932 RICKETTSIA 



crepancy may be due to the circumstance that in pLasma tissue cultures the cells 

 are multiplying actively, whereas in the Maitland medium mitotic division is 

 rarely seen. From these observations Pinkerton (1934) drew the conclusion that 

 typhus rickettsise grow best in cells which are metabolizing slowly. That this 

 explanation is correct is rendered probable by the work of Zinsser and Schoenbach 

 (1937) and Burnet (1938). Thus Zinsser and Schoenbach found that in tissue 

 culture rickettsise underwent no appreciable multiplication till the tissue cells 

 had ceased to grow actively. Viruses, on the other hand, multiplied most abun- 

 dantly during the stage of tissue cell growth. Burnet (1938) made similar observa- 

 tions on the rickettsia of Q fever. It would appear that, in this respect, there 

 is an important difference between the metabolism of Rickettsia and of filtrable 

 viruses. The reason why rickettsise grow better at 32° C. than at 37° C. in plasma 

 tissue cultures, and in the scrotal sac than in the peritoneal cavity, is probably 

 not because they prefer the lower temperature in itself, but because tissue cell 

 metabolism is less rapid at this temperature than at 37° C. Further observations 

 by Burnet and Freeman (1941) on egg membrane cultures suggest that rickettsiae 

 grow only in regions where there is an abundant supply of oxygen. The authors 

 recall the fact that the rickettsiee are essentially parasites of the vascular endo- 

 thelium, which is rich in oxygen, and explain their better growth in the later than 

 in the early stages of tissue cultures on the ground that, not till" active tissue 

 growth is over, does the oxidation-reduction potential of the cells rise sufficiently 

 high to enable rickettsise to multiply. Other factors that favour growth of 

 rickettsise in the animal body, such as riboflavin deficiency, benzol poisoning, 

 and X-ray irradiation, possibly act in the same way, namely by lowering intra- 

 cellular metabolism. 



Though most of the pathogenic species of Rickettsia seem to require intra- 

 cellular conditions for growth, R. nipponica and R. burneti are both said to be 

 capable of growth outside the cells though not, of course, in a cell-free medium. 

 A further difference in the metabolism of different species is shown by the fact 

 that the typhus group of rickettsise multiply exclusively in the cytoplasm, whereas 

 the spotted fever group {R. rickettsi) grow best within the nucleus. 



The Resistance of Rickettsia has not been fully studied. The rickettsise of 

 typhus, Kocky Mountain fever, and heartwater are said to be easily inactivated 

 by heat, drying, and chemical disinfectants, but R. quintana is said to be more 

 resistant (Cowdry 1926). The Trench fever Committee (Bruce 1921), however, 

 found that the infectivity of louse excreta was destroyed by exposure to moist 

 heat for 20 minutes at 60° C, and to dry heat for the same time at 100° C. Ark- 

 wright and Bacot (1923) found that R. prowazeki remained virulent for 11 days in 

 louse excreta which had been kept dry at room temperature. The viability of 

 rickettsise in infected tissues and in tissue cultures, as judged by their infectivity, 

 seems to be considerably affected by the temperature at which they are kept. 

 Spencer and Parker (1924), working with R. rickettsi, found that certain tissues 

 remained infective in pure glycerol for as long as 10 months if preserved at — 10° C. 

 Nigg (1935), working with R. mooseri, found that tissue cultures in a serum-Tyrode 

 mixture remained alive and virulent for several months at 37° C. and at — 20° C, 

 but generally died out in a week or two at the intermediate temperatures of 20° C. 

 and — 4° C. Sterile skim milk is said to be a good suspending agent for rickettsise 

 that are to be preserved in the dried state (Topping 1940) ; and used as a diluent 

 it maintains the virulence of R. prowazeki at 26°-2S° C. for 24 hours (Anderson 



