The invention relates to methods of modulating capsular polysaccharideproduction in pneumococci such as Streptococcus pneumoniae. The inventionfurther relates to methods of alleviating pneumococcal infections in animals,and to methods of identifying both agents capable of modulating pneumococcalinfections in animals. Figure (1) comprising figures 1Ai-1Avi and 1Bi-1Bvi isa series of images indicating the effect of environmental oxygen and carbondioxide concentration on colony morphology and capsule size, as assessed usingthe Quellung reaction.