Characteristics, Classification, Microscopy Methods
Essentially, Monera is a biological kingdom that is made up of prokaryotes (particularly bacteria). As such, it's composed of single-celled organisms that lack a true nucleus.
Based on previous classifications, kingdom Monera includes organisms known as Archaea (Archaebacteria) in addition to blue-green algae and Schizopyta (bacteria). However, further studies identified unique characteristics of Archaea that allowed them to be separated and identified as a distinct kingdom.
Apart from some of the earliest modes of classification (e.g. Linnaeus 2 kingdoms (1735) and the Haeckel 3 kingdoms (1866)), kingdom Monera is recognized in most of the other classifications (e.g. two-empire kingdom system, five-kingdom system and the six-kingdom system) in one form or another.
* Archaebacteria is composed of bacteria that are collectively known as extremophiles. This is because they are capable of surviving harsh environmental conditions.
* The term Archaebacteria comes from the Greek words meaning "ancient things/bacteria".
* The word "Prokaryote" comes from the Greek words "Pro" which means before and "Karyon" that means kernel/nucleus- Before a nucleus.
* The word Monera is derived from the Greek word "Moneres" that means single.
Classification and Characteristics of Kingdom Monera
Several systems of classification were suggested between 1866 and 1977.
- Three kingdom classification system by Haeckel (1866)
- Four kingdom classification system by Copeland (1956)
- Five-kingdom classification system by Whittaker (1969)
- Six kingdom classification system by Woese (1977)
For this section, however, the five-kingdom classification system will be used.
Described in 1969 by Robert H. Whittaker, the 5 kingdom system classification consists of 5 major kingdoms that include:
* For the most part, the five-kingdom classification system described by Whittaker is based on the nutrition model, the structure of the cell, thallus organization, mode of reproduction as well as their phylogenetic relationships.
As the only kingdom that contains bacteria (which are prokaryotic), Monera is the sole prokaryotic kingdom in the five-kingdom classification system.
Generally, within the Whittaker (Five Kingdom Classification) system, kingdom Monera is divided into two major groups (subkingdoms), namely, Archaebacteria and Eubacteria.
As compared to eubacteria, archaebacteria are more primitive being the oldest organisms on earth. While they are also less common in comparison to eubacteria, archaebacteria are capable of surviving such extreme environmental conditions as hot springs and very salty and acidic environments etc.
Within this subkingdom, organisms can be classified/grouped on the basis of the environment in which they reside:
Methanogenic bacteria (methanogens) - Bacteria that can be found in the intestinal tract of animals as well as sewage matter. They are capable of producing methane in hypoxic conditions.
Some examples of methanogenic bacteria include:
- Methanobrevibacter smithii
- Methanococcus maripaludis
- Methanobrevibacter ruminantium
Thermoacidophilic bacteria (thermoacidophiles) - These are bacteria that can be found living in hot springs. They are also capable of surviving in environmental conditions with very low pH. Thermoplasma Picrophilus is a good example of a thermoacidophiles.
Halophilic bacteria - This includes bacteria that can be found living in extremely salty conditions (e.g. bacteria found in the Dead Sea).
Examples of halophilic bacteria include:
- Halomonas elongata
As compared to other bacteria, Archaebacteria have a unique cell wall structure which allows them to survive harsh environmental conditions. However, they can also be found in less extreme environments (fresh and ocean waters and soil).
Cell Structure of Archaea
According to a variety of studies, a majority of archaeal bacteria have been shown to possess flagella used for motility. Compared to the flagella found in other bacteria, archaeal flagella have been found to be more similar to type VI pili of bacteria. In general, they have been described as rotating structures with a filament.
Given that a majority of archaea are either chemotactic or phototactic in nature (or both), these structures, consisting of preproteins, allow them to move from one point to another in their respective environments.
* For a majority of archaea found in extreme environments, glycosylation has been shown to be one of the main components of flagellins. As a result, it has been suggested to contribute to protein stability that allows the organisms to survive.
The flagella of archaea also have the following functions:
- Swarming across surfaces
- Formation of biofilm connecting cells to each other: This may play a role in genetic transfer
Apart from flagella, some of the archaea have been shown to contain structures known as cannulae on their surface. These are hollow tubes consisting of various subunits of glycoprotein. Like several components of the flagella, these structures are also highly resistant to such extreme environmental conditions as heat.
For the most part, cannulae have been identified in newly formed cells. Given that no studies have found these structures to penetrate the cell cytoplasm (they have only been shown to enter the periplasmic region of cells), researchers have concluded that by connecting newly formed cells to each other, these structures allow for nutrient and, in some cases, genetic material between the cells.
