Vaginal flora
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Vaginal flora, vaginal microbiota or vaginal microbiome are the microorganisms that colonize the
vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
. They were discovered by the German gynecologist Albert Döderlein in 1892 and are part of the overall human flora. The amount and type of bacteria present have significant implications for an individual's overall health. The primary colonizing bacteria of a healthy individual are of the genus ''
Lactobacillus ''Lactobacillus'' is a genus of Gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically diver ...
'', such as '' L. crispatus'', and the
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natu ...
they produce is thought to protect against infection by pathogenic species.


Lactobacilli

The primary colonizing bacteria of a healthy individual are of the genus ''
Lactobacillus ''Lactobacillus'' is a genus of Gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically diver ...
'' (90–95%), the most common being '' L. crispatus'', '' L. iners'', '' L. jensenii'', and ''
L. gasseri ''Lactobacillus gasseri'' is a species in the genus ''Lactobacillus'' identified in 1980 by François Gasser and his associates. It is part of the vaginal flora. Its genome has been sequenced. ''L. gasseri'' is a normal inhabitant of the lower ...
''. Since the first description of lactobacilli by Döderlein, lactobacilli have been generally considered the gatekeepers of the vaginal ecosystem. Lactobacilli have been shown to inhibit in vitro growth of pathogenic microorganisms, e.g. ''
Bacteroides fragilis ''Bacteroides fragilis'' is an anaerobic, Gram-negative, pleomorphic to rod-shaped bacterium. It is part of the normal microbiota of the human colon and is generally commensal, but can cause infection if displaced into the bloodstream or surrou ...
'', ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'', ''
Gardnerella vaginalis ''Gardnerella vaginalis'' is a species of Gram-variable-staining facultative anaerobic bacteria. The organisms are small (1.0–1.5 Î¼m in diameter) non-spore-forming, nonmotile coccobacilli. Once classified as ''Haemophilus vaginalis'' ...
'', ''Mobiluncus'' spp., ''
Neisseria gonorrhoeae ''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Ludwig Sigesmund Neisser, Albert Neisser in 1879. It causes the sexually transmit ...
'', ''
Peptostreptococcus anaerobius ''Peptostreptococcus anaerobius'' is a species of bacteria belonging to the ''Peptostreptococcus'' genus of anaerobic, Gram-positive, non- spore forming bacteria. The cells are small, spherical, and can occur in short chains, in pairs or individ ...
'', ''
Prevotella bivia ''Prevotella bivia'' is a species of bacteria in the genus ''Prevotella''. It is gram-negative. It is one cause of pelvic inflammatory disease. Other ''Prevotella'' spp. are members of the oral and vaginal microbiota, and are recovered fro ...
'' and ''
Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...
''. It is generally accepted that this is achieved mainly through the action of lactic acid. Moreover, lactobacilli normally help to prevent long-term colonization of the vagina by adhering to vaginal epithelial cells. This usually reduces pathogens from infecting to the
vaginal epithelium The vaginal epithelium is the inner lining of the vagina consisting of multiple layers of (squamous) cells. The basal membrane provides the support for the first layer of the epithelium-the basal layer. The intermediate layers lie upon the basal ...
. Next to lactic acid production and competition for adherence, other antagonistic mechanisms include
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
(a broad-spectrum antimicrobial) and
bacteriocin Bacteriocins are proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally, and ec ...
s (target-specific antimicrobials) production.


pH and lactic acid

Low pH is generally accepted to be the main mechanism controlling the composition of the vaginal microflora. Although the lactic acid produced by lactobacilli contributes to the vaginal acidity, it is still not proven to be the primary source of low vaginal pH, but the fact remains that most lactobacilli thrive best at a pH < 3.5 . Normal vaginal pH is considered to be under 4.5 with a range of 3.8 to 4.4.


