Author(s):
Neves, Elsa 
; Silva, Ana Carla ; Roche, Sylvie M. ; Velge, Philippe ; Brito, Luisa
Date: 2008
Persistent ID: http://hdl.handle.net/10400.5/5517
Origin: Repositório da UTL
Subject(s): Listeria monocytogenes; cheese; food; virulence
Description
The virulence potential of 51 Listeria monocytogenes isolates, including strains from cheese,
cheese production environments and from human cases of listeriosis, was evaluated in this study.
The isolates were used to infect HT-29 cell monolayers in an in vitro test of virulence, based on a
plaque-forming assay (PFA). Fifteen selected isolates were used for subcutaneous footpad
inoculation in mice and subsequent recovery of the bacterium from the spleen 3 days after
inoculation. In the PFA, two isolates from milk (serovar 1/2a) were not significantly different
(P,0.05) from the low-virulence strain (442) used as reference. Thirty-three isolates were not
significantly different (P,0.05) from the virulent strain (EGDe) used as reference. Nine isolates
were significantly more virulent (highly virulent) than the EGDe strain and seven isolates were
significantly less virulent. The nine highly virulent isolates were either from humans (four), from
cheese dairy environments (two isolates of a strain were found persistently in two dairies), from
cheese (one), from milk (one) and the reference strain for serovar 1/2b (CECT 936). The two milk
isolates with low virulence in the PFA were found to be virulent in mice. In conclusion, all the
isolates from food and food-related environments were potentially virulent or highly virulent. These
results stress the risk of listeriosis associated with the consumption of cheese contaminated with
L. monocytogenes, and once more emphasize the importance of good manufacturing practices
(GMPs) together with sanitation standard operating procedures (SSOPs) throughout the food
chain.
