Abnormal Vaginal Flora as a Biological Risk Factor for Acquisition of HIV Infection and Sexually Transmitted Diseases

    STD Program, Department of Medicine, Division of Infectious Diseases, University of Alabama at Birmingham

    The role that the vaginal flora plays in protecting the host from sexually transmitted infections (STIs), including HIV infection, is becoming increasingly appreciated. Significant alterations in the microbiological environment of the vaginal ecosystem, such as those that occur in bacterial vaginosis (BV), appear to be a biological risk factor for the acquisition and transmission of STIs/HIV, and, therefore, BV has tremendous public health implications.

    BV occurs in 20%25% of women in the general population and in 40%50% of women attending STI clinics [1, 2]. An estimated 3 million symptomatic cases of BV, as well as an equal number of asymptomatic cases, occur in the United States annually [3]. BV is characterized by dramatic changes in the vaginal flora, from an environment in which lactobacilli are predominant to one in which there is a marked decrease in the number of lactobacilli, particularly those that produce H2O2 [46]. In BV, there are large numbers of anaerobic and facultative anaerobic organisms, the majority of which are normally found in the vagina in small amounts [6]. Changes in the vaginal flora are not all or none but comprise a spectrum of patterns [7]. Therefore, we are presently unable to attribute BV to a single pathogenic organism, and this limitation greatly hampers our diagnostic and treatment strategies.

    Lactobacilli become the predominant inhabitant of the vagina at the time of puberty and are believed to be the key defense of the vaginal flora [8]. In vitro, lactobacilli produce various potential microbial toxins, including H2O2 and more poorly defined bacteriocins [8, 9]. Lactobacilli that produce H2O2 have been shown in vitro to inhibit various microorganisms, including Gardnerella vaginalis, anaerobes, Neisseria gonorrhoeae, and HIV [1014]. Other means by which lactobacilli may play a protective role within the vaginal ecosystem include competition for epithelial cell attachment sites and stimulation of the local immune system [15, 16]. Additionally, lactobacilli maintain the acidity of the vagina through the production of lactic acid as a metabolic by-product. The acidic pH, normally <4.5, is inhospitable to many bacteria. The pH of the vagina becomes more alkaline as the flora shifts toward the population present in BV [17].

    The pathogenesis of BV is poorly understood. Epidemiological correlates of BV include a history of STIs, increased numbers of sex partners, a new sex partner within the month preceding symptom onset, and douching [1, 1821]. Epidemiologic data strongly suggest that BV is sexually transmitted, but the agent or agents that may be transmitted are unknown. Data are emerging on the effectiveness of condoms in preventing BV [22, 23].

    BV is frequently present as a coinfection with STIs. Women with lactobacilli that produce H2O2 in vitro have been shown to be less likely than women without such lactobacilli to have BV as well as STI pathogens such as N. gonorrhoeae, Chlamydia trachomatis, and Trichomonas vaginalis [10, 2426]. In addition, incident herpes simplex virus type 2 and human papillomavirus infections have both been associated with abnormal vaginal flora [27, 28]. The leading hypothesis to explain the coexistence of BV and STIs is that the absence of protective lactobacilli found in patients with BV precedes and facilitates the acquisition of an STI.

    One of the first clinical studies to suggest an association between BV and HIV infection was reported from Thailand. In this cross-sectional study of female commercial sex workers, HIV seropositivity was significantly correlated with BV, independently of other behavioral variables [29]. Similar findings were documented in a study of women in Uganda [30]. Interestingly, in this latter study, the highest HIV seropositivity rates were found in those women with the most severe changes in their vaginal flora, as documented by Gram staining. Similar findings were observed in a study of pregnant women in North Carolina, a cohort with a relatively low HIV seroprevalence. The seroprevalence of HIV increased as the Gram stain score for the vaginal flora increased, independently of other demographic and behavioral variables [31].

    A few prospective studies have been conducted that confirmed the earlier findings of the cross-sectional research. In a cohort of pregnant women in Malawi, HIV seroconversion was significantly associated with alterations in the vaginal flora [32]. There was a significant linear association with an increasing score for abnormal vaginal flora determined on the basis of Amsel clinical criteria (pH, whiff test, microscopy) [32]. A prospective study conducted in Kenya followed a population of commercial sex workers for HIV seroconversion and obtained samples of vaginal secretions for Gram staining as well as culturing for lactobacilli. Abnormal vaginal flora detected by Gram staining was significantly associated with HIV seroconversion as well as the acquisition of gonorrhea [33]. Transmission of HIV may also be enhanced by coinfection with the organisms responsible for BV [34].

