Know the terminology describing the general
characteristics of antimicrobial agents and drugs
- Broad spectrum- Antibiotic inhibits or kills lots of things (e.g. tetracycline inhibits Gram positives and negatives, as well as Chlamydia and Rickettsia)
- Narrow spectrum- Inhibits or kills only a few things (e.g. pencillin G only kills Gram positives)
- Bacteriostatic- Inhibits growth, but doesn't kill (e.g. chloramphenicol)
- Bactericidal- Kills microbes (e.g. penicillins)
- Toxic dose- Dose at which the drug becomes too toxic for the host
- Therapeutic dose- Dose needed to treat the infection
- Therapeutic index- Toxic dose divided by therapeutic dose. The larger the therapeutic index, the better
Know the five main mechanisms of antibacterial action
The five main mechanisms are as follows:
- Inhibiting protein synthesis
- Inhibiting cell wall synthesis
- Metabolic antagonists/antimetabolites (i.e. blockers of enzymatic activity etc.)
- Inhibition of nucleic acid synthesis
- Cell membrane disruption
I'll expand on these in the next section...
Be able to describe the effect, mechanism of action,
group members and spectrum of activity for the
antibiotics given as examples in each case
A lot of the drugs that I'm about to mention have already been mentioned here, but time to go into more detail! Yay!
Protein synthesis inhibitors
The main classes of drugs here are aminoglycosides, tetracyclines, macrolides, and chloramphenicol. With the exception of aminoglycosides, which are bactericidal, most protein synthesis inhibitors are bacteriostatic.
Aminoglycosides, such as streptomycin, gentamicin and kanamycin, bind to the 16S rRNA of the 30S ribosomal subunit at the A site. They inhibit translation elongation and make ribosomes error-prone. Aminoglycosides are effective against Gram-negative bacteria, particularly enteric bacteria and Pseudomonas aeruginosa. However, they are quite toxic.
Tetracyclines, such as tetracycline, chlortetracycline (tetracycline with an extra -Cl), doxycycline (extra -OH) and minocycline (extra N(CH3)2) also bind to the 16S rRNA in the 30S subunit. They block the binding of incoming aminoacyl-tRNAs to the A site. Tetracyclines are broad-spectrum drugs that work against Gram-positives and Gram-negatives, as well as Chlamydia, Mycoplasma, and Rickettsia.
Macrolides, such as erythromycin and clindamycin, bind to the 23S rRNA in the 50S ribosomal subunit. They are quite bulky drugs that "plug" the ribosomal tunnel. They are also quite broad-spectrum and are able to act against Gram-positives, mycoplasmas, and some Gram-negatives. A related drug called clindamycin inhibits peptidyl transferase and is good against anaerobes.
Chloramphenicol, like macrolides, bind to the 23S rRNA in the 50S subunit. They affect the binding of aminoacyl-tRNA to the A-site. Chloramphnicol is broad-spectrum but is very toxic, so it is only used in life-threatening situations or in topical treatment of conjunctivitis.
Cell wall synthesis inhibitors
Cell wall synthesis inhibitors have very good selective toxicity as they only target components of cell walls, which humans don't have. Many cell wall inhibitors block transpeptidation, which is the last step in bacterial cell wall synthesis, and are usually bactericidal. During this step, an amino group in one chain attacks the second last D-alanine of the other chain, forming a peptide link. This may form a direct crossbridge (as in E. coli) or a different kind of interbridge (e.g. S. aureus has a pentaglycine crossbridge). Either way, transpeptidation is mediated by transpeptidases, which are also known as penicillin-binding proteins (PBPs).
The most well-known cell wall synthesis inhibitor is probably penicillin. Penicillins contain a beta-lactam ring, which resembles the terminal D-alanyl-D-alanine in peptidoglycans, thus blocking cell wall formation. Cell walls are actually kind of important for bacteria- without it, they can't resist osmotic pressure, so they are easily lysed. Cephalosporins (e.g. cefoxitin) also function in the same way- they are somewhat structurally different to penicillin, but they still have the beta-lactam ring.
Another class of cell wall synthesis inhibitor is the glycopeptides. Glycopeptides bind to the D-alanyl-D-alanine in peptidoglycans. They are relatively narrow-spectrum, limited to Gram-positives, but they can be useful as last-resort drugs in some cases (e.g. MRSA). An example of a glycopeptide is vancomycin (which also happens to be my favourite "nuke" in Microbe Invader).
Metabolic antagonists/antimetabolites
Sulfonamides and trimethoprim are both metabolic antagonists, and are often combined into one drug (trimethoprim-sulfamethoxazole). Sulfonamides block the first step in the folic acid pathway by competing with PABA. Trimethoprim blocks a later step by inhibiting the dihydrofolate reductase enzyme. The end result is that folic acid is not produced, and since bacteria require folic acid to form DNA bases, they're kind of screwed when this pathway is blocked. Both sulfonamides and trimethoprim are bacteriostatic.
Inhibition of nucleic acid synthesis
Nucleic acid synthesis inhibitors, which are mostly bactericidal, have poor selective toxicity because synthesis pathways are pretty similar between eukaryotes and prokaryotes. Quinolones and fluoroquinolones inhibit DNA gyrase (the enzyme that uncoils parent DNA), while rifampin inhibits RNA polymerase. Quinolones and fluoroquinolones vary in specificity, whereas rifampin is a narrow spectrum drug used for tuberculosis and some Gram-negatives.
