Reproduction of microorganisms
Previous chapters looked at different types of microorganisms. Those chapters looked at their habitats and cellular structure. This chapter will look at the ways they reproduce.
Types of reproduction
All living things reproduce. Reproduction is the process of generating offspring. There are two main types of reproduction: sexual and asexual. Some organisms reproduce by only one method of reproduction and others can reproduce using either
method. Microorganisms can reproduce sexually and asexually.
The type of reproduction where cells from only one parent are used, is called asexual reproduction. Only genetically identical organisms are produced by this type of reproduction. In evolutionary terms, asexual reproduction came before sexual reproduction. During sexual reproduction, two cells, one from each parent, fuse to form a new organism. Microbes have survived for billions of years because they can reproduce quickly and in so many different ways.
Archaea and bacteria
Archaea and bacteria mostly reproduce through binary fission. Binary fission is a form of asexual reproduction in which a cell divides into two daughter cells after DNA replication. Bacteria cannot reproduce sexually, but some types of bacteria exchange their genetic information in a process called genetic recombination. During this process, two bacteria exchange their DNA fragments through the following processes:
- by individual contact - conjugation.
- by exposure to DNA of dead bacteria - transformation.
- by exchange of plasmid genes.
- by a viral agent (bacteriophage) - transduction.
Some bacterial cells can divide in about 20 minutes but most need a few hours to reproduce.
Under unfavourable condition, some bacteria form spores with thickened coverings. These spores will return to the bacterium form when conditions improve.
Bacteria grow and reproduce very quickly only when conditions are right. Most bacteria prefer moist, warm surroundings. That is why the human body is their 'favourite' habitat. See image 1.
Cyanobacteria are able to reproduce through a variety of methods: binary fission; budding and fragmentation. These forms of reproduction explain the variety of cyanobacteria colonies that include patches, slimy masses, strings, filaments or branched filaments.
Budding involves the formation of smaller cells from larger ones. Fragmentation involves breaking into fragments, each of which then regenerates into a complete organism.
Photosynthesis plays a large and important role in the reproduction and growth of cyanobacteria. The wavelength of the light available determines what form of cyanobacteria will grow.
Protozoa mostly reproduce by binary fission. Sometimes they reproduce by budding, or a process called schizogony. Schizogony is a multiple cellular fission. During this process the cell's nucleus divides several times before the cell itself divides into multiple new cells, each with one of these new nuclei.
Some protozoans can reproduce sexually. They form sex cells - gametes that fuse together, forming a new organism. Sometimes their gametes look similar. These gametes are called isogametes. Anisogametes are gametes that vary in size and shape.
Most fungi can reproduce both sexually and asexually. Their asexual reproduction includes binary fission, budding, fragmentation and reproduction by spores. The specialised hyphae of fungi, called sporangiophores, produce spores that form in a capsule, called a sporangium. When the sporangium is mature enough it opens up releasing the spores. The spores are the reproductive cells of fungi. Each spore cell has a nucleus and dehydrated cytoplasm surrounded by a protective coating. They can exist for a very long period of time waiting for the right conditions.
Fungi produce sexual and asexual spores. There are no male or female fungi. During sexual reproduction, two mating types, called plus (+) mating type and minus (-) mating type, fuse. These fused hyphae form a specialised structure which produces and scatters genetically-diverse spores. Fungal spores cannot move by themselves, but because they are small and light they can be dispersed by wind, animals, insects or water. Fungal spores can be found almost everywhere. Unlike most eukaryotes, most fungi are haploid (have one set of chromosomes) throughout most of their lives. See image 2.
Viruses can reproduce only in a host cell. When a cell becomes infected by a virus it becomes a virus-making device. The assembly of the viral genome and its capsid does not involve enzymes as is the case during cellular DNA replication. The process is usually spontaneous. When the infected cell is full of newly-created viruses, it is broken by viral enzymes. These new viruses infect more cells.
Viruses mutate easily, creating new forms of the same virus. That ability makes it difficult to fight some viral diseases because antibodies that worked for one viral form do not work for the new one. That is why people get colds or flu every year. See image 3.
Why microbes have not eaten us.
Why are humans not destroyed by bacteria, fungi or protozoa if they reproduce so quickly? These microbes still have not eaten humans because conditions are almost never perfect for them in the real world. Once there are too many microbes in one place, their food runs out. Their waste products accumulate and microbes start crowding each other, eventually dying off.See animation 1.