In this study, 512 milk samples from different sources were
examined .The incidence of Listeria among the different ruminant species
revealed that the percentage of Listeria isolation reached 6.1%. The
isolated Listeria spp. were distributed as 9 strains from cows, 8 strains
from buffaloes, 6 strains from goats, 8 strains from ewes and 4 strains
from she –camels raw milk. The Listeria spp were distributed as 7 strains
of L. monocytogenes, 4 strains L. innocua, 8 strains L. welshimeri, 7
strains L. seelegeri and 5 strains L. grayi in a percentage of 1.4%, 0.8%, 1.6%, 1.4% and 0.98%, respectively.
The results of virulence study revealed that all of the tested Listeria isolates were strong positive for Congo red after 24 hrs. The Listeria monocytogenes isolates induced severe vaculation and detachment in vero cells but the other Listeria spp. did not induce any change. Virulence of the Listeria isolates was also investigated by i/p injection to mice, intraocular distillation in rabbits and intraallantoic inoculation in ECG. Death occurred to mice and chick embryos and eye inflammation in rabbits within 24h after injection in an incidence of 100% (toxigenic). On the other hand the suspected isolates of L. innocua, L. seelegeri, L. welshimeri and L. grayi showed negative results (non toxigenic). The results of biofilm formation showed that the L. monocytogenes from goat and she-camel show high degree to form biofilm to plastic surface than other strains although L. monocytogenes type 4 has a strong degree to form biofilm with glass surfaces than type 1 while L. innocua, L. grayi and L. welshemeri showed a lesser degree to form biofilm to plastics surface. L. seelegeri from cow milk and goat milk showed high degree to form biofilm to plastic surfaces. L. innocua from cow milk and buffaloes milk has strong degree to form biofilm with glass surface while other Listeria spp (L. welshemeri and L. grayi) ranged from moderate to weak.
The antibiogram for Listeria isolates revealed that all isolates showed resistance for the following antibiotics: cloxacillin, oxacillin, pefloxacin, flumoquine, cephalosporin, bacitracin, lincomycin, clindamycin.
It was obvious that the Listeria isolates showed a significant effect of Listeria on the level of nitric oxide and the effect on the lysozyme level was also significant except in buffalo milk.
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