Modes of bacterial reproduction

Modes of bacterial reproduction- Fission, Budding, Fragmentation and Sporulation

Bacteria are prokaryotic organisms that reproduce asexually.  Asexual reproduction is a mode of reproduction in which the new offspring arise from a single parent. The offspring will be physically as well as genetically identical to each other and will be exact copies of their parent cell.  The common modes of asexual reproduction in bacteria are Binary Fission, Budding, Fragmentation and Sporulation.

1.      Binary fission

The most common mode of bacterial reproduction is a kind of cell division called binary fission or transverse binary fission. Binary fission is the process by which a single cell divides to form two cells that are genetically identical to one another.

Most bacteria reproduce by binary fission. In binary fission, initially the bacterial cell reaches a critical mass in its structure and cellular constituents.  The circular double stranded DNA, the genomic DNA, undergoes replication, resulting in the formation of two identical double stranded DNA. The new double stranded DNA molecules move to two poles of the dividing cell. The septum formation is triggered by the completion of DNA replication.   The cytoplasmic membrane grows inward at the middle of the cell, generally associated with a mesosome. The next step is the inward growth of the cell wall and transverse septum develops that ultimately separates the two daughter cells.

In the case of gram positive coccus such as streptococcus faecalis, the new cell wall material synthesized as part of the septum becomes one half of the cell wall of each daughter cell.  In gram positive rod such as Bacillus subtilis, the new cell wall material formed as part of the septum will become only 15% of the new cell wall of the daughter cell.  The remaining part of the cell wall is synthesized along the cylindrical part of the cell during further bacterial growth. 

2.      Fragmentation

In fragmentation, also known as splitting, a cell breaks into several fragments, which later develop into complete organism. Fragmentation is seen in many organisms such as filamentous Cyanobacteria and Nocardia.  Here the bacterial protoplasm undergoes compartmentalization and subsequent fragmentation results in minute bacillary or coccoid cells each of which can grow to a new bacterium.

3.      Budding

The bacterial cell develops small swelling or protuberance at one end which gradually increases in size. Simultaneously the DNA undergoes division, where one remains with the mother and other one with some cytoplasm goes to the swelling. This outgrowth is the bud, which eventually develops into a new cell and gets separated from the parent cell.   e.g., Rhodopseudomonas acidophila. Some bacteria such as Hyphomicrobium form the bud at the end of prosctheca.

Budding has also been observed in some members of the Planctomycetes, Cyanobacteria, Firmicutes and prosthecate Proteobacteria.

4.      Formation of Spores

Spore formation takes place in filamentous bacteria like Actinomycetes etc.  The formation of transverse septum or crosswalls at the hyphal tips results in the formation of spores.  Common types of spores are Conidiopsores, Oidiospores and Sporangiospores.

(a) Conidiospores

Formation of Conidiospores occur in Streptomyces.  The filamentous mycelia of this bacterium forms oval or round spores known as conidia in chains at the tips of special aerial branches known as conidiophores.  In conidiophore the conidiospores are enclosed within a sheath. After detachment from the parent and getting contact with suitable substratum, the spores germinates and gives rise to new mycelium.

(b) Oidiospores

Some Actinomyces undergoes septation throughout its mycelial length to form numerous small reproductive units known as the oidiospores. Each oidiospore, on germination produces, a filamentous bacterium.

(c) Sporangiospores

Some Actinomycetes and Myxobacteria form sporangia-like structures at the end of hyphae. The protoplast of the sporangium then divides to form tiny sporangiospores.  On liberation, the sporangiospores germinate to form filamentous branching bacterium.

Modes of reproduction in bacteria

A.    Transverse binary fission in Bacillus subtilis

B.     Transverse binary fission in Streptococcus faecalis

C.     Transverse binary fission in prosthecate bacterium Prosthecobacter fusiformis

D.    Budding in Rhodopseudomonas acidophila

E.     Budding in prosthecate bacterium Hyphomicrobium

F.      Fragmentation in Nocardia

G.    Formation of conidiospores in Streptomyces

 

References

1. Text Book of Microbiology by Michael J. Pelczar
2. https://www.ck12.org/book/cbse_biology_book_class_x/section/3.3/
3. http://www.biologydiscussion.com/bacteria/how-does-bacteria-reproduce-with-diagram-microbiology/64010
4. Botany for Degree Students Fungi by A K Sinha