Should I stay or should I go: transmission trade-offs in phages and plasmids

Mobile genetic elements (MGEs), like temperate bacteriophages and conjugative plasmids, are major vectors of virulence and antibiotic resistance in bacterial populations. For reproductive success, MGEs must balance horizontal and vertical transmission. Yet, the cost of horizontal transmission (metabolic burden or host death) puts these transmission modes at odds. Using virulence-transmission trade-off (VTT) theory, we identify three groups of environmental variables affecting the balance between horizontal and vertical transmission: host density, host physiology, and competitors. We find that general theoretical predictions of the optimal response to environmental cues align with experimental evidence on the regulation of transmission by phages and plasmids. We further highlight gaps between theory and experiments, differences between phages and plasmids, and suggest areas for future research.