Bacillus subtilis HU58 is a very robust probiotic that forms an outer protective shell, known as an endospore. This near-impenetrable coating protects the bacterium from harsh environments, including heat, acid, pressure, light, and lack of oxygen. The rugged nature of Bacillus subtilis makes it the perfect probiotic to survive manufacturing, packaging, shipping, and ultimately digestion. In fact, this species is so resilient that researchers at California Polytechnic State University reported reviving Bacillus spores from the gut of a 25-million-year-old bee that was fossilized in amber.