Saturday, December 31, 2005
Gram Positive Bacteria
GRAM-POSITIVE BACTERIA are characterised by having as part of their cell wall structure peptidoglycan as well as polysaccharides and/or teichoic acids. The peptidoglycans which are sometimes also called murein are heteropolymers of glycan strands, which are cross-linked through short peptides.
THE BASIS OF THE MUREIN are chains of alternating residues of N-acetylglucosamine and N-acetyl muramic acid which are Beta -1,4-linked. The muramic acid is a unique substance associated with bacterial cell walls. These chains are cross-linked by short polypetide chains consisting of both L- and D-aminoacids. While in Gram-negative bacteria the peptidoglycan is simple in structure and comparatively uniform throughout most genera, in Gram-positive bacteria there is a very big variation in structure and composition. In general the peptidoglycan is multilayered. There have also been recorded some minor variations in composition in some groups. Thus, in Mycobacterium and Nocardia the N-acetyl moiety of the muramic acid is replaced by the oxydised form N-glycolyl. The amino acid composition of the both the cross-linking as well the stem polypeptides can vary extensively with different groups. These differences form the basis for the taxonomy of these organisms.
THE LIPOPOLYSACCHARIDES of the Gram-positive bacteria have not been extensively studied apart from those of the streptococci, where they are the basis of their serological subdivision. They have also been used in investigating the serological relationships between streptococci, enterococci and lactobacilli. The specific polysaccharides associated with the acid-fast bacteria of the Corynebacterium-Mycobacterium-Nocardia group have also been extensively studied. They are arabinogalactans and arabinomannans linked to mycolic acids. The presence in the cell wall of these genera of arabinose, galactose and meso-diaminopimelic acid is characteristic of these genera.
THE TEICHOIC ACIDS , which are polyols consisting predominantly of glycerol, ribitol and mannitol, are covalently linked to the peptidoglycan through phosphodiester bonds and can be substituted by sugars, aminosugars or D-alanine residues. Some rare teichoic acids lack polyols. Teichoic acids are found in some actinomycetes, bacilli, lactobacilli, listeria and staphylococci.
IN ONLY A LIMITED NUMBER of Gram-positive genera including the Corynebacterium-Mycobacterium-Nocardia group are found the mycolic acids. They are 3-hydroxy acids of high molecular weight with a long alkyl branch in position 2.
These are Gram-positive, sporing non-acid fast straight rods. If motile they have peritrichous flagellae. They include aerobes, facultative anaerobes, and strict anaerobes and are generally nonhalophylic with a wide growth range depending on the group. Many will grow on most simple bacteriological media. A variety of biochemical activities are noted in this family, including fermentative, proteolytic activities and the ability to grow on minimal media. Some species can fix nitrogen. A number of species are also characterised by producing specific toxins. Bacillus is the type genus. The two main genera Bacillus and Clostridium are distiguished by the former being aerobic, while the latter is anaerobic. Most of these include a wide variety of species.
This is characteristic of the Bacillus and related groups. They are able to survive in air.
Bacillus: These are peritrichoulsly flagellated, form ellipsoidal or spherical, endospores, which may or may not swell the sporangium. They are aerobic to facultatively anaerobic and generally catalase positive. There are currently very many species in this genus, including, B. anthracis, B. azotoformans, B. cereus, B. coagulans, B. israelensis, B. larvae, B. mycoides, B. polymyxa, B. pumilis, B. stearothormophillus, B. subtilis, B. thuringiensis and B. validus. Many of these species form distinct groups of closely related species. One of these is the B. cereus group, which comprises apart from B. cereus, B. anthracis, B. mycoides and B. thuringiensis as well as newly described species, B. weihenstephanensis and B. pseudomycoides.
B. cereus Group.
This is a group of gram-positive, spore-forming, generally motile, aerobic rod-shaped bacteria, which also grow well anaerobically. Based on recent 16S rRNA analysis the species including B. cereus, B. anthracis, B. mycoides and B. thuringiensis and the more recently identified B. pseudomycoides and B. weihendtephanensis, make up the group. The cells of these six species are all large, being >0.9µm wide. They produce ellipsoidal or cylindrical spores either centrally or subterminally. The spores do not distend the cells. They all form spores readily on most media. Based on detailed 16S and 23S rRNA analyses the members of the group have diverged only recently from a common evolutionary line. Due to its highly virulent pathogenicity B. anthracis has been maintained as a separate species, as has B. thuringiensis, whose strains form the crystalline inclusion (Cry protein) or d-endotoxin, which is highly toxic for certain types of insect. B. anthracis and B. cereus are the only members of the Bacillus genus, which are human pathogens.
This is the characteristic of the Clostridium group related groups and characterised generally by an inability to grow in air, although some may tolerate it.
Clostridium: These include many species, which can be psychrophilic, mesophilic or thermophilic. They are generally Gram-positive with peritrichous flagellation, they degrade organic materials to acids, alcohols, CO2, H2 and minerals. Acids, particularly butyric acid, are a frequent product of clostridial fermentation. They form ellipsoidal or spherical, endospores, which may or may not swell the sporangium. They tend to be grouped into saccharolytic, proteolytic species but some are both and there are also some species, which are specialised in being limited in their biochemical activities.
