30 Microbes and You 



yeasts and water, a relatively easy medium to sterilize. Pouchet, 

 unfortunately, had chosen a hay infusion as his substrate, and we 

 know that such material abounds with spores, those resistant 

 bodies so difficult to kill by mere boiling for the usual time periods. 

 Pouchet's experiments did help eventually to prove that spores are 

 more resistant to heating than are vegetative cells, but he did not 

 have spontaneous generation. These tough endospores help to 

 explain the many irregularities obtained by the pioneers as they 

 fumbled and groped with ideas and technics in their search for 

 proof of one theory or another. The great resistance of these 

 spores to heat was not conclusively demonstrated until 1877 when 

 Ferdinand Cohn described them in the so-called hay bacillus, 

 Bacillus suhtilis. Cohn also showed that spores could be made to 

 germinate into vegetative cells which were not as resistant to 

 boiling and to treatment with chemicals as were the spores. In 

 fact, our present methods for sterilizing materials in bacteriological 

 laboratories and in hospitals are based upon the time, and the 

 temperature, necessary to inactivate these resistant stages in the 

 growth of some, but not of all, bacteria. 



Although Pasteur was essentially a chemist, he had a flair for 

 microscopy, and his ability to see through the clouds of miscon- 

 ceptions to the clear air of reality led him to some of the greatest 

 discoveries in biology up to that time. To overcome all criticism 

 about devitalizing the air by heating it, by passing it through 

 strong chemicals, or by screening it through such an innocuous 

 substance as wool or cotton plugs, Pasteur placed a fermentable 

 substrate in a flask and boiled this medium for a sufficiently long 

 time to insure destruction of all living^ thino[s in this infusion. He 

 then heated the neck of the flask in a flame and drew the glass out 

 into an S-shaped curved capillary tube which he left open to the 

 outside air. Since microbes cannot ambulate, he postulated that 

 it would be impossible for contamination of his infusion to occur 

 unless he tilted the flasks and allowed some of the sterile liquid 

 to come in contact with the tip of the capillary tube containing 

 dust particles. Others had amply demonstrated that boiling in- 

 fusions did not devitalize them, and since ready access to vital 



