Basis of differentiation 

R strain 

S strain

Definition 

Are a type of bacteria that have rough surfaces and well-defined edges

Are a type of bacteria that have smooth rounded surfaces due to its polysaccharide outer surface

Presence of polysaccharide protective coating 

Absent 

Present – in addition to its protective function, it gives S bacteria their distinctive smooth appearance

Virulence 

Is a nonvirulent strain that does not cause death when injected into a mouse

Is a virulent strain capable of causing death in mice – mice injected with S bacteria developed pneumonia and died