Mechanics of Solid Tissue Invasion by the Mammalian Pathogen Pythium insidiosum

The relative significance of mechanical penetration versus the action of substrate-degrading enzymes during solid tissue invasion has not been established for any fungal disease. Pythium insidiosum is an oomycete fungus (or stramenopile) that causes a rare, but potentially lethal infection in humans and other mammalian hosts. Experiments with miniature strain gauges showed that single hyphal apices of this pathogen exert forces of up to 6.9 μN, corresponding to maximum pressures of 0.3 μN μm⁻² or MPa. Samples of cutaneous and subcutaneous tissue from fresh human cadavers displayed a mean strength (resistance to needle puncture) of 24 μN μm⁻², and a mean pressure of 30 μN μm⁻² was necessary to penetrate skin strips from slaughtered horses. These experiments demonstrate that P. insidiosum does not exert sufficient pressure to penetrate undamaged skin by mechanics alone, but must effect a decisive reduction in tissue strength by proteinase secretion.