Bacteria

Bacteria are a biological cell, roughly a millimetre in length, that were among the first living forms on Earth. The biomass of bacteria on Earth is larger than all of the animal and plant biomass combined. Although bacteria have a bad reputation, rightly so due to the illnesses they can cause such as pneumonia, food poisoning and meningitis, bacteria such as probiotics in the gut help food digestion and restore the balance of bacteria in the gut. Fossilised remains of bacteria have been discovered on Mars.