Alaska Natives, specifically Yup’ik Eskimo infants, from the Yukon-Kuskokwim Delta (YKD) region, where 1 in every 40 infants is diagnosed with the invasive disease during their first 2 years of life, a rate of 1,000 per 100,000 per year. This rate is 8 to 10 times higher than for other U.S. population groups (7, 10). Many more cases of pneumococcal disease go undiagnosed because of inaccessibility to culture before empiric antibiotic administration (10). In Alaska since 1986, there has been a 6.5-fold increase in the occurrence of pneumococcal isolates causing invasive disease that have reduced susceptibility to penicillin (MIC $ 0.125 mg/ ml), an 8-fold increase in ceftriaxone resistance (MIC $ 2 mg/ ml), a 12-fold increase in erythromycin resistance (MIC $ 1 mg/ml), and a 26-fold increase in TMP-SMZ resistance (MICs, $4 and 76 mg/ml) (24). The highest rates of penicillin-nonsusceptible and multidrug-resistant pneumococci have been found among Alaska Natives of the YKD region of the state. These isolates now represent $30% of the total S. pneumoniae isolates from the YKD area. In the last few years, these isolates have spread to other areas within Alaska. Of these isolates, a