Plasmid incidence in bacteria from agricultural and industrial soils

Heavy metal contents of agricultural and industrial soils were determined by atomic absorption spectrophotometry. The analysis of the samples collected from two different locations revealed significantly high levels of Fe, Zn, Cu, Cr and Ni. Certain microbiological parameters ( total aerobic heterotrophs, asymbiotic N-2-fixers, total Actinomycetes and fungi) were also monitored from these soils. A total of 70 bacterial isolates from agricultural and industrial soils were examined for plasmid DNA content and resistance to the antibiotics amoxycillin, cloxacillin, chloramphenicol, doxycycline methicillin, nalidixic acid, and tetracycline. Minimum inhibitory concentrations ( MICs) of Cu, Cr, Pb, Cd, Hg, Zn, and Ni for each isolate were also determined. Resistance was most frequent to methicillin (48.5%), cloxacillin (45.7%), and nalidixic acid (40%) for all isolates of bacteria. The highest MICs observed were 100 mug/ml for mercury, 800 mug/ml for Ni and 1600 mug/ml for other metals. The incidences of metal resistance and MICs of metals for bacteria from industrial soil were significantly different to those of agricultural soil. On a percentage basis, 91.4% of the total bacterial isolates from industrial soil were found to harbour plasmids whereas 40% of the isolates from agricultural soil contained plasmids.