2.4. Identification of Listeria species
From the PCOM and MOX plates, two colonies each were
selected based on morphological characteristics typical of Listeria,
and then streaked onto plates of trypticase soy agar supplemented
with 0.6% of yeast extract (TSAYE), incubating at 35 C for 24 h.
From each TSAYE plate, a single colony was transferred to a blood
agar base slant and incubated at 35 C for 24 h to obtain pure iso-
lates. These isolates were then stored at 5 C for further identifi-
cation. Before performing the biochemical identification of Listeria
species, isolates were reactivated onto TSAYE plates and incubated
at 35 C for 24 h and then were subjected to catalase test and Gram
stain. Gram-positive small cocobacilli isolates showing a cata-
laseepositive reaction were further tested for tumbling motility in
wet mount, umbrella-shaped growth in motility test medium
(Difco), H2S production, nitrate reduction, urease production, Voges
Proskauer reaction, and a-methyl-D-mannoside. Cultures
confirmed as members of the genus Listeria were then identified as
to species by performing the Christie, Atkins, Munich-Petersen
(CAMP) test, observation of b-hemolysis on sheep blood agar
(Difco tryptone blood agar base containing 5% sheep blood) and
ability to produce acid from glucose, mannitol, rhamnose, and
xylose fermentation as per recommended methods (Ryser and
Donnelly, 2001).