Amplification of mer A fragments from mercury resistant strains and cloned mer operons using an optimized PCR protocol with primers GC-merAf and merAr on an ethidium bromide-stained agarose gel. On the left hand, environmental isolates (black), biocatalyzer inoculum strains (violet) and the main invader (red) are shown. On the right hand, mer A-containing vectors kindly provided by M. Osborn  are shown. For strain identification of inoculum strains and invader see Fig. 5; environmental isolates were Klebsiella sp. Spi5; P. putida Elb5; Citrobacter freundii Bro62; Sphingomonas sp. Spi7.