إعـــــــلان

تقليص
لا يوجد إعلان حتى الآن.

The Morphology and Fine Structure of Bacteria

تقليص
X
 
  • تصفية - فلترة
  • الوقت
  • عرض
إلغاء تحديد الكل
مشاركات جديدة

  • The Morphology and Fine Structure of Bacteria

    السلام عليكم ورحمة الله و بركاتة.

    Bacterial cells are between 0.3 and 5 um in size. They have three basic
    forms: cocci, straight rods, and curved or spiral rods. The nucleoid consists
    of a very thin, long, circular DNA molecular double strand that is not surrounded
    by a membrane. Among the nonessential genetic structures are
    the plasmids. The cytoplasmic membrane harbors numerous proteins
    such as permeases, cell wall synthesis enzymes, sensor proteins, secretion
    system proteins, and, in aerobic bacteria, respiratory chain enzymes. The
    membrane is surrounded by the cell wall, the most important element of
    which is the supporting murein skeleton. The cell wall of Gram-negative bacteria
    features a porous outer membrane into the outer surface of which the
    lipopolysaccharide responsible for the pathogenesis of Gram-negative infections
    is integrated. The cell wall of Gram-positive bacteria does not possess
    such an outer membrane. Its murein layer is thicker and contains teichoic
    acids and wall-associated proteins that contribute to the pathogenic process
    in Gram-positive infections. Many bacteria have capsules made of polysaccharides
    that protect them from phagocytosis. Attachment pili or fimbriae
    facilitate adhesion to host cells. Motile bacteria possess flagella. Foreign body
    infections are caused by bacteria that form a biofilm on inert surfaces. Some
    bacteria produce spores, dormant forms that are highly resistant to chemical
    and physical noxae.



    Morphological Characteristics of Bacteria

    • Simple staining. In this technique, a single staining substance, e.g.,methylene blue, is used.
    • Differential staining.
      Two stains with differing affinities to different bacteria
      are used in differential staining techniques, the most important of which
      is gram staining. Gram-positive bacteria stain blue-violet, Gram-negative
      bacteria stain red.


    Three basic forms are observed in bacteria: spherical, straight rods, and

    curved rods.

    Fine Structures of Bacteria

    • Nucleoid (Nucleus Equivalent) and Plasmids

    The “cellular nucleus” in prokaryotes consists of a tangle of double-stranded
    DNA, not surrounded by a membrane and localized in the cytoplasm .
    In E. coli (and probably in all bacteria), it takes the form of a single circular
    molecule of DNA. The genome of E. coli comprises 4.63 !106 base pairs (bp)
    that code for 4288 different proteins. The genomic sequence of many bacteria
    is known.
    The plasmids are nonessential genetic structures. These circular, twisted
    DNA molecules are 100–1000! smaller than the nucleoid genome structure
    and reproduce autonomously. The plasmids of human pathogen
    bacteria often bear important genes determining the phenotype of their cells
    (resistance genes, virulence genes).



    • Cytoplasm

    The cytoplasm contains a large number of solute low- and high-molecularweight substances, RNA and approximately 20 000 ribosomes per cell. Bacteria have 70S ribosomes comprising 30S and 50S subunits. Bacterial ribosomes function as the organelles for protein synthesis. The cytoplasm is also frequently used to store reserve substances (glycogen depots, polymerized ****phosphates, lipids).

    • The Cytoplasmic Membrane

    This elementary membrane, also known as the plasma membrane, is typical
    of living cells. It is basically a double layer of phospholipids with numerous
    proteins integrated into its structure. The most important of these membrane
    proteins are permeases, enzymes for the biosynthesis of the cellwall, transfer
    proteins for secretion of extracellular proteins, sensor or signal proteins, and
    respiratory chain enzymes.
    In electron microscopic images of Gram-positive bacteria, the mesosomes
    appear as structures bound to the membrane. How they function and what
    role they play remain to be clarified. They may be no more than artifacts.