    In this issue of the Journal of Infectious Diseases, Myer et el. present additional data on the association between abnormal vaginal flora and HIV seroconversion [35]. The study followed a group of women enrolled in a cervical cancer screening trial near Cape Town, South Africa. Women were tested for the presence of HIV RNA at enrollment and were followed prospectively. Using a case-control study design, the authors compared women who underwent HIV seroconversion during the course of the study with those who remained seronegative. In a multivariate analysis adjusting for demographic characteristics, sexual behaviors, and STIs, women who had a diagnosis of BV by Nugent criteria were significantly more likely to seroconvert than were women with normal vaginal flora (adjusted odds ratio, 2.01 [95% confidence interval, 1.123.62]). The prevalence of BV in this population was 62% in control subjects and 74% in case patients.

    Although production of H2O2 by lactobacilli is the leading explanation for the association seen between abnormal vaginal flora and HIV seroconversion, other mechanisms may play a role. For example, it has been hypothesized that the level of acidity within the vagina may affect CD4 lymphocyte activation. The more alkaline the environment, the more likely it is that CD4 lymphocytes will be activated and thus act as suitable target cells for HIV [36]. Tumor necrosis factor and interleukin-1 are found in high levels in cervical secretions from women with BV, and it has been suggested that these cytokines could up-regulate HIV replication in the vagina [37]. In vitro, certain anaerobic bacteria (Peptostreptococcus asaccharolyticus and Prevotella bivia) that are associated with BV appear to stimulate HIV expression in monocytoid cells and T cells [38].

    STIs are also known to be important biological risk factors for the acquisition and transmission of HIV infection [39]. Rates of BV, however, are significantly higher than rates of STIs worldwide. Thus, the attributable risk of BV with regard to HIV transmission is high [40]. In the Myer et al. study, it is estimated that nearly one-third of all new HIV cases in this South African population might be prevented if all cases of BV could be cured [35]. Widespread control of BV has been suggested as a possible means for decreasing the incidence of HIV infection in the developing world. However, present achievable BV cure rates, combined with high recurrence rates, make this solution impractical. The recommended therapies for BV have cure rates of 70%80%, and recurrence rates are high [41, 42]. Thus, in many women, it is difficult to achieve and maintain normal vaginal flora. For example, in a study of the control of STIs for the prevention of HIV infection in Uganda, therapy for BV was unable to significantly reduce the rates of BV in the treatment communities, compared with those in the control communities. This study also failed to show a difference between the groups in their rates of acquisition of HIV infection [43].

    As a greater understanding of BV unfoldsincluding additional insights into pathogenesis, diagnosis, and therapythere will be more opportunities to consider control efforts as a means to decrease acquisition and transmission of HIV infection. The high prevalence of BV and the emerging prospective data on its association with HIV infection mandate that research efforts be directed at the pathogenesis, diagnosis, and treatment of BV. Until efficacious therapy, as well as an understanding of prevention methods, for BV is available, it will not be feasible to go forward with studies aimed at preventing the complications of this common vaginal infection.

    References

    1.  Amsel R, Totten PA, Spiegel CA, Chen KCS, Eschenbach D, Holmes KK. Non-specific vaginitis: diagnostic and microbial and epidemiological associations. Am J Med 1983; 74:1422. First citation in article

    2.  Rein MF, Holmes KK. "Non-specific vaginitis," vulvovaginal candidiasis, and trichomoniasis: clinical features, diagnosis and management. Curr Clin Top Infect Dis 1983; 4:281315. First citation in article

    3.  Fleury FJ. Adult vaginitis. Clin Obstet Gynecol 1981; 24:40738. First citation in article

    4.  Hill GB, Eschenbach DA, Holmes KD. Bacteriology of the vagina. In: Bacterial vaginosis. Mardh PA, Taylor-Robinson D, eds. Stockholm: Almquist & Wiksell, 1984:2339. First citation in article

    5.  Eschenbach DA, Davick PR, Williams BL, et al. Prevalence of hydrogen peroxide-producing Lactobacillus species in normal women and women with bacterial vaginosis. J Clin Microbiol 1989; 27:2516. First citation in article

