BACTERIA: CHARACTERISTICS, MORPHOLOGY, TYPES, REPRODUCTION - BIOLOGY - 2025

The bacteria are a large group of prokaryotic microorganisms. They are generally only a few microns in size. They have different shapes ranging from cylindrical to spiral to cane shapes.

They are practically ubiquitous organisms and can be located in the soil, marine and freshwater bodies of water, inhabiting the intestinal flora and saliva of many vertebrates, and as parasites of animals and plants. They have also been found in extreme environments such as acidic hot springs, hydrothermal vents, and radioactive waste.

Bacterium Bacterium tartarophtorum, B. manitopoeum and Bacillus sporogenes growing in a stainless steel container. Taken and edited from commons.wikimedia.org

These microbes are an essential part of many nutrient cycles. They are the basic component of the microbiota of all trophic chains and their biomass can be calculated at approximately 5 × 10 ³⁰ bacteria on planet earth.

Another interesting figure is that of the amount of bacteria that inhabit the human body: it is thought that in an average human there are around 39 trillion bacterial cells and most of these are part of the intestinal flora.

The traditional classification of bacteria comprised a polyphyletic taxonomic group. Today that group has been divided into two domains bacteria and archea. Bacteria is recognized as the prokaryotic group with membrane lipids composed of diacyl diesters of glycerol.

On the other hand, archea is the group of prokaryotes whose membrane is composed of isoprenoid lipids (glycerol diether or glycerol tetraether). They also present differences in their ribosomal RNA, being called bacterial rRNA and archaean rRNA, respectively.

Morphology

Bacteria have a great morphological variety and size. These unicellular organisms can measure from 0.3 microns to 0.5 millimeters, however, their measurements are generally between 0.3 and 5.0 microns.

The shape called cocci (spherical) is the most common among bacteria. However, other forms such as bacilli (shaped like a stick or rod) are also relatively common.

Other not so frequent morphs among bacteria are: commas, also called vibrios (shaped like a slightly curved rod or like the punctuation mark “,”), and spirilias or spirochetes (with spiral shapes). Some more unusual are still shaped like stars.

Other non-morphological characteristics

The representatives of the bacterium domain, being prokaryotic unicellular organisms, do not present a defined nucleus or complex membranous organelles. The cell wall of these have a pectidoglycan that contains muramic acid and the membrane lipids contain straight chain fatty acids with ester bonds.

They present gas vesicles. Transfer RNA has thymine (in most tRNAs) and N-formylmethionine (carried by the initiator tRNA). They present polycistronic mRNA, that is, they encode more than one protein.

Ribosomes are 70s in size. They are sensitive to chloramphenicol and kanamycin, they do not show sensitivity to the antibiotic anisomycin.

Bacterial RNA polymerase is a large molecule. It has five subunits of approximately 410 kilodaltons each. Additionally, in its structure the RNA polymerase has a groove 55 Å in length and 25 Å in width. You are sensitive to rifampicin. It does not have type II polymerase promoters.

Bacteria fix nitrogen, carry out photosynthesis based on chlorophyll and also carry out chemolithotrophy (oxidation of inorganic compounds). They do not produce methane nor do they present the enzyme ATPase.

ID

The identification and classification of bacteria is one of the most complex issues in the biology of microorganisms. There are numerous characteristics and methods that are used for the identification and subsequent classification of these individuals.

Classical characteristics include morphology, physiology and metabolism, biochemistry, ecological relationships and functions, and genetics.

The most commonly used analyzes are: fermentation products, type of nutrition, carbon and nitrogen sources, storage inclusions, motility, osmotic tolerance, optimal physical-chemical conditions, photosynthetic pigments, among many others.

Other non-classical features are found at the molecular level. In recent decades, the use of nucleic acids and proteins in the taxonomy of bacteria has gained great momentum.

Comparisons between genes (proteins and nucleic acids) provide vast information on kinship and, of course, similarity between organisms.

Types (classification)

Bacteria, was a term traditionally used to designate all unicellular prokaryotes. However, molecular systematics showed that this ancient group of organisms (prokaryote) diverged into 2 groups or domains.

These two groups were called eubacteria and archebacteria. They were later renamed as bacteria and archea. The Archea are a group most closely related to members of a third domain, called the Eukarya.

This last group is made up of eukaryotic organisms. Together, the 3 domains (bacteria, archea, and eukarya) make up the current classification of life.

