The emergence and transfer of antibiotic-resistant bacteria and antibiotic resistance genes in aquatic environments: effects of plankton interactions

Student thesis: Doctoral Thesis


The environmental dimensions of the global health problem of antibiotic resistance have remained largely uncharted, particularly in regard to biotic interactions of waterborne bacteria. This thesis studied the effect of phytoplankton and zooplankton on horizontal gene transfer (HGT) in Enterococcus faecalis strains in experimental microcosms.
HGT was investigated between an E. faecalis donor strain with the plasmid borne vancomycin resistance gene vanA and rifampicin-resistant recipient strains. Gene transfer frequency (GTF) of vanA in centrifugation-induced liquid mating assays at 37 oC (10-7
) significantly exceeded GTF at 20 oC or 30 oC (10-8). Enhanced nutrient availability increased GTF from 10-7 to 10-5 in liquid mating assays, thus documenting the importance of energy resources to conjugation in E. faecalis.
Active filter feeding increased vanA GTF in Daphnia-bacteria microcosms to a range of 1.5 × 10-8 – 3.3 ×10-7, and ingested enterococci remained viable inside Daphnia for at least four days. Filter feeding zooplankton may thus facilitate the emergence of multi-resistance and disseminate transconjugants in aquatic environments.
Microalgae zooplankton feeds on may further enhance HGT. Microcosm experiments documented that Palmellopsis sp. enhanced vanA GTF by an order of magnitude, indicating that the algal cell mucilage was a suitable attachment substrate and energy source for conjugation.
The ciliate Tetrahymena pyriformis also facilitated HGT in E. faecalis. In the presence of live ciliates vanA GTF was 10-6 – 10-5
, an order of magnitude higher than in ciliate-free controls. GTF was higher within vesicles than in the ambient medium; it increased with time and peaked after 24 h.
Overall, this thesis highlights the potential effects of interactions between eukaryotic freshwater organisms and Enterococcus faecalis as a representative of Gram positive bacteria on the emergence and spread of antibiotic resistance. It also offers a projection of gene transfer frequencies under environmentally relevant conditions in lakes and wastewater treatment plants.
Date of AwardJul 2020
Original languageEnglish
SupervisorJoerg Arnscheidt (Supervisor) & James Dooley (Supervisor)


  • Horizontal gene transfer
  • Conjugation
  • Enterococcus faecalis
  • Biofiltration
  • Zooplankton
  • Daphnia
  • Tetrahymena
  • Microalgae

Cite this