Prevention of horizontal transfer of laboratory plasmids to environmental bacteria: comparison of the effectiveness of a few disinfection approaches to degrade DNA

The routine work of any molecular biology laboratory includes the daily use of microorganisms, including strains of E. coli, transformed with a variety of plasmids expressing at least one antibiotic resistance gene (ARG). Therefore, to avoid the accidental release of ARGs into environmental water, methods for disinfection of liquid laboratory waste must be effective in destroying nucleic acids. In support of this recommendation, the origin of replication of Enterobacteriaceae plasmids has been detected in strains of non-Enterobacteriaceae bacteria isolated from wastewater from laboratories and research institutes, suggesting that interspecific transfer of laboratory plasmids had occurred. Using quantitative polymerase chain reaction, we determined the decimal reduction value (D value, expressed as concentration of disinfectant or length of physical treatment) of several decontamination methods for their DNA degradation effect on cultures of E. coli Top10 transformed with a kanamycin resistant plasmid (pET28A + or pEGFP-C2). The estimated D values were 0.7 M for sulfuric acid, 6.3% for a commercial P3 disinfectant, 25 min for steam sterilization at 121 °C, and 49 min for disinfection by UVC. A 20-min treatment of bacteria cultures with a final concentration of 1–10% sodium hypochlorite was found to be ineffective in completely destroying a bacteria plasmid gene marker (coding for the pBR322 origin of replication). Residual DNA from NaClO-treated cells was 60%, while it decreased under 10% using the commercial disinfectant P3 diluted at 5%. As the degradation was incomplete in both cases, we recommend avoiding discharge of disinfected liquid waste to wastewater (even after chemical neutralization) without additional plasmid destruction treatment, to prevent horizontal transfer of laboratory ARGs to environmental bacteria.