Signature-tagging of a bacterial isolate demonstrates phenotypic variability of the progeny in vivo in the absence of defined mutations

► 10 uniquely tagged wild-type isolates of Haemophilus influenzae were created. ► In vitro, all 10 tagged isolates grew equally well. ► In vivo, individual tagged wild-types showed differing growth kinetics. ► Individual chinchillas select individual tagged wild-types. ► Phase variation and epigenetic mechanisms may be involved.