This study was conducted on120 one day old broiler chicks which were divided into six groups, 20 birds each. Group 1 (control), group 2 (supplemented with probiotic), group 3 (challenged with Salmonella and receive no probiotic), group 4 (challenged with E coli and receive no probiotic), group 5 (challenged with Salmonella and supplemented with probiotic), group 6 (challenged with E coli and supplemented with probiotic). The experiment extended for 30 days starting from one-day-old chicks. Body weights, clinical symptoms, haematological analysis and postmortem lesions were demonstrated on 8^th, 15^th and 30^th day of the experiment. Also, histopathological studies of the intestinal mucosa, liver, spleen, thymus and bursa of Fabricius, as well as immunostaining of surface antigens (CD3A in the thymus and CD79A in the spleen and bursae of Fabricius), were also investigated.  The current study revealed that supplementation of probiotic alone obviously improved weight gains as compared to the control group.
Furthermore, probiotic supplementation decreased the colony forming a unit (CFU) of Salmonella enteritidis and E. coli (strain O2: H45) in the intestinal mucosa. Histopathologically, the intestinal mucosa showed an improvement which indicated by hyperplasia of the lining epithelium and abundance of goblet cells, but this local effect did not extend to other organs in the body that demonstrated mild to severe histopathological changes in challenged groups. The haematological analysis also verified that treatment with probiotics had no significant effect on most blood values (RBCs, WBCs and Hb). However, the differential leucocytic counts were significantly influenced by dietary treatment with probiotics which caused a highly significant decrease in lymphocyte percentage. In conclusion, probiotics obviously improved the growth performance and local immune response in the intestine, however no clear evidence of improvement of the general immune status of the experimental birds.