SEATTLE — Streptococcus pseudoporcinus may be an emerging genital tract pathogen, based on an observational study reported at the annual meeting of the Infectious Diseases Society for Obstetrics and Gynecology.
Research conducted in the 1980s and 1990s suggested that S. pseudoporcinus is rare among humans. However, the microorganism crossreacts with group B Streptococcus (GBS)-typing serums, lead author Kevin A. Stoner of the Magee-Womens Research Institute in Pittsburgh, pointed out. “There is the potential that this organism has been misidentified in the past.” In recent years, S. pseudoporcinus has been increasingly identified in women who experience stillbirth and preterm labor.
S. pseudoporcinus was sought in 663 sexually active women, aged 18-40 years, who participated in a vaccine trial. The women resided in Pittsburgh; Augusta, Ga.; and Houston. Paired rectal and vaginal swabs were obtained from the women at 2-month intervals over an 18-month period and processed in a central laboratory.
The swabs were inoculated in Columbia sheep blood agar and selective enrichment broth. An isolate was preliminarily classified as being S. pseudoporcinus if it was a catalase-negative, gram-positive coccus; had positive agglutination results with group B PathoDx antiserum; and produced a wide zone of beta hemolysis on the blood agar—in contrast to the zone that is typically (though not always) narrower for GBS.
The classification was supported if the isolate did not crossreact with species-specific primers for GBS in polymerase chain reaction analysis, and if the API 20 Strep test indicated the presence of Streptococcus porcinus, a swine pathogen with 94% gene sequence similarity.
“Currently, there are no phenotypic characteristics to determine the difference between these two organisms, but from what we can see, all human isolates are most likely S. pseudoporcinus,” said Mr. Stoner, a researcher at the institute.
Final confirmation was obtained by demonstrating that the isolate's gene sequence was 99% similar to that of S. pseudoporcinus.
Overall, 5.4% of the women were colonized with S. pseudoporcinus at some time during the study, Mr. Stoner reported. Within this group, the organism was recovered in 33% of women at one visit, in 22% at two visits, and in 44% at three or more visits. The greatest number of visits with recovery was six. “So this does indicate that there is persistence in colonization by this organism, and it's not just a random, one-time occurrence,” he said.
Of the affected women, 50% were found to have S. pseudoporcinus in both the vagina and rectum; 42% were cocolonized with GBS.
“We recovered a total of 120 S. pseudoporcinus isolates,” Mr. Stoner said. “That's far more than anybody has ever reported recovering.” Some 56% of the isolates were found in the vagina, and 41% were found in the rectum. In the laboratory, the majority (67%) were detectable in enrichment broth only, indicating that colonization was occurring at very low density levels, he noted.
Of 23 isolates tested for antibiotic susceptibility, none were found to be resistant to penicillin, cefazolin, clindamycin, or erythromycin, according to Mr. Stoner.
Women at the three study sites were equally likely to acquire S. pseudoporcinus during the study. However, the rate of acquisition differed significantly by race, with a much higher rate among black women than among women of other races (11 vs. 0-1 events per 100 woman-years).
In addition, women aged 30-40 years had a significantly higher rate of acquisition than did their younger counterparts (eight events vs. two to three events per 100 woman-years), as did women with 12 or fewer years of education relative to their better-educated counterparts (nine events vs. two to four events per 100 woman-years).
Finally, the rate of S. pseudoporcinus acquisition differed significantly with the number of male sex partners between visits. The rate was about two to three times higher among women who had had at least two partners compared with women who had one on no partners (12 events vs. 4-5 events per 100 woman-years).
Mr. Stoner noted that the organism's distinctive beta hemolysis pattern and DNA sequencing can help avoid misidentification as GBS. “If there is ever a question as to an organism's identity as GBS or S. pseudoporcinus, and you don't have access to DNA sequencing, the API 20 Strep test kits can always be used to confirm identification.”
Mr. Stoner stated that he had no conflicts of interest in association with the study.
The API 20 Strep test kits can always confirm an organism's identity as GBS or S. pseudoporcinus. MR. STONER