Bacterial Concentrations and Water Turbulence Influence the Importance of Conjugation Versus Phage-Mediated Antibiotic Resistance Gene Transfer in Suspended Growth Systems

Despite the abundanceof phage-borne antibiotic resistance genes(ARGs) in the environment, the frequency of ARG propagation via phage-mediatedtransduction (relative to via conjugation) is poorly understood. Weinvestigated the influence of bacterial concentration and water turbulencelevel [quantified as Reynold's number (Re)]in suspended growth systems on the frequency of ARG transfer by twomechanisms: delivery by a lysogenic phage (phage & lambda; carryinggentamycin-resistance gene, genR) and conjugationmediated by the self-transmissible plasmid RP4. Using Escherichia coli (E. coli) as the recipient, phage delivery had a comparable frequency (1.2 & PLUSMN; 0.9 x 10(-6)) to that of conjugation (1.1 & PLUSMN; 0.9 x 10(-6)) in suspensions with lowcell concentration (10(4) CFU/mL) and moderate turbulence(Re = 5 x 10(4)). Turbulence affectedcell (or phage)-to-cell contact rates and detachment (due to shearforce), and thus, it affected the relative importance of conjugationversus phage delivery. At 10(7) CFU/mL, no significant differencewas observed between the frequencies of ARG transfer by the two mechanismsunder quiescent water conditions (2.8 & PLUSMN; 0.3 x 10(-5) for conjugation vs 2.2 & PLUSMN; 0.5 x 10(-5) forphage delivery, p = 0.19) or when Re reached 5 x 10(5) (3.4 & PLUSMN; 1.5 x 10(-5) for conjugation vs 2.9 & PLUSMN; 1.0 x 10(-5) forphage delivery, p = 0.52). Transcriptomic analysisof genes related to conjugation and phage delivery and simulationof cell (or phage)-to-cell collisions at different Re values corroborate that the importance of phage delivery relativeto conjugation increases under either quiescent or turbulent conditions.This finding challenges the prevailing view that conjugation is thedominant ARG transfer mechanism and underscores the need to considerand mitigate potential ARG dissemination via transduction.