Bacterial cellular morphologies are the shapes that are characteristic of various types ofbacteria and often key to their identification. Their direct examination under alight microscope enables the classification of these bacteria (andarchaea).
Generally, the basic morphologies are spheres (coccus) and round-ended cylinders or rod shaped (bacillus). But, there are also other morphologies such as helically twisted cylinders (exampleSpirochetes), cylinders curved in one plane (selenomonads) and unusual morphologies (the square, flat box-shaped cells of the Archaean genusHaloquadratum). Other arrangements include pairs, tetrads, clusters, chains and palisades.
Acoccus (pluralcocci, from the Latincoccinus (scarlet) and derived from the Greekkokkos (berry)), is anymicroorganism (usuallybacteria)[1] whose overall shape isspherical or nearly spherical.[2][3][4] Coccus refers to the shape of the bacteria and can contain multiple genera, such as staphylococci or streptococci. Cocci can grow in pairs, chains, or clusters, depending on their orientation and attachment during cell division. In contrast to many bacilli-shaped bacteria, most cocci bacteria do not have flagella and are non-motile.[5]
Cocci is an Englishloanword of amodern or Neo-Latin noun, which in turn stems from theGreek masculine nounκόκκος (cóccos) meaning 'berry'.[6]
Important human diseases caused by coccoid bacteria includestaphylococcal infections, some types offood poisoning, someurinary tract infections,toxic shock syndrome,gonorrhea, as well as some forms ofmeningitis, throat infections,pneumonias, andsinusitis.[7]
Coccoid bacteria often occur in characteristic arrangements and these forms have specific names as well;[8] listed here are the basic forms as well as representative bacterialgenera:[3]
Thegram-positive cocci are a large group of bacteria with similar morphology. All are spherical or nearly so, but they vary considerably in size. Members of some genera are identifiable by the way cells are attached to one another: in pockets, in chains, or grape-like clusters. These arrangements reflect patterns of cell division and that cells stick together.Sarcina cells, for example, are arranged in cubical pockets because cell division alternates regularly among the three perpendicular planes.Streptococcus spp. resemble a string of beads because division always occurs in the same plane. Some of these strings, for example,S. pneumoniae, are only two cells long. They are calleddiplococci. Species ofStaphylococcus have no regular plane of division. They form grape-like structures.[9]
The various gram-positive cocci differ physiologically and by habitat.Micrococcus spp. are obligate aerobes that inhabit human skin.Staphylococcus spp. also inhabit human skin, but they are facultative anaerobes. They ferment sugars, producing lactic acid as an end product. Many of these species produce carotenoid pigments, which color their colonies yellow or orange.Staphylococcus aureus is a major human pathogen. It can infect almost any tissue in the body, frequently the skin. It often causes nosocomial (hospital-acquired) infections.[9]
Diplococci are pairs of cocci. Examples ofgram-negative diplococci areNeisseria spp. andMoraxella catarrhalis. Examples of gram-positive diplococci areStreptococcus pneumoniae andEnterococcus spp.[10][11] Presumably, diplococcus has been implicated inencephalitis lethargica.[12] The genusNeisseria belongs to the family Neisseriaceae. This genus,Neisseria, is divided into more than ten different species, but most of them are gram negative and coccoid. The gram-negative, coccoid species include:Neisseria cinerea,N. gonorrhoeae,N. polysaccharea,N. lactamica,N. meningitidis,N. mucosa,N. oralis andN. subflava. The most common of these species are the pathogenicN. meningitidis andN. gonorrhoeae.[13]
The genusMoraxella belongs to the familyMoraxellaceae. This genus,Moraxellaceae, comprises gram-negative coccobacilli bacteria:Moraxella lacunata,M. atlantae,M. boevrei,M. bovis,M. canis,M. caprae,M. caviae,M. cuniculi,M. equi,M. lincolnii,M. nonliquefaciens,M. osloensis,M. ovis,M. saccharolytica, andM. pluranimalium.[14] However, only one has a morphology of diplococcus,M. catarrhalis, a salient pathogen contributing to infections in the human body.[15]
