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Growth curves of selected NTHI isolates grown in specific media for 12 h. ⧫, growth in histidine-restricted medium; ▪, growth in histidine-supplemented medium, ▴, growth in Levinthal control medium. (a) Growth curves of isolates possessing the complete his operon; (b) growth curves of isolates missing the his operon. Isolate 23221 (throat isolate) gave inconsistent results in the histidine medium as well as the Levinthal control medium.
