Bacteriology Lecture (1) PDF

LECTURE (1)  Members of the Microbial World, Scope, Relevance and Future of Microbiology Two fundamentally different types of cells exist, Prokaryotic cells having a simpler morphology and lack a true membrane de-limited nucleus. All bacteria...

LECTURE (1)  Members of the Microbial World, Scope, Relevance and Future of Microbiology Two fundamentally different types of cells exist, Prokaryotic cells having a simpler morphology and lack a true membrane de-limited nucleus. All bacteria are prokaryotic. Eukaryotic cells have a membrane-bound nucleus; are more complex morphologically and larger than prokaryotes. Algae, fungi, protozoa, higher plants, and animals are eukaryotes For many years, biologists have divided organisms into five kingdoms; Monera, Protists, Fungi, Animalia and Plantae. In the last few decades, great progress in three areas has been made that affect microbial classification. Detailed structure of microbial cells has been studied using EM  Members of microbial world There are five major members of microorganisms, Archaea, Bacteria, Algae, Protozoa, and Fungi. Archaea and Bacteria are prokaryotic cells. Unicellular algae and protozoa and fungi are eukaryotic cells. Prokaryotes The majority of prokaryotic organisms lack the membrane-bound organelles that are found in eukaryotic organisms (Figure 1). Their DNA is found in the form of one large circular chromosome as well as smaller pieces of circular DNA known as plasmids. Like eukaryotic cells, prokaryotic cells have ribosomes to translate genetic information into proteins. As prokaryotic organisms do not have mitochondria or chloroplasts to generate energy within, they use their cytoplasmic membrane to generate energy. Figure 1: Most prokaryotic cells lack the organelles found in eukaryotic cells. However, they still have ribosomes.  Archea The Archaea are a group of single-celled microorganisms. They have no cell nucleus or any other membrane-bound organelles within their cells. Archaea and bacteria are quite similar in size and shape, although a few archaea have very unusual shapes, such as the flat and square-shaped cells. Similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. The archaea biochemistry are unique, such as presence of ether lipids in their cell membranes.  Bacteria The prokaryotes further split into two distinct groups of microorganisms: bacteria and archaea. Bacteria differ from archaea in several ways including the lipid content of their membranes, the structure of their cell walls, and the structure of their flagella. Some examples of bacteria are Escherichia coli, Salmonella, Neisseria, and Mycoplasma, all of which are human pathogens. However, there are many more bacteria that are completely harmless (and some that are even helpful!) to humans. Bacteria like archaea are prokaryotic – unicellular, and having no cell nucleus or other membrane- bound organelle. Bacteria are microscopic, with a few extremely rare exceptions, such as Thiomargarita namibiensis.  Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. Some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their life cycle, or form clusters in bacterial colonies such as E.coli. Their genome is usually a circular bacterial chromosome – a single loop of DNA, although they can also harbor small pieces of DNA called plasmids. These plasmids can be transferred between cells through bacterial conjugation.  Bacteria have an enclosing cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes by budding, but do not undergo meiotic sexual reproduction. However, many bacterial species can transfer DNA between individual cells by a horizontal gene transfer process referred to as natural transformation. Some species form extraordinarily resilient spores, but for bacteria this is a mechanism for survival, not reproduction. Under optimal conditions bacteria can grow extremely rapidly and their numbers can double as quickly as every 20 minutes.  Bacterial Morphology Bacterial morphology deals with size, shape, and arrangement of bacterial cells. Different Size, Shape and Arrangement of Bacterial Cells Bacteria are prokaryotic, unicellular microorganisms, which lack chlorophyll pigments. The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Due to the presence of a rigid cell wall, bacteria maintain a definite shape, though they vary as shape, size and structure.  Size of Bacterial Cell The average diameter of spherical bacteria is 0.5-2.0 µm. For rod-shaped or filamentous bacteria, length is 1-10 µm and diameter is 0.25-1.0 µm. Thus a few bacteria are much larger than the average eukaryotic cell (typical plant and animal cells are around 10 to 50 µm in diameter).  Shape of Bacterial Cell The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), and spiral (twisted), however pleomorphic bacteria can assume several shapes.  Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they are adjacent to one another.  Bacilli (or bacillus for a single cell) are rod-shaped bacteria.  Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement. A special group of spirilla known as spirochetes are long slender, and flexible.  Arrangement of Bacterial Cell Cocci bacteria can exist singly, in pairs (as diplococci ), in groups of four (as tetrads ), in chains (as streptococci ), in clusters (as staphylococci ), or in cubes consisting of eight cells (as sarcinae). Cocci may be oval, elongated, or flattened on one side. Cocci may remain attached after cell division. These group characteristics are often used to help identify certain cocci.  1. Diplococci The cocci are arranged in pairs. Examples: Moraxella catarrhalis, Neisseria gonorrhoeae, etc. 2. Streptococci The cocci are arranged in chains, as the cells divide in one plane. Examples: Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae,  3. Tetrads The cocci are arranged in packets of four cells, as the cells divide in two plains. Examples: Aerococcus, Pediococcus and Tetragenococcus  4. Sarcinae The cocci are arranged in a cuboidal manner, as the cells are formed by regular cell divisions in three planes. Cocci that divide in three planes and remain in groups cube like groups of eight. Examples: Sarcina ventriculi, Sarcina ureae, etc.  5. Staphylococci The cocci are arranged in grape-like clusters formed by irregular cell divisions in three plains. Examples: Staphylococcus aureus  Arrangement of Bacilli The cylindrical or rod-shaped bacteria are called ‘bacillus’ (plural: bacilli). 1. Diplobacilli Most bacilli appear as single rods. Diplobacilli appear in pairs after division. Example of Single Rod: Bacillus cereus Examples of Diplobacilli: Coxiella burnetii, Moraxella bovis, Klebsiella rhinoscleromatis, etc.  2. Streptobacilli The bacilli are arranged in chains, as the cells divide in one plane. Examples: Streptobacillus moniliformis  Arrangement of Spiral Bacteria Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved shape to a corkscrew-like spiral. Many spirilla are rigid and capable of movement. A special group of spirilla known as spirochetes are long, slender, and flexible. 1. Vibrio They are comma-shaped bacteria with less than one complete turn or twist in the cell. Example: Vibrio cholerae  2. Spirilla They have rigid spiral structure. Spirillum with many turns can superficially resemble spirochetes. They do not have outer sheath and endoflagella, but have typical bacterial flagella. Example: Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi, etc.  3. Spirochetes Spirochetes have a helical shape and flexible bodies. Spirochetes move by means of axial filaments, which look like flagella contained beneath a flexible external sheath but lack typical bacterial flagella. Examples: Leptospira species (Leptospira interrogans), Treponema pallidum, Borrelia recurrentis, etc.