    6.  Spiegel CA, Amsel R, Eschenbach D, Schoenknecht F, Holmes KK. Anaerobic bacteria in non-specific vaginitis. N Engl J Med 1980; 303:6017. First citation in article

    7.  Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of Gram stain interpretation. J Clin Microbiol 1991; 29:297301. First citation in article

    8.  Paavonen J. Physiology and ecology of the vagina. Scand J Infect Dis Suppl 1983; 40:315. First citation in article

    9.  Barefoot SF, Klaenhammer TR. Detection and activity of lactacin B, a bacteriocin produced by Lactobacillus acidophilus. Appl Environ Microbiol 1983; 45:180815. First citation in article

    10.  Saigh JH, Sanders CC, Sanders WE. Inhibition of Neisseria gonorrhoeae by aerobic and facultatively anaerobic components of the endocervical flora: evidence for a protective effect against infection. Infect Immun 1978; 19:70410. First citation in article

    11.  Skarin A, Sylwan J. Vaginal lactobacilli inhibiting growth of Gardnerella vaginalis, Mobiluncus and other bacterial species cultured from vaginal content of women with bacterial vaginosis. Acta Pathol Microbiol Immunol Scand 1986; 94:399403. First citation in article

    12.  Klebanoff SJ, Coombs RW. Viricidal effect of Lactobacillus acidophilus on human immunodeficiency virus type I: possible role in heterosexual transmission. J Exp Med 1991; 174:28992. First citation in article

    13.  Klebanoff SJ, Hillier SL, Eschenbach DA, Waltersdorph AM. Control of the microbial flora of the vagina by H2O2-generating lactobacilli. J Infect Dis 1991; 164:94100. First citation in article

    14.  Zheng H, Alcorn TM, Cohen MS. Effects of H2O2-producing lactobacilli on Neisseria gonorrhoeae growth and catalase activity. J Infect Dis 1994; 170:120915. First citation in article

    15.  Redondo-Lopez V, Cook RL, Sobel JD. Emerging role of lactobacilli in the control and maintenance of the vaginal bacterial microflora. Rev Infect Dis 1990; 12:85672. First citation in article

    16.  Boris S, Suarez JE, Vazquez F, Barbes C. Adherence of human vaginal lactobacilli to vaginal epithelial cells and interaction with uropathogens. Infect Immun 1998; 66:19859. First citation in article

    17.  Morin A, Saheb SA, Bisaillon JG, Beaudet R, Sylvestre M. Effect of culture medium composition on inhibition of growth of Neisseria gonorrhoeae by lactobacilli. Curr Microbiol 1980; 4:2836. First citation in article

    18.  Moi H. Prevalence of bacterial vaginosis and its association with genital infections, inflammation and contraceptive methods in women attending sexually transmitted disease and primary health clinics. Int J STD AIDS 1990; 1:8694. First citation in article

    19.  Barbone F, Austin H, Louv WC, Alexander WJ. A follow-up study of methods of contraception, sexual activity and rates of trichomoniasis, candidiasis and bacterial vaginosis. Am J Obstet Gynecol 1990; 163:5104. First citation in article

    20.  Paavonen J, Miettinen A, Stevens CE, Chen KCS, Holmes KK. Mycoplasma hominis in non-specific vaginitis. Sex Transm Dis 1983; 45:2715. First citation in article

    21.  Wlner-Hanssen P, Eschenbach DA, Paavonen J, et al. Association between vaginal douching and acute pelvic inflammatory disease. JAMA 1990; 263:193641. First citation in article

    22.  McClelland R, Lavreys L, Katingima C, et al. Contribution of HIV-1 infection to acquisition of sexually transmitted disease: a 10-year prospective study. J Infect Dis 2005; 191:3338. First citation in article

    23.  Sanchez S, Garcia PJ, Thomas KK, Catlin M, Holmes KK. Intravaginal metronidazole gel versus metronidazole plus nystatin ovules for bacterial vaginosis: a randomized controlled trial. Am J Obstet Gynecol 2004; 191:18981906. First citation in article

    24.  Wiesenfeld H, Hillier S, Krohn MA, Landers D, Sweet R. Bacterial vaginosis is a strong predictor of Neisseria gonorrhoeae and Chlamydia trachomatis infection. Clin Infect Dis 2003; 36:6638. First citation in article

