The Effect of Chemicals on Microorganisms 209 



various materials cannot be over-emphasized. This is particularly 

 important in the case of disinfectants. Qualitative and quantita- 

 tive tests have been standardized to permit laboratories to dupli- 

 cate results obtained by other testing laboratories when products 

 are subjected to scientific scrutiny. The ramifications of the tests 

 employed for disinfectants are too great to dwell upon, but the 

 underlying principles of some of the accepted procedures might 

 be of interest. 



MARBLE CUP TECHNIC 



One of the earlier methods for evaluating the effectiveness of 

 chemical disinfectants was to inoculate rather heavily a tube of 

 nutrient agar with a test organism such as Micrococcus pyogenes 

 variety aureus. Since this organism is a frequent cause of localized 

 skin infections, it is commonly employed in disinfectant testing. 



After the seeded medium has been poured into a culture dish 

 and before the agar has had an opportunity to solidify, a sterile 

 marble is placed in the center of the agar plate and is left in 

 position until the medium solidifies. By carefully removing this 

 marble with a pair of sterile forceps, a cup-like depression is left 

 in the medium. The test chemical, whether it be a liquid, a cream, 

 or an ointment, is placed in this depression and the plate is in- 

 cubated at body temperature. If the test material has any bac- 

 tericidal or bacteriostatic properties, a zone of clearing will appear 

 adjacent to the depression in the agar. The test organism should 

 grow heavily in all parts of the plate except where the chemical 

 has had an adverse affect on the organisms. In general, the 

 W'ider the zone of clearing around the cup, the more effective is the 

 chemical being tested. This might also be considered to be a 

 semi-quantitative measurement of the penetrating power of the 

 test chemical. 



If one were interested in determining whether the clear zone 

 represented killing (bactericidal effect) or mere inhibition (bac- 

 teriostatic effect) of the organisms, a small piece can be scooped 

 out of the clear area of the agar and can be subcultured into a 

 tube of nutrient broth medium. If the organisms have only been 

 inhibited, the dilution effect of the broth on the test chemical in 



