A NOVEL NICKEL RESISTANCE DETERMINANT FOUND IN SEWAGE-ASSOCIATED BACTERIA

Nickel-resistant bacteria were isolated from effluent discharged from a sewage treatment outfall over an 18-month period. One of these strai ns, Enterobacter cloacae FBA30, was found to harbor a narrow host rang e conjugative plasmid, designated pFBA30, which confers nickel resista nce on its host. A 10.2-kb SstI restriction fragment was cloned from p FBA30 and was shown to specify inducible nickel resistance, both in it s original host and in laboratory strains of Escherichia coli. This DN A fragment, and a 1.75-kb SmaI fragment derived from it, were used as probes to examine other bacterial strains isolated during the study. H omologous nickel resistance genes were detected in enteric bacteria em erging directly from the treatment plant, but not in strains isolated from sediments downstream of the outfall, or in strains harboring well -characterized determinants such as ncc/nre, cnr, or Klebsiella oxytoc a type. Thus, this element constitutes a new nickel resistance determi nant and was named nrf. Nickel-resistant strains were sorted into two groups, based on the sampling date and the size of restriction fragmen ts homologous to probes developed from pFBA30. Group A isolates all ca rried a homologous 6.5-kb PvuII restriction fragment and expressed nic kel resistance constitutively. Group B strains, isolated independently from the same outfall, were different from group A in that the nickel resistance was associated with a 4.7-kb PVuII fragment, and group B s trains displayed a slower growth rate on nickel salts media. Each grou p contained a range of enteric bacterial species, including K. oxytoca , Citrobacter freundii, and Enterobacter spp. Localized genetic exchan ge probably occurs within wastewater treatment effluents, and, once ef fluents are discharged into the environment, enteric bacteria carrying pFBA30-like determinants do not persist downstream.