System of the 3 domains, Bacteria, Archea and Eukarya. Taken and edited from commons.wikimedia.org

-Classifications of bacteria

Bacteria can be classified following various criteria such as:

Cell organization

Bacteria are generally unicellular, however, depending on the cellular organization, they can be classified as "unicellular and multicellular."

Metabolism

Depending on the environment in which they are found and the way in which they carry out their processes to obtain energy and nutrients, bacteria are classified into:

• Anaerobic: those that live and develop in environments without oxygen.
• Aerobic: bacteria that live and thrive in environments with oxygen.
• Facultative: those organisms that live and develop indistinctly in anaerobic or aerobic environments, that is, they can live in environments with or without oxygen.

Cellular wall

Depending on the composition of the cell wall of the bacteria, they react to the Gram stain, either with a dark blue or violet color, or on the other hand with a pink or red color and their classification is as follows:

• Gram-positive: blue or purple coloration and thickened cell wall.
• Gram-negative: pink or red color and thin or thin cell wall.

Growth and development temperature

Depending on the temperatures where these micro-organisms develop, they can be classified into:

• Psychrophils: those bacteria that develop in environments with very low temperatures.
• Mesophilic: bacteria that inhabit and develop at temperatures between 15 and 35 ° C (moderate temperatures), however some researchers consider organisms that develop within a range of 20 to 40 ° C mesophilic.
• Thermophiles: those bacterial cells that develop and live at high temperatures, that is, above approximately 45 ° C.

Shape

Bacteria have long been identified according to their shape, and their classification is as follows:

• Cocaceae: cylindrical or spherical. These shapes have various classifications based on the number of cells they form and the shape they build. For example, with regard to numbers, when cocaceae are observed in pairs they are called "diplococci" and when they are found in numbers of 4 they are called "tetracocci". But when it comes to shape, if these form chains they are called “streptococci”, when they form clusters “staphylococci” and when they are cube-shaped they are called “sarcins”.
• Bacilli: bacteria with elongated shapes, like a rod or rod. When these bacilli form chains they are called "streptobacilli."
• Cocobacilli: semi-cylindrical bacteria but flattened at the poles, showing an oval shape.
• Spirilos: bacteria with spiral shapes, similar to a corkscrew.
• Vibrios: organisms with the shape of a short and bent rod, they are also called commas, like the punctuation mark.

Morphology of bacteria. Taken and edited from commons.wikimedia.org

-Current classification of the bacterium domain

Following the revolutionary classification by Carl Woese and colleagues in 1990, the classification of bacteria radically changed. Currently, according to the LPSN or list of prokaryotic names with standing in the nomenclature (List of Prokaryotic names with Standing in Nomenclature), the bacterium domain is divided into 34 phyla. Among these phyla are:

Spirochaetes

Elongated and helical bacteria. Gram negative. They have an outer cell envelope. They move by means of axial filaments.

Firmicutes

Group of Gram-positive bacteria, mainly with thickened cell wall and low content or percentage of GC. Firmicutes are primarily rod-shaped and sometimes coconut-shaped. Many species produce endospores.

Proteobacteria

Gram-negative bacteria, with varied morphology and a cell wall formed by lipopolysaccharides. Mainly heterotrophic, although some species can photosynthesize. They are very abundant in the oceans and other bodies of water.

Cyanobacteria

Bacterial organisms that have chlorophyll and phycocyanin. They are called blue-green algae. They are Gram-negative and capable of oxygenic photosynthesis.

Bacteroidetes

Bacteria adapted to a great diversity of habitats. Anaerobic metabolism. Gram negative. Some species are opportunistic pathogens.

Chlorobi

Group of bacteria that carry out anoxygenic photosynthesis. Anaerobic metabolism. Gram negative. They are called green sulfur bacteria.

Chloroflexi

Monodermal bacteria, that is, they have a single cell membrane. They have a very thin outer cell wall of peptidoglycan. The group has thermophilic and mesophilic representatives. Some photosynthesize. Mainly aerobic. Gram positive.

Thermotogae

They are bacteria adapted to living in extreme environments. They are considered hyperthermophilic organisms. Anaerobic metabolism and can process carbohydrates. They are Gram-negative.

Reproduction

Binary fission

The main mechanism of reproduction of bacteria is binary fission or bipartition. It is a type of asexual reproduction, where the bacterial cell needs to double in size and then divides, giving rise to two daughter cells.

This type of asexual reproduction allows bacteria to have an exponential population growth rate. In this way, the growing population can make better and faster use of available resources and also expand the possibility of generating organisms or strains resistant to the different environments in which they develop.

