RELATION TO OXYGEN AND OTHER GASES. 41 



many contradictory statements, from which it may be con- 

 cluded that much depends upon the special conditions 

 under which drying occurs. Ficker (Z. H. xxix, 1) has 

 discovered some laws especially for the cholera vibrio. 



According to him, tlie following are of special significance : 



1. Very thin smears were more quickly injured than thick clumps; 

 slight variations in the thickness of the smear are without significance. 



2. Thin smears die more quickly in the exsiccator, and thick ones 

 in room atmosphere. 



3. The lower the temperature, the better the drying is borne. 



4. Virulent cultures are more resistant than non-virulent ones. 



5. Variations of moisture (exsiccator and moist chamber) kill 

 especially rapidly (shown in typhoid and pest). 



6. Older cultures were somewhat more resistant in an exsiccator. 

 Also regarding the variability in a moist chamber, Ficker has 



obtained interesting results. Here old cholera cultures present enor- 

 mously greater resisting power than young ones. For example, in the 

 moist chamber thin smears of cultures seven to twenty-one days old 

 live about thirty to fifty days ; those three days old, fourteen days ; 

 those two days old, as long as seven days ; and those one day old, only 

 one to two days. Yet the old cultures contained no spores, and were 

 equally or more susceptible than the young ones to heat and chemicals. 

 (Compare Vib. chol. in special part.) A complete summary of pre- 

 vious results upon dried cholera, typhoid, diphtheria, and pest organ- 

 isms is given by Ficker, I. c. 



5. RELATION TO OXYGEN AND OTHER GASES. 



Because of their relation to oxygen, bacteria are usually 

 placed in three classes (Fliigge and Liborius): 



I. Obligate aerobes. Growth occurs only when air is 

 admitted, and every limitation to the entrance of air injures 

 the growth. Free oxygen is especially required for spore- 

 formation. With a pressure of three to four atmospheres 

 of pure oxygen, according to Chudiakow, growth of aerobic 

 bacteria ceases ; the lowest limit for aerobes was found to 

 be 5 to 10 mm. of air. According to Roger (C. B. xx, 

 626), a pressure of 500 to 600 atmospheres of air does not 

 kill bacteria. 



II. Obligate anaerobes. Growth and spore-formation 

 occur only when oxygen is comjiletely excluded.'^ In this 



' Regarding the means of preparing anaerobic cultures consult the 

 Technical Appendix. Chudiakow found that the strictest anaerobes 



