Bacterial cellular morphologies

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Bacteria display a large diversity of cell morphologies and arrangements. Bacterial morphology diagram.svg
Bacteria display a large diversity of cell morphologies and arrangements.

Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archaea).

Contents

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 (example Spirochetes ), cylinders curved in one plane (selenomonads) and unusual morphologies (the square, flat box-shaped cells of the Archaean genus Haloquadratum). Other arrangements include pairs, tetrads, clusters, chains and palisades.

Types

Coccus

Arrangement of cocci bacteria Arrangement of cocci bacteria en.svg
Arrangement of cocci bacteria
:Staphylococcus bacteria Staphylococcus aureus Gram.jpg
:Staphylococcus bacteria

A coccus (plural cocci, from the Latin coccinus (scarlet) and derived from the Greek kokkos (berry)), is any microorganism (usually bacteria) [1] whose overall shape is spherical 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 English loanword of a modern or Neo-Latin noun, which in turn stems from the Greek masculine noun κόκκος (cóccos) meaning 'berry'. [6]

Important human diseases caused by coccoid bacteria include staphylococcal infections, some types of food poisoning, some urinary tract infections, toxic shock syndrome, gonorrhea, as well as some forms of meningitis, throat infections, pneumonias, and sinusitis. [7]

Arrangements

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 bacterial genera: [3]

Gram-positive cocci

The gram-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 called diplococci. Species of Staphylococcus 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

Structure b represents diplococcus bacteria Cocci arrangement.png
Structure b represents diplococcus bacteria

Diplococci are pairs of cocci. Examples of gram-negative diplococci are Neisseria spp. and Moraxella catarrhalis . Examples of gram-positive diplococci are Streptococcus pneumoniae and Enterococcus spp. [10] [11] Presumably, diplococcus has been implicated in encephalitis lethargica. [12] The genus Neisseria 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 and N. subflava . The most common of these species are the pathogenic N. meningitidis and N. gonorrhoeae. [13]

The genus Moraxella belongs to the family Moraxellaceae. 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 , and M. pluranimalium . [14] However, only one has a morphology of diplococcus, M. catarrhalis, a salient pathogen contributing to infections in the human body. [15]

The species Streptococcus pneumoniae belongs to the genus Streptococcus and the family Streptococcaceae. The genus Streptococcus 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, like S. pneumoniae, that exhibit pathogenic characteristics in the human body. [17] [11]

The genus Enterococcus belongs to the family Enterococcaceae. 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 from Enterococcus bacteria. [19] [11]

Bacillus

A bacillus (pl.: bacilli), also called a bacilliform bacterium or often just a rod (when the context makes the sense clear), is a rod-shaped bacterium or archaeon. Bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus , capitalized and italicized, refers to a specific genus of bacteria. The name Bacilli, capitalized but not italicized, can also refer to a less specific taxonomic group of bacteria that includes two orders, one of which contains the genus Bacillus. 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 upon Gram staining; there are both gram-positive rods and gram-negative rods. MacConkey agar can be used to distinguish among gram-negative bacilli such as E. coli and salmonella. [23]

Arrangements

Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades. [24]

  • Diplobacilli: Two bacilli arranged side by side with each other.
  • Streptobacilli: Bacilli arranged in chains. [2] [1]

Gram-positive examples

Gram-negative examples

Coccobacillus

Coxiella burnetii (TEM) Coxiella burnetii 01.JPG
Coxiella burnetii (TEM)

A coccobacillus (plural coccobacilli), or bacillococcus, is a type of bacterium with a shape intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). Coccobacilli, then, are very short rods which may be mistaken for cocci. [25] The word coccobacillus reflects an intermediate shape between coccus (spherical) and bacillus (elongated). [2]

Haemophilus influenzae , Gardnerella vaginalis , and Chlamydia 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 causing whooping cough. Yersinia pestis , the bacterium that causes plague, is also coccobacillus. [26]

Coxiella burnetti is also a coccobacillus. [27] [28] Bacteria from the genus Brucella are medically important coccobacilli that cause brucellosis. Haemophilus ducreyi , another medically important Gram-negative coccobacillus, is observed in sexually transmitted disease, chancroid, of Third World countries. [29]

Spiral

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 to shapeshift. Researchers discovered a protein that allows the bacterium Vibrio 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]

Spirillum

Campylobacter jejuni is a common pathogen of bacterial food-related gastrointestinal illness. ARS Campylobacter jejuni.jpg
Campylobacter jejuni is a common pathogen of bacterial food-related gastrointestinal illness.

