Screening halo formation

Screening based on halo formation is not only suited for the identification of single enzyme activities but may also be used to screen for whole metabolic pathways. Aarons et al. recently set up a system to screen for a functional enzyme cascade that is involved in the production of substances that act as antibiotics towards an indicator bacteria strain but not the producer strain. Using the Bacillus overlay-assay [50] they were able to isolate genes capable of complementing P. fluorescens FI 13 gacS and gacA mutants [51]. 

A number of particular semi-quantitative screening methods in agar-plate formats depend on the secretion of (hydrolytic) enzymes. Clones that secrete active enzymes produce a so-called halo , i.e. a zone of clearing in turbid solid growth medium with suspended substrate. In many cases the size of the halo is a good measure for the amount of enzymatic activity released by the growing colony. 

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