Seminar - Prevalence and Diversity of Leuconostoc in Raw Milk

Leuconostoc spp. are heterofermentative, gram-positive lactic acid bacteria generally recognized as safe and nonpathogenic. They are often associated with raw milk and can contribute to spoilage, particularly in sweetened-flavored milks, manifesting as ropy-slimy products with lowered pH. Limited information is available on the prevalence and diversity of Leuconostoc in raw milk. This study examines the prevalence and diversity of Leuconostoc in raw milk silo samples from a Mid-Atlantic dairy processor for 12 months. Milk composition and microbial quality were analyzed. Leuconostoc were isolated and identified by 16S rRNA gene sequencing and further assigned to species and genotype groups. Representative isolates were characterized in terms of carbohydrate utilization, exopolysaccharide (EPS) production, growth at different temperatures, and the ability to survive laboratory pasteurization. Estimated Leuconostoc counts from raw milk range from 20 to 417 cfu/mL with an average of 91 cfu/mL throughout the year. A total of 892 Leuconostoc isolates were identified and isolated from 111 raw milk samples. Six Leuconostoc species were isolated: L. lactis (42.38%), L. mesenteroides (37.33%), L. citreum (11.10%), L. falkenbergense (8.30%), L. pseudomesenteroides (0.56%), and L. holzapfelii (0.34%). These isolates were further classified into 14 different genotypes. Representative isolates from each genotype were further characterized. All representative isolates fermented glucose, N-acetylglucosamine, and sucrose. Mucoid substances (EPS) were produced by all isolates of L. citreum, some L. mesenteroides isolates, and most of L. falkenbergense isolates on sucrose (5% w/v) agar. All representative isolates grew well in unflavored milk incubated at 10°C and 20°C. Only L. lactis and L. mesenteroides representative isolates grew in milk after 15 days of incubation at 4°C. Two L. lactis isolates were able to survive laboratory pasteurization in unflavored milk. This study provides baseline information about the prevalence and diversity of Leuconostoc in raw milk. The identification of six different species and fourteen genotypes highlight the diversity of this genus in raw milk. The ability of certain isolates to grow at refrigeration temperatures and survive pasteurization raises concerns about their persistence during processing and storage, potentially leading to quality defects in fluid milk and in manufactured products.

Join this virtual seminar on Zoom