Polluted Water Can Kill You 237 



presume that coliform bacteria are present in the water. The 

 term coliform refers to all species of the genera Escherichiu and 

 Aerobacter. The Aerohacter is generally of soil or plant origin, 

 but as high as 10% of fecal samples may be found to harbor these 

 organisms. Escherichia coli is considered to be primarily of fecal 

 origin. The interaction of two or more species of bacteria, how- 

 ever, may yield acid and gas from lactose due to synergism, dis- 

 cussed in an earlier chapter. For this reason, it is important that 

 positive presumptive tests be carried at least one step further before 

 the water can be said to contain sewage. 



THE CONFIRMED TEST 

 By employing selective media containing substances bacterio- 

 static for gram positive bacteria, the gram negative species present 

 in the presumptive lactose broth tubes will have a better oppor- 

 tunity to develop when they are streaked on the selective agar 

 media. Endo agar, eosine methylene blue agar, and others may 

 be used to confirm the presence of coliforms as the cause of the 

 positive presumptive test. Typical coliforms appear as colored 

 colonies with or without a metallic-like lustre on the surface when 

 examined in reflected light. A newer medium, Tergitol 7, appears 

 to be better than either Endo or eosine methylene blue agar as a 

 confirmatory medium, and after more research has been con- 

 ducted, Tergitol 7 may be preferred by more laboratories. 



COMPLETED TEST 



Having isolated suspicious-looking colonies on the selective 

 medium, the next step is to select a tvpical colony and inoculate 

 a fresh tube of lactose broth and a standard nutrient agar slant 

 with growth from such a colony. If this pure culture ferments the 

 sugar with acid and gas production, and if a gram stain of the 

 agar slant growth reveals short, gram negative rods, it has been 

 confirmed that the water contains colifoniis. 



Should the analyst desire to determine the species of the coli- 

 form with which he is dealing, a series of additional tests may 

 be run to pinpoint the organism as to genus and species. These 

 tests were discussed in Chapter 6. 



