Fig 2.

Selection of signature-tagged mutations. Input pools of 40 signature-tagged strains were used to inoculate three mice. Forty-eight hours postinfection, bacteria were recovered from the livers and spleens, and aliquots were split to enumerate the bacterial load and to generate output pools of genomic DNA. The tags present in the DNA pools were identified by short-cycle standard PCR and compared to the pool of input DNA prepared from the original inocula. Each PCR mixture contained 1 of 40 ST-specific primers and a primer common to all the mutant strains. Mutants whose tags were absent or present at low levels were considered attenuated.