Classification of Microorganisms: Microbes can be classified into four major groups: 1- Protozoa 2- Bacteria. 3- Fungi. 4- Viruses. 1- The Protozoa: These are unicellular organisms with protoplasm differentiated into nucleus and cytoplasm. Diameters in the range of 2-100 μm. The most important groups of medical protozoa are: A-Amoeba: Entamoeba species. Mode of Motility: pseudopodia.

B- Mastigophora: Mode of Motility: the Flagella. Gastrointestinal flagellates: Giardia intestinalis Urogenital flagellates: Trichomonas vaginalis Tissue and blood flagellates: Trypanosoma, Leishmania . a

Trypanosoma, and Leishmania .

C- Ciliophora: motile by cilia. Example: Balantidium coli.

D- Sporozoa: intracellular infection. Example: Plasmodium that cause Malaria.

1- Filamentous bacteria: Streptomyces: antibiotic producers. 2- The bacteria: Bacteria are unicellular prokaryotic microorganisms that multiply by binary fission. Bacteria can be classified according to morphology, arrangement, and staining reaction into the following groups: 1- Filamentous bacteria: Streptomyces: antibiotic producers. 2- True bacteria: Cocci: Gram positive: Staphylococcus, Streptococcus. Gram negative: Neisseria. Bacilli: Gram positive: Bacillus, Clostridum, Corynebacterium. Gram negative: Enterobacteriaceae, Brucella. 3- Spirochetes: Slender flexuous spiral bacteria. Borrelia, Treponema, Leptospira. 4- Mycoplasma: The Smallest bacteria that lack of a rigid cell wall. 5- Rickettsiae and Chlamydiae: intracellular parasites.

Medical fungi can be divided into: 3- The Fungi: These are saprophytic or parasitic organisms possessing relatively rigid cell walls. Medical fungi can be divided into: 1- Mould: Branching filaments; hyphae, mycelium. Usually 2 to 10 μm in width. Example: Epidermophyton, Trichophyton, Microsporum, Aspergillus. 2- True Yeasts: these are ovoid or spherical cells that reproduce asexually by budding and sexually with formation of spores. Example : Cryptococcus spp. 3- Dimorphic fungi: Produce a vegetative mycelium in artificial media, but are yeast like in infected lesions. Example: Histoplasma. 4- Yeast- like fungi: Example: Candida ( Pseudomycelium).

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4- The viruses: Viruses consist of DNA or RNA enclosed in a simple protein shell known as a capsid. General properties of viruses They are very small in size, from 20-300 m. They contain one kind of nucleic acid (RNA or DNA) as their genome. They are metabolically inert They are obligate intracellular parasites. They are only seen by electron microscope. Depend on the parasitized cell for survival and multiplication

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Structure of bacterial cells: Size, Shape, and Arrangement of bacterial cells: Morphology and arrangement of bacterial cells are criteria used for classification of bacteria into following groups: 1. Cocci (Singular: coccus). 2. Rods (bacilli), (Singular: rod, bacillus). 3. Vibrios (Singular: vibrio). 4. Spirilla (singular :Spirillum) 5. Spirochetes. (Singular: Spirochaete). 1. Cocci: These are round or oval bacteria measuring about 0.5-1.0 micrometer in diameter. When they multiplying, cocci may form pairs, chains, or irregular groups.

Cocci in pairs are called diplococci, for example, meningococci and gonococci. Cocci in chains are called streptococci, for example Streptococcus pyogens. Cocci in irregular groups are called Staphytococci, for example, Staphylococcus aureus.

2. Rods (bacilli): These are stick-like bacteria with rounded, square, or swollen ends. They measure 1-10 micrometer in length by 0.3-1.0 micrometer in width. It may arranged in: A- Chains, for example, Streptobacillus species. B- Branching chains, for example, lactobacilli . C- Mass together, for example, Mycobacterium leprae. D- Remain attached at various angles resembling Chinese letters, for example, Corynebacterium diphtheria.

3-Vibrios: These are small slightly curved rods measuring 3-4 micrometer in length by 0.5 micrometers in width. Most vibrios are motile with a single flagellum at one end. They show a rapid darting motility. For example: vibrio cholerae.

4-Spirochetes: These are flexible, coiled, motile organism, 6-20 micrometer in length. They progress by rapid body movements. Spirochetes are divided into three main groups: A- Treponemes. B- Borreliae. C- Leptospires.