Heavy-metal-tolerant bacteria in agricultural soils contaminated by Pb and Zn

Heavy-metal-tolerant bacteria in the agricultural soils around a lead-zinc smeltery were investigated. The results show that with increase in pollution, the number of culturable bacteria dropped drastically whereas the proportion of heavy-metal-tolerant bacteria rose. The adaptabilities of bacteria to Pb and Zn stresses vary. After a long time of screening, most strains of bacteria became able to stand low concentration of Pb ( < 2 mmol·L ^-1 ) and some grew dependent on low concentration of Pb (0.5 mmol·L ^-1 ). No strains, however, were found tolerant to concentration higher than 4 mmol·L ^-1 Pb. Zn contamination, when reaching 0.5 mmol·L ^-1 in concentration, began inhibiting bacteria activity. But in all the samples investigated, bacteria were found tolerant to Zn, ranging from 4 to 6 in mmol·L ^-1 concentration and some even to Zn of 8 mmol·L ^-1 in concentration. The number of strains tolerant to both Pb and Zn was very limited, so was their tolerance to 2 mmol·L ^-1 and below only.