Yeast are group of non-mycelial unicellular fungi belonging to the group Ascomycetes. They are cosmopolitian fungi which occur in almost all places of the world having organic matter, especially sugars. Most of the species are saprophytes. They grow on the nectar of flowers, surface of sweet fruits, sugarcane, milk and other food stuffs, animal excreta, humus of soil etc.
In general, yeast cells are longer than most bacteria. Yeast vary considerably in size ranging from one to five μm in width and from 5 to 30 μm or more in length. They are commonly egg shaped but are sometimes lemon shaped, pear shaped or elongated into false or true mycelium. Yeast have no flagella and other organelles of locomotion.
Reproduction: Budding is the common type of vegetative reproduction. In budding, cell wall bulges out and softens in the area probably by certain enzymes brought by vesicles. The protoplast bulges out in this region as small protuberance and it gets covered over by a newly secreted wall. Meanwhile, the nucleus undergo intracellular division to form two daughter nuclei. One daughter nucleus along with various cell organelles and a part of vacuole in yeast passes into protuberances. The protuberance grows in size and is called bud. The cytoplasm of bud continues to remain with that of parent cell for a long time through a narrow neck.
When the bud has grown sufficiently, a septum, fungus cellulose and glucan develops in the region of neck and separates the bud from parent cell. Budding generally takes place in Saccharomyces species.
Some yeast reproduce with the help of fission where mature cells elongates and finally nucleus divides into two daughter cells by ‘Karyochoresis’. The daughter nuclei separate ad cytoplasm cleaves in middle till it divides the parent protoplast into two halves.
True yeast: Those yeast that reproduce by means of ascospores, the yeast cell serving as ascus. These yeast are also called ascomycetes.
False yeast: those yeast that do not produce ascospores are called false yeast and are also called ‘fungi imperfecti’.
Molds are the filamentous fungi that grow in the form of tangled mass that spread rapidly and may cover several inches of area in 2 to 3 days. The thallus of mold consists essentially of two parts; the mycellium and the spore. The mycellium is the complex of several filament called hyphae. New hyphae generally arise from a spore which on germination pulls out germ tubes. These germ tubes elongate and are branched to from germ tube.
Each hyphae is about 5 to 10 μm wide. Hyphae are composed of an outer tube like wall surrounding a cavity (lumen). The cavity is filled by protoplasm and is lined by double layer membrane. The hyphal wall is composed of microfibril which is made mostly of cellulose and. Hyphae occurs in three forms
1. Non septate or coenocytic
2. Septate with uniculeate cell
3. Septate with multinucleate cell
Mycellia can be either vegetative or reproductive. Some hyphae of vegetative mycellium penetrate into the medium in order to obtain nutrients. Soluble nutrients are absorbed through the help of hyphae. Reproductive mycellia are responsible for spore production and usually extend from the medium into the air.
Mold reproduce naturally by variety of means. Asexual reproduction called somatic or vegetative reproduction doesn’t involve the union of nuclei, sex cells or sex organelles. It may be accomplished by following methods.
1. Fission of somatic cells, yielding two similar daughter cells
2. Budding of somatic cells or spores. Each cell produce a small outgrowth of parent cell developing into new individual.
3. Fragmentation or disjoining of the hyphal cell where each fragment becomes a new microorganism.
4. Spore formation
Asexual spores whose function is to disperse the species are produced in large number. There are many kind of asexual spores. These are as follows.
1. Sporangiospores: These single celled spores are formed within sacs called sporangia at the end of special hyphae.
2. Conidiospores: These are small single celled conidia (also called microconidia) found at the tip or side of hyphae. Large multicellular conidia are called macro conidia.
3. Oidia: These are single cell spores which are formed by disjoining of hyphal cells.
4. Chlamydospores: This thick walled single celled spores are highly resistant to adverse condition. They are formed from cells of the vegetative hyphae.
5. Blastospores: These are spores formed by budding
Sexual reproduction: It is carried out by compatible nuclei of two parent cell. The process of sexual reproduction begins with the joining of two cells and fusion of their protoplast. This help two haploid nuclei of two mating types to fuse together to form a diploid nucleus.
The sex organelles of fungi, if they are present are called gametangia. They may form differentiated sex cells “gamets” or may contain instead one or more gamete nuclei. If the male and female gametangia are morphologically different, male is called antheridim and female is called oogonium. The sexual reproduction may take place by following method.
1. Gametic copulation
2. Gamate-Gametangial copulation
3. Gametengial copulation
4. Somatic copulation
Protozoa are unicellular eukaryotic microorganism that lack cell wall. They are generally colorless and motile. They are distinguished from prokaryotes by their bigger size and eukaryotic nature from algae by lack of chlorophyll, from fungus by their motility and absence of cell wall and from slime mold by lack of fruiting body formation.
Protozoa usually obtain food from other organism by ingesting it. They are found in marine habitat and variety of fresh water. A large number are parasitic in other animals including humans and some are found growing in soil or arial habitat such as surface of trees.
Mostly protozoa are motile. Their mechanism of motility is the key characteristics used to distinguish them from others. Protozoa that move by ameboid movement are called sarcodina, those using flagella are called mastigophora and those using cilia are called ciliophora. Sporozoa are generally non- motile and are parasitic for higher animals.
Mostly Ricketsia live in obligate, parasitic, intracellular association with eukaryotic host (vertebrates or arthropods). A few can be grown on moderately complex bacteriological media containing mold. Their cell wall contain muramic acid. Mostly these are rod shaped, coccoid and often pleomorphic that stain gram negative and lack flagella. The parasitic species are associated with the reticulo endothelial and vascular endothelial cells or erythrocytes of vertebrates and often with various organs of arthropods which may act as vector or host. Important genera includes Bartonella, Garhamella and Rochalimaea.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.