Direct Measurement of Phage phi29 Stiffness Provides Evidence of Internal Pressure

Using AFM nanoindentation experiments, DNA-full phi29 phage capsids are shown to be stiffer than when empty. The presence of counterions softens full viruses in a reversible manner, indicating that pressure originates from the confined DNA. A finite element analysis of the experiments provides an estimate of the pressure of ∼40 atm inside the capsid, which is similar to theoretical predictions.