PHYSIOLOGY OF MICRO-ORGANISMS 165 



protein. Some important disease-producing bacteria belong in 

 this category, as well as many parasitic spirochetes and some _of 

 the protozoa. Such organisms are not adapted to any natural 

 saprophytic existence, and they grow in the artificial cultures 

 only because the dead medium is made to resemble somewhat 

 their natural parasitic habitat. Finally there are the micro-or- 

 ganisms which have not yet been grown in artificial culture and 

 whose food requirements are essentially unknown. Many of 

 these are parasites, and are called obligate parasites. A few bac- 

 teria, many of the filterable agents, and most of the parasitic 

 protozoa are included in this category. 



Inorganic Salts and Chemical Reaction. Phosphorus, sul- 

 phur, chlorine, calcium, sodium and potassium, in addition to 

 carbon, hydrogen, oxygen and nitrogen, are present as constituents 

 of the microbic protoplasm. Minute quantities of these suffice to 

 supply the food requirements of micro-organisms and it is un- 

 necessary to add them to culture media to serve as food. Com- 

 mon salt, sodium chloride, is ordinarily employed to give the 

 artificial medium an osmotic tension approaching that of the 

 body fluids, and calcium carbonate is sometimes used to neutral- 

 ize the organic acids which may arise in the culture as a result of 

 the bacterial growth. 



The most favorable chemical reaction for most micro-organisms 

 is that of actual slight alkalinity, not sufficiently alkaline to pro- 

 duce a red color with phenolphthalein and not sufficiently acid 

 to produce a red color with litmus. Some bacteria and many of 

 the yeasts and molds will grow well in a weakly acid medium, 

 but most parasitic bacteria and protozoa, which can be cultivated 

 at all, require a reaction slightly alkaline to litmus or rosolic 

 acid. The anaerobic bacteria do best in a medium containing 

 glucose and with a reaction quite alkaline, indeed very close to 

 the point at which phenolphthalein becomes pink. Organisms 

 which produce acid or alkali are usually arrested in their growth as 

 soon as a certain concentration is reached, and the medium may 

 then rapidly kill the micro-organisms. 