The saccharolytic species include: Cl. aerotolerans, Cl. aurantibutyricum, Cl. beijerinckii, Cl. botulinum B,E,F*, Cl. butyricum, Cl. chauvoei, Cl.difficile, Cl. intestinale, Cl. novyi A, Cl. pateurianum, Cl. saccharolyticum, Cl. septicum, Cl. thermoaceticum, and Cl. thermosaccharolyticum.
The proteolytic species include: Cl. argeninense, Cl. ghoni, Cl. limosum, Cl.putrefaciens, Cl. subterminale and Cl. tetani.
The proteolytic and saccharolytic species include: Cl. acetobutylicum, Cl. bifermenans, Cl. botulinum A, B, F (prot.)*, Cl. botulinum C,D*, Cl. cadaveris, Cl. haemolyticum, Cl. novyi B,C,* Cl. perfringens, Cl. putrefaciens, Cl. sordelli and Cl.sporogenes.
The specialist species include: Cl. acidiurici, Cl. irregularis, Cl. kluyveri, Cl. oxalicum, Cl. propionicum, Cl. sticklandii and Cl. villosum.
*Cl. botulinum is subdivided into a number of types according to the serological specificities of the toxins produced. These specificities are based on neutralisation studies. Other Clostridium species can also produce botulinum toxins.
These are Gram-positive, non-sporing non-acid fast cocci. They generally occur in tetrads (groups of four) or clusters and are not motile. They are generally aerobes and produce catalase. Carotenoid pigments are produced by most species. Most will grow on bacteriological media such as nutrient agar at 37°C except some psychrotropic and halophyllic species which require cooler temperatures or 5%NaCl. There are only two genera in this family; Arthrobacter and Micrococcus. The genera Staphylococcus and Planococcus which were once considered part of this family are now excluded, due to marked differences in DNA base composition, cell wall, fatty acids and other compositions.
Arthrobacter: This includes two 'groups of species', known as the A. globiformis / A.citreus group and the A. nicotianae group, based on differences in cell wall structure. They are important soil organisms, often being the numerically largest numbers of isolates on aerobic primary isolation media.
Micrococcus: This genus consists of nine species, of which M.luteus (previously known as M. lysodeikticus), M. lylae, M. roseus are most studied. M. agilis differs from other species in being motile, psychrophilic and producing beta-galactosidase. M. kristinae can ferment glucose anaerobically and M. halobius requires at least 5% NaCl for growth.
These are Gram-positive, non-sporing acid fast straight or slightly curved bacilli. They are non-motile and range in size between 0.2-0.6 x 1.0-10 mµ). Both branching and mycelium-like growth may occur. They are generally aerobes and produce catalase. Many species form white or creamy coloured colonies but some form bright yellow or orange colonies based on carotenoid pigments, in some cases only as a response to light. They can utilise a wide range of carbon compounds. Glycerol is utilised by all cultivable mycobacteria as sole source of carbon and energy, thus media conating this substrate will grow most species. There may also be a requirement for additional CO2 particularly for clinical isolates. There is only one genus in this family; Mycobacterium. Recent studies based on 16S rRNA analyses have suggested that the family Mycobacteriaceae should include the three genera: Mycobacterium, Nocardia, Rhodococcus.
Mycobacterium: On the basis of growth rate, this includes two groups of species, known as the Slow Growers and the Fast Growers. The Slow Growers tend to be associated with human or animal disease, while the Fast Growers tend to be non-pathogenic. They are widely distributed in soil and some marine environments. Over 300 species have been named. The GC content of the DNA of those mycobacterial species studies varies from 66-71 mol%.
These are Gram-positive, non-sporing non-acid fast cocci. They are strict anaerobes and are generally nonhalophylic with a wide growth range depending on the group. Some will grow on the most simple bacteriological media, while special nutrients are required by others. These may include vitamins, other cofactors and amino acids. Many metabolize peptones and amino acids. Some require fermentable carbohydrates. A variety of biochemical activities are noted in this family, including some fermenters, some produce indole, reduce nitrate, produce urease, coagulase or catalase etc. Peptococcus is the type genus. The other genera include Peptostreptococcus, Sarcina and Coprococcus. Most of these include a wide variety of species.
Peptococcus: This includes only one species, P. niger. The cocci are arranged as diplococci, irregular clumps or regular clusters. They are anaerobic and n-caproic acid and butyric acid are the main metabolic products.
Peptostreptococcus: This genus consists of three groups of species. Ps. anaerobius is the only representative of one group. It is biochemically similar to P.niger, but grows confluently on commercial media. It forms short chains. The other two groups, which tend to form clumps and clusters; some species form diplococci, which are distinguished by their ability to form butyrate or not. Each group includes about five-six species, which are differentiated on their morphology and biochemical reactions including fermentation, reduction of nitrate, production of indole, urease, coagulase or catalase etc.