Bacterial, Structure, Function, and Growth Flashcards
Describe the major structural features of bacteria and explain the principal function(s) of each feature.
Cell wall: Present in most bacteria, external to cytoplasmic membrane, contains peptidoglycan. Essential for resisting osmotic lysis and maintaining cell shape.
Cytoskeletal elements: FtsZ (resembles tubulin in eukaryotes), MreB and ParM (resemble actin) and CreS (resembles intermediate filaments). Play essential roles in establishing cell shape.
Capsules: Loose, gelatinous outer surface layers, usually consist of complex polysaccharides. Enhance virulence be enabling bacteria to resist phagocytosis.
Flagella: Appendages originating in the cytoplasmic membrane, function as organs of motility. Can be distributed all over their surface (preitrichous flagella), or polarized to the end of the cell. Chemotaxis involves the control of flagellar rotation.
Pili (aka fimbirae): Long, slender, proteinaceous, antigenic, hair-like structures. Play a role in adherance to surfaces and tissue. Sex pili play a role in bacterial conjugation.
Cytoplasmic membrane (inner membrane in gram-negative bacteria): anatomical and physiological barrier between the inside and outside of the bacterial cell. Selectively permeable (only hydrophic molecules larger than glycerol can pass), prevents metabolites from being lost in the cytoplasm. Also houses the machinery of the electron transport system.
Ribosomes: 70S ribosomes, protein synthesis occurs on the ribosomes. Polyribosomes are formed by the interaction of several ribosomes with a single mRNA. Bacterial mRNAs may be polycistronic.
Nucleoid: Distinct region of bacteria that contains the DNA. DNA is tightly packed and supercoiled. No nuclear membrane is present.
Chromosome: single, double-stranded, circular DNA molecule. (Some bacteria have linear chromosomes, some have more than one). Cytoskeletal components function as a primitive mitotic apparatus during bacterial cell division.
Plasmid: extra-chromosomal, self-replicating DNA molecules, much smaller than bacterial chromosomes. Not essential for viability.
Explain the importance of differences in cell wall structure among bacteria.
Gram-positive and gram-negative bacteria have different cell wall structures and react differently to gram staining procedures.
Peptidoglycan layer is a rigid mesh that surrounds cytoplasmic membrane. Peptidoglycan consists of a polymer with repeating units of 2 hexose sugars: (N-acteylglucosamine and N-acteulmuramic acid). The MurNAc residues are linked to tetrapeptide chains. The tetrapeptides are cross-liked from one chain to amino acids on another chain. The extend of cross-linking of peptidoglycan chains is typically much greater in gram-positive bacteria than in gram-negative bacteria.
Gram-negative bacteria have a thin, sparsely cross-linked peptidoglycan layer, and other major components are located exterior to the peptidoglycan. Outer membrane of gram-neg. bacterica is a lipid bilayer that contains lipopolysaccharide, lipoproteins, and porins. Functions as a barrier to entry of some antibotics and protects the cell against the action of detergents and other toxins.
Gram-positive bacteria have a thick extensively cross-linked peptidoglycan layer that also contains teichoic acids, which are covalently attached to the peptidoglycan layer. Lipoteichoic acids are attached to the underlying cytoplasmic membrane and help to anchor the cell wall to the membrane.
Draw a typical bacterial growth curve and explain the characteristics of each growth phase.
Lag phase: physiologic adjustment for the starting cells (inoculum). Involves induction of new enzymes and the establishment of a proper intracellular environment for optimal growth.
Exponential (logarithmic) phase: rate of increase in cell number/cell mass is proportional to the cell number/mass already present. Rate of cell division is maximal for the available nutritional conditions. (generation time= constant interval of time req’d for doubling cell number/mass).
Stationary phase: Occurs as essential nutrients are consumer and toxic metabolic products accumulate. Cell growth may slow dramatically or cease. Growth that occurs is balanced by cell death. Non-growing/slow growing cells may exhibit resistance to antibiotics.
Death phase: Doesn’t always occur, if so, number of viable bacteria decrease over time. If spontaneous cell lysis (autolysis) occurs, mass of intact bacteria in culture will also decrease.
Describe how bacteria are classified according to their nutritional requirements
Require organic carbon source: heterotrophic
Obtain carbon exlcusively from CO2: autotrophic
Requires oxygen; cannot ferment: Aerobe (strict aerobe)
Killed by oxygen; fementive metabolism: Anaerobe (strict anaerobe)
Ferments in presence or absence of O2: Indifferent (aerotolerant anaerobe)
Respires with O2; ferments in absence of O2: Facultative (Facultative anaerobe)
Grows best at low [O2]; can grow without O2: Microaerophilic
Most bacteria require a nutrient medium that contains several inorganic ions.
Define respiration and fermentation and explain how metabolic “energy currency” is generated.
Respiration: ATP is generated through the electron transport chain and molecular oxygen is used as the final electron acceptor.
Anaerobic respiration: Inorganic substances (nitrate or nitrate, e.g.) are used as the final electron acceptor, instead of O2.
Fermentation: Organic compounds serve as both electron donors and electron acceptors, and no net oxidation of substrates occurs.
There are two forms of energy currency: ATP and electrochemical gradients (the proton motive force). Reducing power via NADH and NADPH is required to interconvert these two forms of energy, as is ATPase.
Explain why unique bacterial components are important as potential targets for antimicrobial therapy.
Antimicrobials must target unique bacterial componsents, as to not inadvertently damage host eukaryotic components during treatment.
Identify the principal targets for the major groups of antibiotics used in human medicine.
Cell wall-active antimicrobials:
- Beta-lactams inhibit the final transpeptidation rxn in the cross-linking of peptidoglycan.
- Vanomycin inhibits utilization of lipid-linked intermediate step in peptidoglycan synthesis.
- Cycoserine inhibits alanine racemase, prevents formation of an early intermediate in peptidoglycan synthesis.
Outer and cytoplasmic membrane antimicrobials: Polymixins disrupter outer and cytoplasmic membranes, less active on mammalian cell membranes
Inhibited of protein synthesis at ribosomal level:
Aminoglycosides: bind target proteins in 30S ribosomal subunit.
Tetracylines: Reversibly bind 30S ribosomal subinit, inhibit binding of aminoacyl tRNA
Chloramphenicol: 50s ribosomal subunit
Macrolide and lincomycins: 23S and 50 S ribosomal subunits
Inhibitors of nucleic acid synthesis:
Quinolones: inhibit DNA gyrase and topoisomerase
Rifampicin: inhibits RNA polymerase
Metabolic inhibitory antimicrobials:
Sulfonamides: inhibit formation of folic acid, preventing nucleic acid synthesis.
Trimethoprim: interferse with folate metabolism by inhibiting dihydrofolate reductase.
Isoniazid: inhibits lipid synthesis
Metronidazole: interfers with anaerobic metabolism, specifically.
