Introduction to Microbiology Lecture Notes PDF

Introduction to microbiology General Microbiology Introduction to microbiology Microbiology is the study of all living organisms that are too small to be visible with the naked eye The major fields of microbiology: Bacteriology: Stu...

Introduction to microbiology General Microbiology Introduction to microbiology Microbiology is the study of all living organisms that are too small to be visible with the naked eye The major fields of microbiology: Bacteriology: Study of bacteria Parasitology: The study of parasites Mycology: The study of fungi Virology: The study of viruses Immunology: The study of human immune response to infections. The scale of invisible world The scale of the “invisible world’; There are a thousand millimeters in a meter. There are a thousand microns (micrometers) in a millimeter, an E. coli bacterium is about a micron long – so a million of them lined up form a line a meter long, a cell of bakers yeast (a fungus) is about 10- 15 microns in diameter. Introduction to microbiology History of infectious diseases Key figures in the history of microbiology Robert Hooke (1635 - 1703) was a “polymath’ he made many scientific discoveries in the 17th century, including making one of the first microscopes and also using a copy of one of Leeuwenhoek’s microscopes to see and draw details of the structure of plant cells and some microbes. Antony van Leeuwenhoek (1632-1723) made the first useful microscopes in the 17th century, they were fiendishly difficult to make and use, they were essentially a lens held in a metal clip, the lens was made from a tiny drop of molten glass, and he used such a microscope to see the first microscopic cells. Ilya Metchnikoff (1845-1916) was the first to realize that animals such as us had a defence system against infection, what we now call the immune system Paul Ehrlich (1854-1915), searched for the “magic bullet” against infectious disease, he synthesized the first successful (but very toxic) drug against a disease – syphilis, it was an arsenic derivative he called salvarsan. Gerhardt Domagk (1895-1964) developed the first useful drug against a variety of bacterial infections, the first sulfa drug –prontosil. Ironically, he died of an infection! Sir Alexander Fleming (1881-1955) and Selman Waksman (1888-1973) discovered Introduction to microbiology the first relatively safe and effective antibiotics (of natural origin) – isolated from microorganisms. Fleming discovered penicillin, Waksman discovered streptomycin and a number of other antibiotics. Louis Pasteur (1822-1895) was a chemist, he made many great discoveries, and he performed a crucial experiment using a swan necked flask that proved that new life did not just spontaneously arise from substances like rotting meat. For centuries before Pasteur, many people believed in Spontaneous Generation- the belief that life isgenerated spontaneously from dead organic matter. Robert Koch (1843-1910) and his colleagues made many important discoveries in microbiology, Koch initiated the use of the seaweed polysaccharide gel called agar as a stable material for the formation of a gel on which separated and pure (single species) colonies of bacteria and fungi could be grown (actually it was the wife of a colleague of his who suggested it), this was a critical advance, and he also stated and used his Koch’s postulates, required to prove that a given organism caused a given disease. Introduction to microbiology There are two basic types of cell: prokaryotic and eukaryotic. (Viruses are not cellular and most biologists consider them to be biological entities of course, but as not being alive in the generally accepted sense of the word. Do not make the common mistake of thinking of viruses as being cells, they are not! ) Prokaryotic cells do NOT have a discrete membrane bound nucleus that contains chromosomes, this is the defining feature of a prokaryotic cell. All bacteria are prokaryotic , while Fungi and parasites are eukaryotic We will discuss the structure of bacteria as a prokaryotic organism Cell Wall Nearly all prokaryotes have a rigid cell wall, which determines the shape of the cell, and in most cases the wall contains peptidoglycan. The bacterial cell wall serves to resist the very high internal osmotic pressure that would burst (lyse) the cell unless the wall was present. Peptidoglycan is made up of a polysaccharide backbone consisting of alternating N Acetylmuramic acid (NAMA) and N-acetylglucosamine (NAGA) residues in equal amounts. Peptidoglycan strands have a helical shape ,Peptidoglycan chains are crosslinked by peptides for strength interbridges that form interconnected networks. While all bacterial cell walls contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in eukaryotes, several antibiotics notably the (penicillins and cephalosporins) stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cells which have no cell wall only a cell membrane. There are two main types of bacterial cell walls, those of Gram- positive bacteria and those of Gram-negative bacteria, which are differentiated by their Gram staining characteristics. Introduction to microbiology There are a few bacterial genera that do not have cell walls, Mycoplasma is a classic example, they are parasites of animals such as cattle and are able to exist at the same osmotic pressure as the animal cells that they live in and among. The Gram-positive cell wall Gram-positive cell walls are thick and the peptidoglycan (also known as murein) layer constitutes almost 95% of the cell wall matrix substances in the walls of Gram positive bacteria may be polysaccharides or teichoic acids. One of its purposes is providing an antigenic function. The