Pili have been identified in many archaea species across the globe. As is the case with flagella, pili of archaea have been shown to be different from those of other bacteria. Depending on the species, they also play a number of functions ranging from aggregation to motility.
For instance, in a study where Sulfolobus cells were exposed to UV light treatment, pili formation allowed for aggregation of the cells before conjugation occurred (the transfer of genetic material from a donor to recipient).
While archaea and other bacteria share various characteristics with regards to the plasma membrane, the plasma membrane of archaea has a number of unique characteristics that contribute to their general characteristics.
In archaea, the glycerol linkage between the phospholipid head and side-chain has been shown to be of the L-isomeric form which is different from the D-isomeric form found in other bacteria and eukaryotes.
Also, the ether-linkage located between the glycerol and side chain in archaea provide better chemical stability to the membrane of these organisms which also contributes to their overall ability to survive extreme environmental conditions.
With regards to the plasma membrane, some of the other unique characteristics include:
- Contain isoprenoid chains that may contain branching side chains
- Plasma membrane exist as monolayers
As is the case with other bacteria, the cell wall of archaea also plays an important role in protecting the internal components of the cell from the environment. In addition, the cell wall also serves to withstand turgor pressure exerted against the plasma membrane.
While some archaea lack a cell wall, these structures vary from one species to another depending on their environment. Moreover, they display characteristics that are unique and different from the cell wall of other bacteria.
In some of the species, the cell wall has been shown to contain a proteinaceous S-layer. This, in some species, acts as the sole component of the cell wall.
Although they lack peptidoglycan found in bacteria, some archaea possess pseudomurein, which has a similar chemical structure. Also, they contain N-acetylalosaminuronic acid that is linked to the N-acetylglucosamine thus increasing the overall strength of the structure.
Some of the other components of the cell wall in archaea include:
Methanochondroitin - Lattice structure that forms the protein sheath
Plasmids - Small DNA molecules consisting of between 5 and 100 genes
- Ribosome - spherical particles involved in protein synthesis
- Cytoskeleton - proteins involved in cell division as well as influencing the shape of the cell
Commonly referred to as "true bacteria" or simply "bacteria", eubacteria is the more complex domain (described as a subkingdom in some books) of the kingdom Monera.
As compared to archaebacteria, members of eubacteria are more common and widely distributed in most habitats (water, soil, inside and on extracellular organisms, etc.) across the globe.
As members of the kingdom Monera, eubacteria are prokaryotes and thus do not have membrane-bound organelles. While some species exist as parasites that cause diseases to both plants and animals (including human beings), some bacteria are beneficial and are therefore used for food and drug production among many other uses.
Given that all bacteria (except archaebacteria) fall under the domain/subkingdom eubacteria, they may be grouped into the following categories:
Also known as blue-green algae, cyanobacteria contain chlorophyll that allows them to manufacture their own food. As such, they, like plants, are photosynthetic autotrophs.
In nature, they may exist as unicellular, colonial, or filamentous in fresh or marine water. However, they can also be found in terrestrial environments where they use water, carbon dioxide, and solar energy to manufacture their own food.
While they can manufacture food on their own, some species in this group also form symbiotic relationships with fungi thus forming lichens. In this mutualistic relationship, the bacteria provides organic nutrients that the fungi require while the fungi provides inorganic material as well as protection to the bacteria.
* Cyanobacteria are the only prokaryotes capable of photosynthesis and oxygen production.
With over 2,000 species in this division, cyanobacteria come in many shapes and sizes with varying cellular structures. Some of the species produce toxins in water as well as noxious blooms and are thus of significance in water quality management.
Some of the most popular cyanobacteria include:
* Apart from photosynthetic autotrophic bacteria, cyanobacteria also includes chemosynthetic autotrophic bacteria that convert inorganic molecules (e.g. nitrates and ammonia etc) to organic substances. As such, they obtain energy from the oxidation of inorganic molecules.
Heterotrophs are organisms that obtain energy by consuming organic material.
Unlike photosynthetic or chemosynthetic autotrophs, these organisms are unable to manufacture their own food/organic material and thus depend on organic material/food in their environments. Heterotrophic bacteria are abundant in nature with a majority existing as decomposers.
As such, they feed on dead plants and animals in their environment thus breaking them down. This contributes to soil humus that in turn contributes to proper plant development. These types of bacteria are collectively known as saprophytic bacteria.
Apart from heterotrophic bacteria found in the environment (terrestrial habitats etc), some of these bacteria are also part of the normal flora on human skin and do not generally cause harm. However, some are pathogens and cause diseases not only in human beings, but also in plants and animals. A good number of these bacteria depend on the host for their nutrition and thus exist as parasites.
