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Streptococcus pneumoniae R6 is a pathogenic bacterial strain but it is avirulent. How can a pathogen be avirulent. What does it mean if a pathogenic bacterium is avirulent?

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Streptococcus pneumoniae is a pathogenic species. Streptococcus pneumoniae R6 is an avirulent strain of this species. Presumably this means that specific virulence genes that are present in virulent strains of Streptococcus pneumoniae are missing or mutated in the R6 strain. In fact the most likely difference would be that virulence factors are encoded by a plasmid that has been lost in the avirulent strain.

The use of avirulent strains is a common strategy for studying pathogenic bacteria, although obviously it precludes studying mechanisms of pathogenicity directly.

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    $\begingroup$ Details as to what render the strain avirulent detailed here. Namely, the lack of a capsule. $\endgroup$ – CKM May 18 '17 at 18:56

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