Tolerance of Xanthomonas campestris for heavy metals typically associated with brownfield land

Heavy metals as environmental contaminants often exhibit high latency and persistence in soil systems. The capacity of Xanthomonas campestris to tolerate 1mM – 15mM concentrations of lead, cadmium and chromium(VI) which are typically associated with soil pollution due to industrial activities was assessed in this study. Bacterial abundance and enzymatic activity were seen to decrease with increasing heavy metal concentration and contact time. Cadmium was the strongest inhibitor of growth while chromium(VI) was identified as the most toxic to X. campestris with regards to enzyme activity. The viable bacterial counts of Xanthomonas campestris after exposure to the heavy metals dropped from 0.97 – 1.03 Log CFU/mL on day 0 to mean values ranging from 5.92 – 0.00 Log CFU/mL on day 7 of the study at the different concentrations applied representing reductions of 26.49% – 100%, 33.76% – 100% and 33.38% – 100% for Pb, Cd and Cr(VI) respectively. The changes in growth differed significantly (p≤0.05) between groups from one heavy metal to the other and within groups from one heavy metal concentration to the other. Pb, Cd and Cr(VI) all inhibited oxidase, catalase and protease activity at higher concentrations. Only Cd exerted an inhibitory effect against α-amylase activity but only at 10 mM and 15 mM concentrations after 5 – 7 days exposure. Pb and Cr(VI) only exhibited inhibition against cellulase activity by the end of the study at the highest concentration of 15 mM. The study established that acute exposure to the heavy metals, lead, cadmium and chromium (VI) had potential to inhibit growth and certain enzymatic activities in X. campestris.