Therefore the number of Cytosine bases equals the number of Guanine bases and likewise A=T. In a double helix of DNA, Adenine pairs with Thymidine and Guanine pairs with Cytosine. The DNA of all living things is made up of four nucleotide bases Adenine (A), Cytosine (C), Guanine (G) and Thymidine (T). White scale bar, lower right, represents 100nm. Scanning electron micrograph of Mycoplasma mobile. This group contains several very important human pathogens that can cause pneumonia, urethritis, carditis, and gingivitis, to name a few. They also have special lipids called sterols and lipoglycans, to help protect against osmotic stress. Therefore Mollicutes are often closely associated with a host that can provide them with a stable environment. Without a cell wall these bacteria tend to be very delicate. Mollicutes such as the Mycoplasma are very small cells lacking a peptidoglycan cell wall. Currently, there are seven recognized Classes of Firmicutes: the Erysipelotrichia, the Negativicutes, the Limnochordia, the Tissierellia, the Thermolithobacteria, the Clostridia and the Bacilli. Groups of Firmicutes have been classified based on characteristics like type of cell envelope, endospore formation and aerotolerance (how well they live and grow in oxygen). Other distinguishing features of these cell envelopes include porins, S layers, teichoic and lipoteichoic acids, and lipoproteins. The Firmicutes and Gram-negative Proteobacteria cell envelopes differ most notably by the presence or absence of an outer membrane. The function of the S layer is not known but it is believed to prevent predation in the environment. Many members of the Firmicutes have an outermost envelope layer of protein called the S layer. Underneath the peptidoglycan there is a phospholipid bilayer and its associated proteins that act as a selective barrier. The typical Firmicutes cell envelope consists of a layer of peptidoglycan, which is a polymer of protein and carbohydrate that gives structure and shape to the cell and protects the bacterium from osmotic stress. It is evident that Firmicutes are as diverse as they are important. Certain Firmicutes make resistant progeny called endospores, while others can only reproduce through binary fission. Exceptions have been identified and some Firmicutes have G+C content as high as 55% (e.g.
In fact, some Firmicutes have no cell wall at all! They are called "low G+C" because their DNA typically has fewer G and C DNA bases than A and T bases as compared to other bacteria. While many Firmicutes stain Gram-positive, some do not. This procedure identifies cells that have a thick cell wall of peptidoglycan. Gram-positive organisms stain purple with a differential staining procedure developed in 1884 by Christian Gram. Many have certain distinct cellular characteristics. The group name refers to a phylum of Bacteria, also known as the Firmicutes, its members share a common evolutionary history. This is an ecologically and industrially important group of microorganisms.
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