Hydrogen peroxide

Production of hydrogen peroxide (H2O2) is a well-known mechanism for bacterial antagonism, inhibiting growth of microorganisms via direct interaction or via human myeloperoxidase. Hydrogen peroxide-producing lactobacilli have been shown to inactivate
HIV-1 The subtypes of HIV include two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in the sooty mangabey ...
, herpes simplex virus type 2 (HSV-2), ''
Trichomonas vaginalis ''Trichomonas vaginalis'' is an anaerobic, flagellated protozoan parasite and the causative agent of a sexually transmitted disease called trichomoniasis. It is the most common pathogenic protozoan that infects humans in industrialized countri ...
'', '' G. vaginalis'', '' P. bivia'' and '' E. coli''. O'Hanlon and Baeten found that 96% of ''
Lactobacillus ''Lactobacillus'' is a genus of Gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically diver ...
'' species from a healthy vaginal ecosystem produced H2O2 ('' L. jensenii'' and '' L. vaginalis'' produce the highest levels of H2O2), whereas only 6% of the lactobacilli recovered from women with BV produced H2O2. In agreement with this, '' L. iners'', most frequently associated with disturbed vaginal microflora, is a poor producer of H2O2. Vaginal colonization by H2O2-producing lactobacilli has been associated with a decrease in the occurrence of
bacterial vaginosis Bacterial vaginosis (BV) is a disease of the vagina caused by excessive growth of bacteria. Common symptoms include increased vaginal discharge that often smells like fish. The discharge is usually white or gray in color. Burning with urination ...
(BV). However, more recently O'Hanlon et al. demonstrated that cervicovaginal fluid and semen have a significant H2O2-blocking activity and they later demonstrated that physiological concentrations of H2O2 below 100 μM fail to inactivate any of the 17 tested BV-associated bacteria, e.g. '' A. vaginae'', '' G. vaginalis'', ''Mobiluncus'' spp., '' P. bivia'', '' P. corporis'', ''
Mycoplasma hominis ''Mycoplasma hominis'' is a species of bacteria in the genus ''Mycoplasma''. ''M.hominis'' has the ability to penetrate the interior of human cells. Along with ureaplasmas, mycoplasmas are the smallest free-living organisms known. They have no ...
'', even in the presence of human
myeloperoxidase Myeloperoxidase (MPO) is a peroxidase enzyme that in humans is encoded by the ''MPO'' gene on chromosome 17. MPO is most abundantly expressed in neutrophil granulocytes (a subtype of white blood cells), and produces hypohalous acids to carry ou ...
, known to increase the microbicidal activity of H2O2. Only supraphysiologic concentrations of exogenous H2O2 (0.34% w/v, 100 mM) were sufficient to inactivate BV-associated bacteria at which concentration it more potently inactivated vaginal lactobacilli ('' L. crispatus'', ''
L. gasseri ''Lactobacillus gasseri'' is a species in the genus ''Lactobacillus'' identified in 1980 by François Gasser and his associates. It is part of the vaginal flora. Its genome has been sequenced. ''L. gasseri'' is a normal inhabitant of the lower ...
'', '' L. iners'' and '' L. jensenii''). A concentration of 100 mM H2O2 is approximately 50-fold higher than lactobacilli are capable of producing even under optimal aerobic, low-antioxidant conditions, and approximately 5,000-fold higher than the estimated H2O2 concentration in vivo. Even more remarkable, the addition of only 1% vaginal fluid blocked the microbicidal activity of 1 M H2O2. Possible explanations may be that cervicovaginal fluid and semen contain proteins, glycoproteins, polysaccharides, lipids, and other molecules with the potential to react with and inactivate H2O2. In addition, the vagina is hypoxic most of the time, whereas lactobacilli require oxygen to produce hydrogen peroxide. It is also remarkable that catalase, which provides bacteria protection against toxic H2O2, is absent in lactobacilli, and as such they would be unprotected against their own H2O2 production. In contrast, under optimal anaerobic growth conditions, physiological concentrations of lactic acid inactivated the BV-associated pathogens without affecting the vaginal lactobacilli. In summary, although the hydrogen peroxide production of lactobacilli has been considered as an important antimicrobial component, contributing to the colonization resistance provided by lactobacilli, and although there seems to be a link between H2O2-producing lactobacilli and normal vaginal microflora, recent data do not support this role for H2O2.