Heterotrophic bacteria may be divided into the following groups:
See more on Gram positive and Gram negative bacteria
General Characteristic of Heterotrophic Bacteria
Although a few bacteria are large enough to be seen with the naked eye (e.g. Epulopiscium fishelsoni bacteria that can grow to about 600um in length), a majority of bacteria are microscopic and can only be observed with the use of a microscope. However, they vary in size, ranging from 100 to 200 nm in diameter.
These bacteria also vary in shape which has allowed them to be classified according to their general appearance. For instance, whereas some species are rod-shaped (bacillus), others are more spherical in shape (coccus). Other bacteria have more complex shapes including spirillum which are spiral-shaped and vibrios that look like curved rods.
Depending on the species, these bacteria may exist as single cells, pairs or as colonies, etc. For instance, whereas staphylococci are cocci bacteria in clusters, streptobacilli are bacilli bacteria that occur in chain-form. The shape of heterotrophic bacteria also plays an important role in motility.
The shape of spirochaetes and vibrio bacteria (in addition to the presence of flagella) contribute to better mobility of these organisms in their environment.
Different species of heterotrophic bacteria also depend on certain environmental conditions for their survival. As such, they can only thrive under certain conditions in their environment. These characteristics have also allowed bacteria to be classified on the basis of the conditions they require to grow and reproduce.
Whereas some bacteria need oxygen for normal cellular respiration (Bacillus subtilis and Azotobactor), others like Clostridium tetani thrive in very low oxygen during their vegetative phase.
Some of the other species are capable of switching from aerobic to anaerobic respiration depending on the presence or absence of oxygen in their environment. For instance, whereas facultative anaerobes can produce energy if oxygen is present, they can switch to anaerobic respiration in the absence of oxygen (energy is produced through fermentation).
* While true bacteria are not extreme as is the case with archaebacteria, they are capable of forming spores that are resistant to harsh environmental conditions. This allows them to survive through such conditions.
Cell Surface Structures of Eubacteria
As previously mentioned, there are several differences between the cell surface of archaebacteria and eubacteria. Whereas the glycerol linkage between the phospholipid head and side-chain is of D-isomeric form in eubacteria, it is of L-isomeric form in archaebacteria.
However, eubacteria have ester-linked lipids present between glycerol and side chains compared to the ether-linkage that are found in archaebacteria. As with many other cells, the plasma membrane of eukaryotes (eubacteria) is characterized by lipid bilayers that separate the internal and external environments of the cells.
A majority of eukaryotes also possess a cell wall. This is a semi-rigid structure that serves to protect and maintain the shape of cells. Typically, the cell wall of eubacteria is characterized by the presence of peptidoglycan that makes it possible to distinguish between Gram-positive and Gram-negative bacteria.
To observe and study members of the Kingdom Monera, a number of microscopic techniques can be used. Whereas some of the techniques are used to study and differentiate the genetic material found in these organisms, some are simply used to study general morphology which also makes it possible to differentiate the different members of the kingdom.
FISH (Fluorescent in Situ Hybridization)
Unlike microscopy techniques used to observe the general morphology of different types of bacteria, FISH is a molecular technique that can be used to detect the presence of given genetic elements of an organism making it possible to differentiate between various species. Generally, the technique largely depends on the hybridization of a probe with a fluorescent tag that is complementary to a given DNA sequence.
Here, the probe is applied to the sample under favorable conditions to allow for the attachment of the probe to the sequence of interest. The sample is then observed under the microscope (fluorescent microscope). Using this technique, it's possible to differentiate the different species or archaebacteria as well as eubacteria based on their genetic material.
* In fluorescence microscopy, fluorescent dyes are also used to identify given cellular components. For instance, using phalloidin, it is possible to identify the actin filaments of bacteria.
This is a technique that has been used to observe the swimming behavior of archaea. First, cells in a sample were transferred into glass capillaries (rectangular in shape). The capillaries were then sealed (both sides) and placed on an electrically heated stage on the microscope table. The thermo-microscope was then used for the purposes of analyzing the swimming behavior of the archaea.
* For this technique, heating the stage provided favorable conditions for M. thermoautotrophicus.
For bacteria referred to as "true bacteria" a variety of microscopic techniques can be used.
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Gina Hamilton. (2006). Kingdoms of Life: Monera.
Johannes P. Schneider and Marek Basler. (2016). Shedding light on biology of bacterial cells. ncbi.
Sandy Y. M. Ng, Behnam Zolghadr, Arnold J. M. Driessen, Sonja-Verena Albers, Ken F. Jarrell. (2008). Cell Surface Structures of Archaea. Journal of Bacteriology.