-------------------------------------------------------------------------------------------------------------------------------------------------------------------Generally the early Instars of silkworm larvae (I,II,III Instars) were more susceptible to bacterial infection than the IV and V Instar stages. Studies on bacterial pathogenicity were conducted on IV Instar mulberry silkworm Bombyx mori. Assessment of pathogenicity and identification of bacterium of E.coli were isolated from flacherie infected silkworm larvae. It is most common and deadly diseases in seri field. The present study showed that LD50 dose of E.coli infected to IV Instar larvae was 2.4x106 cells/ml and determined the LD50 value. When the inoculated worms were treated with the probionts Bifidobacterium bifidum and yeast at different concentrations. Bifidobacterium bifidum showed significant enhancement in larval weight, cocoon weight, shell weight, shell ratio, filament length, filament width, Denier and fibroin content. Bifidobacterium bifidum with a concentration of 6% was very effective. Mulberry leaves treated with Bifidobacterium bifidum (6%) fed larvae recorded a maximum shell ratio (11.87) filament length (956.33±47.18) Denier (13.3%) and fibroin content (0.29±0.03) Shell ratio in this treatment (6%) showed an increase of 11.87% whereas yeast treatment (6%) also increase of 13.8% and over the control, Various growth parameters like relative consumption rate (RCR) relative growth rate (RGR), weight gain, approximate digestibility (AD), efficiency of conversion of digested food (ECD) and efficiency of conversion of ingested food (ECI) were enhanced by the Amway supplementation. Determined the quantity of protein in Bifidobacterium bifidum (6%) 67.5%. whereas the yeast treatment (6%) had 28.57% over the control. Feed supplementation not only enhanced economic and nutritional parameters but also prevent bacterial infection in B.mori. --------------------------------------------------------------------------------------------------------------------------------------------------------------------