Characterization of Listeria monocytogenes Originating from the Spanish Meat-Processing Chain

Using agglutination techniques, 118 Listeria monocytogenes isolates from red meat and poultry were serotyped. Strains were ascribed to the serotypes 4b/4e (44.1% of the strains), 1/2 (a, b or c; 28.0%), 4c (6.8%), 4d/4e (5.9%) and 3 (a, b or c; 2.5%). Among these are the serotypes most frequently involved in cases of human listeriosis. The susceptibility of 72 strains to 26 antibiotics of clinical importance was determined by disc diffusion (Clinical and Laboratory Standards Institute; CLSI). High levels of resistance were observed to cefoxitin (77.8% of the strains showed resistance), cefotaxime (62.5%), cefepime (73.6%), nalidixic acid (97.2%), nitrofurantoin (51.4%) and oxacillin (93.1%). Less than 3% of the strains showed resistance to the antibiotic classes used in human listeriosis therapy (i.e., ampicillin, gentamicin, rifampicin, chloramphenicol, enrofloxacin, vancomycin, trimethoprim-sulfamethoxazole, erythromycin, and tetracycline). The influence of species and serotype on the growth kinetics (modified Gompertz equation) and on the adhesion ability (crystal violet staining) of nine isolates of L. monocytogenes (serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, and 4d), and one strain of Listeria ivanovii were investigated. The maximum growth rate (ΔOD(420)-(580)/h) varied between 0.073 ± 0.018 (L. monocytogenes 1/2a) and 0.396 ± 0.026 (L. monocytogenes 4b). The isolates of L. monocytogenes belonging to serotypes 3a and 4a, as well as L. ivanovii, showed a greater (p < 0.05) biofilm-forming ability than did the remaining strains, including those that belong to the serotypes commonly implied in human listeriosis (1/2a, 1/2b, 1/2c and 4b). The need for training in good hygiene practices during the handling of meat and poultry is highlighted to reduce the risk of human listeriosis.