Streptococci are non-motile, Gram-positive cocci with widely varying pathogenic potential that occur in pairs or chains. Streptococcus pneumoniae is the most common bacterial cause of acute respiratory infection and otitis media. The parental strain for R6 is R36A, itself a derivative of the virulent capsular type 2 clinical isolate D39; the lack of a polysaccharide capsule in R6 renders it avirulent and a safe strain with which to work. Comparison with D39 has shown there are 81 differences between D39 and R6, enough perhaps to indicate that not all physiological and metabolic properties studied in strain R6 will extrapolate to virulent strain D39. Additionally strain R6 does not carry the cryptic plasmid pDP1 (see PubMed 17041037).