Abstract
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 (ΔOD420-580/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.
Highlights
Twenty species of Listeria have been described [1]
Investigations of growth kinetics and the ability to form biofilm were undertaken with strains from the American Type Culture Collection (ATCC) and the Spanish Type Culture Collection (STCC): L. monocytogenes ATCC 19111, L. monocytogenes STCC 936, L. monocytogenes
Typing of the 118 strains of L. monocytogenes obtained from red meat and poultry was carried out by the agglutination method with the Seiken Listeria antisera set (Denka Seiken Co., Tokyo, Japan), in accordance with the manufacturer’s instructions
Summary
Twenty species of Listeria have been described [1]. Of these, the most prominent is Listeria monocytogenes, the species responsible for most cases of human listeriosis, an infection with a high mortality rate, at between 20% and 40% [2]. Listeria monocytogenes is a ubiquitous Gram-positive zoonotic bacterium displaying remarkable resistance to a variety of adverse environmental conditions This microorganism tolerates high salinity (it can grow in the presence of 10% NaCl) [10] and strong acidity It should be noted that it can cope with a wide range of temperatures and still grow (from 1 ◦ C to 45 ◦ C, its optimum growth temperature is between 30 ◦ C and 37 ◦ C), and that it is a facultative anaerobe [10] These characteristics encourage the appearance and persistence of L. monocytogenes in food-processing environments, where the bacterium can remain for long periods of time [11,12]. Microbial growth parameters and ability to form biofilm was compared for different Listeria species and serotypes
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