PROBLEM 1. Hoiv We Arc Frotcctcd A\iainst Microorgaiihnis 





Fu;. 289 rypl.ioid hactciia. Only a fcii' ihir- 

 teria are shozvn. The ivaz'v Jifies are flagella. 

 (general biological supply) 



laboratory, bacteria arc raised on a cul- 

 ture ?ned'unn. One kind of medium is a 

 broth containing proper food materials, 

 such as beef extract and peptones. Man^' 

 kinds of bacteria will grow and repro- 

 duce in this medium if thev are kept at a 

 suitable temperature. Incubators can pro- 

 vide a suitable temperature. B\' doing 

 Exercise i \'ou will learn how to pre- 

 pare a culture medium for raising many 

 kinds of bacteria. To raise pathogenic 

 bacteria, it is sometimes necessary to use 

 a special culture medium that contains 

 some substance taken from the tissue of 

 the host. 



To be useful for stud\ a culture me- 

 dium must be free from bacteria. In 

 other words, it must be sterile. Since 

 bacteria are present practically every- 

 where, a bacteriologist must provide for 



gcrring rid of all bacteria which ma\- 

 tind their w a\ into the culture medium. 

 He sterilizes evervthing used in the prcp- 

 araticm. He uses ver\- high tempera- 

 tures for sterilization. An object to be 

 sterilized nuu l>e p.issed throuL;h a tlanie 

 oi" exposed to steam under pressure. 



Solid media arc also used. An especially 

 useful de\ ice was deveU)ped In a famcnis 

 Ciermai\ bacteriologist, Robert Kocl\ 

 (184^-1010). The ditricuitN- with liijuid 

 cultui-e media is that in them the ditl'er- 

 ent kinds of bacteria are all nuxed to- 

 gether. It is impossible to stud\ a single 

 kind because the different kinds cannot 

 be separated from one another. To get 

 around this difhcultN', Koch mixed gela- 

 tin with the broth. Now in place of gela- 

 tin \\e often use atiar, a substance ob- 

 tained from certain seaweeds. Agar, like 

 gelatin, is licjuid while it is hot but be- 

 comes solid w hen it cools. To provide a 

 large surface for bacteria to grow on, the 

 culture medium, while still liquid, is 

 poured into a flat glass dish known as a 

 Petri dish, where it hardens. Sometimes it 

 is poured into a test tube which is kept 

 in a slanting- position while the mediiuu 

 hardens. This is called :\n agar slant. 

 Your teacher nun be able to show vou 

 examples of both. See Kxkrcise :. Since 

 the culture medium is solid the bacteria 

 do not mix. Thev increase in number 

 and soon there is a huc^e number of them 

 in one spot. This mass of bacteria is 

 called a colony. All the bacteria in a 

 colony are of the same t\pe since they 

 all came from a sinnle cell. You can raise 

 colonies \ourself. See I'xkrcise 3. 



If a bacteriologist w ishes to studN one 

 kind he can raise this kind. He touches a 

 sterile needle to a colons ot the kind 



