162 PURE CULTURES FROM PATHOLOGIC MATERIAL 



media, and by placing part of it in the incubator to see whether it 

 will become cloudy and whether anything will develop in it. 



Such filtrates are commonly said to be germ-free or sterile. The 

 former term is correct provided it refers to visible germs of known 

 type only. The term sterile, however, is objectionable, because it 

 is known today that there are ultramicroscopic invisible filterable 

 organisms which are evidently able to multiply and cause diseases, 

 such as hog cholera, rinderpest, pleuropneumonia of cattle, etc. 

 Since at present it is not known whether bacteria-free filtrates may not 

 contain other ultramicroscopic live substances which so far cannot be 

 demonstrated and recognized, the term sterile should not be used in 

 connection with filtrates. 



The fluids which are to pass through Pasteur or other filters must 

 be subjected to pressure because gravity alone will not force them 

 through rapidly enough. Therefore, such bacteria filters are generally 

 connected with a suction pump screwed to a faucet, or they are 

 attached to a vacuum apparatus which exhausts the air by a steam 

 or gas engine or electrical device. In any case a partial vacuum 

 is formed in the vessel which is to receive the filtrate, and the external 

 air pressure acting upon the fluid to be filtered, presses it through 

 the pores of the filtering device. It is, of course, understood that all 

 parts of a filtering apparatus must be connected with each other in an 

 absolutely air-tight manner; otherwise, filtration is not perfect and the 

 filtrate may become contaminated with bacteria from aspirated air. 



QUESTIONS. 



1. Describe the method of obtaining a pure culture of a bacterium suspected 

 of being the cause of an abscess which has not yet broken through or ulcerated. 



2. Describe the same process in the case of an ulcerated abscess. 



3. What is meant by pouring plates? What has superseded the original Koch 

 method of pouring plates ? 



4. Describe a Petri dish. How are these prepared for use? 



5. What is meant by naming the upper end of a culture tube? When prac- 

 tised? 



6. Describe methods of preparing culture tubes in order to pour plates from 

 their contents. 



7. Describe procedure of inoculating a set of three tubes from which plates 

 are to be poured. 



8. How are the Petri dishes treated after the liquefied agar medium has been 

 poured into them? 



9. What is the object of pouring plates (Petri dishes)? 



10. What is the difference in development of colonies in Petri dish No. 1, 

 No. 2, and No. 3, respectively? 



11. How can it be ascertained when several different types of colonies are 

 present; which is the causative pathogenic and which are the accidental contam- 

 inating microorganisms? 



12. How are young small colonies found in a Petri dish? 



13. What kind of contamination is frequently found in Petri dishes? 



14. If the environment makes the use of Petri dishes impossible, how can a 

 pure culture be obtained ? 



15. What is Esmarch's method of obtaining pure cultures? 



16. What is an impression or "Klatsch" preparation? 



