Brevibacterium linens

Brevibacterium linens is a Gram-positive, non-spore-forming bacterium renowned for its pivotal role in the ripening and flavor development of surface-ripened cheese. This coryneform organism, originally isolated from the rind of Limburger cheese, is characterized by its distinctive orange-red pigmentation and its ability to thrive in high-salt, alkaline environments. Its metabolic activities are essential for producing the characteristic pungent aromas and flavors of washed-rind cheese varieties, making it a cornerstone of traditional dairy microbiology.

Description and characteristics

Brevibacterium linens is a rod-shaped bacterium that exhibits a characteristic coryneform morphology, often appearing as irregular rods or club-shaped cells. Cells are typically non-motile and stain Gram-positive, possessing a cell wall rich in meso-diaminopimelic acid. A defining feature is its production of carotenoid pigments, which impart a distinctive orange to reddish-brown hue to colonies and the cheese rinds it colonizes. The organism is halotolerant, capable of growth in media containing up to 15% sodium chloride, and is alkaliphilic, preferring a pH range of 6.5 to 9.5. It is chemoorganotrophic and strictly aerobic, requiring oxygen for growth. Key diagnostic tests include positive reactions for catalase and the hydrolysis of casein, which contributes to its proteolytic activity in cheese.

Habitat and ecology

Naturally, Brevibacterium linens is found in saline environments such as marine sediment and brine, but its most significant ecological niche is the surface of matured cheese. It is a ubiquitous component of the microbial community in dairy plant environments, particularly those producing smear-ripened cheese. The bacterium is often introduced onto cheese curd from the environment, equipment, or brining solutions. It forms part of a complex surface microbiota that can include other bacteria like Staphylococcus species and yeasts such as Debaryomyces hansenii. These microorganisms create a synergistic ecosystem where yeast metabolites help establish the alkaline, aerobic conditions favorable for its proliferation.

Role in cheese production

In the cheese making process, Brevibacterium linens is the principal agent in developing the flavor, aroma, and appearance of washed-rind cheese and smear-ripened cheese. Notable varieties reliant on its activity include Limburger cheese, Munster cheese, Époisses de Bourgogne, and Port-du-Salut. During ripening, the surface of these cheeses is regularly washed with a brine solution, promoting the growth of a bacterial smear. The bacterium secretes potent extracellular proteases and lipases that break down casein and milk fat, respectively. This proteolysis and lipolysis generates a suite of volatile compounds, including ammonia, methanethiol, and various sulfur compounds, responsible for the cheese's strong, pungent smell and savory, umami taste.

Genomics and metabolism

The complete genome sequence of Brevibacterium linens strain ATCC 9172 has been elucidated, revealing a single circular chromosome of approximately 4.3 megabase pairs. Genomic analysis highlights an extensive repertoire of genes encoding proteases, peptidases, and amino acid catabolic enzymes, underpinning its strong proteolytic capacity. The genome also contains pathways for the synthesis of carotenoid pigments like lycopene and for the metabolism of sulfur-containing amino acids such as methionine, leading to volatile sulfur compound production. Its metabolic flexibility allows it to utilize various amino acids, lactic acid, and citric acid as carbon sources, which are abundant in the cheese matrix.

Industrial and biotechnological applications

Beyond traditional cheese ripening, Brevibacterium linens has garnered interest for various biotechnological applications. Its potent proteases are studied for use in detergent formulations and leather processing. The bacterium's ability to produce glutaminase and other flavor enhancers is relevant to the food industry for creating savory taste compounds. Furthermore, its capacity to degrade heterocyclic compounds and other pollutants positions it as a candidate for bioremediation projects. Research into its carotenoid biosynthesis pathways also explores potential in producing natural food coloring agents and antioxidants for the nutraceutical sector.

Category:Actinobacteria Category:Cheese microbes Category:Industrial microbiology