for example) are often predisposing factors. Although not a common
infection in AIDS patients, its incidence is around 300 times that in the
general population. Other conditions such as alcoholism, diabetes and
cirrhosis can act as predisposing factors, but illness does occur in
otherwise healthy individuals who account for about 18% of adult cases
in England and Wales.
Adult listeriosis has a high mortality rate, figures calculated using data
for 1989 gave values around the world of between 13 and 34%. Early
treatment with antibiotics, normally ampicillin, with or without an
aminoglycoside, or chloramphenicol, is essential but in the most severe
forms, the prognosis remains poor.
L. monocytogenes is a facultative intracellular pathogen which like
Mycobacterium, Brucella,and others can survive and multiply in cells of
the monocyte–macrophage system. The organism penetrates the gut
either by crossing the Peyer’s patches or by invading enterocytes. This
process of endocytosis is promoted by a number of virulence factors
including internalin, an 800 amino acid bacterial surface protein encoded
by a chromosomal geneinlA, other products of the so-calledinlfamily of
genes, and p60, a 60 kDa extracellular protein. Internalization results in
the bacteria being enclosed in a phagosome. In order to multiply
intracellularly the organism must survive in the phagosome and escape
rapidly before it fuses with a lysosome, a process which would kill the
bacteria. PathogenicL. monocytogenesproduce listeriolysin O (LLO) a
58 kDa haemolysin which breaks down the lipid bilayer of the phagoso-
mal membrane allowing the bacteria to escape from the phagosome.
MostL. monocytogenes infecting epithelial-type cells do this and are
released into the cytoplasm. Only 10% of those invading monocytes are
successful, the remaining 90% being destroyed.
During intracellullar replication, actin polymerizes around the bacterial
surface, a process which propels the bacterial cell around the host cell and
into adjacent cells, thereby spreading the organism while avoiding the
host’s immune system. The bacterial cells reach the mesenteric lymph
nodes and are disseminated around the bodyviathe blood. The liver plays
an important role in eliminating the organism and controlling the infec-
tion. Infection of hepatocytes causes an intense inflammatory reaction.
Polymorphonuclear cells (neutrophils) destroy infected hepatocytes forcing
them to release the bacteria which are in their turn destroyed. Where
infection is not controlled in the liver, it can be further disseminated by the
blood to the central nervous system or placenta causing more severe illness.
7.9.4 Isolation and Identification
Low-temperature enrichment at 4 1 C is the traditional technique for
isolating L. monocytogenes from environmental samples, but the
228 Bacterial Agents of Foodborne Illness