Characterisation of Listeria monocytogenes strains isolated from soft and semi-soft cheeses sampled in a region of Italy

Abstract

A total of 47 Listeria monocytogenes strains isolated in a survey of cheeses sampled from retail outlets were characterised. Five cheeses (Gorgonzola, Taleggio, Asiago, Crescenza and Brie) were chosen from the most popular soft and semi-soft cheeses consumed in Italy and most commonly contaminated with L. monocytogenes. The serotype and antibiotic resistance pattern were determined for each strain and their macrorestriction profile was analysed with pulsed-field gel electrophoresis (PFGE). The main serotypes detected were 1/2a (76.6%) and 1/2c (21.3%). Serotype 1/2b was found in only one sample. A total of 97.9% of strains were resistant to oxacillin (OX), 80.9% to lincomycin (L) and 78.7% to clindamycin (CC). Of these strains, 17% were found to be resistant to two antibiotics (OX-CC or OX-L) while 70.2% were resistant to three antibiotics (OX-CC-L). No strains were susceptible to all the compounds tested. A combined analysis of the macrorestriction profiles AscI and ApaI identified eleven pulsotypes divided into three clusters. Two pulsotypes predominated, accounting for 57.4% and 21.3% of the isolated strains. Analysis of the PFGE profiles did not reveal any correlation between pulsotype and type of cheese, producer or retail outlet. A temporal analysis revealed that one pulsotype was persistent throughout the study period, with the exception of August and September, in which time a different pulsotype was detected. This variability suggests the influence of factors affecting the dynamics of the contamination of these products. Large-scale studies could help clarify this phenomenon.