Bacteriocins

Vaginal lactobacilli produce antimicrobial peptides, i.e. bacteriocins such as lactocin 160 and crispasin. with inhibitory activity ranging from narrow (closely related ''Lactobacillus'' species) to broad (diverse groups of bacteria, including '' G. vaginalis'' and '' P. bivia''), and bacteriocin-like substances, with a broader spectrum of activity than bacteriocins (e.g. a heat-resistant peptide produced by ''Ligilactobacillus salivarius'' subsp. ''salivarius'' CRL 1328). Several studies have indicated that the activity of bacteriocins is favored by low pH. The inhibitory substances produced by vaginal ''
Lactobacillus ''Lactobacillus'' is a genus of Gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically diver ...
'' is a primary factor in protecting the vaginal microbiota, with organic acids, bacteriocins, and hydrogen peroxide. These act synergistically against infection by pathogens. Not all ''Lactobacillus'' spp. and not all strains within one ''Lactobacillus'' species exhibit all 3 mechanisms. ''Lactobacillus'' species differ in premenopausal women, i.e. '' L. crispatus'', '' L. jensenii'', '' L. iners'', ''
L. gasseri ''Lactobacillus gasseri'' is a species in the genus ''Lactobacillus'' identified in 1980 by François Gasser and his associates. It is part of the vaginal flora. Its genome has been sequenced. ''L. gasseri'' is a normal inhabitant of the lower ...
'' (and possibly '' L. vaginalis''), as assessed through cultivation-dependent and cultivation-independent techniques. Vaginal lactobacilli have been shown to display a pronounced vaginotropism, and their pili act as ligands for attachment to receptors of vaginal epithelial cells. The limited number of ''Lactobacillus'' spp. found in the human vagina is remarkable, which leads to the possibility that there are host factors that select for specific organisms, that these species have unusual characteristics that allow them to successfully colonize the vagina, or both . However, the vaginotropism, does not only apply to this selected group of lactobacilli that stand for a healthy vagina, but also for the bacterial species associated with BV. The microbiota detected in the human genital and gut econiche do not appear to grow outside their host and probably are likely to rely on the close contact between parents and their children for transmission, e.g. mother to neonate transmission of genital microflora, most probably also with gut microflora homogenously distributed over the baby's body including skin, the oral cavity, nasopharynx, and feces.


Other microbiota

Healthy, normal vaginal microbiota that is dominated by lactobacilli may differ among some ethnic groups. Non-pathogenic vaginal species are part of the normal microbiota of some women. Several studies have demonstrated that a significant proportion (7–33%) of healthy asymptomatic women (especially black and Hispanic women) lack appreciable numbers of Lactobacillus species in the vagina, and instead have a vaginal microbiota that consist of other lactic acid-producing bacteria, i.e. species from the genera '' Atopobium'', '' Leptotrichia'', ''
Leuconostoc ''Leuconostoc'' is a genus of gram-positive bacteria, placed within the family of Lactobacillaceae. They are generally ovoid cocci often forming chains. ''Leuconostoc'' spp. are intrinsically resistant to vancomycin and are catalase-negative (w ...
'', ''
Megasphaera ''Megasphaera'' is a genus of Bacillota bacteria classified within the class Negativicutes. This classification has been reexamined.Yutin N, Galperin MY (2013) A genomic update on Clostridial phylogeny: Gram-negative spore formers and other mi ...
'', ''
Pediococcus ''Pediococcus'' is a genus of gram-positive lactic acid bacteria, placed within the family of Lactobacillaceae. They usually occur in pairs or tetrads, and divide along two planes of symmetry, as do the other lactic acid cocci genera '' Aerococc ...
'', ''
Streptococcus ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...
'' and '' Weissella'', All ethnic populations have vaginal microflora communities containing lactic acid producing bacteria. This implies that not all communities may be equally resilient, so that if the resilience of a vaginal community is low then transitory changes in the structure of these communities may occur more readily in response to disturbances of various kinds, including
menses Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of horm ...
, sexual intercourse, douching and contraceptive practices. These differences in the structure and composition of microbial communities may underlie well-known differences in the susceptibility of women in these racial groups to BV and various vaginal infections. Though vaginal microflora may be disturbed, and lactic acid is produced by other species, vaginal pH and acid production establish normal vaginal microbiota. The pH further decreases during pregnancy.


