Male factor is attributable in up to 50% of cases of infertility. In-vitro studies demonstrate that bacteria can negatively impact sperm function. The use of next-generation sequencing (NGS) techniques have provided a better understanding of the human microbiome, and dysbiosis has been reported to impact health. Evidence regarding the impact of the semen microbiome on sperm function and fertility remains conflicting.
A systematic search was conducted in accordance with the Preferred Reporting Items for Reviews and Meta-analysis (PRISMA) statement. The databases MEDLINE, OVID and PubMed were searched to identify English language studies related to the identification of bacteria in the semen of infertile and fertile men, between 1992-2019. 55 observational studies were included, with 51299 subjects. We included studies identifying bacteria using NGS, culture or polymerase chain reaction (PCR).
The semen microbiome (SM) was rich and diverse in both fertile and infertile men. Three NGS studies reported clustering of the seminal microbiome with a predominant species. Lactobacillus and Prevotella were dominant in respective clusters. Lactobacillus was associated with improvements in semen parameters. Prevotella appeared to exert a negative effect on sperm quality. Bacteriospermia negatively impacted sperm concentration and progressive motility (PM), and DNA fragmentation index (DFI) (MD 3.518,95%CI 0.907 to 6.129, p=0.008). There was an increased prevalence of Ureaplasma urealyticum(UU) in infertile men(OR 2.25, 95% CI 1.47-3.46). UU negatively impacted concentration and morphology. There was no difference in the prevalence of chlamydia trachomatis (CT) between fertile and infertile men and no significant impact on semen parameters. Enterococcus faecalis (EF) negatively impacted total motility and Mycoplasma hominis (MH) negatively impacted concentration, PM and morphology.

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