SIZE AND WEIGHT OF BACTERIA 39 



as clubs many times larger than the ordinary, or they may appear 

 as crosses or stars. The formation of large club-shaped organisms 

 is very characteristic of the organism which causes diphtheria, 

 whereas the formation of crosses, stars, and the like is characteris- 

 tic of the organism which grows in the roots of alfalfa. Some writers 

 have considered them degenerate forms and compared them to the 

 " lame and halt" in the human species. This, however, is hardly an 

 apt illustration, for these peculiar shaped organisms have all of the 

 powers possessed by others and if they found their way into the 

 body of an animal, they would be just as likely to produce the dis- 

 ease which is characteristic of the organism as would the ones with 

 the normal shape. 



In some cases this characteristic has served as a valuable aid to 

 the differential diagnosis of the organism. This is especially true 

 with the plague bacillus which, when grown on nutrient agar con- 

 taining from 2.5 to 3.5 per cent, of sodium chlorid, is prone to give 

 rise to involution forms. 



Size and Weight. The unit of measurement in microscopy is the 

 micron (/A), or micromillimeter. This is O.OQ1 of a millimeter or 

 approximately 2Tijo^ of an inch. The majority of the organisms 

 vary from 0.2/z up to 30 jii or 40ju. They are smallest in the case of 

 the cocci and largest in the case of the spirilla. 



Although there is a great variation in the size of bacteria, all are 

 extremely small; even the largest are not visible to the naked eye. 

 The smallest are beyond the range of our most powerful micro- 

 scopes, and others appear as mere dots. The Pfeiffer bacillus, the 

 one which was thought to cause influenza, are rod-shaped organisms, 

 and if they be placed end to end it would take fifty thousand of 

 them to reach one inch, or it would require about fifteen thousand of 

 the bacteria which cause typhoid to form a line one inch in length. 

 Of the very largest known it would require seven thousand to reach 

 an inch. We often magnify bacteria one thousand times and then 

 they appear as dots under the microscope, but if we would magnify 

 a man to that extent he would appear to be six thousand feet tall 

 and fifteen hundred feet wide. Bacteria are so small that at times 

 we find five millions in a small drop of milk and yet they have 

 plenty of room to move about, for it would require one hundred and 

 twenty-five billion to weigh the same as a drop of milk. 



A person may wonder, since bacteria are so small, how they can 

 bring about such enormous changes, for it takes but a short time 

 for them to tear to pieces the body of a large animal that has died. 

 All know how fast various plants and fruits decay under appro- 

 priate conditions. Decay is due to bacteria. One organism could 

 of itself bring about only a small change, but they multiply with 

 almost inconceivable rapidity. The bacilli grow until they have 

 reached a certain length, then divide into two, and these in turn 