    • Cell Wall

    The tasks of the complex bacterial cellwall are to protect the protoplasts from
    external noxae, to withstand and maintain the osmotic pressure gradient between
    the cell interior and the extracellular environment (with internal pressures
    as high as 500–2000 kPa), to give the cell its outer form and to facilitate
    communication with its surroundings.


    Murein (syn. peptidoglycan). The most important structural element of
    the wall is murein, a netlike polymer material surrounding the entire cell
    (sacculus). It is made up of polysaccharide chains crosslinked by peptides.

    The cell wall of Gram-positive bacteria. The murein sacculus may
    consist of as many as 40 layers (15–80 nm thick) and account for as much as
    30% of the dry mass of the cell wall. The membrane lipoteichoic acids are
    anchored in the cytoplasmic membrane, whereas the cell wall teichoic acids
    are covalently coupled to the murein. The physiological role of the teichoic




    acids is not known in detail; possibly they regulate the activity of the autolysins
    that steer growth and transverse fission processes in the cell. Within
    the macroorganism, teichoic acids can activate the alternative complement
    pathway and stimulate macrophages to secrete cytokines. Examples of cell
    wall-associated proteins are protein A, the clumping factor, and the fibronectin-
    binding protein of Staphylococcus aureus or theMprotein of Streptococcus
    pyogenes. Cell wall anchor regions in these proteins extending far beyond the
    murein are bound covalently to its peptide components. Cell wall-associated
    proteins frequently function as pathogenicity determinants (specific adherence;
    phagocyte protection).

    The cellwall of Gram-negative bacteria. Here, the murein is only about 2 nm
    thick and contributes up to 10% of the dry cell wall mass . The outer
    membrane is the salient structural element. It contains numerous proteins
    (50% by mass) as well as the medically critical lipopolysaccharide.


    • Outer membrane proteins.

    — OmpA (outer membrane protein A) and the murein lipoprotein form a
    bond between outer membrane and murein.
    — Porins, proteins that form pores in the outer membrane, allow passage of
    hydrophilic, low-molecular-weight substances into the periplasmic space.
    — Outer membrane-associated proteins constitute specific structures that
    enable bacteria to attach to host cell receptors.
    — A number of Omps are transport proteins. Examples include the LamB
    proteins for maltose transport and FepA for transport of the siderophore
    ferric (Fe3+) enterochelin in E. coli .
    • Lipopolysaccharide (LPS).
    This molecular complex, also known as endotoxin,
    is comprised of the lipoid A, the core polysaccharide, and the O-specific
    polysaccharide chain .


    • Lipoid A

    is responsible for the toxic effect. As a free substance, or bound up
    in the LPScomplex, it stimulates—by binding together with the LPS binding
    protein (LBP) to the CD14 receptor of macrophages—the formation and
    secretion of cytokines that determine clinical endotoxin symptomatology.
    Interleukin 1 (IL-1) and tumor necrosis factor (TNF) induce an increased
    synthesis of prostaglandin E2 in the hypothalamus, thus setting the “thermostat”
    in the temperature control center higher, resulting in fever. Other
    direct and indirect endotoxin effects include granulopoiesis stimulation,
    aggregation and degeneration of thrombocytes, intravasal coagulation due
    to factor VII activation, a drop in blood pressure, and cachexia. LPS can also
    activate the alternative complement pathway. Release of large amounts of
    endotoxin can lead to septic (endotoxic) shock. Endotoxin is not inactivated
    byvapor sterilization. Therefore, the parent materials usedinproduction of
    parenteral pharmaceuticals must be free of endotoxins (pyrogens).
    • The O-specific polysaccharide chain
    is the so-called O antigen, the fine
    chemical structure of which results in a large number of antigenic variants
    useful in bacterial typing (e.g., detailed differentiation of salmonella
    types) .
    • L-forms (L = Lister Institute).
    The L-forms are bacteria with murein defects,
    e.g., resulting from the effects of betalactam antibiotics. L-forms are highly
    unstable when subjected to osmotic influences. They are totally resistant to
    betalactams, which block the biosynthesis of murein. The clinical significance
    of the L-forms is not clear. They may revert to the normal bacterial form when
    betalactam therapy is discontinued, resulting in a relapse.
    • Capsule

