Basic Microbiology Lecture 12 PDF

Lecture – 12 Farzana Hossain, Ph.D. Assistant Professor Dept. of Biochemistry and Microbiology *  * Most bacteria are free-living or parasitic forms that can metabolize and reproduce by independent means.  Two groups of bacteria—the rickettsias and chlamydias— have adapted to...

Lecture – 12 Farzana Hossain, Ph.D. Assistant Professor Dept. of Biochemistry and Microbiology *  * Most bacteria are free-living or parasitic forms that can metabolize and reproduce by independent means.  Two groups of bacteria—the rickettsias and chlamydias— have adapted to life inside their host cells, where they are considered obligate intracellular parasites.  * Rickettsias:  Rickettsias are distinctive, very tiny, Gram-negative bacteria.  Although they have a somewhat typical Red-stained Rickettsia rickettsii visible bacterial morphology, in cells of a vector tick they are atypical in their life cycle and other adaptations.  Important Characteristics of Rickettsias: 1. Most are pathogens that alternate between a mammalian host and blood-sucking arthropods, such as fleas, lice, or ticks. These arthropods feed on the blood or tissue fluids of their mammalian hosts, but not all of them transmit the rickettsial pathogen through direct inoculation with saliva. 2. They are obligate intracellular parasites. Rickettsias cannot survive or multiply outside a host cell and cannot carry out metabolism completely on their own, so they are closely attached to their hosts. 3. Rickettsias possess a Gram-negative cell wall, binary fission, and DNA and RNA. 4. They are among the smallest cells, ranging from 0.3 to 0.6 m wide and 0.8 to 2.0 m long. 5. They are non-motile pleomorphic rods or coccobacilli. 6. Several important human diseases are caused by rickettsias. Among these are  Rocky Mountain spotted fever, caused by Rickettsia rickettsii (transmitted by ticks), and  epidemic typhus, caused by Rickettsia prowazekii (transmitted by lice).  An exceptionally resistant rickettsia, Coxiella burnetti (the cause of Q fever), is transmitted in air and dust by arthopods. Chlamydia s Because of their tiny size and obligately parasitic lifestyle, they were at one time considered a type of virus (Figure 2). Later studies indicated that their structure was that of a Gram-negative cell and that their mode of cell division (binary fission) was clearly prokaryotic. Chlamydia trachomatis in brown Important Characteristics of Chlamydias 1. Bacteria of the genus Chlamydia are similar to the rickettsias in that they require host cells for growth and metabolism, but they are not closely related and are not transmitted by arthropods. 2. Like rickettsias, the chlamydias are obligate parasites that depend on certain metabolic constituents of host cells for growth and maintenance. 3. They show further resemblance to the rickettsias with their small size, Gram-negative cell wall, and pleomorphic morphology, but they are markedly different in several aspects of their life cycle. Important Characteristics of Chlamydias 4. They multiply by binary fission. 5. The species of greatest medical significance are Chlamydia trachomatis, a very common pathogen involved in sexually transmitted, neonatal, and ocular disease (trachoma).  Mycoplasmas (Cell-Wall-Deficient Bacteria) Mycoplasmas the smallest self- microorganisms. are All replicating naturally lack (Figure of them 3), and except for genus, a all cell species wall one parasites of animals and are plants. Figure 3: Scanning electron micrograph of Mycoplasma pneumoniae (62,000x). Cells like these that naturally lack a cell wall exhibit extreme pleomorphism. Important Characteristics of Mycoplasmas 1. Mycoplasmas are bacteria that naturally lack a cell wall. 2. Although other bacteria require an intact cell wall to prevent the bursting of the cell, the mycoplasmal cell membrane is stabilized by sterols and is resistant to lysis. 3. They are extremely tiny, pleomorphic cells are considered the smallest cells, ranging from 0.1 to 0.5 m in size. 4. They range in shape from filamentous to coccus or doughnut-shaped. Important Characteristics of Mycoplasmas 5. They are not obligate parasites and can be grown on artificial media, although added sterols are required for the cell membranes of some species. 6. Mycoplasmas are found in many habitats, including plants, soil, and animals. 7. The two most clinically important genera are Mycoplasma and Ureaplasma. Disease of the respiratory tract has been primarily associated with different species of Mycoplasma and Ureaplasma sp. M. hominis and Ureaplasma urealyticum are implicated in urogenital tract infections.  Free-Living Non-Pathogenic Bacteria: Photosynthetic  Bacteria Most bacteria are heterotrophic, meaning that they derive their nutrients from other organisms. Photosynthetic bacteria are independent cells that contain special light-trapping pigments and can use the energy of sunlight to synthesize all required nutrients from simple inorganic compounds.  The two general types of photosynthetic bacteria are those that produce oxygen during photosynthesis and those that produce some other substance, such as sulphur Cyanobacteria: Blue-Green Bacteria The cyanobacteria were called blue-green algae for many years and were grouped with the eukaryotic algae structure (Figure 4). However, further study verified that they are indeed bacteria with a Gram-negative cell wall and are prokaryotic. Figure 4: Structure and examples of cyanobacteria: Two species of Oscillatoria, a gliding, filamentous form (100x). Important Characteristics of Cyanobacteria 1. Cyanobacteria are prokaryotic and can carry out oxygenic photosynthesis. 