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Lecture 10 Bacterial morphology – structure and arrangement of cells Learning objectives: This lecture gives you basic understanding of how miniature the bacterial cell is in comparison with other life and molecules that are even smaller than a virus. You are also expected to know different shapes of bacteria with their arrangements and an outline structure with its various components present in a bacteria. This lecture will be a foundation for future classes to know all about bacteria. Size, shape and arrangement Size: Bacteria are very small, most being approximately 0.5 to 1.0 m diameter. Smallest bacteria are Mycoplasmas, as small as 0.2 micrometers (almost as small as largest poxviruses!) Accepted wisdom is that bacteria are smaller than eukaryotes. But certain cyanobacteria are quite large; Oscillatoria cells are 7 micrometers diameter, size of red blood cells. And certain eukarotes (e.g. Nanochlorum eukaryotum) are very small, only 1 to 2 micrometers, but true eukaryotes (nucleus, chloroplast, mitochondrion are present). So size difference is, like many generalizations, only a useful yardstick, not an absolute truth. Epulopiscium fishelsoni, discovered in 1985 in intestinal tract of sturgeonfish, is an enormous, cigar-shaped cell, as large as 80 x 600 micrometers (that's 0.6 mm, large enough to be seen by the naked eye). Amazingly, this cell is prokaryotic! Initial evidence by EM was hard to believe, but confirmed rRNA comparisons with other organisms, a cousin of Gram-positive Clostridium genus. Later, largest cells of the colorless sulfur bacteria, Thiomargarita namibiensis, with a diameter of 750 m was discovered from the Namibian coast is considered as BIG bacteria. Shape: The shape of the bacterium is governed by rigid cell wall. Typical bacterial cells are spherical (Cocci and Coccus) or straight rod shaped (Bacilli Bacillus) or helically curved rods (Spirilli and Spirillum). Few bacteria can exhibit verity of shapes that are referred as Pleomarphic. (Ex. Arthrobacter). its and Cocci appear in several characteristic arrangements, depending on the plane of cellular division and whether the daughter cells stay together or not. Spherical is called coccus. Division along the same plane forms chains; 2 cocci together – Diplococcus 4 - 20 in chains - Streptococcus. Division along 2 different planes – Tetrads or Tetracoccus Division along 3 planes regularly – Sarcinae Division along 3 planes irregularly – Staphylococcus Bacilli are not arranged in patterns as complex as those of cocci and mostly occur in single or in pairs. The following are some arrangements of rod shaped bacteria. Most bacilli appear as single rods. Diplobacilli appear in pairs after division. Streptobacilli appear in chains after division. Some bacilli are so short and fat that they look like cocci and are referred to as coccobacilli. Palisades - Rods side by side arrangement like match sticks or in X, V or Y figures. Ex. Corynebacterium diphtheria Spiral bacteria have one or more twists called Spirillium. (Azospirillum). Spirals with less than one complete twist are called as Vibrioid shapes (Ex. Vibrio). . If the organism is flexible and undulating, it is called Spirochete. Other shapes: Stella are star-shaped. Haloarcula, a genus of halophilic archaea, are rectangular. Filamentous shaped – Streptomyces sp. form long, multinucleated, branched filaments called as hyphae Actinomycetes - hyphae Pear shaped – Ex. Pasteuria Lobed spheres - Ex. Sulfolobus Rod with square end – Ex. Bacillus anthracis Bacteria - Morphology Plasmid Mesosome Bacteria are unicellular organisms of relatively simple construction, especially if compared to eukaryotes. Whereas eukaryotic cells have a preponderance of organelles with separate cellular functions. Prokaryotes carry out all cellular functions as individual units. A prokaryotic cell has five essential structural components: a genome (DNA), ribosomes, cell membrane, cell wall and a surface layer. Other than enzymatic reactions, all the cellular reactions incidental to life can be traced back to the activities of these macromolecular structural components. Thus, functional aspects of prokaryotic cells are related directly to the structure and organization of the macromolecules in their cell make-up, i.e., DNA, RNA, phospholipids, proteins and polysaccharides. Diversity within the primary structure of these molecules accounts for the diversity that exists among bacteria.