Pollution-Induced Community Tolerance (PICT) of Bacteria: Evaluation in Phenol- and Antibiotic Polluted Soil

Abstract: The effects of organic pollutants on the soil microbial community were studied in this work. The main aim was to evaluate the use of pollution-induced community tolerance (PICT) in studying the effects of pollution. The leucine incorporation technique (indicating bacterial growth) was used to monitor changes in tolerance. The impact of toxic substances on bacterial and fungal growth (acetate-in-ergosterol incorporation) and microbial community structure (PLFA) was also studied. Increased PICT was found at increasing concentrations of both phenols and antibiotics (tylosin and sulfamethoxazole, SMX) in soil. However, at higher concentrations of phenol and tylosin no further increase in PICT was observed. The extent of PICT varied depending on the toxicant. Re-development of PICT was found in tylosin-polluted soil, indicating that PICT can be used to monitor soil remediation. The specificity of PICT was studied by examining co-tolerance to other phenols (2-chlorophenol, 2,4-dichlorophenol and 2,3,6-trichlorophenol) and metals (Cu and Zn) in phenol-polluted soil. Co-tolerance to other phenols was observed, however, always to a lower extent than to phenol. No co-tolerance to metals was seen. Direct inhibiting effects on bacterial growth were correlated to PICT. In phenol-polluted soil bacterial growth recovered over time and growth rates at high phenol concentrations were higher than in the unpolluted controls. In tylosin-polluted soil bacterial growth rates recovered to those in the unpolluted soil, while in SMX-polluted soil bacterial growth recovered only slightly over time. PICT to phenol and SMX was reflected in structural changes of the soil microbial community. In SMX-polluted soil a decrease in bacterial PLFAs was found, together with a corresponding increase in fungal PLFAs. A dramatic decrease in bacterial growth and increase in fungal growth were also observed after adding different bacterial antibiotics to soil, indicating competition between fungi and bacteria in soil. The leucine incorporation technique was shown to be a fast and sensitive method of estimating PICT and the direct inhibition of soil bacteria exposed to organic pollutants. The PICT concept was found to be a valuable tool in determining the toxic effects of organic pollutants and in distinguishing the toxic effects from other environmental factors. However, co-tolerance between different toxicants can complicate the interpretation of PICT.