The speciesStreptococcus pneumoniae belongs to the genusStreptococcus and the familyStreptococcaceae. The genusStreptococcus has around 129 species and 23 subspecies[16] that benefit many microbiomes on the human body. There are many species that show non-pathogenic characteristics; however, there are some, likeS. pneumoniae, that exhibit pathogenic characteristics in the human body.[17][11]
The genusEnterococcus belongs to the familyEnterococcaceae. This genus is divided into 58 species and two subspecies.[18] These gram-positive, coccoid bacteria were once thought to be harmless to the human body. However, within the last ten years, there has been an influx of nosocomial pathogens originating fromEnterococcus bacteria.[19][11]
Abacillus (pl.:bacilli), also called abacilliform bacterium or often just arod (when the context makes thesense clear), is a rod-shaped bacterium orarchaeon. Bacilli are found in many different taxonomic groups of bacteria. However, the nameBacillus, capitalized anditalicized, refers to a specific genus of bacteria. The nameBacilli, capitalized but not italicized, can also refer to a less specific taxonomic group of bacteria that includes two orders, one of which contains the genusBacillus. When the word is formatted with lowercase and not italicized, 'bacillus', it will most likely be referring to shape and not to the genus. Bacilliform bacteria are also often simply called rods when the bacteriologic context is clear.[20]
Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades.[21]
There is no connection between the shape of a bacterium and its color uponGram staining; there are both gram-positive rods and gram-negative rods.MacConkey agar can be used to distinguish among gram-negative bacilli such asE. coli andsalmonella.[23]
Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades.[24]
Acoccobacillus (pluralcoccobacilli), orbacillococcus, is a type of bacterium with a shape intermediate betweencocci (spherical bacteria) and bacilli (rod-shaped bacteria). Coccobacilli, then, are very short rods which may be mistaken for cocci.[25] The wordcoccobacillus reflects an intermediate shape betweencoccus (spherical) andbacillus (elongated).[2]
Haemophilus influenzae,Gardnerella vaginalis, andChlamydia trachomatis are coccobacilli.Aggregatibacter actinomycetemcomitans is a Gram-negative coccobacillus prevalent in subgingival plaques.Acinetobacter strains may grow on solid media as coccobacilli.Bordetella pertussis is a Gram-negative coccobacillus responsible for causingwhooping cough.Yersinia pestis, the bacterium that causesplague, is also coccobacillus.[26]
Coxiella burnetti is also a coccobacillus.[27][28] Bacteria from the genusBrucella are medically important coccobacilli that causebrucellosis.Haemophilus ducreyi, another medically important Gram-negative coccobacillus, is observed in sexually transmitted disease, chancroid, of Third World countries.[29]
Spiral bacteria are another major bacterial cell morphology.[2][30][31][32] Spiral bacteria can be sub-classified as spirilla, spirochetes, or vibrios based on the number of twists per cell, cell thickness, cell flexibility, and motility.[33]
Bacteria are known to evolve specific traits to survive in their ideal environment.[34] Bacteria-caused illnesses hinge on the bacteria's physiology and their ability to interact with their environment, including the ability toshapeshift. Researchers discovered a protein that allows the bacteriumVibrio cholerae to morph into a corkscrew shape that likely helps it twist into — and then escape — the protective mucus that lines the inside of the gut.[34]
Aspirillum (plural spirilla) is a rigid spiral bacterium that is gram-negative and frequently has externalamphitrichous orlophotrichous flagella.[33] Examples include:
Aspirochete (plural spirochetes) is a very thin, elongate, flexible, spiral bacteria that is motile via internal periplasmicflagella inside the outer membrane.[33] They comprise the phylumSpirochaetes. Owing to their morphological properties, spirochetes are difficult to Gram-stain but may be visualized usingdark field microscopy orWarthin–Starry stain.[35] Examples include:
Helicobacter species arehelically shaped, the most common example of which isHelicobacter pylori.[36] A helical shape is seen to be better suited for movement of bacteria in a viscous medium.[37]
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