    Many pathogenic bacteria make use of extracellular enzymes to synthesize a
    polymer that forms a layer around the cell: the capsule. The capsule protects
    bacterial cells from phagocytosis. The capsule of most bacteria consists of a
    polysaccharide. The bacteria of a single species can be classified in different
    capsular serovars (or serotypes) based on the fine chemical structure of this
    polysaccharide.
    • Flagella

    Flagella give bacteria the ability to move about actively. The flagella (singular
    flagellum) are made up of a class of linear proteins called flagellins. Flagel
    lated bacteria are described as monotrichous, lophotrichous, or peritrichous,
    depending on how the flagella are arranged . The basal
    body traverses the cell wall and cytoplasmic membrane to anchor the flagellum
    and enables it to whirl about its axis like a propeller.
    In Enterobacteriaceae, the flagellar antigens are called H antigens. Together
    with the O antigens, they are used to classify bacteria in serovars.
    • Attachment Pili (Fimbriae), Conjugation Pili

    Many Gram-negative bacteria possess thin microfibrils made of proteins (0.1–
    1.5 nm thick, 4–8 nm long), the attachment pili. They are anchored in the
    outer membrane of the cell wall and extend radially from the surface. Using
    these structures, bacteria are capable of specific attachment to host cell receptors
    (ligand—receptor, key—keyhole).
    The conjugation pili (syn. sex pili) in Gram-negative bacteria are required
    for the process of conjugation and thus for transfer of conjugative plasmids.
    • Biofilm

    A bacterial biofilm is a structured community of bacterial cells embedded in a
    self-produced polymer matrix and attached to either an inert surface or living
    tissue. Such films can develop considerable thickness (mm). The bacteria located
    deep within such a biofilm structure are effectively isolated from immune
    system cells, antibodies, and antibiotics. The polymers they secrete are
    frequently glycosides, from which the term glycocalyx (glycoside cup) for the
    matrix is derived.
    • Bacterial Spores

    Bacterial spores (endospores) are purely dormant life forms. Their development
    frombacterial cells in a “vegetative” state does not involve assimilation
    of additional external nutrients. They are spherical to oval in shape and are
    characterized by a thick spore wall and a high level of resistance to chemical
    and physical noxae. Among human pathogen bacteria, only the genera Clostridium
    and Bacillus produce spores. The heat resistance of these spores is
    their most important quality from a medical point of view, since heat ster
    ilization procedures require very high temperatures to kill them effectively.
    Potential contributing factors to spore heat resistance include their thickwall
    structures, the dehydration of the spore, and crosslinking of the proteins by
    the calcium salt of pyridine-2,6-dicarboxylic acid, both of which render protein
    denaturing difficult. When a spore’s milieu once again provides favorable
    conditions (nutrient medium, temperature, osmotic pressure, etc.) it returns
    to the vegetative state in which spore-forming bacteria can reproduce.

    النهايــــــــــــــة :sm188:

    والسلام عليكم

    مجلـة مختبرات العرب
    أطلس مختبرات العرب

    تابعونا على :

  • #2
    نسيت ذكر المصدر وهو من الكتاب : Kayser, Medical Microbiology

    اتمنى ذكر الاقتراحات و الملاحظات , أسلوب طرح الموضوع ما هو تقييمك له ؟

    وشكراً
    مجلـة مختبرات العرب
    أطلس مختبرات العرب

    تابعونا على :

    تعليق


    • #3
      فعلا موضوع رائع ودائم افضل المورفولوجي كونها
      سهلة الاستيعاب وتقديمك للموضوع رائع جدا
      والصور كذلك

      تحياتي

      تعليق


      • #4
        طرح رائع و أسلوب عرض شيق جدا ...

