The effect of the selected heavy metals on the growth and proliferation of Streptomyces spp. isolated from soils

Mając na uwadze wzrost zanieczyszczenia środowiska naturalnego metalami ciężkimi i ich oddziaływanie na mikroorganizmy w glebie, podjęto próbę określenia wpływu wybranych metali ciężkich: miedzi, kadmu, cynku i ołowiu na wzrost promieniowców wyizolowanych z gleb pobranych z terenu huty ArcelorMittal w Krakowie. Do badań wykorzystano 5 gleb, z których do dalszych badań wyselekcjonowano 10 izolatów promieniowców należących do rodzaju Streptomyces. Przeprowadzone analizy wykazały, że obecność huty przyczynia się do zanieczyszczenia środowiska glebowego poprzez emisje metali ciężkich. Ma to wpływ także na populację promieniowców w glebie nie tylko poprzez ograniczenie ich liczebności, lecz również poprzez selekcję szczepów opornych na zanieczyszczenia gleb, a przez to na ograniczenie ich bioróżnorodności. Pośrednim skutkiem ograniczenia różnorodności promieniowców może być wyginięcie szczepów ważnych pod względem funkcjonalnym dla środowiska, zubożenie mikroflory gleby, a także zmiany właściwości fizykochemicznych gleby. Niniejsza praca pokazuje, że skażenie środowiska metalami ciężkimi jest poważnym problemem środowiskowym. Zjawisko to należy monitorować i kontrolować, aby zapobiec nieodwracalnym zmianom w ekosystemie.People have changed their environment since the beginning of the civilization - it is an inevitable process. However, this phenomenon is accompanied by emission of many harmful chemicals to the environment, and recently there has been an increase in the amount of pollution. Having regard to the increase in environmental contamination with heavy metals due to the expanding industry and its impact on soil microorganisms, this study attempted to determine the effect of the selected heavy metals: cadmium, copper, lead and zinc, on the growth of actinobacteria (belonging to the genus Streptomyces) isolated from soils of ArcelorMittal steelworks in Cracow. Five soil samples were selected for the analysis, from which 2 isolates of Streptomyces spp. were selected for further analyses. The number of Streptomyces spp. in the soil samples was determined using the serial dilutions method. The systematic affiliation of microorganisms was confirmed by macroscopic as well as microscopic observations of Gram-stained smears, catalase reaction test and the sequencing of 16S rDNA region. Additionally, soil physical and chemical properties were examined - pH, moisture and heavy metal content - using atomic spectrometry. The heavy metal resistance of the examined Streptomyces spp. strains was analyzed using a well method with increasing concentrations of cadmium, copper, lead and zinc. After the incubation results were recorded as the growth inhibition zones (diameters) and the minimum inhibitory concentration (MIC) of the heavy metals were determined. The conducted analyses revealed that heavy metal concentrations in two out of five examined soils exceeded the admissible values given by the Regulation of the Minister of Environment on soil quality standards. The number of actinobacteria in soils was varied (from 6700 to almost 250 thousand CFU per gram of soil dry weight). There was a negative correlation between the heavy metal concentrations in soils and the number of the studied microorganisms. The analyzed soils were alkaline - the lowest pH value was 7.7. Regarding the heavy metal resistance of the analyzed Streptomyces spp. strains, diverse in vitro susceptibility was recorded. The bacterial reactions differed even between strains isolated from the same soil samples. Minimum inhibitory concentrations of the examined heavy metals were as follows: Cd and Cu - 100 μg • cm−3, Zn - 3000 μg • cm−3 and Pb - 8000 μg • cm−3. There was a negative correlation between the concentration of heavy metals in soils and the growth inhibition zones (in the case of cadmium this correlation was high), indicating that the soil contamination with cadmium may have modified the soil population of Streptomyces spp., causing extinction of the susceptible strains while the ones with reduced sensitivity survived. The results of the presented research indicated that the ArcelorMittal steelworks contaminates the soil environment, which may affect the microbial community of soils, not only by reducing the population, but also by decreasing biodiversity of soil microorganisms, which was particularly visible in the case of cadmium - a strongly toxic heavy metal. This study demonstrates that the heavy metal contamination is a serious environmental problem. This phenomenon needs to be monitored and controlled to prevent irreversible changes to the ecosystem