Other vaginal bacterial species

Other bacterial species are frequently found in the vagina, such as the Gram positive cocci: ''
Atopobium vaginae ''Fannyhessea vaginae'' is a species of bacteria in the family ''Atopobiaceae''. It is a facultative anaerobic, Gram-positive rod-shaped or elliptical coccobacilli found as single elements or in pairs or short chains. It is typically isolated fr ...
'', ''Peptostreptococcus'' spp., ''Staphylococcus'' spp., ''Streptococcus'' spp., and ''Bacteroides'' spp., ''Fusobacterium'' spp., ''
Gardnerella vaginalis ''Gardnerella vaginalis'' is a species of Gram-variable-staining facultative anaerobic bacteria. The organisms are small (1.0–1.5 Î¼m in diameter) non-spore-forming, nonmotile coccobacilli. Once classified as ''Haemophilus vaginalis'' ...
'', '' Mobiluncus'', ''Prevotella'' spp., and Gram-negative enteric organisms, such as ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
''. '' Mycoplasma'' and ''
Ureaplasma ''Ureaplasma'' is a genus of bacteria belonging to the family Mycoplasmataceae. As the name imples, ''Ureaplasma'' is urease positive. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenc ...
'' are frequently found in the vagina. Some of the obligate and facultative anaerobic bacteria are associated with BV.


Pregnancy

The effect of
tampon A tampon is a menstrual product designed to absorb blood and vaginal secretions by insertion into the vagina during menstruation. Unlike a pad, it is placed internally, inside of the vaginal canal. Once inserted correctly, a tampon is held in ...
use on vaginal flora is debated, but application of tampons appears not to significantly modify the balance of bacterial presence. Pregnancy alters the microbiota with a reduction in species/genus diversity.


Disease prevention

A healthy vaginal microbiome aids in the prevention of bacterial vaginosis, yeast infections and other possible problems by maintaining an acidic pH (< 4.5) that is unfavourable for the growth of common pathogens, such as ''
Gardnerella vaginalis ''Gardnerella vaginalis'' is a species of Gram-variable-staining facultative anaerobic bacteria. The organisms are small (1.0–1.5 Î¼m in diameter) non-spore-forming, nonmotile coccobacilli. Once classified as ''Haemophilus vaginalis'' ...
''. The lactobacilli present in a healthy vaginal microbiome also occupy the
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
that would otherwise be available for exploitation by pathogenic organisms. However, harmful bacteria or an imbalance in bacteria can lead to infection.
Bacterial vaginosis Bacterial vaginosis (BV) is a disease of the vagina caused by excessive growth of bacteria. Common symptoms include increased vaginal discharge that often smells like fish. The discharge is usually white or gray in color. Burning with urination ...
is associated with the presence of ''
Gardnerella vaginalis ''Gardnerella vaginalis'' is a species of Gram-variable-staining facultative anaerobic bacteria. The organisms are small (1.0–1.5 Î¼m in diameter) non-spore-forming, nonmotile coccobacilli. Once classified as ''Haemophilus vaginalis'' ...
'' and ''
Peptostreptococcus anaerobius ''Peptostreptococcus anaerobius'' is a species of bacteria belonging to the ''Peptostreptococcus'' genus of anaerobic, Gram-positive, non- spore forming bacteria. The cells are small, spherical, and can occur in short chains, in pairs or individ ...
'' and a decrease in the number of ''
Lactobacillus ''Lactobacillus'' is a genus of Gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically diver ...
'' species that comprise the healthy vaginal microbiota.


Research

Investigations have found that the presence of lacto-bacillus dominated bacteria in the vagina is associated with a lower incidence of sexually transmitted infections.


See also

*
Human Microbiome Project The Human Microbiome Project (HMP) was a United States National Institutes of Health (NIH) research initiative to improve understanding of the microbiota involved in human health and disease. Launched in 2007, the first phase (HMP1) focused on ...
*
Lactic acid bacteria Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped ( bacilli) or spherical ( cocci) bacteria that share common metabolic and physiological characteristics. These bact ...
*
List of microbiota species of the lower reproductive tract of women This is the list of healthy vaginal microbiota (VMB), which is defined as the group of species and genera that generally are found to have lack of symptoms, absence of various infections, and result in good pregnancy outcomes. VMB is dominated ...
* Skin flora *
Vaginal microbiota in pregnancy The vaginal flora in pregnancy, or vaginal microbiota in pregnancy, is different from the vaginal flora (the population of microorganisms that resides in the vagina) before sexual maturity, during reproductive years, and after menopause. A descrip ...


References


External links

*
Döderlein's bacteria
{{Authority control Bacteriology Microbiology Feminine hygiene Vagina Microbiomes