        ارابز لاب ممكن اضيف صورك هذي في الاطلس لو تكرمت .....

        تعليق


        • #5
          المشاركة الأصلية بواسطة the forget_1 مشاهدة المشاركة
          فعلا موضوع رائع ودائم افضل المورفولوجي كونها
          سهلة الاستيعاب وتقديمك للموضوع رائع جدا
          والصور كذلك

          تحياتي
          الله حي المنسي

          هذي مقدمة General Bacteriology وفيها هذا الجزء الصغير من المرفولوجي

          ان شاء الله لو تابعنا راح تشوف الاجمل
          مجلـة مختبرات العرب
          أطلس مختبرات العرب

          تابعونا على :

          تعليق


          • #6
            المشاركة الأصلية بواسطة الواقع والحياد مشاهدة المشاركة
            طرح رائع و أسلوب عرض شيق جدا ...

            ارابز لاب ممكن اضيف صورك هذي في الاطلس لو تكرمت .....
            حياك الله اخي الواقع والحياد

            براحتك اضيفي زي ما تحبي وان شاء الله الصور الباقية راح تكون دايركت بالاطلس
            مجلـة مختبرات العرب
            أطلس مختبرات العرب

            تابعونا على :

            تعليق


            • #7
              موضوع ممتاز وطرح رائع


              شكرا ارابز لاب



              هذا ملف بوربوينت يعرض نفس الموضوع وفيه صور حلوة وكلام مختصرر


              حمل من هنا


              اتمنى انه ينفعكم

              تعليق


              • #8
                هلا اخوي.....
                مشكوووووور للموضوغ...
                الموضوع سهل بالنسبه لي انا افضل الصور اكثر من الكتابه ...لان الصور اذا تشوفها ماتنساها

                وعلى هالمناسبه ذي راح اعطيك صوره


                مبين فيها البيسك استركشر حق البكتيريا...
                لان نعرف بعض البكتيريا تحتوي على فلاجيلا وبعضها لا وبعضها فقط بيلاي وبعضها لا

                سلااااام
                http://www.tntup.com/photo/img2/1663...5a9acb/hiv.jpg

                تعليق


                • #9
                  مقدمة رائعة لفهم علم البكتيريا

                  عجبني سهولة وترتيب طرح المعلومات في الكتاب


                  شكراً على جهدك الرائع

                  تعليق


                  • #10
                    وايت فالكون ..

                    تسلم تسلم

                    و مشكور على الملف , جمع المختصر و المفيد بس وراك داسه ليه ما طلعته من أول

                    HIV

                    العفو

                    الصور بالنسبة لي أفضل و أحسن للفهم , لكن بالاختبار راح تسأل بالدتيلز ولا راح توفي معك الصوره بالاضافة الي انك تحتاج لمعرفه بعض المعلومات عن المرفولوجي غير الشكل في اللاب دايقنوزيز ... حياك الله

                    راعي مايكروبات

                    العفو

                    و إن شاء الله أجد الوقت لاكمل بقية المواضيع ..

                    حياكم الرحمن
                    مجلـة مختبرات العرب
                    أطلس مختبرات العرب

                    تابعونا على :

                    تعليق


                    • #11
                      بارك الله فيك

                      تعليق


                      • #12
                        موضوع اكثر من رائع

                        عرض ممتاز جدا

                        شكرا لك
                        LO MEJOR سابقا..

                        تعليق


                        • #13
                          ماشاء الله موضوع حلو
                          http://www.m5zn.com/uploads/3df9c32955.jpg

                          تعليق


                          • #14
                            طرح سلس يتفوق ع كثير من دكاترة الجامعات ...بارك الله فيك
                            لا تنتقد دورك في مسرحية الحياة بل افهمه و اده بأفضل ما تستطيع و1

                            تعليق


                            • #15

                              تعليق

                              يعمل...
                              X