an epidemiological study of listeriosis in dairy cattle

L. monocytogenes was the only recognised species within the genus Listeria
until 1961 when Listeria denitrificans was included in the genus. Listeria grayi was
included in 1966 and Listeria murrayi in 1971 (Roucort and others 1982). In their
taxonomic investigation of L. monocytogenes Seeliger and Hohne (1979) divided it
into 16 different serogroups (Table 1. 2.). All serotype 5 strains were strongly β-
haemolytic and in 1975 Ivanov, a Bulgarian scientist proposed a separate species,
Listeria bulgarica. L. bulgarica was later named L. ivanovii by Seeliger and colleques
(1984) after Ivanov. Two years later, non haemolytic and non pathogen serotype 6
strains were proposed as a new species, Listeria innocua by Seeliger (1981). Listeria
welshimeri and Listeria seeligeri were included in the genus in 1983 (Rocourt and
Grimont 1983).
The original taxonomic classification of Listeria was based on biochemical and
physical characteristics of the organism. The species are biochemically very similar to
each other but differ in DNA sequences. Recent developments in DNA technology have
extended our knowledge of the bacteria at molecular level. DNA-DNA hybridisation of
Listeria has confirmed that Listeria monocytogenes, innocua, welshimeri, seeligeri and
ivanovii are closely related to each other but still differ at the DNA sequence level
(Rocourt and others 1982, Seeliger and Jones 1986, Farber and Peterkins 1991). The
classification of L. denitrificans , L. murrayi and L. grayi became problematic after the
study of the numerical taxonomic, DNA base composition and DNA-DNA
hybridisation studies (Stuart and Welshimer 1973 and 1974, Rocourt and others 1982).
These studies concluded that these three species were distinct from L. monocytogenes.
However Fiedler and Seger (1983) examined the amino acid sequences of the cell wall
of these species and concluded that the murein variation found in L. grayi and L.
murrayi was also found in L. monocytogenes. The murein content of L. denitrificans
was significantly different from those of other Listeria species which led to exclusion of
6