Importance of the outer cell membrane in bacterial classification The (low G + C) Bacillota, have a 45–60% GC content, but this is lower than that of the Actinomycetota. The Actinomycetota include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera. The high G + C phylum was made up of the Actinobacteria, and the low G + C phylum contained the Firmicutes. Two of these were gram-positive and were divided on the proportion of the guanine and cytosine content in their DNA. Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla. Based on 16S ribosomal RNA phylogenetic studies of the late microbiologist Carl Woese and collaborators and colleagues at the University of Illinois, the monophyly of the gram-positive bacteria was challenged, with major implications for the therapeutic and general study of these organisms. Historically, the kingdom Monera was divided into four divisions based primarily on Gram staining: Bacillota (positive in staining), Gracilicutes (negative in staining), Mollicutes (neutral in staining) and Mendocutes (variable in staining). Species identification hierarchy in clinical settings Such staining, together with growth requirement and antibiotic susceptibility testing, and other macroscopic and physiologic tests, forms the full basis for classification and subdivision of the bacteria (e.g., see figure and pre-1990 versions of Bergey's Manual of Systematic Bacteriology). Classification Īlong with cell shape, Gram staining is a rapid method used to differentiate bacterial species. Some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan. Specific to gram-positive bacteria is the presence of teichoic acids in the cell wall. Gram-negative bacteria's S-layer is attached directly to the outer membrane. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Both gram-positive and gram-negative bacteria commonly have a surface layer called an S-layer. Also, only some species are flagellates, and when they do have flagella, have only two basal body rings to support them, whereas gram-negative have four. Only some species have a capsule, usually consisting of polysaccharides.
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