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Test of Bacteria: Total count, Coli Form count

The term BACTERIUM -COLL (B-coll) used to indicate a group of organisms that include. The whole 'Coli - aero gens' group.


a.     Total count


b.     Coli - Form count


Total count = More popularly known as „PLATE COUNT? or „AGAR COUNT?. Test procedure consists of placing '1ml' or some other portion of water to be tested on a sterialelish '100m' in diameter. To this a sterile nutrient medium is added and the plate is incuvated at 200C for 48 hours' or at 370C for 24 hours Each bacterium would have formed am colony which can be seen on the plate and counted using a colony counter for POTABLE WATER the total count should not exceed 100 colonies/cc of water.


COLIFORM COUNT (Determination of coliform or B-coli)


COLIFORMS are defined as that group of organisms which includes all the aerobic facultative, grain stain negative, non-spore forming bacteria. This group of organisms ferments lactose with gas. Formation at 310C within 48hours.


The multiple tube fermentation Technique is used for the determination of coli forms. It consists of

a.     Presumptive test


b.     Confirmatory test


c.      Completed test




The preferred method for bacterial enumeration is membrane filter techniques in this procedure.


i)                   A measured volume of water is drawn through a cellulose acetate of glass fibre filter with the openings less than 0.5ill


ii)                The bacteria present in the sample will be retained upon the filter


iii)              The filter is rinsed with sterile buffer solution placed upon a pad saturated with suitable nutrient medium and incubated at an appropriate temperature.


iv)              The bacteria which are able to grow upon the nutrient medium will produce visible calories which can be counted.


v)                Each colony departs one bacterium in the original sample.



In order to arrive at the number of coli forms in a water sample, it is necessary to know the positive results from various size portions of a sample. The MPN of coli forms in the water is obtained by applying the laws of statistics to the results of test.





MPN of coli forms or B-coli is defined as that bacterial density which if actually present in the sample under examination would requently give the observed analytical results

In other words MPN indicates the bacterial density which is most likely to be present in water. The standard sample for potable water is 5 of 10mc portions. If all these are negative, the MPN


is zero. If only 1 out of 5 is true, then MPN is two.




Thomas has suggested the following approximate equation for calculation of MPN

MPN  = 100xNumber of tve portions / SqRt(ml in all -ve portionsml of all portions)


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