Classification of Micro-organisms of Medical Importance and Bacterial Cell Structure PDF

CLASSIFICATION OF MICRO-ORGANISMS OF MEDICAL IMPORTANCE AND BACTERIAL CELL STRUCTURE. Dr Ogban. BACKGROUND  Microbiology is the study of microorganisms, a diverse group of microscopic organisms some of which are unicellular (prokaryotic) and others mult...

CLASSIFICATION OF MICRO-ORGANISMS OF MEDICAL IMPORTANCE AND BACTERIAL CELL STRUCTURE. Dr Ogban. BACKGROUND  Microbiology is the study of microorganisms, a diverse group of microscopic organisms some of which are unicellular (prokaryotic) and others multicellular (Eukaryotic). It also includes viruses, which are microscopic but not cellular.  They play unique roles in cycling the chemical elements essential for life, including carbon, nitrogen, sulfur, hydrogen, and oxygen; more photosynthesis is carried out by microorganisms than by green plants.  A useful survival strategy for prokaryotes is to enter into consortia, arrangements in which the physiologic characteristics of different organisms contribute to survival of the group as a whole.  QUORUM SENSING, a cell-cell communication to regulate the transcription of genes involved in diverse physiologic processes that ensure the production of one or more diffusible signal molecules termed autoinducers or pheromones that enable a bacterium to monitor its own cell population density.  Prokaryotes have the ability to exchange small packets of genetic information carried on plasmids, small and specialized genetic elements that are capable of self-replication.  Of concern is the carriage of genes that code for antibiotic resistance.  They also have the ability to establish a range of interaction with other organisms including Symbiosis as exemplified by flora in human.  On the other hand parasitic interaction may be deleterious to the host.  Advanced symbiosis or parasitism can lead to loss of functions that may not allow growth of the symbiont or parasite independent of its host.  Eg. Mycoplasmas, are parasitic prokaryotes that have lost the ability to form a cell wall.  The organism thus incorporates a substantial quantity of cholesterol into its cell membrane from the host.  In the same light, chlamydiae and rickettsiae depend on the host cell for many essential metabolites and coenzymes.  This loss of function is reflected by the presence of a smaller genome with fewer genes. CLASSIFICATION OF PROKARYOTES  This refers to the division of organisms into specific groups based on established characteristics which may be structural, physiologic, biochemical and or genetic. The essence is for ease of understanding, identification, management and control.  Presence of spores has been used to divide prokaryotes into spore-forming and non-spore forming organisms.  Ability to ferment certain sugars classifies organisms into related groups. An example is lactose fermenting and non-lactose fermenting bacteria. Others include citrase, urease, indole as well as other biochemical positivities and negativities.  The Gram stain, is an effective criterion for classification because response to the stain reflects fundamental and complex differences in the bacterial cell surface that divide most bacteria into gram negative and positive groups  Bacteria are classified based on shape into bacilli and cocci.  Based on oxygen utilization into aerobic and anaerobic bacteria.  Presence or absence of locomotive organnels into motile or non motile bacteria.  Based on nutrition into fermenters and non fermenters.  Based on temperature preference into psycrophiles (cold-loving) mesophiles (moderate heat –loving) and thermophiles (high heae-loving).  Advances in DNA-based technologies, Genetic criteria are increasingly being used in bacterial classification.  Comparison of DNA sequences for some genes led to the elucidation of phylogenetic relationships among prokaryotes. Ancestral cell lines can be traced, and organisms can be grouped on the basis of their evolutionary affinities. PROKARYOTIC CELL STRUCTURE  Prokaryotic cell is simpler in structure than Eukaryotic cell.  Prokaryotes have no true nuclei; instead they package their DNA in a structure known as the nucleoid which can be seen with light microscope on stained slide.  In bacteria, the number of nucleoids depend on the growth conditions. Rapidly growing bacteria have more nucleoids per cell than slowly growing ones.  Electron micrographs of a typical prokaryotic cell reveal the absence of a nuclear membrane and a mitotic apparatus.  Prokaryotic cells lack autonomous plastids, such as mitochondria and chloroplasts; the electron transport enzymes are localized instead in the cytoplasmic membrane.  The photosynthetic pigments (carotenoids, bacteriochlorophyll) of photosynthetic bacteria are contained in intracytoplasmic membrane systems of various morphologies.  Prokaryotic cells are surrounded by complex envelope layers. These structures protect the organisms from hostile environments, such as extreme osmolarity, harsh chemicals, and even antibiotics.  The bacterial cell membrane (cytoplasmic membrane) is a typical “unit membrane” composed of phospholipids and upward of 200 different kinds of proteins far higher in proportion than that of mammalian cell membranes.  The membranes of prokaryotes are distinguished from those of eukaryotic cells by the absence of sterols, the only exception being mycoplasmas that incorporate sterols, such as cholesterol, into their membranes.  The prokaryotic CM functions as :  i. Semipermeable membrne ii. Pathogenicity Proteins/exoenzymes excretion. Iii. Biosynthesis  iv. Electron transport/ oxidative phoshorylation system v. Chemotactic system.  The bacterial cell wall has high tencil strength such that it protects the cell from being damaged by external and internal osmotic pressure. The bacterial cell wall owes its strength to a layer composed of a substance variously referred to as murein, mucopeptide, or peptidoglycan.  The cell wall also plays an essential role in cell division.  Various layers of the wall are the sites of major antigenic determinants of the cell surface, and one component—the lipopolysaccharide of gram-negative cell walls—is responsible for the nonspecific endotoxin activity of gram-negative bacteria.  The cell wall is, in general, non- selectively permeable; however the outer membrane of the gram negative bacteria hinders the passage of relatively large molecules.  The bacterial cell wall also serves as site for some antibiotic actions.  Most bacteria are classified as gram- positive or gram negative according to their reaction to the Gram-staining procedure, invented by Hans Christian Gram.  The Gram stain depends on the ability of certain bacteria ( gram-positive) to retain a complex of crystal violet dye (purple in color) and iodine after a brief wash with alcohol or acetone. This because the Gram +ve bacteria has large bundles of peptidoglycans contrary to Gram –ve bacteria which has just few bundles.  Gram-negative bacteria do not retain the dye–iodine complex and become translucent after washing, but they can then be counterstained with safranin or neutral red or dilute carbol fuchsin (a red dyes).  Thus, gram-positive bacteria appear purple under the microscope and gram- negative bacteria look red.  The explanation is that the Gram positive bacterial cell wall has far more bundles of peptidoglycans than does the Gram negative bacterial cell wall. This implies that the GPB cell wall takes up more of the primary stain ( crystal violet) than the GNB cell wall so much that even after decolorization with acetone or alcohol enough of the primary stain is still retained whereas the GNB cell wall is totally depleted of the primary stain and has to retain the secondary stain.