    25.  Hawes S, Hillier S, Benedetti J, Stevens C, et al. Hydrogen peroxideproducing lactobacilli and acquisition of vaginal infections. J Infect Dis 1996; 174:105863. First citation in article

    26.  Wlner-Hanssen P, Krieger JN, Stevens CE, et al. Clinical manifestations of vaginal trichomoniasis. JAMA 1989; 261:5716. First citation in article

    27.  Cherpes T, Meyn L, Krohn MA, Lurie J, Hillier SL. Association between acquisition of herpes simplex virus type 2 in women and bacterial vaginosis. Clin Infect Dis 2003; 37:31925. First citation in article

    28.  Watts D, Fazarri M, Minkoff H, et al. Effects of bacterial vaginosis and other genital infections on the natural history of human papillomavirus infection in HIV-1infected and high-risk HIV-1uninfected women. J Infect Dis 2005; 191:112939. First citation in article

    29.  Cohen CR, Duerr A, Pruithithada N, et al. Bacterial vaginosis and HIV seroprevalence among female commercial sex workers in Chiang Mai, Thailand. AIDS 1995; 9:10937. First citation in article

    30.  Sewankambo N, Gray R, Waiver MJ, et al. HIV-1 infection associated with abnormal vaginal flora morphology and bacterial vaginosis. Lancet 1997; 350:54650. First citation in article

    31.  Royce R, Thorp J, Granados J, Savitz D. Bacterial vaginosis associated with HIV infection in pregnant women from North Carolina. J Acquir Immune Defic Syndr Hum Retrovirol 1999; 20:3826. First citation in article

    32.  Taha TE, Hoover DR, Dallabetta GA, et al. Bacterial vaginosis and disturbances of vaginal flora: association with increased acquisition of HIV. AIDS 1998; 12:16991706. First citation in article

    33.  Martin H, Richardson B, Nyange P, et al. Vaginal lactobacilli, microbial flora, and risk of human immunodeficiency virus type 1 and sexually transmitted disease acquisition. J Infect Dis 1999; 180:18638. First citation in article

    34.  Cu-Uvin S, Hogan J, Caliendo A, Harwell J, Mayer K, Carpenter C. Association between bacterial vaginosis and expression of human immunodeficiency virus type 1 RNA in the female genital tract. Clin Infect Dis 2001; 33:8946. First citation in article

    35.  Myer L, Denny L, Telerant R, de Souza M, Wright TC Jr, Kuhn L. Bacterial vaginosis and susceptibility to HIV infection in South African women: a nested case-control study. J Infect Dis 2005; 192:137280 (in this issue). First citation in article

    36.  Hill J, Anderson D. Human vaginal leukocytes and the effects of vaginal fluid on lymphocyte and macrophage defense functions. Am J Obstet Gynecol 1992; 166:7206. First citation in article

    37.  Sturm-Ramirez K, Gaye-Diallo A, Eisen G, Mboup S, Kanki P. High levels of tumor necrosis factor and interleukin-1 in bacterial vaginosis may increase susceptibility to human immunodeficiency virus. J Infect Dis 2000; 182:46773. First citation in article

    38.  Hashemi F, Ghassemi M, Faro S, Aroutcheva A, Spear G. Induction of human immunodeficiency virus type 1 expression by anaerobes associated with bacterial vaginosis. J Infect Dis 2000; 181:157480. First citation in article

    39.  Wasserheit J. Epidemiological synergy: interrelationships between human immunodeficiency virus infection and other sexually transmitted diseases. Sex Transm Dis 1992; 19:6177. First citation in article

    40.  Schmid G, Markowitz L, Joesoef R, Koumans E. Bacterial vaginosis and HIV infection. Sex Transm Infect 2000; 76:34. First citation in article

    41.  Joesoef MR, Schmid GP, Hillier SL. Bacterial vaginosis: review of treatment options and potential clinical indications for therapy. Clin Infect Dis 1999; 28 (Suppl):S57S65. First citation in article

    42.  Blackwell AL, Fox AR, Phillips I, Barlow D. Anaerobic vaginosis (non-specific vaginitis): clinical, microbiological and therapeutic findings. Lancet 1982; 2:137982. First citation in article

    43.  Waiver MJ, Sewankambo NK, Serwadda D, et al. Control of sexually transmitted diseases for AIDS prevention in Uganda: a randomized community trial. Rakai Project Study Group. Lancet 1999; 353:5135. First citation in article