Moisture is known to favour the survival of Gram-negative bacilli in the environment, while dry conditions favour the survival of Gram-positive cocci (Hellat, 1948; Bardsley, 1948; Lidwell & Lowbury, 1950). Lowbury & Fox (1953) found that the proportion of bacteria which died when their suspending medium evaporated to dryness was much greater with a suspension of Pseudomonas aeruginosa than with one of Staphylococcus aureus or Streptococcus pyogenes; the death-rate of the Gram-negative bacilli which had survived drying and which were exposed for a further period under atmospheric conditions was, however, no greater than that of the survivors in dried suspensions of streptococci or staphylococci.Among the questions left unanswered in the earlier studies were (1) the consistency, on replicate testing, of differences in survival of different strains of Ps. aeruginosa, (2) the relative capacity of bacterial cells, which have survived one exposure to drying, to survive a second drying, and (3) the relative capacities of Ps. aeruginosa and of Staph. aureus to survive in identical moist environments. These questions are considered in this paper. The relevance of the survival of Ps. aeruginosa and Staph. aureus under different environmental conditions is also discussed. SURVIVAL OF ORGANISMS ON ATMOSPHERIC DRYING Methods and materials Bacterial strainsThe Gram-positive cocci tested were three strains of Staph. aureus (phage types 52A/80, 3C and 80/81) and two strains of coagulase-negative staphylococci (micrococci). The Gram-negative bacilli included four strains of Ps. aeruginosa isolated from burns (phage types 21/68/F 7, 119x, 21/68/col. 21

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