Bugbuster: Survivability of Living Bacteria Upon Shock Compression

Survivability of bacteria during impact events has implications both for the transport of life between planets and development of organisms on Hadean Earth and other planets during the period of heavy bombardment which ended 3.5 Gyr before the present [1]. We envision that life existed within internal rock surfaces immersed in the early ocean. We performed shock recovery experiments on live E. coli bacteria to determine survival rate vs. shock pressure. Samples of 2x10⁷ cells were suspended in ∼10^-5 l of a buffer solution (TE: a 10:1 solution of Tris and EDTA), sealed into stainless steel chambers that are impacted by 1.5 mm thick flyer plates at 670-760 m s^-1 using a 20 mm gun. Recovered liquid was mixed with a nutrient broth (LB: growth medium containing tryptone, yeast extract and NaCl) and spread on a Petrie dish containing agar (a polysaccharide growth medium extracted from marine algae Rhodophyceae). Recovered samples were cultured for ∼16 hours at 37° C. In addition, sample bacteria studied under an optical microscope with DAPI fluorescent stain to verify presence of bacteria in shock recovered samples. Initial and reverberated shock pressures in H₂O varied from 0.2 to 2.0 and 2.4 to 14.9 GPa respectively. We modeled the bacteria cell walls with stilbene, ρ ₀=1.16 g cm^-3, U_S=2.866+1.588u_P and the cell interiors as water. Upon initial loading the net strain imposed on E. coli that just caused non-survival for 10^-6 s duration stress was 2.8. If this strain is characteristic of that tolerable by E. coli, we predict that shock stresses of 25 MPa, 25 kPa and 25 Pa are sustainable upon shock loading by 0.1 ms, 0.1 s and 100 s shock duration pulses. Such shock durations are induced by 2.5 m, 2.5 km and 2,500 km diameter silicate impactors. References: [1] Maher K.A. & Stevenson D.J., Nature, 331, pp.612-614, 1988