The chapter deals with both types of bacteria, i.e. Gram positive and Gram negative with emphasis o n the organisms that cause post operative endophthalmitis or severe keratitis, the organisms that have profound systemic involvement, i.e. tuberculosis, leprosy, syphilis too have been delta in detail with their ocular ramification. Atypical mycobacteria that are becoming more common infection have been given due importance like their classification, methods of growth, Propioni bacterium acnes and B. cerus which were not accorded sufficient importance in the past have been given sufficient space as per their preset role in ophthalmology. Various types of chlamydiae which were not considered as bacilli have been discussed at length with special reference to trachoma.