Firmicutes is the main bacterial phylum, comprising over 250 genera, including Lactobacillus, Streptococcus, Mycoplasma, and Clostridium, most of which have gram-positive cell wall structure. The phylogeny is based on 16S rRNA-based. Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are obligate or facultative aerobes, and the Mollicutes, which are parasites.