It has been agreed by most workers in water bacteriology that the plain Lactose Broth usually used for presumptive test is not altogether satisfactory, in that it gives many false positive tests. A number of modifications have been suggested to eliminate as far as possible, these false positive tests. In most of the modifications, dyes are used to restrain the growth of gram-positive organisms, which are cause of many of the false positive presumptive tests obtained in plain Lactose Broth. Addition of basic fuchsin in plain Lactose Broth has been advocated by Ritter (2).
Fuchsin Lactose Broth is a selective medium, which may be used in parallel with Lactose Broth (M026) in the control of water filtration plant operation (1). Basic fuchsin inhibits many gram-positive organisms, which are responsible for false positive results. However Fuchsin Lactose Broth may not be used as Lactose Broth with all waters, but could be used as a confirmatory medium. This was studied by McCrady while studying procedures for the detection of the presence of coliforms in water (3).
Acid production is observed by the formation of pink to red medium whereas non-fermenters will show no change in the color of the medium.
Peptone special and meat extract in the medium provides nitrogen and other nutrients necessary to support bacterial growth. Basic fuchsin inhibits many gram-positive organisms, which are responsible for false positive results. Lactose is the fermentable carbohydrate.
Storage and Shelf-life:
Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label.
References:
1. Standard Methods for the Examination of Water and Sewage, 1946, 9th Ed., p. 226.
2. Ritter, 1932, J. Am. Water Works Assoc., 24:413.
3. McCrady, 1937, Am. J. Publ. Health, 27:1243.