Cell membrane disruption
Cell membrane disruptors are also bactericidal and have poor selective toxicity. The main class here are polymixins, such as Polymixin B and colistin (a.k.a. Polymixin E). They are narrow-spectrum and are mainly used topically for Gram-negative infections.
Macrolides, such as erythromycin and clindamycin, bind to the 23S rRNA in the 50S ribosomal subunit. They are quite bulky drugs that "plug" the ribosomal tunnel. They are also quite broad-spectrum and are able to act against Gram-positives, mycoplasmas, and some Gram-negatives. A related drug called clindamycin inhibits peptidyl transferase and is good against anaerobes.
Chloramphenicol, like macrolides, bind to the 23S rRNA in the 50S subunit. They affect the binding of aminoacyl-tRNA to the A-site. Chloramphnicol is broad-spectrum but is very toxic, so it is only used in life-threatening situations or in topical treatment of conjunctivitis.
Cell wall synthesis inhibitors
Cell wall synthesis inhibitors have very good selective toxicity as they only target components of cell walls, which humans don't have. Many cell wall inhibitors block transpeptidation, which is the last step in bacterial cell wall synthesis, and are usually bactericidal. During this step, an amino group in one chain attacks the second last D-alanine of the other chain, forming a peptide link. This may form a direct crossbridge (as in E. coli) or a different kind of interbridge (e.g. S. aureus has a pentaglycine crossbridge). Either way, transpeptidation is mediated by transpeptidases, which are also known as penicillin-binding proteins (PBPs).
The most well-known cell wall synthesis inhibitor is probably penicillin. Penicillins contain a beta-lactam ring, which resembles the terminal D-alanyl-D-alanine in peptidoglycans, thus blocking cell wall formation. Cell walls are actually kind of important for bacteria- without it, they can't resist osmotic pressure, so they are easily lysed. Cephalosporins (e.g. cefoxitin) also function in the same way- they are somewhat structurally different to penicillin, but they still have the beta-lactam ring.
Another class of cell wall synthesis inhibitor is the glycopeptides. Glycopeptides bind to the D-alanyl-D-alanine in peptidoglycans. They are relatively narrow-spectrum, limited to Gram-positives, but they can be useful as last-resort drugs in some cases (e.g. MRSA). An example of a glycopeptide is vancomycin (which also happens to be my favourite "nuke" in Microbe Invader).
Metabolic antagonists/antimetabolites
Sulfonamides and trimethoprim are both metabolic antagonists, and are often combined into one drug (trimethoprim-sulfamethoxazole). Sulfonamides block the first step in the folic acid pathway by competing with PABA. Trimethoprim blocks a later step by inhibiting the dihydrofolate reductase enzyme. The end result is that folic acid is not produced, and since bacteria require folic acid to form DNA bases, they're kind of screwed when this pathway is blocked. Both sulfonamides and trimethoprim are bacteriostatic.
Inhibition of nucleic acid synthesis
Nucleic acid synthesis inhibitors, which are mostly bactericidal, have poor selective toxicity because synthesis pathways are pretty similar between eukaryotes and prokaryotes. Quinolones and fluoroquinolones inhibit DNA gyrase (the enzyme that uncoils parent DNA), while rifampin inhibits RNA polymerase. Quinolones and fluoroquinolones vary in specificity, whereas rifampin is a narrow spectrum drug used for tuberculosis and some Gram-negatives.
Cell membrane disruption
Cell membrane disruptors are also bactericidal and have poor selective toxicity. The main class here are polymixins, such as Polymixin B and colistin (a.k.a. Polymixin E). They are narrow-spectrum and are mainly used topically for Gram-negative infections.
Understand the three methods of antibacterial
susceptibility testing described
Antibacterial susceptibility tests are often used to determine MIC (minimal inhibitory concentration), which is the lowest concentration of a drug required to prevent bacterial growth. There are three main methods used: disk diffusion tests, Etests, and broth and agar dilution tests.
Disk diffusion tests (a.k.a. Kirby-Bauer tests)
In disk diffusion tests, the microbe is spread onto an agar plate. Sterile paper disks impregnated with an antibiotic are placed onto the surface of the plate. If the antibiotic kills off the microbe, there will be a clear zone around that disk, also known as an inhibition zone. The size of the inhibition zone can be used to determine MIC.
Etests
Etests are kind of like disk diffusion tests in that the microbe is spread onto an agar plate. Instead of disks, Etests use plastic strips that have a concentration gradient of an antibiotic, which is labelled with a scale. The strips are placed on the agar plate so that the lowest concentration of antibiotic is at the centre of the disk. The MIC can be determined by finding the place where the inhibition zone intersects with the strip.
Broth and agar dilution tests
In a broth dilution test, the microbe is added to a bunch of different broths, each containing a different concentration of the antibiotic. The MIC is the tube with the lowest concentration of antibiotic without any bacterial growth. Agar dilution tests are similar, but they use a concentration gradient of antibiotic across the agar (I *think*).
No comments:
Post a Comment