2 MISCELLANEOUS PUBLICATION 955, U.S. DEPT. OF AGRICULTURE 



Virus particles generally are less than 0.1 micron in diameter. On 

 the other hand, rickettsiae are generally more than 0.1 micron in 

 diameter, are retained by bacterial filters, and may be seen with the 

 light microscope. 



Both viruses and rickettsiae are parasites, depending on other living 

 organisms for their growth and multiplication. As far as is known, 

 none of them are free living. All hosts of rickettsiae are in the animal 

 kingdom, but viruses find hosts among both plants and animals. 

 Even bacteria and actinomycetes are attacked by viruses, which are 

 designated bacteriophages and actinophages, respectively. Among 

 the common virus diseases of man and animals are poliomyelitis, 

 mumps, measles, foot-and-mouth disease, and rabies. Rickettsiae are 

 causative agents of typhus, scrub typhus (tsutsugamushi disease) , Q 

 fever, and Rocky Mountain spotted fever, and are often transmitted 

 by arthropod vectors- 



Although phages may interfere with some industrial fermentations 

 by destroying or weakening the fermentative agent, other viruses are 

 industrially useful. Some are grown to make vaccines useful in 

 the prevention and control of disease, such as the Salk polio vaccine 

 and the rabies vaccine. 



BACTERIA \ 



Micro-organisms of the class Schizomycetes (fission fungi) are 

 generally referred to as bacteria. To laymen they are better known 

 as germs or microbes, terms that are less definitive because they may 

 include viruses, molds, and yeasts. 



Bacteria are larger than viruses and rickettsiae. Cells of the most 

 important kinds in medicine, industry, and agriculture are from 1 to 



3 microns in their largest dimension. Moreover, most bacteria are able 

 to grow without living hosts or body fluids. They require essentially 

 the same nutrients for growth as do higher forms of life ; i.e., sources 

 of carbon and nitrogen, essential mineral elements, and water. Many 

 must also be provided with one or more vitamins, but many others 

 make their own from ingredients at hand. Bacteria lack chlorophyll, 

 the green matter of higher plants that enables them to utilize the 

 sun's energy for the manufacture of food. A few varieties of bacteria 

 have photosynthetic pigments chemically related to chlorophyll, which 

 permit them to function like higher plants. Most bacteria can only 

 utilize soluble nutrients, which enter cells through the semipermeable 

 cell membrane. 



Reproduction of bacterial cells is by simple cell division, or fission, 

 rarely by budding. This characteristic differentiates bacteria from 

 viruses, whose reproductive mechanism is so intimately associated with 

 the living protoplasm of the host cells that the exact method is un- 

 known. It also differentiates them from yeasts and molds, which 

 reproduce by budding and full-fledged sexual processes. 



Under the microscope bacterial cells are seen in three basic forms. 

 Some are more or less spherical and are referred to as cocci. Others 

 are cylindrical and are called rods. Curved rods are vibrios if comma 

 shaped, and spirilla if helicoidal. The cells may occur in natural 

 associations. Thus cocci may appear as strings of beads (streptococci) , 