Gram-negative cell wall Gram-negative cell walls are thin and unlike the Gram-positive cell walls, they contain a thin peptidoglycan layer adjacent to the cytoplasmic membrane. It also contains an outer membrane The chemical structure of the outer membrane's lipopolysaccharides is often unique to specific bacterial sub-species and is responsible for many of the antigenic properties of these strains. Lipopolysaccharides, also called endotoxins, are composed of polysaccharides and lipid A which are responsible for much of the toxicity of Gram-negative bacteria. As a lipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. Plasma membrane The plasma membrane or bacterial cytoplasmic membrane is composed of a phospholipid bilayer and thus has all of the general functions of a cell membrane such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules and enzymes into the cell and energy transport chain. Introduction to microbiology Glycocalyx Many bacteria secrete extracellular polymers outside of their cell walls called Glycocalyx. These polymers are usually composed of polysaccharides and sometimes protein. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteria from phagocytosis and desiccation. Capsules is a form of glycocalyx that is more thick and firmly attached to the cell wall. Capsules can help the bacteria to evade immune detection and make them more virulent (able to cause disease more aggressively). Ex Klebsiella Fimbrae and pili Fimbrae (sometimes called "attachment pili") are protein tubes that extend out from the outer membrane in many members of the Proteobacteria. Fimbrae usually function to facilitate the attachment of a bacterium to the surfaces. Also can help in motility in a Twitching movement. Pili are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. Pili are involved in the process of bacterial conjugation where they are called conjugation pili or "sex pili". Type IV pili (non-sex pili) also aid bacteria in gripping surfaces. Flagella Flagella are whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility (i.e. movement). The arrangement of flagella about the bacterial cell is unique to the species observed. Common forms include: A. Monotrichous - Single flagellum B. Lophotrichous - A tuft of flagella found at one of the cell pole C. Amphitrichous - Single flagellum found at each of two opposite poles D. Peritrichous - Multiple flagella found at several locations about the cell Internal structures The bacterial DNA and plasmids Unlike eukaryotes, the bacterial DNA is not enclosed inside of a membrane-bound nucleus but instead resides inside the bacterial cytoplasm. This means that the transfer of cellular information through the processes of translation, transcription and DNA replication all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosomes. The bacterial DNA is not packaged using histones to form chromatin as in eukaryotes. Most bacterial DNA are circular Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode for traits that are advantageous but not essential to their bacterial host. Plasmids can be easily gained or lost by a bacterium and can be transferred Introduction to microbiology between bacteria as a form of horizontal gene transfer. So plasmids can be described as an extra chromosomal DNA in a bacterial cell. “ Ribosomes and other multiprotein complexes In most bacteria the most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms. All prokaryotes have 70S ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA. Inclusions Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Gas vacuoles Gas vacuoles are membrane-bound, spindle-shaped vesicles, found in some planktonic bacteria and Cyanobacteria. They are made up of a shell of protein that has a highly hydrophobic inner surface, making it impermeable to water but permeable to most gases. Endospores Some bacterial adaptation to stress is the formation of endospores. Endospores are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria in environments that would be lethal for these cells in their normal vegetative form.Ex bacillus SPP and Clostridium spp Bacterial growth The time it takes for a bacterium to undergo one cell division is called the generation time. There will often be a short lag phase in the culture before one sees an increase in bacterial numbers. This lag phase is generally seen while the inoculated bacteria adjust to the new culture fluid. The growth will then be exponential – a straight line on a semi log graph indicates this, this phase is also referred to as logarithmic or log phase, and it is in this phase that microbes are growing rapidly in numbers and are most metabolically active, and it is in this phase that they are most susceptible to an antimicrobial agent. Introduction to microbiology Later, the graph will begin to curve, this is a transition phase as the culture fails to provide needed nutrients to allow bacterial cell division, then the graph will show a horizontal line for some time plateau, this is stationary phase – as many bacteria are dying as are being generated, Finally the curve descends, this indicates death phase in the culture. It is possible for some bacteria to be reproducing in the death phase, but remember that the growth curve represents the behavior of a huge population of bacteria, in the death phase many more bacteria are dying than are reproducing. Bacterial Morphology Bacteria display a wide diversity of shapes and sizes, called morphologies. Most bacterial species are either spherical, called cocci (single coccus,), or rod-shaped, called bacilli (sing. bacillus).. Some bacteria, called vibrio, are shaped like slightly curved rods or comma shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes.