Reaction shown in the illustration is seen for:

Ans. C. Pneumococcusa. The illustration shows Quellung reaction. This method involves testing a pneumococcal cell suspension with pooled and specific antisera directed against the capsular polysaccharide. The antigen-antibody reactions are observed microscopically.b. A positive Quellung reaction is the result of the binding of the capsular polysaccharide of pneumococci with type specific antibody contained in the typing antiserum.The protocol has three main steps: -a. Preparation of a bacterial cell suspension,b. Mixing of cells and antisera on a glass slide, andNote: -a. It is recommended to initially test with pooled antisera in succession until a positive reaction is observed.b. Typing should then proceed by testing with individual group and serotype-specific antisera included in the antisera pool that gave a positive reaction to determine the serogroup and serotype.c. Reading the Quellung reaction using a microscope.Principle: -a. An antigen-antibody reaction causes a change in the refractive index of the capsule so that it appears "swollen" and more visible. After the addition of a counter stain (methylene blue), the pneumococcal cells stain dark blue and are surrounded by a sharply demarcated halo which represents the outer edge of the capsule.b. The light transmitted through the capsule appears brighter than either the pneumococcal cell or the background. Single cells, pairs, chains, and even clumps of cells may have positive Quellung reactions.