30 BACTERIA 



tions. The presence of deleterious agents, such as arsenic, seems to 

 favor the development of capsules. 



Flagella. All bacteria show passive movements when suspended 

 in fluids and watched under the microscope. Such motion is known 

 as molecular or Brownian and is common to all finely divided par- 

 ticles when suspended in fluid. Apart from this, certain bacteria are 

 capable of active motion, which is accomplished by the contractions 

 of flagella or whips. Those without flagella have no active motion. 

 There may be only one whip at one end, as in the cholera bacillus; 

 a single whip at each end, as in many saprophytic vibrios; a bunch of 

 whips at one end as in certain large saprophytic vibrios ; and many 

 whips distributed over the cell, as in the typhoid bacillus. The flagella 

 seem to be outgrowths from the ectoplasm and they may be removed 

 from certain bacteria by shaking and then centrifuging, or by filtra- 

 tion through porcelain. Their presence favors agglutination, which 

 however, is not wholly dependent on them. 



Oxygen Need. Pasteur observed that some bacteria grow best 

 when supplied with oxygen, while others have their optimum growth 

 when this element is excluded. The former he designated as aerobic 

 and the latter as anaerobic. Further study has divided bacteria accord- 

 ing to their need of oxygen into the following classes : 



1. Obligate aerobes are those which will not grow save in the 

 presence of free oxygen. This does not mean that they must have 

 an abundant air-supply, because some do grow when the oxygen 

 tension is only one-hundredth that of the air. Among the pathogenic 

 bacteria, the plague and influenza bacilli and the pneumococcus and 

 gonococcus belong to this group. An excess of oxygen may prove 

 harmful to even the obligate aerobes. 



2. Facultative anaerobic bacteria are those that grow best in the 

 presence of oxygen, but may grow when this element is wholly 

 excluded. Many pathogenic organisms belong to this class, such as 

 the cholera vibrio, anthrax and typhoid bacillus and pus cocci. 



3. Obligate anaerobes are bacteria which grow only in the absence 

 of free oxygen. In this group are the bacilli of tetanus, symptomatic 

 anthrax and malignant edema. These bacteria obtain their oxygen 

 if they utilize this element at all, from the combined oxygen in their 



