40 Biology of Bacteria 



air unless the surface of the colony becomes roughened or 

 broken. 



Most of the bacteria carried about by the air are what 

 are called saprophytes, and are harmless; but not all belong 

 to this class, nor will they do so while tuberculous patients 

 are allowed to expectorate upon the sidewalks, and typhoid 

 patients' wash to dry upon the clothes-line, and their dejecta 

 to be spread upon the ground. 



The growth of bacteria is profoundly influenced by 

 environment, so that a consideration of the conditions 

 favorable and unfavorable to their existence becomes 

 necessary. 



CONDITIONS ESSENTIAL TO THE GROWTH OF BACTERIA* 



Oxygen. As all micro-organisms must have oxygen 

 in order to live, the greater number of them grow best 

 when freely exposed to the air. Some will not grow at all 

 where uncombined oxygen is present, but secure all they 

 need by severing it from its chemic combinations. These 

 peculiarities divide bacteria into the 



Aerobes, which grow in the presence of uncombined 

 oxygen, and the 



Anaerobes, which do not grow in the presence of uncom- 

 bined oxygen. 



As, however, some of the aerobic forms grow almost as 

 well without oxygen as with it, they are known as optional 

 (facultative) anaerobes. 



As examples of strictly aerobic bacteria Bacillus subtilis, 

 Bacillus aerophilus, Bacillus tuberculosis, and Bacillus 

 diphtheriae may be given. These will not grow if oxygen 

 is denied them. The cocci of suppuration, the bacillus of 

 typhoid fever, and the spirillum of cholera grow almost 

 equally well with or without oxygen, and hence belong to 

 the optional anaerobes. The bacilli of tetanus and of 

 malignant edema, and the non-pathogenic Bacillus butyricus, 

 Bacillus muscoides, and Bacillus polypiformis, will not 

 develop at all where any free oxygen is present, and hence 

 are strictly anaerobic. 



Nutriment. The bacteria grow best where diffusible 

 albumins are present, the ammonium salts being less fitted 

 to support them than their organic compounds. Proskauer 

 and Beck * have succeeded in growing the tubercle 



*" Zeitschrift fur Hygiene," etc., Aug. 10, 1894, vol. xvm, No. i. 



