Infectivity ofPasteurella tularensisclouds

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Abstract
The infectivity ofPasteurella tularensisclouds decreases with age, for guineapigs challenged by the peritoneal and respiratory routes. No decrease occurs in mice challenged peritoneally. The loss of infectivity depends to some extent on age of culture but more especially on culture medium used. Cultures grown in a partially hydrolysed casein medium and aged 2–3 weeks lose infectivity severely in clouds held 20 hr. at 85%r.h., whereas if grown in cysteine broth with catalase (CBC) no loss occurs in this time until cultures are aged several months. Chloride ion, yeast extract and casein hydrolysate are shown to be infectivity depressors when used in culture media or when added to suspensions before spraying. Sodium chloride in CBC can be replaced adequately by sodium sulphate or sodium phosphate for growth of the organism.