Introduction To Bacteriology

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GENERALIZED STRUCTURE OF A BACTERIAL CELL:

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*Lack a nuclear membrane
*inside = cytoplasm w/ SS circular DNA, no membrane-bound organelles; contains independently-replicating plasmids
*PM = functions like "organelle"--energy production, DNA replication
*CW = (except mycoplasma) Support/Protection
*Capsule = secreted outside CW; gelatinous, for evasion of phagocytosis; only some have these
*Flagella, fimbriae/pili = Mvmt and Attachment

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Fimbriae

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(or pili): short hair-like filaments that extend from the cell surface. Pili mediate the attachment of bacteria to specific receptors present on the cells of certain tissues, a step that is essential in the initiation of infection for some organisms.

They are antigenic.

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Flagella:

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filamentous, long, whiplike protein structures attached to the cell surface and provide motility for bacteria. They express antigenic and strain determinants.

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Different types of flagella and examples of bacteria with each:

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vibrio - monotrichous
spirochetes - amphitrichous
helicobacter - lophotrichous
shigella - peritrichous

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Key points about the bacterial CW:

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*Essential structure for viability.
*Main structural component is peptidoglycan (a.k.a murein; this is unique to bacteria)
-polymer of sugars cross-linked by short chains of amino acids
-glycan backbone made up of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

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What makes the chemical ∆ b/t a gram + and a gram - ?

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*Usually l-lysine linked to 5 glycines in a gram +
*Gram negatives have DAP; this is a unique molecule.

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How do antibiotics tgt the CW?

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What effects do the 4 dyes have on gram + and gram - bacteria?

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Generally describe a gram + :

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*Thick multilayered peptidoglycan CW prevents the complex of crystal violet and iodine from diffusing out of the bacterium in etoh or acetone. Thus, they stain purple.
*Associated with Teichoic acid/Lipoteichoic acid
*Lipoteichoic acid links peptidoglycan to PM
*Teichioc acid exists within the peptidoglycan layer.
*Both can be antigenic and play a role in promoting immune response.

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Generally describe a gram neg :

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*The peptidoglycan layer is thin and is overlaid by an outer membrane. In the stain, the decolorizer disperses the outer membrane and washes the crystal violet from the thin layer of peptidoglycan. Bacteria are visualized by the red counterstain (safranin).
*Bi-layer
*Porins help sneak nutrients into the bacterium.
*LPS are unique to gram neg bacteria; they are antigenic.

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What is the structure of LPS in a gram neg bacterium?

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*Lipid A region contains endotoxin (xs causes sepsis); is attached to outer membrane
*O antigens are repeating polysaccharides; most external component of LPS.
*The core is a conserved region; consists of 2 sugars.

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to identify

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Left: gram neg (thin membrane, red color)

Right: gram + (thick membrane, purple) and gram neg

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Shapes of gram + bacteria?

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*cocci; can come in different number combos
*rods

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Shapes of gram neg bacteria?

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*diplococcus (prototype is gonorrhea)
*rods (various shapes; vibrio shown here is curved)

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WHAT ARE THE ATYPICAL BACTERIA?

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Left: Actinomycetes
Middle: Mycoplasma shown w/ acid fast stain.
Right: Spirochetes; thin

*Special shapes/staining properties
-Acid-fast bacteria
-Spirochetes
-Mycoplasma (no CW)
-Chlamydiae and Rickettsiae (obligate intracellular parasites)
-Actinomycetes (filamentous)

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How do bacteria replicate?

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*binary fission, freak.
*occurs every ~20 mins.

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Growth curve phases of BACTERIAL GROWTH AND METABOLISM:

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*Lag: metabolic activity without division.
*Exponential: rapid cell division = steady state growth
*Stationary: nutrients exhausted = growth slows and ceases

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Bacterial requirements for growth: 5

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*energy
*nutrients (Fe is most important)
*optimal temperature
*optimal pH
*oxygen requirements (TB is strictly aerobic; C. diff is strictly anaerobic; most bacteria are facultative)

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What factors determine the O2 requirements of bacteria?

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*Depends on ability to respire or ferment.
*Respiration: molecular oxygen is used as final electron acceptor of the electron transport chain.
*Fermentation: an organic metabolic intermediate serves as final electron acceptor.

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How can fermentation in bacteria be used clinically?

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*Fermentation produces large amounts of organic acids and alcohols, unique to type of bacteria.
*These products can be diagnostic in the clinical laboratory.

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What effect do toxic effects of O2 have on bacteria? How does this determine which bacteria are aerobic?

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*Only some bacteria can protect themselves from O2's toxic effects. These are the ones that have special enzymes to deal with ROS.
*Superoxide dismutase breaks down superoxide radicals.
*H2O2 is broken down by peroxidase OR catalase.

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Summary of aerobic vs anaerobic bacteria:

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KEY: +/+ = SOD and catalase; + = SOD; -/- = lacks all enzymes.
*Aerotolerant ones can only tolerate a little O2.

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Describe the normal flora in the human body:
What spots are heavily colonized, colonized, or sterile?

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*Mixture of organisms that inhabit the body surfaces exposed to the environment, such as the skin, oropharynx, intestinal tract, and vagina.
-Body distribution is directly related to their metabolism.
-Can be affected by the lifestyle of the host.
-Most are commensals, but can cause opportunistic infections.

