482 Physiology 



pyriformis on Serratia marcescens also was about the same between pH 

 4.5 and 8.6, but yields were greatest at about pH 5.0 and 7.4 in similar 

 suspensions of Klebsiella pneumoniae, Pseudornoans fluorescens and 

 Proteus vulgaris (253). 



Temperature 



The biothermal range, or range of temperature permitting growth, 

 extends from about 54 to aproximately 0° C. for Protozoa. Adaptation 

 to the higher temperatures within this range is rare, although certain 

 flagellates (at 54°), shelled rhizopods (at 51°), amoebae (at 50-52°) and 

 cilia tes (at 46°) have been reported from hot springs (232). 



Except for the unusual thermophilic species, active stages are killed 

 by temperattnes approaching or exceeding 45°. Euglena gracilis, at pH 

 7.0, is killed within eight minutes at 44° (239); Entamoeba gingivalis, 

 within 20 minutes at 45° (299); Paramecium caudatum, within nine 

 seconds at 40° (451); Spirostomum ambigmun, at 36° (524); Colpoda 

 cuciillus, at 37-45° after exposures of 0.5-10.0 minutes (17). Termite 

 flagellates are eliminated from their hosts after 24 hours at 36° (67), 

 and gregarine trophozoites from Tenebrio larvae after six days at 37.5° 

 (369). Lethal exposures depend upon time as well as temperature, and 

 the thermal death time at a given temperature also varies with pH of 

 the mediimi. The resistance of Euglena gracilis to high temperatures is 

 greatest at pH 5.0 and is less above pH 7.0 than below (239). Paramecium 

 caudatum, on the other hand, shows greater resistance to 40° above and 

 below pH 7.0 than at the neutral point (53). Both E. gracilis (239) and 

 P. midtimicroniLcleatum (98) have shown increasing resistance with in- 

 creasing density of population. After the maximum is reached, however, 

 resistance decreases gradually in older cultures of the latter. 



Cysts are generally more resistant than corresponding active forms. 

 Dried cysts of Colpoda cucullus, for example, resist 100° dry heat for three 

 hours, although moistened cysts die within 30 minutes at temperatures of 

 49-55°. Excystment is retarded by non-lethal exposures to 37-48° (17). 

 Somewhat higher lethal temperatures, in 5-minute exposures to moist 

 heat, have been reported for intestinal parasites: Entamoeba coli, 76°; 

 E. histolytica, 68°; Endolimax nana, 64°; Giardia lamblia, 64°; Chilo- 

 mastix 7nesnili, 72°; lodaynoeba biischlii, 64° (18). Unsporulated oocysts 

 of Eimeria miyairii are quickly killed at 53° (476). 



Short exposure to temperatures below 0° C. is often not lethal to 

 active stages (111, 239, 576). Cultures of Leishmania donovani have re- 

 mained viable after intermittent exposure to —12° over a period of 10 

 days (200) and Entamoeba gingivalis may live almost 18 hours at 0° (299). 

 Fission may continue slowly — for example, a fission every two weeks in 

 Paramecium caudatum (111) — at temperatures just below zero. Cyto- 

 plasmic division is more susceptible than nuclear division to extremes of 



