Physiology 451 



ditions (61). On the other hand, conditions in the vertebrate intestine 

 suggest that intestinal parasites are anaerobes. Experimental evidence in- 

 dicates that the rumen ciliate, Eudiplodinium neglectum, is an obligate 

 anaerobe (218). Such is true also for flagellates of termites (217, 555). 

 Entamoeba histolytica, in contrast to many other intestinal parasites, 

 normally invades the wall of the colon. Yet this species grows as an 

 anaerobe in cultures (56). 



Relationships between oxygen tension and growth of laboratory popu- 

 lations have been investigated in a few cases. Aeration of flask cultures 

 increases giowth of Tetrahymena pyriformis (245, 440), reduction of the 

 oxygen supply (pyrogallol technique) decreases populations to about half 

 the normal density (171), and complete anaerobiosis prevents growth 

 (340). Growth of Chilomonas Paramecium, on the other hand, is retarded 

 by aeration of cultures (245). Quantitative data also have been reported 

 for Trichomonas vaginalis (255) and for C. Paramecium and T. pyri- 

 formis (428). Growth of T. vaginalis is heaviest in complete anaerobiosis 

 and is inhibited progressively by increasing oxygen tensions. Oxygen pres- 

 sures of 0.5 to 500 nnn Hg permit growth of C. para?necium, with an 

 optimum at about 75 mm Hg (about half the normal atmospheric con- 

 centration of oxygen), while pressures of 600 mm Hg and higher are 

 lethal. Growth of T. pyriformis increases from 10 mm Hg to a maximum 

 (about twice the growth with atmospheric concentrations of oxygen) in 

 an atmosphere of pure oxygen (739 mm Hg). 



Oxidation-reduction potentials^ 



The oxidation-reduction potential of the culture medium is an- 

 other factor related to the giowth of microorganisms. In a general sense, 

 this potential is a measure of the reducing intensity or oxidizing intensity 

 of a given system. Examples of such systems — each of which consists of a 

 more reduced and a less reduced substance — are leuco-methylene blue/ 

 methylene blue, lactate/pyruvate, and reduced cytochrome a/oxidized 

 cytochrome a. If a platinum electrode and a calomel electrode, in a po- 

 tentiometer hookup, are immersed in such a system, a potential difference 

 can be measured. Since the calomel electrode is standardized against the 

 hydrogen electrode, measurements are expressed in millivolts in terms of 

 the hydrogen electrode potential. The more negative the potential, the 

 greater is the reducing power of the system; the more positive, the greater 

 the oxidizing power (and the lower the reducing power). Each system has 

 a characteristic Ef/ value at which it is half reduced at a particular tem- 

 perature and pH. Consideration of pH is necessary because the potential 



" Compact discussions of oxidation-reduction potentials have been published by John- 

 son (257) and Stephenson (538). In addition, there is available a table of potentials for 

 more than 200 different systems (6). Early literature on Protozoa has been reviewed by 

 Jahn (246). 



