Biofilm formation by Listeria monocytogenes. Resistance to industrial biocides and crossresponse caused by adaptation to benzalkonium chloride.

Resumen

Listeria monocytogenes is one of the most important food-borne pathogens, since it has a high incidence in different kinds of food products, including fish products, and can cause severe illness and even death in susceptible individuals. In general terms, this work is therefore justified by the need of reducing the risk of the presence of L. monocytogenes in food. Within this global objective three main reasons support the appropriateness of the specific objectives raised in this Ph.D. thesis: 1. That the formation of biofilms is one of the principal strategy for L. monocytogenes to persist in food processing plants. Persistence implies a high risk of cross-contamination of food by biotransfer (contact, aerosols). 2. That the persistence of L. monocytogenes biofilms in the food industry may be related to the resistance to external physical or chemical stimuli, including biocides. 3. That the transfer of biocide-resistant cells from biofilms to food can imply cross-responses to preservation techniques. Additionally, experimental systems were developed with the aim of simulating realistic industrial conditions, which is not taken into consideration in many previous studies. Biofilms were thus formed under experimental conditions simulating conditions found in mussel processing plants. The use of mussels and mussel soils as experimental systems is justified by the fact that Spain, particularly Galicia, is the number one mussel-producing European country. In second term, single and dual species mature biofilms were used in this work to assess the biocide activity of widely-used disinfectants as well as the potential of further biocides as new disinfectants, whereas planktonic cells and young biofilms are commonly used as experimental systems in previous works. Lastly, the structural complexity of biofilms was measured in terms of the resistance to biocides instead of the number of cells. Taking this into account, the main objectives of this work provided the following articles: 1. Effects of mussel processing soils on the adherence of Listeria monocytogenes to polypropylene and stainless steel. P. Saá, M. L. Cabo M. and J. Herrera. Reference: J. Food Prot 72(9), 1885-90 (2009). 2. An efficient method to obtain benzalkonium chloride-adapted cells of Listeria monocytogenes CECT 5873. P. Saá Ibusquiza, J. R. Herrera, D. Vázquez-Sánchez and M. L. Cabo. Reference in preparation. 3. Resistance to benzalkonium chloride, peracetic acid and nisin during formation of mature biofilms by Listeria monocytogenes. P. Saá Ibusquiza, J. R. Herrera and M. L. Cabo. Food Microbiol. doi:10.1016/j.fm.2010.09.014 4. Adherence kinetics, resistance to benzalkonium chloride and microscopic analysis of mixed biofilms formed by Listeria monocytogenes and Pseudomonas putida. P. Saá Ibusquiza, J. R. Herrera, D. Vázquez-Sánchez and M. L. Cabo. Reference submitted to Food Control. 5. Comparison between the resistance of benzalkonium chloride-adapted and non-adapted biofilms of Listeria monocytogenes to modified atmosphere packaging (MAP) and nisin once transferred to mussels. P. Saá Ibusquiza, J. R. Herrera and M. L. Cabo. Accepted in J. Food Prot.