Impact of Long-Term Application of Treated Tannery Effluents on the Emergence of Resistance Traits in Rhizobium sp. Isolated from Trifolium alexandrinum

A total of 35 Rhizobium sp. were isolated from the root nodules of Trifolium alexandrinum (Egyptian clover) irrigated with treated tannery effluents and characterised on the basis of morphological, cultural and biochemical characteristics. Rhizospheric soils and plant parts were also analysed for metal concentrations by atomic absorption spectrophotometry. The test soil samples were contaminated with a high level of chromium and also with other heavy metals, i.e. Ni, Zn, Cu, and Cd. The heavy metal analysis of Trifolium alexandrinum plant parts revealed different accumulation of these metals in different plant parts, such as root, stem, and leaf. Trifolium alexandrinum roots accumulated the highest amount of these metals and this was followed by leaves. All the isolates of Rhizobium sp. were tested for their resistance against Cr3+, Cr6+, Cd2+, Cu2+, Zn2+, and Ni2+. The highest minimum inhibitory concentration (MIC) of 1600 µg/ml was observed against Cr3+ in 37.1% of the isolates. Some of the metal resistant isolates that showed maximum resistance were also tested for their resistance against 6 commonly used antibiotics, namely tetracycline, ampicillin, gentamycin, kanamycin, chloramphenicol, and nalidixic acid. Forty percent of Rhizobium sp. isolates were resistant against nalidixic acid and 33.3% were resistant to chloramphenicol and tetracycline.