Effects of temperature and pH on conjugal transfer of zinc-resistant plasmids residing in Gram-negative bacteria isolated from industrial effluents

Two zinc (Zn)-resistant strains, AnZn-1 and AnZn-2, which were resistant to ZnSO 4 up to 12.5 mg ml −1 were isolated from industrial effluents. Both were Gram-negative with motile cells. They exhibited tolerance to Ba 2+, Ni +, Co 2+ Mn 2+, Cu 2+, Fe 2+, Ni 2+, Cd 2+, kanamycin, chloramphenicol, ampillicin and tetracycline, but were sensitive to Hg 2+ and streptomycin. For AnZn-1 and AnZn-2, the optimum pH for growth was 7. Both were facultative anaerobes and had cytochrome oxidase and urease enzymes, while catalase was present only in AnZn-2. Both strains had the ability to hydrolyse gelatin, reduce nitrate, and yield acid from arabinose and rhamnose. The two strains shared maximum characters with Vibrionaceae. Each strain carries a single Zn-resistant conjugative plasmid. The plasmid residing in AnZn-1 (pSH1211) displayed a lower level of resistance than the plasmid of AnZn-2 (pSH1212). Both required a minimum of 24 h for mating and showed highest transfer frequency at 25°C. pSH1211 preferred pH 7 and pSH1212 pH 8.5 for their transfer. Both plasmids, when allowed to mate with Escherichia coli at 25°C, alkaline pH values of 8–8.5 (pSH1211) of pH 7.5 (pSH1212), showed increased transfer frequency.