FIG. 4.

Identification of avirulent mutants. Two identical filters containing the same amount of individually amplified DNA tags (from A1 to L5) were hybridized to labeled DNA tags from the in vitro pool and the in vivo pool. The filters shown exemplify the many analyzed. Tags that weakly hybridized to the probes from both the in vitro pool and the in vivo pool (D1, E1, F1, G1, D2, C5, and E3) probably corresponded to slow-growing mutants and were not considered. Tags that hybridized to the probe from the in vitro pool but not to the probe from the in vivo pool (I5, L5, F5, L1, A2, and C4) corresponded to mutants with attenuated virulence and therefore were unable to survive in the host.