210 Microbes and You 



the agar will permit the dormant cells to grow and produce a 

 turbidity in the tube o£ broth. However, failure of the organisms 

 to grow in the broth would indicate a bactericidal action of the 

 test chemical on the specific microorganisms. 



PENICYLINDERS 

 There are definite objections and limitations to the marble cup 

 technic of measuring the effectiveness of disinfectants. The tend- 



o 



ency of the agar to split when removing the marble is one of the 

 more serious of these drawbacks. Cracks in the agar allow 

 seepage of the test chemical and a false measurement of pene- 

 trating power may result. 



An outgrowth of this marble method has been the development 

 of what are termed penicylinders— cylinders of glass, porcelain, or 

 metal about '^'ig of an inch long with an inside diameter of % inch 

 and open at both ends. By having one end beveled the peni- 

 cylinders can be imbedded in the agar which has been previously 

 seeded with the test organisms. A measured amount of the chemi- 

 cal is placed in the open end of the penicvlinders that are left in 

 the agar. As is true with the marble technic, zones of clearing 

 adjacent to the penicylinders are measured after the plates have 

 undergone prescribed incubation. This is a routine method 

 employed for determining the potency of antibiotics. 



FILTER PAPER DISC METHOD 

 Another technic for testing sensitivities of organisms to various 

 disinfectants and antibiotics is the use of standardized circles of 

 special filter paper which have been impregnated with the mate- 

 rial to be tested. The surface of an agar plate is heavily seeded 

 with specific organisms, and the paper discs are asepticallv trans- 

 ferred to the agar surface. Zones of clearing are interpreted in 

 the usual way. 



Clinical laboratories employ such discs as an aid to phvsicians 

 who are interested in knowing which antibiotic has the best chance 

 of combatting a given infection. A half-dozen or more antibiotics 

 may be tested on a series of plates, and with rapidly growing or- 



