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Chapter 28 : What Is Virulence?
Analysis of virulence gene expression by Northern dot blotting revealed that transcription of the genes for pneumolysin and for synthesis of type 3 capsule is more than doubled. Screening with tissue-cultured host cells is superior to lifeless culture media. Nevertheless, it fails to reveal all virulence genes because such cells reflect neither the sophisticated architecture of the target organ nor the complex interactions between pathogens and host immune defenses. It was initially validated using Salmonella and a mouse model of typhoid fever by the isolation of many genes now known to be important for virulence. The Compton team also used in vivo tests to confirm that the putative hcp mutant was attenuated in calves but remained virulent in mice. This, in turn, is the first piece of work showing that the hcp locus influences virulence and potentially serotype-host specificity. Practical and semantic difficulties remain in distinguishing so-called virulence factors from the many determinants promoting general fitness.