CharacteristicsIn general, the following characteristics are present in gram-positive bacteria: # Cytoplasmic lipid membrane # Thick layer # and lipoids are present, forming s, which serve as agents, and also for certain types of adherence. # Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme . # A much smaller volume of than that in gram-negative bacteria. Only some species have a capsule, usually consisting of . Also, only some species are s, and when they do have , have only two rings to support them, whereas gram-negative have four. Both gram-positive and gram-negative bacteria commonly have a surface layer called an . In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Gram-negative bacteria's S-layer is attached directly to the outer membrane. Specific to gram-positive bacteria is the presence of s in the cell wall. Some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan.
ClassificationAlong with cell shape, Gram staining is a rapid method used to differentiate bacterial species. 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''). , the kingdom was divided into four based primarily on Gram staining: (positive in staining), (negative in staining), (neutral in staining) and Mendocutes (variable in staining). Based on phylogenetic studies of the late microbiologist and collaborators and colleagues at the , the of the gram-positive bacteria was challenged, with major implications for the therapeutic and general study of these organisms. Based on molecular studies of the 16S sequences, Woese recognised twelve . Two of these were gram-positive and were divided on the proportion of the and content in their . The high G + C phylum was made up of the and the low G + C phylum contained the . The Actinobacteria include the '' '', '' '', '' '' and '' '' genera. The (low G + C) Firmicutes, have a 45–60% GC content, but this is lower than that of the Actinobacteria.
Importance of the outer cell membrane in bacterial classificationAlthough bacteria are traditionally divided into two main groups, gram-positive and gram-negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species. The gram-positive and gram-negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups. However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane. All gram-positive bacteria are bounded by a single-unit lipid membrane, and, in general, they contain a thick layer (20–80 nm) of peptidoglycan responsible for retaining the Gram stain. A number of other bacteria—that are bounded by a single membrane, but stain gram-negative due to either lack of the peptidoglycan layer, as in the s, or their inability to retain the Gram stain because of their cell wall composition—also show close relationship to the Gram-positive bacteria. For the bacterial cells bounded by a single cell membrane, the term ''monoderm bacteria'' has been proposed. In contrast to gram-positive bacteria, all typical gram-negative bacteria are bounded by a cytoplasmic membrane and an outer cell membrane; they contain only a thin layer of peptidoglycan (2–3 nm) between these membranes. The presence of inner and outer cell membranes defines a new compartment in these cells: the or the periplasmic compartment. These bacteria have been designated as diderm bacteria. The distinction between the monoderm and diderm bacteria is supported by conserved signature indels in a number of important proteins (viz. DnaK, GroEL). Of these two structurally distinct groups of bacteria, monoderms are indicated to be ancestral. Based upon a number of observations including that the gram-positive bacteria are the major producers of antibiotics and that, in general, gram-negative bacteria are resistant to them, it has been proposed that the outer cell membrane in gram-negative bacteria (diderms) has evolved as a protective mechanism against selection pressure. Some bacteria, such as '' '', which stain gram-positive due to the presence of a thick peptidoglycan layer and also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (gram-positive) and diderm (gram-negative) bacteria. The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide, the archetypical diderm bacteria where the outer cell membrane contains lipopolysaccharide, and the diderm bacteria where outer cell membrane is made up of .
ExceptionsIn general, gram-positive bacteria are monoderms and have a single whereas gram-negative bacteria are diderms and have two bilayers. Some taxa lack peptidoglycan (such as the domain , the class , some members of the , and the insect-endosymbionts of the ) and are gram-variable. This, however, does not always hold true. The '' Deinococcus-Thermus'' bacteria have gram-positive stains, although they are structurally similar to gram-negative bacteria with two layers. The Chloroflexi have a single layer, yet (with some exceptions) stain negative. Two related phyla to the Chloroflexi, the TM7 clade and the Ktedonobacteria, are also monoderms. Some Firmicute species are not gram-positive. These belong to the class (alternatively considered a class of the phylum ), which lack peptidoglycan ( gram-indeterminate), and the class Negativicutes, which includes Selenomonas and stain gram-negative. Additionally, a number of bacterial taxa (viz. Negativicutes, Fusobacteria, Synergistetes, and Elusimicrobia) that are either part of the phylum Firmicutes or branch in its proximity are found to possess a diderm cell structure. However, a conserved signature indel (CSI) in the HSP60 (GroEL) protein distinguishes all traditional phyla of gram-negative bacteria (e.g., Proteobacteria, Aquificae, Chlamydiae, Bacteroidetes, Chlorobi, Cyanobacteria, Fibrobacteres, Verrucomicrobia, Planctomycetes, Spirochetes, Acidobacteria, etc.) from these other atypical diderm bacteria, as well as other phyla of monoderm bacteria (e.g., Actinobacteria, Firmicutes, Thermotogae, Chloroflexi, etc.). The presence of this CSI in all sequenced species of conventional LPS (lipopolysaccharide)-containing gram-negative bacterial phyla provides evidence that these phyla of bacteria form a monophyletic clade and that no loss of the outer membrane from any species from this group has occurred.
PathogenesisIn the classical sense, six gram-positive genera are typically pathogenic in humans. Two of these, ''Streptococcus'' and ''Staphylococcus'', are Coccus, cocci (sphere-shaped). The remaining organisms are bacilli (rod-shaped) and can be subdivided based on their ability to form endospore, spores. The non-spore formers are '' '' and ''Listeria'' (a coccobacillus), whereas ''Bacillus'' and ''Clostridium'' produce spores. The spore-forming bacteria can again be divided based on their Cellular respiration, respiration: ''Bacillus'' is a Facultative anaerobic organism, facultative anaerobe, while ''Clostridium'' is an obligate anaerobe. Also, ''Rathybacter'', ''Leifsonia'', and ''Clavibacter'' are three gram-positive genera that cause plant disease. Gram-positive bacteria are capable of causing serious and sometimes fatal neonatal infection, infections in newborn infants. Access provided by the University of Pittsburgh. Novel species of clinically relevant gram-positive bacteria also include ''Catabacter hongkongensis'', which is an emerging pathogen belonging to .
Bacterial transformationTransformation (genetics), Transformation is one of three processes for horizontal gene transfer, in which exogenous genetic material passes from a donor bacterium to a recipient bacterium, the other two processes being bacterial conjugation, conjugation (transfer of plasmid, genetic material between two bacterial cells in direct contact) and transduction (genetics), transduction (injection of donor bacterial DNA by a bacteriophage virus into a recipient host bacterium). In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium. As of 2014 about 80 species of bacteria were known to be capable of transformation, about evenly divided between gram-positive and ; the number might be an overestimate since several of the reports are supported by single papers. Transformation among gram-positive bacteria has been studied in medically important species such as ''Streptococcus pneumoniae'', ''Streptococcus mutans'', ''Staphylococcus aureus'' and ''Streptococcus sanguinis'' and in gram-positive soil bacterium ''Bacillus subtilis, Bacillus cereus''.
Orthographic noteThe adjectives ''Gram-positive'' and ''Gram-negative'' derive from the surname of Hans Christian Gram; as Eponym#Orthographic conventions, eponymous adjectives, their initial letter can be either capital ''G'' or lower-case ''g'', depending on which style guide (e.g., that of the Centers for Disease Control and Prevention, CDC), if any, governs the document being written. This is further explained at ''Gram staining#Orthographic note, Gram staining § Orthographic note''.