The Rickettsiae 383 



been injected with virulent rickettsiae definitely affords the ani- 

 mals protection, since their survival rate is significantly higher than 

 that found in groups of like animals kept at "ordinary" tempera- 

 ture. Any treatment or process which slows down the metabolism 

 of cells, including the use of chemicals, irradiation of tissues, and 

 withholding of certain vitamins, will generally favor better growth 

 of rickettsiae. Unless some means is discovered for growing these 

 organisms in the absence of cells, it will be necessary to continue to 

 investigate their metabolism through indirect approaches, such as 

 studies concerned with the acceleration and the inhibition of 

 enzymes. 



The usual technics employed for killing vegetative bacteria and 

 other microorganisms— application of heat, use of chemicals, drying, 

 etc.— are generally effective for the destruction of rickettsiae. Such 

 preservatives as glycerol may keep these microbes viable for 

 months when they are stored at 10° C. 



Animals to which sulfa drugs have been administered are usu- 

 ally more susceptible to rickettsiae due, perhaps, to the direct ac- 

 tion that sulfa drugs have on metabolism. The antagonist of sulfa 

 — para-amino benzoic acid (PABA)— has been employed to combat 

 typhus fever. It speeds up cellular respiration, and if the drug is 

 administered early enough and in large enough doses, it appears 

 to be effective. The B complex vitamins, vitamin Bo (riboflavine) 

 in particular, may be fed to animals to speed up their metabolism 

 and to aid in combattinor rickettsial infections. If this vitamin is 

 deficient in the diet, the animals are made more susceptible. 

 This observation may explain in part why typhus fever spreads so 

 rapidly during wartime and in periods of famine when dietary 

 deficiencies are prevalent. 



CLASSIFICATION 



Bergetjs Manual provisionally places the order Rickettsiales as 

 a supplement following the bacteria. The main discussion in this 

 chapter deals with members of the family Rickettsiaceae. The 

 more common vectors, genera, species, and diseases of this group 

 mav be summarized in tabular form as follows: 



