Budding is the process of the formation of buds, which is often associated with a method of vegetative reproduction. Buds are described as “thin filaments with terminal knobs” by Stutzer & Hartleb while studying Hyphomicrobium vulgare. In general terms, buds can be described as protrusion of local tissue of the cell, which leads to the formation of filament-like structure. These are also known as prosthecae. The bud is generally smaller than the cell forming it. Budding is often used to describe vegetative reproduction in the budding bacterium, but it has a broad spectrum of applications. There are various types of budding that do not result in vegetative reproduction. In budding the cell grows from the polar end (the site where budding starts) which allows the cell to develop complex structures such as stalks in Caulobacter, hyphae in Hyphomicrobium, and appendages in Stella.
These are the bacteria that undergo budding to reproduce. The budding bacteria are also known as appendaged bacteria. These are heterogeneous groups of unicellular microbes that share a unified mode of reproduction. These microbes differ from each other in life forms, life cycle morphology as well as biochemical pathways. Budding and appendaged bacteria are vastly chemoheterotrophic, that is they derive their energy by metabolizing the organic compounds found in nature. Examples of organic compounds include compounds having one carbon, but some bacteria tend to utilize complex organic sources of food such as humic acid.
Some budding bacteria belong to the photolithotrophic category which means they use photosynthesis to produce energy.
Budding in Bacteria
Budding is the term used to describe the asexual reproduction in which progeny develops from, the generative tissue or cell of the parent organism. The budding is marked by the formation of protuberance in the parental cell of an organism. The protuberance is known as the bud, the growth in the protuberance leads to the development of the mirror image of the prenatal organism the only condition is that the bud should not detach from the parental cell before maturation. Once the budding process has started the development of the cell wall is seen. An important point is that the cell wall synthesized is vastly de-novo, this is an important difference between fission and budding as in fission a large portion of the cell envelope is contributed by the parental cell. The parent organism or cell can also be known as the mother cell. There are the following criteria that should be considered to determine the true budding process.
Morphological - The cell must be smaller in size compared to the parental cell. Even after being detached as the progeny will undergo developmental stages.
Developmental - The progeny cell must have synthesized its own cellular envelope. The de-novo synthesis of the cell membrane is an important feature of budding.
The individual identity of progeny and parental cell is maintained, unlike fission where cell division leads to the formation of two sister cells which results in loss of identity of the parent in budding the parent cell retains its own identity. For example, in Pasteuria sensu the mother cells do not have flagellum and are sessile (attached to a surface) whereas the daughter cell has flagellum and is motile.
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Types of Budding in Bacteria
There are various types of budding that occur in bacteria, some of them are classified as
1. Budding for Multiplication
This a type of budding where the bacteria reproduce through the budding process. As the name suggests this process leads to an increase in the cell number. The progeny formed can remain attached to the parent cell or can travel to another site. This is one of the most common types of budding found in nature. An example of it includes, budding in Pasteuria strains.
2. Budding for Hyphal Branching
In this type of budding cytoplasmic extrusions such as hyphae and filaments are formed. The examples of it include Pelodietyon or hyphal extensions, such as in Hyphomicrobium, Rhodomicrobium and, Pedomicrobium
3. Budding for Spore Formation
Spore formation is known as the process of sporulation. This process ensures the survival of a species even in a drastically harsh environment. This process that is sporulation is not an obligate process in the life cycle of an organism. It can be defined as the dormant stage which ensures the survival of the organism. These dormant structures can be called endospores, cysts or spores. Nutritional deprivation is one of the major causes that lead to spore formation, because of these conditions cells need to preserve their nutrition hence the cell spore is metabolically inactive. The spores germinate only in the presence of nutrition that is in the vegetative phase. The budding bacteria form a coat around a small part of the genome, which then surrounded by the thin spore membrane and thick cortex of peptidoglycan, the thick cortex forms the outermost layer, which provides the mechanical strength and protection. The examples of it include Micromonospora chalcea, Hyphomicrobium and, Dermatophilus dermatomes.
Classification of Budding and Appendaged Bacteria.
This classification includes various budding bacterium which is classified into the following groups
1. Group I
This class includes budding and appendaged bacteria that are oval, rod-shaped, or spindle-shaped cells with prosthecae, the filament at which bud formation takes place. The buds are motile and the mortality of the cell is maintained by monotrichous flagella at the polar end. This can be further divided into cells that have prosthecae at the singular pole; examples of such type includes Pedomicrobium and cells which have it on non specifically which includes the following Hyphomonas, Hyphomicrobium and, Thiodendron.
2. Group II
This class includes the cells in which daughter cells are not formed at prosthecae. Some cells contain more than one prosthecae, examples of such organisms are as follows, Prosthecomicrobium, Stella and Ancalomicrobium.
3. Group III
This class includes the organisms that have a single polar to subpolar prosthecae, this includes the following, Caulobacter, and Asticcacaulis.
4. Group IV
This group is characterized by having an ovoidal, spherical, or rod-shaped structure of the cells. This represents the budding and appendaged bacteria. It produces holdfast at one pole and the buds are produced at the opposite pole. An example of members of this group includes genus Planctomyces, which have acellular appendages, like stalks, with apical holdfasts.
5. Group V
This group is characterized by a distinctive feature of secreting stalks at the lateral surface of the cell, an example of this group includes Gallionella.