SEWAGE CONTAMINATION 



97 



10. Evidence of Sewage Contamination. General Methods. 



It may be assumed that the presence of any or all of the 

 large group of colon bacilli in water or in food substances is indi- 

 cative of sewage contamination or contamination with fecal 

 matter. The colon bacilli are aerobic, nonsporeforming, motile, 

 short and produce acid and gas in dex- 

 trose and lactose media and develop 

 best at a comparatively high tempera- 

 ture (37° C). A practical presumptive 

 colon bacillus test depends upon the 

 characteristics thus indicated and is car- 

 ried out as follows: 



I. Presumptive Colon Bacillus Test. 

 — Add the substances to be tested 

 (water, sewage, mineral water, shellfish 

 liquor, washings from vegetables, etc.) 

 in o.oi cc, o.io cc, i cc, 5 cc. and 10 

 cc. quantities (or these equivalents in 

 dilutions) into fermentation tubes hold- 

 ing at least 40 cc. of lactose bile, incu- 

 bate at 37° C. and look for the forma- 

 tion of gas. If gas formation is observed 

 the presence of colon bacilli may be sus- 

 pected. If, in the case of water supplies 

 for example, the o.io cc. tubes show gas 

 formation then it may be reasonably as- 

 sumed that colon bacilli are present. If 



two out of five of such tubes give positive gas reactions, the 

 test may be considered conclusive. To test the gas formed, 

 fill the tubes showing gas formation with a 2-per cent, solution 

 of sodic hydrate, hold thumb firmly over the opening of the fer- 

 mentation tube and mix contents by tilting back and forth 

 carefully. The volume of gas absorbed is CO? whereas the un- 



FiG. 25. — Fermentation 

 tube. This type of fermenta- 

 tion tube is especially conven- 

 ient for making the gas deter- 

 mination with the colon bacil- 

 lus. Other forms of fermenta- 

 tion tube may be used. — (Fitt- 

 field.) 



