STUDIES IN MICROBIOLOGY (4) 



Viruses: Their Identification, Mode of 

 Reproduction, and Filterability * 



(Readings: Weisz, pp. 32-34. S.P.T., pp. 43, 316. Villee, pp. 138-141. F. M. 

 Burnet, "Viruses," Sci. Am. 184, No. 5, 43-51, May 1951, Reprint No. 2. 

 G. S. Stent, "The Multiplication of Bacterial Viruses," Sci. Am. 188, No. 5, 

 36-39, May 1953, Reprint No. 40. Other readings listed at the end.) 



Viruses are particles, smaller than most cells, 

 composed of protein and nucleic acid. Although 

 they are unable to grow or carry out any of the 

 processes characteristic of living things by them- 

 selves, they have the curious ability to divert the 

 machinery of a cell so that in place of its normal 

 activity it begins to mass-produce the virus. 

 Some of the most dread diseases of man, small- 

 pox, polio, and rabies, are caused by viruses. 



Bacteriophages, viruses which infect bacterial 

 cells, have been intensely studied in recent years 

 and much has been learned of their structure 

 and mode of operation. As an example we shall 

 take the phage called T4 which attacks cells of 

 the bacterium E. coli. Observations made with 

 the electron microscope show the virus to con- 

 sist of a polyhedral body containing DNA, to 

 which a tubelike structure is appended. It looks 

 like a bulb bearing a tube. The virus attaches 

 to the cell by the end of the tube, and injects 

 its DNA into the cell through the tube. For 

 about 10 minutes, though in this interval viral 



*Directions for setting up these experiments are 

 in Appendix A. 



DNA and protein constituents begin to accumu- 

 late, no new virus is formed. Then, during the 

 next 20 minutes, more and more virus particles 

 form until, about a half-hour after infection, 

 the cell bursts and releases over 100 new virus 

 particles. 



We shall follow such a growth cycle. Virus 

 particles can be counted by spreading a suspen- 

 sion of them on an agar surface which is covered 

 with a dense population of susceptible bacteria. 

 The bacteria grow except in the areas surround- 

 ing each virus particle, where they have been 

 killed by the multiplying virus. Such blank 

 areas, or plaques, can be counted in the same 

 manner as bacterial colonies, and from such 

 counts the density of infective virus particles in 

 the original suspension can be calculated. 



Under proper conditions, a given phage pro- 

 duces plaques of quite uniform and reproducible 

 morphology. An experimenter can often decide 

 with which bacteriophage he is dealing from the 

 character of the plaques, just as one can often 

 identify a bacterium from the character of its 

 colonies. In the second part of this exercise you 

 will be given samples of three known phages 



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