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Bacteriology Basics

Main Bacterial Functions

  • Bacterial DNA is made via folic acid production with p-aminobenzoic acid (PABA) as an intermediate.

  • Bacterial genome is unwound by DNA gyrase and helicase allowing it to be read.

  • DNA is transcribed forming mRNA.

  • mRNA combines with the bacterial 70S ribosome (composed of 50S and 30S components) and translation occurs, forming proteins including cell wall components.

  • Penicillin-binding proteins (PBPs) catalyze construction of cell wall peptidoglycan including transpeptidase, which binds d-Ala-d-Ala chain of peptidoglycan for cross-linking.

  • Gram-negative (GN) bacteria have an outer cell wall, whereas gram positive (GP) do not have.

  • Porins (outer membrane proteins [omps] of different subtypes) allow entry of particular molecules of various sizes, charges, and shapes through the cell wall.

  • Efflux pumps (with different subtypes) are ATPase pumps that expel molecules from within the cell.

Main Bacterial Resistance Mechanisms

  1. Enzymatic inhibition—modification of the antibacterial material

    1. Beta-lactamase—hydrolysis of beta-lactam—affects penicillins and cephalosporins.

      1. Number one resistance pattern

      2. Many with particular beta-lactamase are coresistance to tetracyclines, sulfonamides, aminoglycosides, even fluoroquinolones (GN only)

    2. Aminoglycoside modifying enzymes reduce affinity for 30S ribosome.

  2. Penicillin-binding proteins (PBPs) modification—changing binding site of beta-lactam, decreasing its affinity for PBP

    1. Low affinity for binding beta-lactam antibiotic requiring clinically unattainable drug concentrations to overcome inhibition.

    2. PBP2a—confers penicillin and cephalosporin resistance via plasmid gene mecA in methicillin-resistant Staphylococcus aureus (MRSA).

    3. Use of d-Ala-d-Lactate instead of d-Ala-d-Ala confers resistance to cephalosporins and vancomycin.

  3. Porin (omp) modification

    1. Decreased porin production—Pseudomonas aeruginosa—decreased susceptibility to beta-lactams

    2. Loss of porin type—may confer particular or multidrug resistance

  4. Efflux pump in GN organisms

    1. Increased production—Escherichia coli—may produce major facilitator tetracycline transporter (tetB) for tetracycline only versus small multidrug resistance (SMR) emrE-gene transport protein which provides resistance against broad range of antibiotics.

  5. Modification of antibiotic target

    1. Ribosome—antibiotic binding site may be altered.

      1. Point gene mutation

      2. Methylation

    2. Cell wall—modification of cell phospholipid bilayer leads to repulsion of daptomycin-calcium molecule.

    3. Mutated DNA gyrase and topoisomerase IV genes confers resistance to fluoroquinolones.

    4. RNA polymerase—rifampicin cannot bind.

    5. Biofilm production—production of extracellular polymeric substance which may account for nine times greater mass than the bacteria itself. Reduced penetration of antibiotic.

      1. Bacteria enter slow growth state.

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DNA
Classification Name Spectrum Mechanism of Action Side Effects
Quinolones (bactericidal) Ciprofloxacin GN bacilli, Pseudomonas, atypicals, no anaerobic or GP coverage Inhibit DNA gyrase and bacterial topoisomerase IV that promotes DNA strand breakage  Stevens-Johnson syndrome, QTc prolongation, arthropathy, tendonitis, tendon rupture  
Levofloxacin MSSA, Streptococcus sp., GN bacilli, atypicals
Moxifloxacin MSSA, Streptococcus sp., anaerobes, atypicals    
Nitrofuran Nitrofurantoin (bacteriostatic, can be bactericidal at high concentrations) Broad spectrum—GP and GN: E. coli and S. saprophyticus Damages bacterial DNA Pulmonary fibrosis, hemolytic anemia, neuropathies
Nitroimidazole (bactericidal) Metronidazole Anaerobes, GN—Bacteroides fragilis, C. difficile, protozoa; penetrates blood-brain barrier Binds to cellular proteins and DNA Headache, nausea, vaginitis, metallic taste, disulfiram-like reaction with alcohol, peripheral ...

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