120
But in all cases Saccharose and Dulcit have been tested so that the coli groups can be divided up into MacConkey's four sub-groups according as they ferment or not these two sugars.
THE VALUE OF B. COLI AS AN INDICATOR.
It must be admitted that the ordinary standards for B. Coli in drinking water used in England do not appear to be strictly applicable to a Tropical Water.
Not much work has been done as regards Tropical Waters, but as far as it goes the evidence seems to indicate that a much stricter series of tests must be used for B. Coli in the Tropics than is usually required at Home.
Using group tests such as the "Excretal
*
or Flaginac" and using a Home standard, many tropical waters which are in use would be at once condemned.
In the absence of sufficient investigation, reliance appears to have been placed on the absence of water-borne epidemics among he people drinking a water-supply with the Coli Group present in small quantities of the water.
It is hardly necessary to point out how very unsafe it is to base an opiniou about the potability of the water on the absence of water-borne epidemics for a time.
It is impossible here to go thoroughly into the question of Coli in water at Home for that reference must be made to the long series of investigations made by Houston, Savage, MacConkey and others; but a few points may be noted about which there does not seem to be any great difference of opinion.
Houston has shown that domestic sewage usually contains 100,000 B. Coli per cc. and that in fæces they are 10 to 100 times as numerous. Thus with even one B. Coli in lcc. of water a very large dilution has taken place.
n
There are unfortunately no means of differentiating between B. Coli of man and those of animals; or between B. Coli from a case of Typhoid and from a normal healthy individual.
The Pathogenicity of the isolated coli is also no help in determining whether a water is dangerously infected or not. "Virulence as a property of B. Coli is a very variable element" (Savage, 1903).
It is sometimes stated that the pollution of water is due to Birds and Fish but as has been pointed out by Houston polluted waters do not have a monopoly of these and in non-polluted areas B. Coli is relatively absent.
Of course the chief value of B. Coli as an indicator turns on the point as to whether it is really confined to focally contaminated waters or whether it is present generally
where there is no chance of such contamination.
On this point Savage (1906 p. 141) considers that it is not too much to state "that there is no evidence or observations which have ever shown that B. Coli, reasonably defined, is present in any numbers in sources which have not been exposed to some form of focal contamination."
This statement appears to represent the general opinion on the subject at Home.
A further point is that under conditions such as are met with in connection with the examination of water supplies, there is no evidence of multiplication of true coli in the water, but on the contrary, gradual decrease and extinction. Under similar conditions, no multiplication of pathogenic organisms would take place; on the contrary they would die out much more quickly than the B. Coli do.