SUMlClARY:The growth of Bacterium coli on agar media containing bile salts is conditioned by a number of factors. The proportion of total cells able to grow on a medium not containing bile salts rapidly declines at temperatures of incubation above 43". On a medium containing bile salts and lactose a distinctly inhibitory effect is observed at 37'; with most strains this effect is still more pronounced a t 4 4 ' . Some brands of bile salts are appreciably more inhibitory than others. Inclusion of phosphate in a bile salts medium introduces a markedly inhibitory factor, the severity of which varies with the strain of organism; some strains are virtually unable to grow on such a rnediuni.When a culture of Bact. coli is suspended in water containing only small concentrations of inorganic salts an iticreasing proportion of the population becomes attenuated so that the cells are unable to grow on bile salts lactose agar at 44'. An occasional strain may also exhibit sensitiveness to the presence of neutral red. This attenuating effect may be largely decreased if, before the inoculum is mixed with the bile salts and agar, it is subjected t o a short period of incubation with lactose broth. This treatment has been made the basis of a technique for obtaining a colony count of Bact. coli which is applicable t o polluted waters. (1939). Disadvantages of the method are the large quantity of medium required and the large error involved in the computation of the most probable number. Thus Halvorson & Ziegler (1933) calculated that, with five tubes of medium to each dilution, the count obtained will be between 70% below and Z60y0 above the true value.Enumeration of this organism provides a simple method of following the eff'ect of different treatments of sewage on the numbers of faecal bacteria discharged into a river and of the rate a t which these numbers decrease as the organisms are carried downstream. A colony count of Bact. coli would introduce into sanitary surveys of this character a welcome improvement in accuracy. Clegg & Sherwood (1947) introduced a roll-tube method of counting faecal coli in shellfish in which inocula were mixed with a modified MacConkey agar and the tubes were incubated a t 44". Preliminary tests showed, however, that the ability of cells of Bact. coli to form colonies on bile salts lactose agar
SUMMARY: The effect of various conditions on the rate of death of Bacterium coli and of Streptococcus faecalis in dilute buffer solutions was followed by making colony counts at intervals during prolonged periods of incubation and then constructing mortality curves. The form of these curves was characteristic for each organism. Of the pH values investigated (approximately 5, 6, 7 and 8) both organisms were least viable in the range 6 to 7. Strep. faecalis was not affected by the degree of oxygenation of the water but Bad. coli died much more rapidly under anaerobic than under aerobic conditions. Growth of Bact. coli was observed with as little as 0.28 part per million organic matter in solution. A much higher concentration of nutriment was required for growth of Strep. faecalis; with a concentration slightly below that required for growth the viable population remained virtually stable for a long period. The rate of death depended on the age of the cells a t the time of immersion in water.Factors affecting the longevity in water of organisms used as indices of pollution are of importance in the sanitary survey of rivers or in assessing the suitability of water for drinking purposes. Reports in the literature suggest that the period of survival may be conditioned by a number of factors but the conclusions drawn are often open to doubt owing to failure t o arrange for conditions to be the same in comparable experiments. Moreover, in earlier work there is often some doubt of the precise characters of the organism referred to as Bacterium coli. \'ery few observations appear to have been made on the longevity of Streptococcus fa
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