A spirillum (plural spirilla) is a rigid spiral bacterium that is gram-negative and frequently has external amphitrichous or lophotrichous flagella. [33] Examples include:

Spirochetes

Thin spirochete Treponema pallidum bacteria, the causative agent of syphilis magnified 400 times. Treponema pallidum.jpg
Thin spirochete Treponema pallidum bacteria, the causative agent of syphilis magnified 400 times.

A spirochete (plural spirochetes) is a very thin, elongate, flexible, spiral bacteria that is motile via internal periplasmic flagella inside the outer membrane. [33] They comprise the phylum Spirochaetes. Owing to their morphological properties, spirochetes are difficult to Gram-stain but may be visualized using dark field microscopy or Warthin–Starry stain. [35] Examples include:

Helical

Helicobacter species are helically shaped, the most common example of which is Helicobacter pylori . [36] A helical shape is seen to be better suited for movement of bacteria in a viscous medium. [37]

See also

Related Research Articles

<i>Bacillus</i> Genus of bacteria

Bacillus is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum Bacillota, with 266 named species. The term is also used to describe the shape (rod) of other so-shaped bacteria; and the plural Bacilli is the name of the class of bacteria to which this genus belongs. Bacillus species can be either obligate aerobes which are dependent on oxygen, or facultative anaerobes which can survive in the absence of oxygen. Cultured Bacillus species test positive for the enzyme catalase if oxygen has been used or is present.

<span class="mw-page-title-main">Gram-positive bacteria</span> Bacteria that give a positive result in the Gram stain test

In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.

<span class="mw-page-title-main">Gram-negative bacteria</span> Group of bacteria that do not retain the Gram stain used in bacterial differentiation

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. These bacteria are found in all environments that support life on Earth.

<i>Streptococcus</i> Genus of bacteria

Streptococcus is a genus of gram-positive or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales, in the phylum Bacillota. Cell division in streptococci occurs along a single axis, thus when growing they tend to form pairs or chains, which may appear bent or twisted. This differs from staphylococci, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are facultative anaerobes.

<i>Enterococcus</i> Genus of bacteria

Enterococcus is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the intestines of humans: E. faecalis (90–95%) and E. faecium (5–10%). Rare clusters of infections occur with other species, including E. casseliflavus, E. gallinarum, and E. raffinosus.

<span class="mw-page-title-main">Bacillota</span> Phylum of bacteria

Bacillota is a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature.

<span class="mw-page-title-main">Bacilli</span> Class of bacteria in the phylum Firmicutes

Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as Bacillus anthracis. Bacilli are almost exclusively gram-positive bacteria.

<span class="mw-page-title-main">Bacterial capsule</span> Polysaccharide layer that lies outside the cell envelope in many bacteria

The bacterial capsule is a large structure common to many bacteria. It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases.

<i>Moraxella catarrhalis</i> Species of bacterium

Moraxella catarrhalis is a fastidious, nonmotile, Gram-negative, aerobic, oxidase-positive diplococcus that can cause infections of the respiratory system, middle ear, eye, central nervous system, and joints of humans. It causes the infection of the host cell by sticking to the host cell using trimeric autotransporter adhesins.

The oxidase test is used to determine whether an organism possesses the cytochrome c oxidase enzyme. The test is used as an aid for the differentiation of Neisseria, Moraxella, Campylobacter and Pasteurella species. It is also used to differentiate pseudomonads from related species.

In microbiology, pleomorphism, also pleiomorphism, is the ability of some microorganisms to alter their morphology, biological functions or reproductive modes in response to environmental conditions. Pleomorphism has been observed in some members of the Deinococcaceae family of bacteria. The modern definition of pleomorphism in the context of bacteriology is based on variation of morphology or functional methods of the individual cell, rather than a heritable change of these characters as previously believed.

