Star Anise (Illicium verum Hook. f.) as Quorum Sensing and Biofilm Formation Inhibitor on Foodborne Bacteria: Study in Milk

What is it about?

Bacteria use quorum sensing (QS) systems to communicate with each other and regulate microbial group behavior, such as the secretion of virulence factors, including biofilm formation. In order to explore safe, edible agents, the potential of star anise (SA) as an anti-QS and antibiofilm agent and its possible application in milk safety were investigated. Staphylococcus aureus, Salmonella Typhimurium, Pseudomonas aeruginosa, and biosensor strain Chromobacterium violaceum were selected as test strains for QS, biofilm, and exopolysaccharide assays. The percent acidities and total plate counts were determined to evaluate the quality of biofilm-inoculated and noninoculated milk. The yield of SA extraction was 25.90% ± 0.2% (w/w). At sub-MIC, SA extract did not show any effect on bacterial growth. The production of violacein was inhibited by 89% by SA extract. The extract also inhibited the formation of biofilm by up to 87% in a dose-dependent manner. Inhibition rates of 70.45%, 42.82%, and 35.66% were found for exopolysaccharide production. The swarming motility of S. aureus was reduced by about 95.9% by SA extract. Confocal laser scanning microscopy analysis confirmed that the development of biofilm architecture was hampered. It was found that SA extract could delay the spoilage of milk. In the endeavor to avoid drug resistance, pathogenesis, and resistance to biocides while improving food safety and avoiding health hazard issues arising from synthetic chemicals, SA extract could be used as a potential QS and biofilm inhibitor.

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Why is it important?

Many bacterial species regulate gene expression in response to changes in cell population density by the production, release, and subsequent detection of chemical signaling molecules called autoinducers through a mechanism called quorum sensing (QS) (3, 10, 35). By using QS mechanisms, bacteria work like multicellular organisms and express different virulence phenotypes and factors, such as biofilm formation, bioluminescence, sporulation, pathogenesis, etc. (28). Biofilms are three-dimensional, architecturally complex structures that protect microbes from hostile environmental factors and generally consist of exopolymeric substances and glycoprotein matrix. It is now known from many lines of evidence that QS coordinates the transition to a biofilm lifestyle when the population density reaches a threshold level (23, 30). Biofilm-forming pathogens frequently colonize certain types of fish, dairy, poultry, meat, and ready-to-eat foods and food packaging materials and even all types of equipment surfaces in brewing and food processing plants (4, 26, 31). Microorganisms (foodborne pathogens in biofilm matrix) produce saccharolytic, proteolytic, lipolytic, and pectinolytic enzymes whose metabolic products are associated with microbial spoilage of food (4). Therefore, the QS-regulated formation of biofilm is a safety problem in food industries that needs to be solved. A large number of anti-QS and antibiofilm agents are available on the market, but most of them are synthetic. According to the regulations for producing and labeling natural food products set forth by the U.S. Department of Agriculture, the application of artificial flavorings and chemical and synthetic preservatives in foods labeled as “natural” is prohibited in the United States (33). At present, all over the world, consumers are becoming more concerned about the issue of carcinogenicity. Considering the harmful effects of many synthetic compounds on human health, it is necessary to seek out QS and biofilm inhibitors from natural sources as natural/green products. Many of these green products are generally recognized as safe (GRAS) (5, 19). Star anise (SA) (Illicium verum Hook. f.) has been widely applied as an ingredient of the traditional five-spice powder of Chinese cooking, as a flavoring agent, and as a medicine for well over 3,000 years. Few reports have been published characterizing SA and its efficacy as an antifungal, antimycotoxigenic, antioxidant, and antidiarrheal agent. It is a major industrial source of shikimic acid, a primary ingredient used to prepare the anti-influenza virus drug Tamiflu, which mitigates the severity of the influenza virus (1, 12, 15). However, the development of novel therapy and food preservation strategies based on the anti-QS and antibiofilm properties of crude extract of SA has not been undertaken. The main objective of this study was to evaluate the anti-QS and antibiofilm efficacies of SA extract. Then, SA extract was applied in a food matrix to evaluate its suitability for ensuring a balance between food safety and quality concerns.

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