INTRODUCTION 3 



Many organisms can accommodate themselves equally well to both 

 these modes of existence, exposed to the air they are ' aerobic," 1 in its 

 absence they become ' anaerobic.'' Bacteria possessing this faculty are 

 known as ' aero-anaerobic ' or 'facultative "* organisms. 



The most favourable temperature for the development of bacteria 

 pathogenic for warm-blooded animals is that of the human body, viz., 

 37*5 C., while the so-called normal water bacteria grow best at about 

 20 C., and the ' thermophilic ' bacteria develop, according to some 

 authorities, as low as 34 to 44 C., and as high as 70 to 75 C. 

 From this it will be seen that the range of the activity of bacterial 

 growth depends upon specific differences and may have wide limits : 

 some growing best at low, others at high, and others at medium tem- 

 peratures. Corresponding to this we find that the limits are defined by 

 minimum, optimum, and maximum temperatures for growth. The 

 minimum, when growth is just possible ; optimum, when the growth is 

 most luxurious ; and maximum, the highest temperature at which the 

 organism will develop. 



Besides a suitable temperature, bacteria require for their develop- 

 ment moisture and a medium of suitable composition and reaction. 



Bacteria are also influenced in a varying degree by light most forms 

 develop by far the best in the dark. It is therefore important to 

 have a dark closet in the laboratory for the storing of cultures, and it 

 is, moreover, known that cultures retain their vitality and virulence 

 longer when maintained at low temperatures. 



4. The Protozoa are one-celled animal organisms, usually consider- 

 ably larger than the largest bacteria, on which they frequently feed. 

 The cell body is differentiated as a homogeneous ectoplasm and a 

 granular entoplasm containing vacuoles, also one or more nuclei. 

 Some protozoa possess motile organs or flagella, others possess 

 pseudopodia, others cilia. They develop by fission or by spore 

 formation. They may coalesce and form so-called plasmodia, and as 

 in the bacteria the spores may be more resistant than the active 

 organism. 



THE CLASSIFICATION AND MORPHOLOGY 

 OF BACTERIA. 



The following simple classification of bacteria has been found con- 

 venient by medical bacteriologists, though perhaps not quite correct 