<i>Bacillus mycoides</i> Species of bacterium

Bacillus mycoides is a bacterium of the genus Bacillus. Like other Bacillus species, B. mycoides is Gram positive, rod-shaped, and forms spores. B. mycoides is distinguished from other Bacillus species by its unusual growth on agar plates, where it forms expansive hairy colonies with characteristic swirls.

Moraxella is a genus of gram-negative bacteria in the family Moraxellaceae. It is named after the Swiss ophthalmologist Victor Morax. The organisms are short rods, coccobacilli, or as in the case of Moraxella catarrhalis, diplococci in morphology, with asaccharolytic, oxidase-positive, and catalase-positive properties. M. catarrhalis is the clinically most important species under this genus.

Neisseria elongata is a Gram negative bacterium and is different from the other cocci shaped members of the genus Neisseria as it is rod shaped. Unlike other Neisseria it is catalase negative. N. elongata is the most ancestral of the human Neisseria.

Neisseria flava is a bacterium belonging to a group of species under the genus Neisseria that is considered non-pathogenic. Along with its other members of the non-pathogenic group, Neisseria flava is often found in the upper respiratory tract surface in humans. On rare occasions, it can cause rheumatic heart disease and ventricular septal defect aortic insufficiency.

Neisseria bacilliformis is a bacterium commonly found living as a commensal in the mucous membranes of mammals. However, depending on host immunocompetence, there have been documented cases of N. bacilliformis infections of the respiratory tract and oral cavity thus making it an opportunistic pathogen. It was originally isolated from patients being treated in a cancer center. Rarely, a more serious infection such as endocarditis can occur often as a result of a predisposing condition.

Lautropia mirabilis is a Gram-negative, facultatively anaerobic, oxidase- and catalase-positive, motile bacterium of the genus Lautropia and family Burkholderiaceae, isolated from the mouth of children who were infected with human immunodeficiency virus.

<span class="mw-page-title-main">Lancefield grouping</span> System for classifying streptococci bacteria

Lancefield grouping is a system of classification that classifies catalase-negative Gram-positive cocci based on the carbohydrate composition of bacterial antigens found on their cell walls. The system, created by Rebecca Lancefield, was historically used to organize the various members of the family Streptococcaceae, which includes the genera Lactococcus and Streptococcus, but now is largely superfluous due to explosive growth in the number of streptococcal species identified since the 1970s. However, it has retained some clinical usefulness even after the taxonomic changes, and as of 2018, Lancefield designations are still often used to communicate medical microbiological test results.

ESKAPE is an acronym comprising the scientific names of six highly virulent and antibiotic resistant bacterial pathogens including: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. The acronym is sometimes extended to ESKAPEE to include Escherichia coli. This group of Gram-positive and Gram-negative bacteria can evade or 'escape' commonly used antibiotics due to their increasing multi-drug resistance (MDR). As a result, throughout the world, they are the major cause of life-threatening nosocomial or hospital-acquired infections in immunocompromised and critically ill patients who are most at risk. P. aeruginosa and S. aureus are some of the most ubiquitous pathogens in biofilms found in healthcare. P. aeruginosa is a Gram-negative, rod-shaped bacterium, commonly found in the gut flora, soil, and water that can be spread directly or indirectly to patients in healthcare settings. The pathogen can also be spread in other locations through contamination, including surfaces, equipment, and hands. The opportunistic pathogen can cause hospitalized patients to have infections in the lungs, blood, urinary tract, and in other body regions after surgery. S. aureus is a Gram-positive, cocci-shaped bacterium, residing in the environment and on the skin and nose of many healthy individuals. The bacterium can cause skin and bone infections, pneumonia, and other types of potentially serious infections if it enters the body. S. aureus has also gained resistance to many antibiotic treatments, making healing difficult. Because of natural and unnatural selective pressures and factors, antibiotic resistance in bacteria usually emerges through genetic mutation or acquires antibiotic-resistant genes (ARGs) through horizontal gene transfer - a genetic exchange process by which antibiotic resistance can spread.

References

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