A phylum of chiefly Gram-positive bacteria characterized by having DNA with a relatively low ratio of G-C base pairs compared to A-T base pairs. It includes bacteria that form resistant endospores, such as Clostridium spp. and Bacillus spp., and several important lactic acid-producing species, such as Lactobacillus spp., which are used in cheese and yoghurt manufacture. Other significant members include Streptococcus and Staphylococcus, species of which are common pathogens of humans, and the minute mycoplasmas, which lack cell walls.