PUB - Publikationen an der Universität Bielefeld

Artificial laboratory evolution was used to produce mutant strains of Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA) able to survive on antimicrobial metallic copper surfaces. These mutants were 12- and 60-fold less susceptible to the copper-mediated contact-killing process than their respective parent strains. Growth of the mutant and its parent in complex growth medium was similar. Tolerance to copper ions of the mutants was unchanged. The mutant phenotype remained stable over about 250 generations under non-stress conditions. The mutants and their respective parental strains accumulated copper released from the metallic surfaces to a similar extent. Nevertheless, only the parental strains succumbed to copper stress when challenged on metallic copper surfaces, suffering complete destruction of the cell structure. Whole genome sequencing and global transcriptome analysis were used to decipher the genetic alterations in the mutant strains; however, these results did not explain the copper-tolerance phenotypes on the systemic level. Instead, the mutants shared features with those of stressed bacterial sub-populations entering the early or "shallow" persister state. In contrast to the canonical persister state, however, the ability to survive on solid copper surfaces was adopted by the majority of the mutant strain population. This indicated that application of solid copper surfaces in hospitals and elsewhere has to be accompanied by strict cleaning regimens to keep the copper surfaces active and prevent evolution of tolerant mutant strains.Significance Microbes are rapidly killed on solid copper surfaces by contact-killing. Copper surfaces thus have an important role to play in preventing the spread of nosocomial infections. Bacteria adapt to challenging natural and clinical environments through evolutionary processes, for instance by acquisition of beneficial spontaneous mutations. We wishes to address the question whether mutants can be selected that have evolved to survive contact-killing on solid copper surfaces. We isolated such mutants from E. coli and S. aureus MRSA by artificial laboratory evolution. The ability to survive on solid copper surfaces was a stable phenotype of the mutant population and not restricted to a small sub-population. As a consequence, standard operation procedures with strict hygienic measures are extremely important to prevent emergence and spread of copper-surface-tolerant persister-like bacterial strains if copper surfaces are to be sustainably used to limit the spread of pathogenic bacteria e.g., to curb nosocomial infections. Copyright © 2020 American Society for Microbiology.