Fennel (Foeniculum vulgare) is an edible spice with edible value and is cultivated in both tropical and temperate regions. It is a traditional spice with important economic value and extensive medical application value. The purpose of the research: Antibacterial tests were conducted on four types of opportunistic bacteria in the gastrointestinal tract to determine their biochemical indicators. Fennel oil was used as raw material in this study. The object of study were opportunistic microorganisms of the gastrointestinal tract Escherichia coli, Proteus mirabilis, Staphylococcus aureus, Bacillus cereus. The study groups were fennel essential oil produced by Botanika LLC, Russian Federation (FEN-B) and IP Saules Sapnis, Republic of Belarus (FEN-SS). Collection microorganisms of the same species were used as a control. Research results have shown that fennel essential oil (Foeniculum vulgare) inhibits the growth of opportunistic microflora. E. coli, B. cereus and P. mirabilis exhibited proteolytic and amylolytic activities. In S. aureus cultured cells, amylolytic enzymes were not present.