Bacteriology is the branch of microbiology that deals with the morphology, structure, classification, and biochemistry of bacteria. The discipline of bacteriology arose during the 19th century from scientific attempts to prove the “germ theory of disease,” namely that diseases were caused by microscopic organisms invading host cells. Bacteria are prokaryotic Prokaryotic Prokaryotes are unicellular organisms that include 2 of the 3 domains of life: bacteria and archaea. Prokaryotic cells consist of a single cytoplasm-filled compartment enclosed by a cell membrane and cell wall. Cell Types: Eukaryotic versus Prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission Binary fission Cell Types: Eukaryotic versus Prokaryotic. Some of these organisms play a significant role in the pathogenesis of diseases. Management of bacterial disease is generally with antibiotics; however, the choice of antibiotics may vary depending on the bacterial structure and metabolism.
Last updated: 16 May, 2022
Process of laboratory
identification
Identification
Defense Mechanisms:
Identification
Identification
Defense Mechanisms of bacterial pathogens follows a stepwise process that usually begins with Gram staining and is followed by growth in isolated culture.
Structure of a prokaryote cell:
The cell
envelope
Envelope
Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there.
Virology comprises a
plasma
Plasma
The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation.
Transfusion Products membrane, seen here in green, and a thick
peptidoglycan
Peptidoglycan
Penicillins-containing
cell wall
Cell wall
The outermost layer of a cell in most plants; bacteria; fungi; and algae. The cell wall is usually a rigid structure that lies external to the cell membrane, and provides a protective barrier against physical or chemical agents.
Cell Types: Eukaryotic versus Prokaryotic (yellow layer). No outer lipid membrane is present, as seen in gram-negative bacteria. The red layer, known as the
capsule
Capsule
An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides.
Bacteroides, is distinct from the cell
envelope
Envelope
Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there.
Virology.
Structure | Chemical composition | Function |
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Appendages | ||
Flagellum | Protein | Motility Motility The motor activity of the gastrointestinal tract. Gastrointestinal Motility |
Pili Pili Filamentous or elongated proteinaceous structures which extend from the cell surface in gram-negative bacteria that contain certain types of conjugative plasmid. These pili are the organs associated with genetic transfer and have essential roles in conjugation. Normally, only one or a few pili occur on a given donor cell. This preferred use of ‘pili’ refers to the sexual appendage, to be distinguished from bacterial fimbriae, also known as common pili, which are usually concerned with adhesion. Salmonella/ fimbriae Fimbriae Thin, hairlike appendages, 1 to 20 microns in length and often occurring in large numbers, present on the cells of gram-negative bacteria, particularly enterobacteriaceae and Neisseria. Unlike flagella, they do not possess motility, but being protein (pilin) in nature, they possess antigenic and hemagglutinating properties. They are of medical importance because some fimbriae mediate the attachment of bacteria to cells via adhesins. Bacterial fimbriae refer to common pili, to be distinguished from the preferred use of ‘pili’. Escherichia coli | Glycoprotein | Adherence to cell surface |
Specialized structures | ||
Spore Spore The reproductive elements of lower organisms, such as bacteria; fungi; and cryptogamic plants. Microsporidia/Microsporidiosis |
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Cell envelope Envelope Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there. Virology | ||
Capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides | Organized polysaccharide layer | Protects against phagocytosis Phagocytosis The engulfing and degradation of microorganisms; other cells that are dead, dying, or pathogenic; and foreign particles by phagocytic cells (phagocytes). Innate Immunity: Phagocytes and Antigen Presentation |
Slime layer | Loose network of polysaccharides Polysaccharides Basics of Carbohydrates | Mediates adherence to surfaces |
Outer membrane |
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Periplasm | Peptidoglycan Peptidoglycan Penicillins in middle | Accumulate components exiting gram- cells |
Cell wall Cell wall The outermost layer of a cell in most plants; bacteria; fungi; and algae. The cell wall is usually a rigid structure that lies external to the cell membrane, and provides a protective barrier against physical or chemical agents. Cell Types: Eukaryotic versus Prokaryotic | Peptidoglycan Peptidoglycan Penicillins in sugar backbone | Net-like structure gives rigid support. |
Cytoplasmic membrane | Phospholipid bilayer sac |
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Gram staining is a technique named after the bacteriologist Hans Christian Joachim Gram, and is used to differentiate between groups of bacteria based on the differences in the constituents of their cell walls.
Differences between the cell membranes of
gram-positive
Gram-Positive
Penicillins and gram-negative bacteria:
While both
gram-positive
Gram-Positive
Penicillins and gram-negative bacterial cell walls contain
peptidoglycan
Peptidoglycan
Penicillins layers, the layer in gram-negative bacteria is much thinner. Gram-negative bacteria make up for this deficit by having another membrane layer outside the
peptidoglycan
Peptidoglycan
Penicillins layer.
Gram staining helps distinguish between gram-positive Gram-Positive Penicillins and gram-negative bacteria by staining cells red or violet.
Process of Gram staining:
Staining of gram-positive Gram-Positive Penicillins bacteria:
Staining of gram-negative bacteria:
Staining plays important role in diagnostics/pathology:
Morphology | Arrangements |
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Cocci |
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Bacilli Bacilli Shigella |
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Spirilla |
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Spirochetes Spirochetes An order of slender, flexuous, helically coiled bacteria, with one or more complete turns in the helix. Treponema |
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Genus Selenomonas | Cylinders curved in a plane |
Genus Haloquadratum |
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Vibrions |
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Bacterial cells with various morphologies and arrangements:
Bacteria exist in a wide variety of morphologies and arrangements. Cocci and
bacilli
Bacilli
Shigella in pairs and clusters are the most common.
