Alcohol‐Mediated Hemolysis in Dairy Yeast Isolates and Hemolytic Activities on Blood Agar Media Containing Milk and Cheese

Abstract

Of a total of 143 presumptive yeast isolates obtained from Galician (north-west Spain) short-ripened raw cow's milk starter-free cheeses, 27 exhibited microbial alcohol-conferred hemolytic (MACH) activity. Among the MACH-positive isolates, eight were identified as Yarrowia lipolytica and four as Kluyveromyces lactis. These isolates of potential technological interest were subjected to spot agar assays on special blood agar media prepared with skim milk, whole milk and cheese. The hemolytic activities of the K. lactis isolates were generally more evident on skimmilk blood agar. However, the hemolytic activities of the Y. lipolytica isolates were strongest on whole-milk blood agar and cheese blood agar, and all of these isolates displayed total or predominantly beta-hemolysis reaction after 72-h incubation. Despite the fact that the MACH-positive yeast isolates assayed in the special media produce extracellular hydrolytic enzymes, particularly proteases and esterases, the hemolytic activity in these yeasts might be predisposed or triggered by environmental conditions which could be common in dairy products. PRACTICAL APPLICATIONSYeasts displaying alcohol-conferred hemolytic activity (MACH) were isolated from traditional raw-milk cheeses. Among these yeast isolates, some species of technological interest for the food industry, Yarrowia lipolytica and Kluyveromyces lactis, were identified. Despite the fact that the MACH-positive yeast produce proteases and esterases, these dairy yeasts were able to develop alcohol-induced hemolysis on standard media and as well as hemolysis in conditions designed to simulate dairy products. Currently, the specific virulence factors that differentiate pathogenic yeasts from innocuous ones are not conclusively known and little research has been made in relation to microbial hemolytic activities expressed both in the absence and in the presence of alcohol vapor. Hemolysis is generally associated with higher virulence in microorganisms, so new research might be needed to understand these hemolytic events predisposed/triggered by environmental growth conditions in relationship to food safety.