By Ike, Melanie A.c
A total of 82 samples of meat, surface swabs, and soil samples obtained from
retail outlets, butchers' tables and soil in the Nsukka area, were examined for the
presence of Listeria species using standard methods. Listeria species were isdated
from all samples examined except the pork and goat meat samples. The highest
frequency of occurrence (100%) was observed in the soil and in the surface swab
samples, followed by the vegetable samples (85%) and the beef samples (8090). Fish
samples gave the lowest frequency (40%). Heat resistance studies indicated that L.
monocytogenes could survive heating for 1 minute at 80°C, but not at 85°C. The
Listeria isolates were sensitive to Ampicillin, Gentamicin, and Sulfamethoxazole-
Trimethoprim, but were resistant to Penicillin (go%), Neomycin (go%), Streptomycin
(79%), Tetracycline (68%), and Erythromycin (53%). One isolate of L. monocytogenes
was resistant to Gentamicin but not to Ampicillin and Sulfamethoxazole-Trimethoprim.
All isolates of L. monocytogenes and L. ivanovii were pathogenic for mice, haemolytic
on sheep blood agar and lecithinase positive. L. seeligeri isolates were haemolytic but
not pathogenic to mice. Quantitation of haemolytic activity of L. monocytogenes and
L. ivanovii using washed human erythrocytes showed that L. ivanovii produced higher
titers (CHU=l : 16) than L. monocytogenes (CHU = 1 :4). Haemolysin production was
shown to be a function of pH, incubation time ,and cell density. At 30°C L.
monocytogenes lost it's viability after 14 days at pH 4.8 but only after seven days at
pH 4.4. At 5"C, the rate of death at the two pH levels was slower. Viability was still
detected after 63 days of incubation at pH 4.8 but at pH 4.4, viability was lost after 20
days. Histopathological examination revealed the liver as the major organ of attack by
L. monocytogenes, as evidenced by mild degeneration of the hepatocytes, scattered
foci characterized by the necrosis of the hepatocytes and dense infiltration of
mononudear cells. Accumulation of mononuclear cells in the connective tissues
around the bile ducts was equally evident. The heart showed mild pericarditis in the
region of the coronary groove while effects on the kidneys was characterized by frank
haemorrhage and mononuclear infiltration of the cortical interstitium