For centuries, people have produced mushrooms by solid-state fermentation, which results in the accumulation of a large amount of waste. As an alternative, the production of mushrooms by submerged liquid fermentation is used today because it is more environmentally friendly. The purpose of this study was to investigate the antimicrobial activity of medicinal mushrooms produced by submerged liquid fermentation towards selected pathogenic microorganisms. Two medicinal mushroom samples were used—Ganoderma lucidum from Asia, Malaysia (Kuala Lumpur) and Ganoderma lucidum from Europe, Serbia (Belgrade). The mycelium of both samples was dried to a constant mass and powdered. An initial concentration of the water solution of 80 mg/mL was prepared from the powder. Using the microdilution method and determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), antimicrobial activity was tested on Gram-positive bacteria (Enterococcus faecalis) and Gram-negative bacteria (Salmonella enterica) at sample concentrations of 40, 20, 10, 5, 2.5, and 1.25 mg/mL. The antibiotic gentamicin was used as a control. An inhibitory effect against E. faecalis was determined in both tested samples (MIC - 20 mg/mL). Despite the different conditions under which they were grown, the mycelia of both tested mushrooms show a similar antimicrobial potential, which could be due to the presence of ganoderic acid, a compound known for its bioactivity. S. enterica was more resistant to the extracts of the mycelia examined (MIC - 80 mg/mL). The difference in the structure of the cell wall of Gram-positive and Gram-negative bacteria directly affects their resistance to bioactive substances such as antibiotics or mushroom extracts.