Phenotypic Plasticity in Bacterial Plasmids

Abstract

Plasmid pB15 was previously shown to evolve increased horizontal (infectious) transfer at the expense of reduced vertical (intergenerational) transfer and vice versa, a key trade-off assumed in theories of parasite virulence. Whereas the models predict that susceptible host abundance should determine which mode of transfer is selectively favored, host density failed to mediate the trade-off in pB15. One possibility is that the plasmid's transfer deviates from the assumption that horizontal spread (conjugation) occurs in direct proportion to cell density. I tested this hypothesis using Escherichia coli/pB15 associations in laboratory serial culture. Contrary to most models of plasmid transfer kinetics, my data show that pB15 invades static (nonshaking) bacterial cultures only at intermediate densities. The results can be explained by phenotypic plasticity in traits governing plasmid transfer. As cells become more numerous, the plasmid's conjugative transfer unexpectedly declines, while the trade-off between transmission routes causes vertical transfer to increase. Thus, at intermediate densities the plasmid's horizontal transfer can offset selection against plasmid-bearing cells, but at high densities pB15 conjugates so poorly that it cannot invade. I discuss adaptive vs. nonadaptive causes for the phenotypic plasticity, as well as potential mechanisms that may lead to complex transfer dynamics of plasmids in liquid environments.

Evolution of early male-killing in horizontally transmitted parasites

“…1). Also, there might be a trade-off between the VT efficacy j and HT rate l [15,16], which we model as…”

Competition favours reduced cost of plasmids to host bacteria

“…Simulation design and programming Our model was inspired by the mass-action chemostat models of Levin et al (1979) and Hansen and Hubbell (1980), but adapted to serial-transfer experiments such as those performed by Turner (2004) and Dahlberg and Chao (2003). Our model considered the growth of bacterial populations in the presence of a limiting nutrient C, with periodic dilution into fresh medium with a nutrient concentration C 0 every 24 h. Simulated cultures contained up to four distinct populations: plasmidfree cells (N), cells hosting a tra-repressed plasmid P fin þ (P 1b ), new recipients of P fin þ not yet repressed by fin (P 1a ), and cells hosting a tra-derepressed plasmid P finÀ (P 2 ).…”

“…Furthermore, increased horizontal transfer might reduce vertical transmission by slowing host growth; such tradeoffs have been predicted and observed for diverse parasites (Bull et al, 1991;Kover et al, 1997;Kover and Clay, 1998;Messenger et al, 1999;Dahlberg and Chao, 2003;Jensen et al, 2006). Plasmids display a tradeoff between conjugation rate and vertical transfer (Turner et al, 1998;Turner, 2004), and have been shown to evolve reduced cost to the host at the expense of horizontal transfer when the environment limits conjugative spread (Dahlberg and Chao, 2003).…”

What traits are carried on mobile genetic elements, and why?

“…Infectious elements such as plasmids are likely to face a tradeoff between horizontal transfer and vertical transmission, mediated by the costs that they impose on their hosts (Turner et al, 1998;Turner, 2004;Haft et al, 2009). Any tradeoff between HGT and costs to the host can be viewed as a form of the much-discussed virulence-transmission tradeoff (Alizon et al, 2009).…”