Pseudomonas are a group of 191 species of bacteria that do not have a cell wall and thrive in an oxygen environment. Most are harmless and are commonly used in medical research, but one species, pseudomonas aeruginosa, is an opportunistic pathogen that thrives in damaged tissue and the immunocompromised, as well as being tolerant of oxygen poor environments. It also thrives in moist environments, making it a threat to most types of hospital equipment and warm wet environments like hot tubs.