2. A specialized adaptation of cyanobacteria are extensive internal membranes called thylakoids, which contain granules of chlorophyll a and other photosynthetic pigments. 3. These bacteria range in size from1 m to 10 m, and they can be unicellular or can occur in colonial or filamentous groupings. Some species occur in packets surrounded by a gelatinous sheath. 4. They also have gas inclusions, which permit them to float on the water surface and increase their light exposure. 5. They (such as Nostoc, Anabaena) also contain heterocysts that convert gaseous nitrogen (N2) into a form usable by plants, and therefore they are important providers of nitrogen fertilizer in the cultivation of rice and other crops. 6. This group is sometimes called the blue- green bacteria in reference to their content of phycocyanin pigment that tints some members a shade of blue, although other members are colored yellow and orange. 7. Cyanobacteria are widely distributed in nature. They can occur as planktonic cells or form biofilms in fresh water and marine environments. Some types of lichens are symbiotic associations between fungi and cyanobacteria that assist in the breakdown of rock and soil formation. 8. Many species of cyanobacteria (Anabaena, Aphanizomenon, and Microcystis) form water blooms, where they often impart an unpleasant smell and due to large amounts of suspended organic matter water become toxic and causes mortality among livestock. 9. Certain cyanobacteria produce cyanotoxins including microcystin LR, nodularin R (from Nodularia) which can be toxic and dangerous to humans and animals.  Green and Purple Sulphur Bacteria These bacteria are named for their predominant colours, but they can also develop brown, pink, purple, blue, and orange coloration. They can carry out anoxygenic photosynthesis (Figure 5). Figure 5: Microscopic view of purple sulphur bacteria (Chromatium vinosum) carrying large, yellow sulphur granules internally. Important Characteristics of Green and Purple Bacteria 1. The green and purple bacteria are also photosynthetic and contain pigments. They differ from the cyanobacteria in having a different type of chlorophyll called bacteriochlorophyll and by not giving off oxygen as a product of photosynthesis. 2. They live in sulphur springs, freshwater lakes, and swamps that are deep enough for the anaerobic conditions they require yet where their pigment can still absorb wavelengths of light. 3. They exist as single cells of many different shapes and frequently are motile. 4. Both groups utilize sulphur compounds (H2S, S) in their metabolism, and some can deposit intracellular granules of sulphur or sulphates. 5. Purple sulphur bacteria are anaerobic or microaerophilic,. On the other hand, green sulphur bacteria are obligately anaerobic photoautotrophic bacteria, are found in anaerobic and sulphide-containing fresh or marine waters, and wetlands.  Gliding Fruiting Bacteria: Myxobacteria Probably the most intriguing and exceptional members of this group are the slime bacteria, or myxobacteria Figure 6: Myxobacterium: A photograph of an actual mature fruiting body of Chondromyces. Important Characteristics of Myxobacteria 1. The myxobacteria, such as and Myxococcus gliding Chondromyces, bacteria are an fruiting that produce interesting family bodies in of starvation conditions. 2. They are common in animal dung and organic-rich soils of neutral or alkaline pH. 3. The name is derived from the tendency of members to glide over moist surfaces. The gliding property evidently involves rotation of filaments or fibers just under the outer membrane of the cell wall. 4. They do not have flagella. 5. They have complexity and advancement in their life cycle. During this cycle, the vegetative cells swarm together and differentiate into a many-celled, colored structure called the fruiting body, which produces myxospores.  The fruiting body is a survival structure that makes spores by a method very similar to that of certain fungi.  These fruiting structures are often large enough to be seen with the unaided eye on tree bark and plant debris.  Appendage containing Bacteria Appendaged bacteria contain appendages that outward protrude from the surface of the microorganism. Some are highly anchored to the surface, whereas others, like the glycocalyx, are loosely associated with the surface. Figure 7: Budding and stalked bacterium, Caulobacter rosette, showing the budding and stalked phases.  Appendaged Bacteria The appendaged bacteria are quite varied in their structure and life cycles, but all of them produce an extended process of the cell wall in the form of a bud, a stalk, or a long thread (Figure 7). Figure 7: Budding and stalked bacterium, Caulobacte rosette, showing the budding and stalked r phases.  Stalked They live Bacteria attached to the surface of objects in aquatic environments. One type can even grow in distilled water or tap water. The stalks evidently help them trap minute amounts of organic materials present in the water. Fig 8: Scanning electron micrographs of wild type Caulobacter  Budding They Bacteria reproduce entirely by budding; that is, they form a tiny bulb (bud) at the end of a thread. The bud then breaks off, enlarges, develops a flagellum, and swarms to another area to start its own cycle. These bacteria can also grow in very low-nutrient habitats. Fig 9: In the budding bacteria Hyphomonas polymorpha, the bud grows out of the end of a filament called a prostheca.