Semen, the biological fluid that carries sperm, is not a sterile environment as once thought. Modern molecular techniques, such as next-generation sequencing, have shown that semen contains a diverse community of microorganisms, primarily bacteria. This microbial population, known as the seminal microbiome, is a natural component of the male reproductive system, similar to communities found in the gut or on the skin. Understanding its composition is important because imbalances can affect reproductive health and male fertility.
The Seminal Microbiome
The seminal microbiome consists of microorganisms consistently residing within the male reproductive tract. This community is highly diverse, varying widely among individuals, suggesting a unique and personalized bacterial environment in each person’s semen. Common bacterial genera identified in the semen of healthy men include Lactobacillus, Staphylococcus, and Corynebacterium.
These bacteria are generally considered commensal, meaning they are non-harmful inhabitants existing in a balanced state with the host. The microbes primarily originate from the surrounding urogenital tract, likely entering the seminal fluid through retrograde flow from the urethra. Contributions from accessory glands, such as the prostate and epididymis, also create a mixture of bacteria from different anatomical sites.
Advanced gene sequencing studies have identified over 20 different bacterial genera in seminal fluid. Proteobacteria and Actinobacteria are often predominant in healthy samples, along with Bacteroidetes and Firmicutes. Microscopy often reveals bacterial cells in high numbers, sometimes exceeding the concentration of sperm.
Distinguishing Natural Flora from Pathogens
The natural flora must be distinguished from pathogenic bacteria, which cause disease or infection. Pathogens are typically introduced through sexual transmission or an ascending infection from the urinary tract. Sexually transmitted pathogens, such as Chlamydia trachomatis and Neisseria gonorrhoeae, can cause severe inflammation in the male reproductive organs.
Non-STI pathogens, such as Escherichia coli and Enterococcus faecalis, commonly originate from the intestines or urinary tract and can cause infections like bacterial prostatitis. The presence of commensal bacteria is normal, but the colonization or overgrowth of virulent pathogens represents an active infection. These infections often trigger a robust immune response, leading to the accumulation of white blood cells in the semen, a condition known as leukocytospermia.
Even the natural flora can become problematic in a state called dysbiosis, which is an imbalance in the microbial community. Dysbiosis involves a significant shift in the ratios of existing bacteria, such as a decrease in beneficial Lactobacillus species or an overgrowth of opportunistic organisms like Corynebacterium or Anaerococcus. This imbalance can create a pro-inflammatory environment that negatively affects sperm health.
How Bacteria Affect Fertility
An abnormal presence of bacteria, whether due to infection or dysbiosis, compromises male fertility through direct and indirect mechanisms. A significant effect is the induction of inflammation within the reproductive tract, which generates high levels of reactive oxygen species (ROS). This oxidative stress is damaging, leading to peroxidation of the sperm cell membrane and fragmentation of sperm DNA.
Bacteria can also directly interfere with sperm function by reducing motility, a condition called asthenozoospermia. Certain bacteria, such as E. coli and Corynebacterium, can physically attach to the surface of the sperm head or tail. This mechanically impedes the sperm’s ability to swim effectively and can lead to the physical clumping of sperm, known as agglutination, making fertilization impossible.
Bacterial toxins and enzymes released into the seminal fluid further contribute to sperm damage. Pathogens like E. coli produce lipopolysaccharide (LPS), a molecule that triggers cell death pathways in sperm, resulting in increased apoptosis and necrosis. Treating the underlying bacterial imbalance or infection is a necessary first step in improving semen parameters and addressing male factor infertility.