MYCOPLASMA HOMINIS: CHARACTERISTICS, MORPHOLOGY, PATHOGENESIS - BIOLOGY - 2025

Mycoplasma hominis is a very small bacterium that can be found in the genitourinary tract of men and women. The colonization rate is between 0 to 31%, being directly related to sexual activity with multiple partners.

Therefore, this microorganism is considered a sexually transmitted microorganism. Although it may be colonizing asymptomatically, its finding is important in patients with infertility, as it has been related to this condition.

This photograph shows a series of gram-negative Mycoplasma hominis

It is also associated with pelvic inflammatory disease in women and non-gonococcal urethritis in men. Another important aspect of M. hominis is that it does not have a rigid cell wall, therefore it is not susceptible to penicillins and other antibiotics that act on this structure.

However, they are susceptible to a variety of other broad-spectrum antibiotics. But in this sense we must be careful, since M. hominis has acquired resistance to many of them.

characteristics

Biochemical characteristics

Mycoplasma hominis does not use glucose, but it does use arginine and forms basic end products from it. This characteristic differentiates it from M. pneumoniae and M. genitalium.

It grows at an optimum pH of 5.5 to 8 with a CO ₂ atmosphere at 35 ° C, although it also grows anaerobically. On the other hand, all species of the genus Mycoplasma are demanding from a nutritional point of view, requiring the addition of sterols, purines and pyrimidines for their growth in vitro.

However, M. hominis is the least demanding of all. For this reason, it can sometimes be isolated in routine culture media such as Columbia agar and chocolate agar, as long as it does not contain SPS like some blood culture bottles.

Virulence factors

Mycoplasma hominis has polypeptides on its surface called P50, P100, P140, P110, MG218 and MG317 that help it to adhere to eukaryotic cells, that is, they act as adhesins.

Likewise, M. hominis has a special affinity for sulfated glycolipids present in sperm and in the urogenital tract of men and women.

This explains the tropism that this microorganism has to urogenital tissue and the rapid adhesion to sperm cells, which in in vitro studies has occurred in just 10 minutes of exposure.

Taxonomy

Domain: Bacteria

Phylum: Firmicutes

Class Mollicutes

Order: Mycoplasmatales

Family: Mycoplasmataceae

Genus: Mycoplasma

Species: hominis

Morphology

The Mycoplasma hominis bacterium measures approximately 0.2-1 μm in diameter. It lacks a cell wall and contains a three-layered (trilaminar) plasma membrane.

The absence of the cell wall gives the bacteria excessive plasticity and flexibility, managing to adopt various forms (pleomorphism).

In addition, the lack of a cell wall makes this bacterium unable to stain with the Gram stain. Its inability to form the cell wall is believed to be due to its DNA being made up of 500,000 base pairs. In other words, it is extremely small.

The typical morphology of the M. hominis colony is fried egg, measuring 50 to 300 µm in diameter and growing for 5 - 7 days.

Colonies can be stained with Dianes' stain as a visualization aid. In liquid cultures such as M broth, it produces a slight turbidity in addition to a color change.

Pathology

The role of M. hominis as a pathogenic microorganism is controversial, since it has been found in asymptomatic people, therefore it is believed that it may act as an opportunist.

In this sense, Mycoplasma hominis has been associated with cases of bacterial vaginosis. If it is accompanied by anaerobic germs and Gardnerella vaginalis as co-pathogens, they produce pelvic inflammatory disease and infertility.

This microorganism alone or associated with other bacteria is a risk element for human fertility and therefore should be investigated whenever the reason for consultation is the impossibility of procreation.

Pelvic inflammatory disease, vaginitis, and infertility

It can cause infertility if they persist for a long time without treatment. Mycoplasmas rise through the mucosa and settle in the epithelia of the female or male reproductive system.

They produce changes in vaginal pH, alter the characteristics of the cervix and cervical mucus, which thins the endocervical epithelium and increases capillary fragility that facilitates bleeding.

All of this interferes with fertilization (mucocervical-semen interaction).

Endometriosis and abortions in women

Through sexual intercourse, the infected sperm reach the woman's uterus, generating alterations such as endometriosis and disorders in pregnancy that can cause embryo loss.

M. hominis has also been isolated from the blood of 10% of women with postpartum or post-abortion fever.

Non-gonococcal, nonchlamydial urethritis in men

M. hominis has been isolated from many patients with this condition who have tested negative for N. gonorrhoeae and C. trachomatis.

Infertility in men

Many in vitro investigations have revealed that Mycoplasma hominis is capable of adhering to any part of the sperm, damaging the membrane and the acrosome, modifying their morphology.

The morphological changes observed in sperm consist of tail curls and vesicles on the neck. All of this diminishes its viability.

Motility is affected by damage to the inner membrane of the sperm. This is due to the formation of hydrogen peroxide and reactive oxygen species (ROS) that induce peroxidation of sperm lipids.

The decrease in motility and viability influences the ability to penetrate the oocytes, being a cause of infertility. In addition, the bacterium also increases the rate of sperm DNA fragmentation.

Diagnosis

Any spermogram with a high percentage of morphological abnormalities and an increase in leukocytes per field is a candidate for Mycoplasma hominis studies.

Although this is not the only bacterium to be investigated, as other similar bacteria such as Mycoplasma genitalium and Ureaplasma urealyticus are important in patients who complain of infertility.

These bacteria are not visible in a bright field microscope, which makes their diagnosis difficult with the naked eye, so it is necessary to have methodologies that allow their detection and identification.

These include culture methods and molecular biology tests for rapid and efficient identification. Currently there is the Mycoplasma System Plus KIT, among others.

This system consists of 24 wells containing desiccated antibiotics and biochemical substrates. It is used for the semi-quantitative identification and execution of the antibiogram to the urogenital mycoplasmas isolated by vaginal swabs.

This assay enables the detection of antimicrobial susceptibility to tetracycline, pefloxacin, ofloxacin, doxycycline, erythromycin, clarithromycin, minocycline, clindamycin, and azithromycin.

It is important to highlight that its detection even in asymptomatic patients can prevent the appearance of diseases at the gynecosbtetric level.

Treatment

Antibiotic therapy in patients with Mycoplasma hominis is recommended, since it improves seminal quality and increases the probability of pregnancy.

Among the antibiotics that can be used are: fluoroquinolones, tetracyclines and chloramphenicol. On the other hand, azithromycin and ofloxacin are also effective.

However, some studies have revealed strains of Mycoplasma hominis resistant to macrolides (clarithromycin, azithromycin and erythromycin), in addition cases of resistance to tetracycline have been reported.

In persistent infections, the combination of doxycycline and azithromycin has been recommended. Likewise, M. hominis has shown high sensitivity to minocycline and clindamycin.

For obvious reasons, Mycoplasma hominis cannot be treated with antibiotics that have the cell wall as their target binding site, nor those that intervene with the synthesis of folic acid, such as beta-lactams and rifampicin, respectively.

References

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