Mycoplasma pneumoniae is the smallest (range from 0.2-0.8 micrometers) free-living bacteria and it is member of the class Mollicutes. This class of organisms lack of peptidoglycan cell wall. Instead, it has a cell membrane which incorporates sterol compounds, similar to eukaryotic cells. It obtains sterols from host serum, allowing it to retain the cell’s structure. Since it has no cell wall, it is resistance to penicillins and other beta-lactam antibiotic which are designed to disrupt the bacterial cell wall. And so, they can be parasitic and saprophytic.
Mycoplasma pneumonia are extracellular, mucous membrane pathogen that do not invade other tissues. They have a low GC-content (18-40 mol%). The analysis of 16S ribosomal RNA sequences suggest that this kind of bacteria is classified under gram-positive since they are too small to be stained. They may induce cellular changes including changes in metabolism and cell growth and infection.
M. pneumonia has the smallest genome size and as a result, lack of many metabolic pathways and so they need to extract nutrient especially glucose from host’s cells and metabolize it in order to get energy for survival and reproduction. M. pneumonia is strict aerobes. They grow slowly by binary fission and the colonies may take up to 3 weeks to develop and so the physical sign of infection of the patient will present for several days to weeks.
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