Gram-positive
Gram-Positive
Penicillins bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thick layer of
peptidoglycan
Peptidoglycan
Penicillins retain the crystal violet stain utilized in Gram staining but are not affected by the safranin counterstain. These bacteria appear as purple-blue on the stain, indicating that they are
gram positive
Gram positive
Bacteria which retain the crystal violet stain when treated by gram’s method.
Nocardia/Nocardiosis. The bacteria can be further classified according to morphology (branching filaments,
bacilli
Bacilli
Shigella, and cocci in clusters or chains) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The cocci can also be further identified. Staphylococci can be narrowed down on the basis of the presence of the enzyme
coagulase
Coagulase
Enzymes that cause coagulation in plasma by forming a complex with human prothrombin. Coagulases are produced by certain staphylococcus and yersinia pestis. Staphylococci produce two types of coagulase: staphylocoagulase, a free coagulase that produces true clotting of plasma, and staphylococcal clumping factor, a bound coagulase in the cell wall that induces clumping of cells in the presence of fibrinogen.
Staphylococcus and on their
sensitivity
Sensitivity
Binary classification measures to assess test results. Sensitivity or recall rate is the proportion of true positives.
Blotting Techniques to the antibiotic novobiocin. Streptococci are grown on
blood agar
Blood agar
Nocardia/Nocardiosis and classified on the basis of which form of hemolysis they employ (α, β, or γ). Streptococci are further narrowed on the basis of their response to the pyrrolidonyl-β-naphthylamide (PYR) test, their
sensitivity
Sensitivity
Binary classification measures to assess test results. Sensitivity or recall rate is the proportion of true positives.
Blotting Techniques to specific antimicrobials (optochin and bacitracin), and their ability to grow on
sodium
Sodium
A member of the alkali group of metals. It has the atomic symbol na, atomic number 11, and atomic weight 23.
Hyponatremia
chloride
Chloride
Inorganic compounds derived from hydrochloric acid that contain the Cl- ion.
Electrolytes (NaCl) media.
Gram-negative bacteria:
Most bacteria can be classified based on a lab procedure called Gram staining.
Bacterial cell walls having a thin layer of
peptidoglycan
Peptidoglycan
Penicillins do not retain the crystal violet stain used for Gram staining. However, gram-negative bacteria retain the safranin counterstain and appear pinkish-red. These bacteria can be further classified according to morphology (diplococci, curved rods,
bacilli
Bacilli
Shigella, and coccobacilli) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). Gram-negative bacteria can be accurately identified by culturing on specific media (triple sugar
iron
Iron
A metallic element with atomic symbol fe, atomic number 26, and atomic weight 55. 85. It is an essential constituent of hemoglobins; cytochromes; and iron-binding proteins. It plays a role in cellular redox reactions and in the transport of oxygen.
Trace Elements (
TSI
TSI
Shigella) agar), where their
enzymes
Enzymes
Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules.
Basics of Enzymes can be identified (
urease
Urease
An enzyme that catalyzes the conversion of urea and water to carbon dioxide and ammonia.
Nocardia/Nocardiosis,
oxidase
Oxidase
Neisseria) and their ability to ferment lactose can be determined.
* Stains poorly on
Gram stain
Gram stain
Klebsiella
**
Pleomorphic
Pleomorphic
Bacteroides rod/coccobacillus
*** Require special transport media
Bacteria are heterotrophic organisms that need organic substances to survive.
Classified based on oxygen requirements:
Bacteria can exchange genetic material:
Virulence Virulence The degree of pathogenicity within a group or species of microorganisms or viruses as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenic capacity of an organism is determined by its virulence factors. Proteus is the ability of an organism to infect the host and cause disease. Virulence Virulence The degree of pathogenicity within a group or species of microorganisms or viruses as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenic capacity of an organism is determined by its virulence factors. Proteus factors are molecules that assist the bacterium in colonizing the host and can be either secretory, membrane associated, or cytosolic in nature.
Mechanism | Virulence Virulence The degree of pathogenicity within a group or species of microorganisms or viruses as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenic capacity of an organism is determined by its virulence factors. Proteus factors | Function |
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Colonization |
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Avoiding the immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs |
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Creates physical barrier blocking opsonization and phagocytosis Phagocytosis The engulfing and degradation of microorganisms; other cells that are dead, dying, or pathogenic; and foreign particles by phagocytic cells (phagocytes). Innate Immunity: Phagocytes and Antigen Presentation |
Bacterial nutrition | Siderophores Siderophores Low-molecular-weight compounds produced by microorganisms that aid in the transport and sequestration of ferric iron. Klebsiella |
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Antigenic variation |
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Camouflage of surface molecular markers that allow evasion of the immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs |
Intracellular survival |
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Prevents intracellular destruction of the bacteria |
Type III secretion system Type III secretion system Multiprotein transmembrane complexes in gram-negative bacteria involved in either the secretion of effector proteins from the bacterial cytoplasm into host cells, or the secretion and assembly of flagellar components. The major component of the t3sss is called the injectosome or needle complex. Yersinia spp./Yersiniosis | Injectisome | Allows bacteria to inject toxins into host cells |
Inflammatory response |
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