Multiple fission

It is a type of cell division in which the nucleus is divided into several equal parts and then the division of the cytoplasm occurs, giving rise and simultaneously to several daughter cells.

Budding or budding

This type of asexual bacterial reproduction occurs in a nonspecific location of the parent bacterium. It begins with a bulge in the cytoplasm called a bud, which then doubles to the size of the parent and separates as a new individual (daughter cell). This type of reproduction has been observed in the phyla Planctomycetes, Firmicutes and Cyanobacteria.

Baeocyte production

This type of reproduction, also called atypical binary fission, consists of a small circular cell (baeocyte), which subsequently increases in mass or size, forming a vegetative cell.

During the increase in size, this vegetative cell replicates its DNA multiple times, later it goes on to the reproductive phase where it undergoes cytoplasmic fissions, which will later become tens or even hundreds of baeocytes. This type of reproduction has been studied in cyanobacteria.

Nutrition

Bacteria have multiple types of nutrition:

Lithotrophs

Bacteria that use inorganic substrates such as nitrites, nitrates, iron or sulfates for biosynthesis or preservation of energy through anaerobiosis or aerobiosis.

Organotrophs

Bacterial organisms that obtain hydrogen or electrons from organic sources such as carbohydrates, hydrocarbons, or lipids. These organisms can be aerobic or anaerobic, even heterotrophic or autotrophic.

Autotrophic bacteria

Organisms that develop by synthesizing inorganic substances that could be carbon, but inorganic such as carbon dioxide.

Heterotrophic bacteria

Those organisms that synthesize chemical substances whose carbon source is organic, such as polysaccharides.

Mixotrophs

Bacteria that require synthesizing inorganic substances for the conservation and obtaining of energy, but also require organic compounds to fulfill their biosynthetic metabolic needs.

Diseases caused

Of the great diversity of bacteria known to man, only a few (in proportion) cause disease. The pathologies caused by these microorganisms in humans can be classified according to their origin, that is, according to the mechanism of transmission or acquisition of this:

- Airborne

Bacteria that cause airborne illnesses usually affect the respiratory tract or respiratory system, and in other cases can cause skin conditions. Here are some airborne diseases:

Diphtheria

In most cases this disease is transmitted by Corynebacterium diphtheriae, although C. ulcerans can produce similar clinical manifestations.

The disease is transmitted from a sick person to a healthy person through particles transmitted during respiration. It can also occur through contact with the secretion of skin lesions. Diphtheria can affect almost any mucous membrane and the most common clinical forms are:

• Pharyngeal: it is the most common manifestation. Symptoms include general malaise, mild fever, sore throat, and even anorexia.
• Nasal anterior: it is the least frequent clinical manifestation. It presents as a nosebleed. A purulent mucous discharge may also be present or a pseudomembrane may develop in the nasal septum.
• Laryngeal: This clinical manifestation of diphtheria produces fever, hoarseness, shortness of breath, barking cough, and high-pitched sounds when breathing. If it is not controlled in time, death may occur due to obstruction of the airways.
• Cutaneous: presents as a scaly rash on the skin or as well-defined ulcers. Depending on the location of the affected area (membrane) and its extent, complications such as pneumonia, myocarditis, neuritis, airway obstruction, septic arthritis, osteomyelitis, and even death can occur.

Legionellosis or Legionnaires' disease

The disease is caused by an aerobic Gram-negative bacterium native to soil and aquatic ecosystems called Legionella pneumophila. This bacterium has also been isolated in air conditioning systems and shower enclosures.

The disease is the result of the spread of the bacteria via the air from a reservoir to the human respiratory system. Men over 50 years of age who are compromised with smoking, alcoholism, or immunodeficiencies are more likely to contract the disease.

The bacterium lodges in the phagosomes of the alveolar macrophages, from where it multiplies and causes tissue damage. The symptoms of this disease are: cough without expulsion of respiratory secretions, fever, severe bronchopneumonia and neurological problems may manifest.

Meningitis

This disease consists of an inflammation of the meninges of the brain and spinal cord. It can have an aseptic or bacterial origin. The pathology of bacterial origin comes from the respiratory secretions of carriers of the disease or from active cases.

The bacteria that cause meningitis initially colonize the nasopharynx, from where they cross the mucosa and enter the bloodstream, and from there to the cerebrospinal fluid from where they inflame the meninges.

The symptoms of this infection are: a respiratory illness or sore throat, followed by confusion, vomiting, headache (in some cases severe), stiff neck and back.