*Compartments of the human body can be divided into three categories depending on degree of colonization:
-Heavily colonized: large bowel, mouth, skin, vagina
-Colonized: small bowel, trachea, stomach
-Sterile: CSF, blood, lung, deep tissue

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What are some examples of normal flora in the nose, mouth, throat, large intestine, vagina, urethra, and skin?

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*Don't need to memorize these
*Know that staph and strep are common on skin.

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3 relationships we can have with bacteria:

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Exposure of an individual to an organism, including normal flora, can lead to one of three outcomes. The organism can:
*Transiently colonize the person
*Permanently colonize the person
*Produce disease

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What roles do normal flora play?

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*Common source of infection (most infections are from normal flora)
*Constitute a protective host defense mechanism (they occupy receptors and use nutrients; called microbial antagonism)
*Serve a nutritional function (produce vitamins)
*Stimulate the production of natural antibodies (we make Abs to them that may cross-react and protect against a pathogen).

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How do we get sick from infectious diseases?

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*The balance between the organism and the host shifts in favor of the organism.
*Pathogens are pathogens because they have found ways to tip the balance.

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Tasks of pathogenic bacteria:

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1. Encounter and Entry
2. Establish an infectious niche
3. Evasion of host defenses or penetration
4. Damage to host cells or tissues
5. Transmission to a new host

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Describe the encounter and entry of pathogenic bacteria:

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*Requires a breach in innate defense mechanisms and barriers or bacteria may have the means to directly compromise defense barriers and invade the body.

*Dependent on the infectious dose (ID50) entering a preferred portal of entry.
*The smaller the ID50, the more virulent the organism!

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Summary of which bacteria enter by ingestion, inhalation, trauma, needlestick, arthropod bite, or sex:

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Describe bacterial establishment of an infectious niche:

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*Attachment to a cell surface requires receptor on host cell and adhesin on invading pathogen.

*Examples of adhesins:
-pili
-outer membrane proteins (M protein of Streptococci)
-polysaccharide capsules
-lipoteichoic acid

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Adherence of gonococci to the surface of a human urethral epithelial cell.

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Microbial attachment in the intestinal tract.

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Adherence of Vibrio cholerae to M cells in human ileal mucosa.

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How do bacterial capsules help bacteria evade host defenses or penetrate into tissue to stay away from defenses?

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*Several factors contribute to invasiveness by limiting host defenses, especially phagocytosis.
*capsule keeps C3b from binding; degrades it
*bacteria proliferate while immune system is trying to make viable antibodies. Stays one step ahead.

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Describe the importance of CW factors in evasion/penetration by bacteria:

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*Cell wall factors
-Protein A - blocks opsonization by stealing the Fc portion of Ab
-M protein- acts in a similar fashion

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Describe the importance of invasive factors in evasion/penetration by bacteria:

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*Invasins
-Can be delivered by bacterial injection secretion systems.Important in enteric bacteria (gastroenteritis)
*Forces rearrangement of actin in host cell; allows mvmt from cell to cell without exposure to immune system.

*Promote intracellular survival

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Describe the importance of secreted enzymes in evasion/penetration by bacteria:

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*Extracellular enzymes secreted by invasive bacteria hide the organism and allow for deeper penetration into tissue:

*Coagulase; forces formation of a clot, which the bacterium lives inside of.
*Kinases; dissolve the clot to allow invasion into deeper tissues
*Collagenase and hyaluronidase; allows invasion thru deeper tissue
*IgA protease
*Leukocidins

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How do bacteria cause damage to host cells or tissues?

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1) Direct damage: caused by by-products of bacterial growth (fermentation, degradative enzymes; example is clostridium perfringens).

2) Toxin production: toxins are bacterial products that directly disrupt host function or trigger destructive biological activities; example is

Two general types of toxins: exo (secreted) and endo (intrinsic to bacterium).
*endotoxins are ONLY made by gram neg bacteria.

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3 classes of exotoxins:

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A-B Exotoxins
Membrane-disrupting toxins
Superantigens

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Describe the A-B exotoxins:

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*toxins with enzymatic activity; act intracellularly.
*A inactivates signaling proteins like G proteins.
*B (binding): binds to receptors; shuttles and releases A

*Inhibition example: diphtheriae
*Hyperactivation example: cholera

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Describe the membrane-disrupting toxins:

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*Similar to MAC, perforin.
*Tgt leukocytes, RBCs

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Describe Superantigens:

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-Toxins that stimulate the immune system.
-Bind to TCRs and MHCII in the ABSENCE of antigen
-xs cytokines hype up inflammation, T cells, macrophages
-systemic effects

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DESCRIBE ENDOTOXINS:

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*The lipopolysaccharides (LPS), the lipid A component of the cell wall of gram-negative bacteria.

*Powerful activator of acute-phase cytokines (IL-1, TNF-a, IL-6, and prostaglandins) and inflammatory reactions.

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Compare exotoxins and endotoxins:
-bacterial source
-location in bacterium
-chemical nature
-ability to form toxoid
-heat stability
-mechanism
-toxicity

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-Toxoid = antigenic form of a toxin

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Describe the role of the host immune response in damaging its own cells/tissues in response to bacterial infection:

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*Host immune response to infection:
-Excessive inflammation
-Misdirected immune response
(i.e. granuloma, etc.)

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Summary of the tasks of pathogenic bacteria:

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Summary of the gram-positive bacteria:

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Summary of the gram-negative bacteria:

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Summary of the atypical bacteria:

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*mycoplasma = wall-less
*obligate intracellular = chlamydia and rickettsia