Heterotrophic Plate Count (HPC)
The heterotrophic plate count (HPC), formerly known as the standard plate count, is a procedure for estimating the number of live, culturable heterotrophic bacteria in water and for measuring changes in swimming pools or during water treatment and distribution. Colonies may arise from pairs, chains, clusters, or single cells—all of which are included in the term colony-forming units (CFU). The final count also depends on interaction among developing colonies.
Enzyme substrate tests use hydrolyzable chromogenic and fluorogenic substrates to simultaneously detect enzymes produced by total coliforms and Escherichia coli (E. coli). In this method, total coliform bacteria produce the enzyme β-d-galactosidase, which cleaves the chromogenic substrate in the medium to release chromogen. Most E. coli strains produce the enzyme β-glucuronidase, which cleaves a fluorogenic substrate in the medium to release fluorogen. The release of chromogen indicates that coliform bacteria are present, and the release of fluorogen indicates that E. coli are present.
The membrane filter (MF) technique is reproducible, can be used to test relatively large sample volumes, and usually yields numerical results more rapidly than the multiple-tube fermentation procedure. It is useful in monitoring drinking water and various natural waters.
The colorimetric method is useful for the determination of hexavalent chromium in a natural or treated water in the range from 100 to 1000 ug/L. This range can be extended by appropriate sample dilution or concentration and/or use of longer cell paths.