Pneumonia

Several species of bacteria are related to pneumonia, however Mycobacterium avium and M. intracellulare species are the main causes of this disease. These bacteria have a worldwide distribution and infect not only man but other vertebrates and insects.

The respiratory and digestive systems are thought to be the entry point for these bacilli to colonize patients. The disease manifests itself in people as a lung infection, much like that caused by tuberculosis.

Other diseases

Many other diseases are transmitted by airway bacteria, among which we can mention: tuberculosis, produced by the Koch bacillus (Mycobacterium tuberculosi); whooping cough, caused by the bacteria Bordetella pertussis, and diseases caused by streptococci.

-Transmission by arthropods

Bacterial diseases caused by these invertebrates are considered rare, however they are the subject of much interest. Some of these diseases are:

Ehrlichiosis

Pathology caused by the bacterium Ehrlichia chaffeensis, which is transmitted by animal reservoirs such as the tick. Once the bacteria enter the bloodstream, it causes a nonspecific febrile illness called Human Monocytic Ehrlichiosis (HEM). The disease is characterized by symptoms such as: fever, chills, headache and myalgia.

Epidemic typhus

Bacterial disease transmitted to man by lice. The bacillus causing this disease is Rickettsia prowasekii. When the louse feeds on an infected person, the bacteria infect the arthropod's gut and spread.

Before long, large amounts of rickettsiae appear in the lice feces, and when the louse sucks the blood of another healthy individual they defecate.

When the irritation from the sting causes the individual to scratch, it contaminates the damaged site and allows the rickettsiae to enter their bloodstream, where they subsequently cause inflammation of the blood vessels by infection of their endothelial cells. The symptoms of this disease are fever, severe headache, and myalgia.

Lyme's desease

Lyme disease is a bacterial infection transmitted to man by a tick bite whose natural hosts are voles and deer. The causative bacteria are spirochetes of the genus Borrelia.

Clinically the disease has three stages: first it usually begins with skin lesions that expand like rings. This stage is often accompanied by fever, chills, fatigue, malaise, and headaches.

The second stage is characterized by bouts of arthritis, heart inflammation, and neurological problems. The third and last stage can be observed years later, and is characterized in that individuals develop demyelination of neurons and present symptoms similar to Alzheimer's or multiple sclerosis.

Other diseases

Although bacterial infections transmitted by arthropods are considered rare, some have caused massive deaths in humanity, such as the Black Death or the bubonic plague, caused by Yersinia pestis.

Another disease not as deadly as the black plague is Q fever caused by the bacterium Coxiella burnetii and infecting livestock, domestic animals and man.

Bacteria that cause the Black Plague. Yersinia pestis. Taken and edited from

-Direct contact diseases

These bacterial diseases are mainly associated with skin infections and underlying tissue. Some of these pathologies are:

Carbuncle

Disease transmitted by being in direct contact with infected farm animals or their products. The bacteria causing the disease is Bacillus anthrasis and its endospores can remain viable for many years in the soil or in animals.

Infection in humans occurs mainly due to damage or cuts to the skin (skin condition), it can also affect the respiratory (pulmonary anthrax) and gastrointestinal (gastrointestinal anthrax) systems.

An eschar (an ulcerated skin papule) forms on the skin, and accompanying symptoms include fever, headache, and nausea.

Bacterial vaginosis

It is a polymicrobial STD (sexually transmitted disease), that is, produced by several bacteria. Such bacteria are Gardnerella vaginalis, species of the genus Mobiluncus, and Mycoplasma hominis.

It is a disease considered mild but very contagious and its symptoms are: foamy, copious vaginal discharge with odors similar to fish, there is no pain, burning or itching.

Gonorrhea

Another bacterial sexually transmitted disease. It is caused by Neisseria gonorrhoeae. This diplococcus, once it enters the body, attaches itself to mucous cells through pili and protein II. This adhesion prevents it from being expelled from the vagina by normal secretions or urine.

Symptoms in men are: yellow to green pus draining from the urethra, with frequent urination, accompanied by pain and a burning or burning sensation. Only 10-20% of those exposed to the bacteria develop in women, and if they do develop the disease, it can cause ectopic pregnancies and even sterility.

Other diseases

Bacterial diseases by direct contact are very diverse both in their origin and in their development, the most mentioned are STDs and of these we can mention: the genitourinary diseases produced by the mycoplasmas Ureaplasma urealyticum and Mycoplasma hominis; and canker, produced by Haemophilus lucreyi.

Other diseases of non-sexual contact and caused by bacteria are: inclusion conjunctivitis, leprosy, cat scratch disease, gas gangrene and many others.

References

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