Does the semen microbiome affect male fertility?

This study investigated the effect of the semen microbiome (SM) in male infertility (MI). They found that the SMB had negative effects on semen parameters.

Male infertility (MI) is a common cause of infertility in couples. It accounts for 30-50% of infertility cases. Understanding factors that reduce fertility is important. This could improve the treatment of MI. MI is commonly diagnosed by measuring semen parameters. These include sperm count (SC) and motility (SM; movement). Semen is made up of sperm and seminal fluid. 

It also has its own microbiome (MB). The MB is a collection of tiny bacteria, fungi, and viruses. They can only be detected using sensitive techniques. Next-generation sequencing (NGS) detects bacteria in the MB. Studies show the MB is important in body function. Each area of the body has its own MB. A lot of research has focused on the gut MB. There is some interest in the semen MB (SMB). The SMB could have an impact on fertility. It is unclear if the SMB affects MI. 

This study included 24 clinical reports and 29358 men. The SMB was investigated in each study. The authors pulled all the information from these studies and analyzed the combined results. The main outcomes were any differences in the SMB. This was compared to sperm parameters including sperm concentration, SC, SM, and DNA fragmentation (also leads to MI). 

Levels of Ureaplasma urealyticum (UU) bacteria were 2.25-times higher in infertile men. UU was associated with reduced concentration and abnormally shaped sperm. Mycoplasma genitalium (MG) was associated with a reduced sperm concentration. Enterococcus faecalis (EF) bacteria was associated with reduced SM.

Lactobacillus was associated with improvements in sperm quality, while Prevotella had a negative effect on sperm quality. Overall, bacteria in the sperm was associated with higher DNA fragmentation. High DNA fragmentation in the sperm leads to MI.

The authors concluded that SMB had negative effects on semen parameters.

There were a lot of differences in these studies. Different methods and protocols were used. The quality of the evidence was considered to be low. More studies on the SMB are needed.