66 



LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



duced into water sterilized by autoclaving or by boiling, the results shown 

 in Figure 2 are obtained. In the autoclaved water the death rate of the 

 coliform bacteria is very slow. The maximum decrease observed was to one- 

 fifth of the total population in a period of seven days. In the boiled water 

 the death rate is more rapid and is similar to the rate found in the untreated 

 sea water. It is clear from these results that the bactericidal action of sea 

 water is destroyed by the heat of autoclaving, but is unaffected by the 

 milder boiling treatment. 



Another observation shown in this figure is that a second or subsequent 

 inoculum of E. coli dies off more rapidly than the first. In these experi- 



T" 



(8 V~-».^ Aoloelo»»4 



{ ^ ft '~'~-»^ S«a Wol«f 



\ 



J i__L 



_i I I i_ 



\ \ 



—I Llfc-i I 1 lA 



J I ■ 



Time in Days 



Tim« in Days 



Fig. 2. The viability of Escherichia coli in autoclaved and in boiled sea water. The 

 original inoculation of E. coli provided a population between SxlO'^ and 5x10^ cells/ml. 

 The surviving fraction is plotted. Different symbols represent different experiments. 



ments the water receiving the first inoculum was stored until no viable cells 

 were found. This took about 20 days for the autoclaved sea water, 5 

 to 10 days for the boiled water. When another inoculum of E. coli was 

 added to the water, the bacterial counts decreased as shown by the heavy 

 lines. The death rate in the reinoculated autoclaved water is eight times 

 as great as the rate observed for the first inoculum. A threefold increase 

 in rate was observed with the boiled water. It may be presumed that the 

 greater death rates observed with reinoculated water correspond to what 

 would be obtained in polluted estuaries. 



The coefiicients of death rate, as shown in Table I, summarize the 

 results of these laboratory investigations. This coefficient is the reciprocal 

 of the time (in days) for the population to decrease to one-tenth its original 



