One of the problems facing the production of biosensors
for direct detection of bacteria is the sensitivity of
assay in real samples. The infectious dosages of pathogens
such as Salmonella or E. coli O157:H7 is 10 cells
and the existing coliform standard for E. coli in water is
4 cells/100 ml. The Environmental Protection Agency
regulations specify the minimum frequency of water
sampling and the maximum number if coliform organisms
allowed; treated drinking water should contain no
coliforms in 100 ml (Federal Register, 1991; Greenberg
et al., 1992). Hence, a biosensor must be able to
provide a detection limit as low as single coliform
organism in 100 ml of potable water, with a rapid
analysis time at a relatively low cost. Only in this case
will the biosensor be convenient for on-line testing of
bacterial pathogens in real samples. Thus, sensitivity is
another issue that still requires improvement. There is
also a problem of distinguishing between live and dead
cells.