Endogenous Bacteria Outnumbering Human Cells Biology Essay

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With endogenous bacteria outnumbering human cells ten to one, it has become increasingly apparent that human health is reliant on commensal microorganisms2, 10. Widely inhabiting the skin, oral cavity, nasal passages, gastro-intestinal tract and genital tract, interactions between the human host and its resident bacteria must be rigorously explored to transform healthcare into a new age of personalized medicine13. The replacement of antibiotic/antiviral treatment to drugless therapy to combat sexually transmitted infections (STIs) has been a recently trending topic. As the number of STI cases skyrocket among teenagers and elders21, new advances in the field are critical to provide preventative measures, as well as innovative treatments, against common STIs. Furthermore, concentrating on using natural immunity of the vaginal mucosa to enhance protection against STIs such as bacterial vaginosis and genital herpes is imperative. Natural immunity of the vaginal mucosa encompasses the roles of glycogen derived from vaginal epithelial cells, commensal bacteria colonizing the vaginal tract and vaginal pH2. The intimate relationship between these elements is yet to be definitively characterized, and thus we are interested in determining their interdependence. The eventual goal of this project is to manipulate and enhance natural immunity and provide preventative measures, as well as treatments, against common STIs.


The human vaginal ecosystem is dominated by Lactobacillus species and Lactobacillus spp. are consistently reported to be the predominant species found in the vagina in the majority of healthy menstruating women12. Lactobacilli are typically non-pathogenic, Gram positive, rod-shaped, facultative anaerobic bacteria that, as well as inhabiting the vaginal tract, are often endogenous to the gut and the oral cavity1, 4. The genus is large and comprises more than 100 described species. In recent years, L. iners, L. crispatus, L. gasseri and L. jensenii have been reported to be the most common species found in the vagina of healthy females9. The protective role of lactobacilli is gradually being accepted and clinical studies have been carried out in order to evaluate the use of promising probiotic bacteria, which are defined as “live microorganisms which when administered in adequate amounts confer a health benefit on the host” 21.

Vaginal acidity is imperative for the maintenance of vaginal health. Moreover, the presence of lactic acid in the vaginal tract allows for a healthy pH under 4.53. Glycogen, which is thought to be deposited by vaginal epithelial cells, is recovered in high amounts during estrogen production16. Through the conversion of glycogen to lactic acid, lactobacilli employ anaerobic respiration to produce this acidic pH16. Additionally, vaginal pH undergoes fluctuations from birth to menopause, according to changes in o varvaoccurring during woman's life.

Herpes simplex virus type 2 (HSV-2) is the major cause of genital ulcer disease worldwide and a significant co-factor in the transmission and acquisition of human immunodeficiency virus type 1 (HIV-1)7. Anti-viral drugs are typically used to treat genital herpes infections, however, even with chronic use these drugs do not result in permanent viral clearance or long-term control7, 18. An alternate approach for prophylaxis and treatment of HSV-2 infection may be the delivery of vaginal probiotics such as Lactobacillus spp. to prevent or alleviate viral pathogenesis. Moreover, administering glycogen as a prebiotic factor for endogenous Lactobacilli may induce similar effects.

Although the vaginal microenvironment and its natural immunity undoubtedly influence the composition of the microbial ecosystem at that site, the magnitude of this microenvironment in effecting protection from invasion by pathogenic bacteria or viruses, especially those that are sexually transmitted, remains an open question8. The eventual goal of this project is to manipulate and enhance natural immunity and provide preventative measures, as well as treatments, against common STIs. To do so, fundamental knowledge of the vaginal mucosa must be understood first. The purpose of this study is manyfold. pH fluctuations in the vaginal mucosa throughout lifetime are well characterized, however, total carbohydrate concentrations are unknown. Elucidation of changing vaginal pH and vaginal carbohydrate concentrations in different reproductive stages in murine models is therefore necessary. Furthermore, the second objective is to establish the inter-relationship of pH, bacterial concentrations and carbohydrate concentrations in the vaginal mucosa as determined by endogenous estrogen. Manipulation of the vaginal microenvironment would succeed these findings. Moreover, determination of fluctuations in pH and bacterial concentrations after artificial delivery of glycogen or Lactobacillus spp. to the vaginal mucosa would be approached. Lastly, investigating the role of glycogen and Lactobacillus spp. as possible determinants in prevention of HSV-2 infection is required to provide insight into the development of targeted therapies.

Preliminary Results

In the Ashkar lab to date, vaginal pH, total carbohydrate and bacterial concentrations have been identified in many reproductive stages. Reproductively capable mice in the estrus stage have been identified as having the lowest vaginal pH but highest total carbohydrate and Lactobacillus spp. concentrations. In contrast, reproductively capable mice in the diestrus stage had a significantly higher vaginal pH and low total carbohydrate and Lactobacillus spp. concentrations. Mice which were administered Depo-Provera contraceptive hormone, ovariectomized mice, young mice (<5 weeks old) and old mice (>18 months old) had the same trends as the mice in diestrus. Lastly, characterization of an estrogen-receptor knockout strain (ERKO) demonstrated similar trends to mice in diestrus. These data suggest a positive association between total vaginal carbohydrate and Lactobacillus spp. concentrations, as well as a negative association of total vaginal carbohydrate and Lactobacillus spp. concentrations with vaginal pH.

Currently, we are working on determining how artificial delivery of glycogen and Lactobacillus spp. obtained from a mouse in estrus will alter the vaginal microenvironment in terms of pH and the bacterial load itself. Pilot studies have shown that delivering a glycogen lavage to the vaginal tract increases the bacterial load and decreases pH. In contrast, bacterial delivery did not show any difference in the vaginal microenvironment. These studies will give insight into the growth constraints of the vaginal microbiota, as well as providing information about utilizing glycogen and Lactobacillus spp. delivery to prevent and treat HSV-2.


Hormone treatments. Inbred 16-18 week old C57Bl/6 mice were injected with Depo-Provera hormone subcutaneously.

Vaginal pH. The pH values of the vaginal mucosa were measured using a pH meter equipped with combination microelectrode (Model MI-413E, Microelectrodes, Inc.). The electrode was first calibrated with pH 4.0, 7.0 and 10.0 solutions. The microelectrode was rinsed in MilliQ H2O each time prior to insertion into the vagina. Presented data are averages from at least two independent measurements on each mouse.

Vaginal lavage collection and estrus cycle staging. The mice were physically restrained and 30 µl of MilliQ H2O was pipetted into the vaginal lumen in and out before collection. Each secretion was smeared onto a glass slide, dried and stained with Hema-3 Manual Staining System (Fisher Healthcare).