Mycoplasma. Composition coloured scanning electron micrograph (SEM) of a dendritic cell infected with Mycoplasma pneumoniae (green). Mycoplasmas are bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders. They commonly occur as parasites and pathogens (disease-causing organisms) in humans and other animals. They are the smallest bacteria known, and also have a very small genome (total genetic material). Magnification:x4000 when printed at 10cm high.