Should I stay or should I go: Transmission trade-offs in mobile genetic elements

Mobile genetic elements (MGEs), including temperate bacteriophages and conjugative plasmids, are major vectors of virulence and antibiotic resistance in bacterial populations. To maximize reproductive fitness, MGEs have to optimize horizontal and vertical transmission. Yet, the cost of horizontal transmission (e.g. phage lysis) puts these transmission modes at odds. Using virulence-transmission trade-off 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 closely align with experimental evidence on the regulation of transmission by MGEs. We further highlight gaps between theory and experiments, differences between phage and plasmids, and suggest areas for future research.