Literature review > Issue_3 > Review Wiggins et al. 

Bacterial vaginosis (BV) remains an enigmatic condition on more than one front. It is clearly related to sex, having many risk factors in common with straightforward sexually transmitted infections, like chlamydia and gonorrhea, including report of a new sex partner and - intriguingly - male partner's lack of condom use [1-3]. However, what precipitates the loss of hydrogen peroxide-producing lactobacilli that characterizes BV is not known, nor is it clear, as Wiggins and colleagues point out, exactly how BV impacts and appears to promote important adverse events, including preterm delivery, or indeed, what predisposes only a subgroup of women with BV to suffer these outcomes. One candidate group of mediators include mucinases, which are enzymes that degrade mucins present in the adherent, protective gel layer that coats the vaginal and cervical epithelium. Mucinases are produced by BV-associated anaerobes in abundance, and are thought in part to be candidates for mediating some of BV's more disruptive effects, including degradation of protective endocervical mucous [4]. This may be particularly important in pregnancy, when the mucous plug is tenacious and designed to protect the intrauterine pregnancy. Thus, the degree of mucinase activity may be directly related to the pathogenic potential of BV.

Mucinase activity is typically measured using a variety of synthetic substrates, and the assays are not clearly standardized so that values can be easily compared across studies. The study by Wiggins and colleagues describes the investigators' efforts to measure the degree of mucinase activity using human cervical mucous as a substrate. The reasoning behind this effort is that cervical mucous would be a logical innate target for BV-associated mucinase activity, and that the degradation of cervical mucous might cause or potentiate some of the more serious outcomes associated with BV, particularly those that require ascension of BV-associated bacteria to the upper genital tract.

These authors focused on one intriguing mucinase, sialidase. Sialidase is one of the more prominent mucinases whose concentration in vaginal fluid increases markedly as the vaginal environment shifts from Lactobacillus to BV-related anaerobe predominance. As the authors note, sialidase effects removal of sialic acid residues from the oligosaccharide chains of mucins, and in doing so may modify the negative charge of these compounds, allowing bacterial adhesion and exposure of other mucous components to further degradation.

To test their assay using endocervical mucous from women, the investigators obtained vaginal fluid from 100 pregnant women (mean age 28.8 y) who were seeking pregnancy termination, and classified their vaginal flora by Gram stain using a standard scale. Women were not eligible if they had taken antibiotics in the previous month, but there appeared to be no other exclusion critera. To compare the results of their novel assay, they used measurements of glycosidase activity obtained from standard assays performed in the same group of women. Investigators were blinded to the findings of Gram stain.

The cervical mucin substrate was generated by pooling cervical mucous from 60 of the 100 women enrolled. We are not told how the investigators arrived at this number, but we are told that none of the 60 had "a sexually transmitted disease" (STD; not defined) or abnormal vaginal flora as determined by Gram stain. Presumably, that is how they were selected, a choice made to favor a more "pure" sample of mucin unadulterated or less likely to be degraded by the presence of other bacterial enzymes that might be increased in the presence of BV or other STD. This is curious, since the number of women with normal vaginal flora shown in Table 1 is only 46. Further, the fact that we are not told how the presence of STD was ruled out is a problem, since cervical infection with Chlamydia trachomatis, Neisseria gonorrhoeae, and human papillomavirus may alter the endocervical milieu considerably, even when these infections do not result in clinical signs consistent with cervicitis [5]. One would suspect that aggregation of the samples from 60 women would abrogate any differences that undiagnosed STD might impose; however, because we are told very little about these women other than their reason for clinic visit and their age, it is difficult to say much about this. The relatively old median age of this group does reduce the likelihood of infection with several important STD, notably chlamydia and gonorrhea, but does not rule out presence of other potentially common infections, such as trichomoniasis, vulvovaginal candidiasis, or genital herpes.

After elaboration and purification of the mucin substrate, the investigators used an ELISA plate assay to validate the presence of mucin activity, adapted the assay to apply to vaginal swab samples, and compared the results of the assays using this test to standard tests for other glycosidases. By this comparison, mucinase assay did not differ by grade of vaginal flora; however, sialidase and - galactosidase activity did, and predictably increased with increasing abnormality of vaginal flora. As the authors note, cervical mucin activity measured in aggregate may mask more subtle effects operative within the cervicovaginal environment, including the activity of individual enzymes acting on different domains of mucin or, potentially, at different sites (such as the cervix vs. vagina).

The authors claim that the new assay they describe is important because 1) it uses a natural human substrate, 2) it is rapid, and might be used for screening of vaginal swab samples in clinical practice, and 3) it is sensitive for the detection of significant enzyme activity associated with BV. Clearly, the first claim is valid, but the paper as written did not convince me that this is a test that could clearly be exported to the clinical microbiology laboratory or truly useful in predicting adverse outcomes associated with enhanced enzymatic activity. To satisfy the first criterion, the assay would need to be fully automated, and a fresh supply of human cervical mucin would need to be assured. The authors do not suggest what the source of this material might be. More critically, there are no data to support the use of this assay-or any other measurement of glycosidase activity, for that matter-in a large number of women as a public health measure. Although data do support that elevated sialidase levels in vaginal fluid of pregnant women with BV are associated with a higher likelihood of adverse pregnancy outcomes, demonstrating that this assay can benefit such women would require considerably more analysis. Currently, the standard of therapy is to treat BV in pregnancy if it is symptomatic (however imprecise the designation of "symptomatic" may be). Assuming that BV treatment favorably impacts the likelihood of adverse pregnancy outcomes, the value in an assay that assesses glycosidase activity lies in prioritizing asymptomatic pregnant women for treatment. One way for these investigators to expand the relevance of their novel findings would be to apply this new assay to saved specimens from women from several studies assessing BV and associated adverse outcomes. If they could demonstrate the predictive value of their assay in these outcomes, as well as its superiority to standard assays that do not involve human cervical mucous, then contemplating a sustained source of such mucous for their purposes might be worth entertaining.

References:

1. Schwebke JR, Richey CM and Weiss HL. Correlation of behaviors with microbiological changes in vaginal flora. J Infect Dis 1999;180:1632-6.

2. Shoubnikova M, Hellberg D, Nilsson S and Mardh PA. Contraceptive use in women with bacterial vaginosis. Contraception 1997;55:355-8.
3. Smart S, Singal A and Mindel A. Social and sexual risk factors for bacterial vaginosis. Sex Transm Infect 2004;80:58-62.

4. Olmsted SS, Meyn LA, Rohan LC and Hillier SL. Glycosidase and proteinase activity of anaerobic gram-negative bacteria isolated from women with bacterial vaginosis. Sex Transm Dis 2003;30:257-61.

5. Mittal A, Kapur S and Gupta S. Host immune response in chlamydial cervicitis. Br J Biomed Sci 1996;53:214-20.

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