/:/-7-7-;rr OF DELETERIOUS i\ ri. i 'i-:\< 'ES i 'r< >\ n. i ( ' 



A 105 



by Siivna and Alessi gives some idea of the extent and effect of such 

 influences. In the experiments silk threads were saturated with bouillon 

 cultures or aqueous suspensions of the bacteria, and some then enclosed 

 in tubes containing sulphuric acid or calcium chloride, w r hile others 

 were left exposed to various outside influences: 



With sulphuric With calcium In incubator In dry room in i In moist 

 Desiccation. acid, killed at chloride, killed at 37, killed shade, killed room, killed 



end of at end of at end of at end of at end of 



The results of all investigators, however, would seem to indicate 

 that the greatest possible care must be exercised in desiccation experi- 

 ments to come to any positive conclusions ; but recently most astonishing 

 results have been obtained with regard to many species usually sup- 

 posed to be particularly sensitive to desiccation, showing that under 

 certain conditions they may retain their vitality in a dry state for a 

 very long time. Thus, Koch found that cholera spirilla lived only a 

 few hours when dry; Kitasato determined their life duration at four- 

 teen days at most; while various French observers have found that 

 they may, under favorable conditions, live 150 to 200 days. The vary- 

 ing results sometimes reported by different observers in such experi- 

 ments may be explained by the fact that the conditions under which 

 they were made were different, depending upon the desiccator used, 

 the medium upon which the cultures were grown, and the use of silk 

 threads or cover-glasses. In all these experiments, of course, it should 

 be previously determined that in spore-bearing species there are no 

 spores present. Even when a dried culture lives for a long time the 

 majority of the organisms die in a few hours after drying. We have 

 found 1,500,000 colon bacilli to be reduced to 100^000 after three 

 hours' drying. \Vhen protected by a covering of mucus, as in expec- 

 toration, they live much longer than when unprotected. 



Behavior toward Oxygen and Other Gases. As already noted under 

 the nutritious substances required by bacteria, it is customary to divide 

 bacteria into three classes, according to their behavior toward oxygen. 



1. AEROBIC BACTERIA. Growth only in the presence of oxygen: 

 the slightest restriction of air inhibits development. Spore formation 

 especially requires the free admission of air. 



2. ANAEROBIC BACTERIA. Growth and spore formation only in the 

 total exclusion of oxygen. Among this class of bacteria are the bacillus 

 of malignant oedema, the tetanus bacillus, the bacillus of symptomatic 

 anthrax, and many soil bacteria. Exposed to the action of oxygen, 

 the vegetative forms of these bacteria are readily destroyed ; the spores, 



