Microbiota in Fish Production Facilities and Impact on Growth and Biofilm Formation of Listeria monocytogenes

Introduction: Listeria monocytogenes remains a problem for the food production industry, especially those producing Ready-To-Eat food. The bacterium may establish itself in certain niches in food production facilities and some clones may persist for years in the same plant. Thorough cleaning is a recommended measure, but it is sometimes speculated that removing the natural competing bacterial background flora increases the problem.

Purpose: Determine the impact of the bacteria representing the background microbiota in salmon production plants on biofilm production of L. monocytogenes.

Methods: Bacteria were collected from typical harborage sites for L. monocytogenes after regular cleaning and disinfection in four salmon production plants. The temperature in the production facilities was measured during one week. L. monocytogenes and a non-monocytogenes Listeria sp. (isolated together with L. monocytogenes) was attached and grown as biofilm on steel coupons at 12°C and 20°C with a thin layer of salmon broth (“listeriaculture”). The same experiment was also done adding a mixture of ten different species of bacteria commonly found in the production environment together with listeria (“multiculture”). The experiment was repeated three times. In one occasion, the composition of the microbial flora was determined by 454 pyrosequensing.

Results: The listeriaculture biofilm multiplied through the incubation period reaching about 10⁹ CFU/cm³ after nine days. The Listeria sp. outcompeted L. monocytogenes at 20°C (P = 0.011). A steady biofilm of about 10⁹ CFU/cm³ was obtained after two days for the multiculture. The background microbiota suppressed L. monocytogenes (P = 0.001) and it only represented 0.1 - 0.01% of the total biofilm population. The multispecies biofilms were dominated by Pseudomonas sp. followed by Flavobacterium sp., Chryseobacterium sp., and Acinetobacter sp.

Significance: Listeria monocytogenes may grow and form biofilm at conditions simulating salmon production facilities and the natural background microbiota may inhibit but not eliminate it in biofilms. Proper cleaning and disinfection is recommended to combat L. monocytogenes in the food production environment.