Pharm Antibiotics Moa W Exp

Learning objective: Explain the mechanism of action common to ampicillin, ceftriaxone and
vancomycin.
Answer: E
The signs and symptoms of the patient strongly suggest the diagnosis of meningitis. When the
patient is very ill and the offending organism is unknown an empiric triple antibiotic therapy is
usually given, pending CSF test results. Third generation cephalosporins like ceftriaxone are active
against most gram positive and gram negative bacteria. Since many MR resistant cocci are
increasingly prevalent, vancomycin is usually added. Ampicillin is added to cover Listeria species.
All these antibiotic act by inhibiting the synthesis of peptidoglycan. Ampicillin and ceftriaxone inhibit
the transpeptidation reaction so blocking the cross link formation. Vancomycin inhibits
transglycosylase, so blocking further elongation of peptidoglycan chain.

Learning objective: explain the mechanism of action of metronidazole.
Answer: D
Metronidazole is an antibiotic active against most microaerophile and anaerobic bacteria and
anaerobic protozoa. These microorganisms , unlike their aerobic counterparts, contains electron
transport components, called ferredoxins, that can donate electrons to metronidazole. This donation
forms a highly reactive nitro radical anion that damages bacterial DNA. and. The ultimate effect is
bactericidal.

Learning objective: identify the site of action of clarithromycin.
Answer: G
Macrolide antibiotics act by binding reversibly to the 50S subunit of the bacterial ribosome which is
therefore their primary cellular site of action.
A, B; C; D) Since macrolides act inside the bacterial cell, they must cross all the above mentioned
structures. These site are important for the pharmacokinetic of these drugs but they are not the site
of action of macrolides.
E, F) Macrolides do not affect mitochondria or ribosomes.

Learning objective: identify the site of action of tetracyclines.
Answer: C
Tetracyclines bind reversibly to the 30S subunit of the bacterial ribosome and block the access of
aminoacyl-tRNA to the acceptor side of the mRNA ribosome complex. This prevent the addition of
amino acids to the growing peptide chain and therefore inhibits bacterial protein synthesis

Learning objective: identify the specific step of the synthesis and breakdown of bacterial cell
wall which is inhibited by beta-lactam antibiotics.
Answer: C
Beta-lactam antibiotics inhibit cell wall synthesis by preventing cross-linking between the peptide
chains of the acetylmuramic acid. This linking is catalyzed by transpeptidase enzymes and beta
lactam antibiotics specifically inhibit these enzymes.
A) This step is inhibited by fosfomycin
B) This step is inhibited by vancomycin
D) This step is enhanced (not inhibited) by beta-lactam antibiotics and this action, together with the
inhibition of transpeptidase, contributes to the final bactericidal effect of these drugs.

Learning objective: identify the site of action of linezolid.
Answer: D
Linezolid binds to 50S ribosomal subunit at a site near to the 30 S ribosomal subunit so preventing
the formation of the initiation complex

Learning objective: explain the mechanism of action of fluoroquinolones
Answer: B
Fluoroquinolones Inhibit bacterial DNA synthesis by blocking the following enzymes:
1) Topoisomerase II (also called DNA gyrase). The blockade prevent the relaxation of supercoiled
DNA which is required for normal transcription (prevalent mechanism in gram-negative bacteria)
2) Topoisomerase IV. The blockade interferes with separation of replicated chromosomal DNA
during cell division (prevalent mechanism in gram-positive bacteria).

Learning objective: explain the mechanism of action of sulfonamides
Answer: C
Sulfonamides inhibit folic acid synthesis which in turn inhibits the synthesis of tetrahydrofolate.
Since tetrahydrofolate is needed for the synthesis of purines and pyrimidines, the synthesis of these
molecules is blocked.
A, B, D, E) Sulfonamides have no effect on the function of already formed DNA or NRA

Learning objective: explain the mechanism of action of tetracyclines.
Answer: D
Mediterranean spotted fever (also called fiebre boutonneuse) is a disease caused by Rickettsia
conorii. Doxycycline is a tetracycline antibiotic that has became the drug of choice for most
rickettsial diseases, because of its efficacy, long half-life and very good oral bioavailability.
Tetracyclines bind reversibly to the 30s ribosomal subunit and block the access of aminoacyl-tRNA
to the acceptor side of the mRNA ribosome complex. This prevent the addition of amino acids to the
growing peptide chain and therefore inhibits bacterial protein synthesis

Learning objective: explain the mechanism of action of trimethoprim-sulfamethoxazole
combination.
Answer: A
By inhibiting dihydropteroate synthase, sulfonamides inhibit the formation of folate. By inhibiting
dihydrofolate reductase, trimethoprim inhibits the formation of dihydrofolate and tetrahydrofolate.
Inhibition of these consecutive steps in the formation of tetrahydrofolate constitute a sequential
blockade and results in antibacterial synergy.
B, C, D, E) The synthesis of DNA and RNA from already formed tetrahydrofolate are not inhibited by
trimethoprim sulfamethoxazole.

Learning objective: explain the mechanism of action of chloramphenicol
Answer: B
Chloramphenicol enters the cell by facilitated diffusion and binds reversibly with the 50s ribosomal
subunit. In this site it prevents binding of the amino acid containing end of the aminoacyl tRNA to
the acceptor site of the 50S ribosomal subunit. Thus the peptide at the donor site cannot be
transferred to its amino acid acceptor, that is the transpeptidation reaction cannot occur. This
reaction is catalyzed by peptidyl transferase that is therefore indirectly inhibited by chloramphenicol.

Learning objective: list the main classes of bacteriostatic antibiotics.
Answer: C
Among the listed classes only macrolides are mainly bacteriostatic antibiotics. All beta-lactam drugs
(including cephalosporin and carbapenems) as well as aminoglycosides and fluoroquinolones are
bactericidal.

Learning objective: identify the site of action of quinupristin and dalfopristin.
Answer: A
Quinupristin and dalfopristin are antibiotics of the streptogramin group. They bind to tow separate
sites on the 50S bacterial ribosome, forming a stable ternary complex. Thus they synergistically
interrupt protein synthesis.

Learning objective: describe the antibacterial action of beta-lactam antibiotics.
Answer: D
Beta-lactam antibiotic exert aa bactericidal effect which is time dependent, that is the killing
continues as long as the serum concentration of the drug is grater than the minimum bactericidal
concentration ()MBC).
A) All beta-lactam antibiotics are mainly bactericidal.
B) The antibacterial action of beta-lactam antibiotics is not concentration dependent.
C) Among beta-lactam antibiotics only carbapenems haves a postantibiotic effect.
E) The antibacterial action involves an activation, not an inhibition, of bacterial autolysins which are
able to destroy the bacterial cell wall.

Learning objective: explain the mechanism of action of macrolides
Answer: C.
Macrolide and lincosamides antibiotics block the translocation reaction, i.e. the transfer of peptidyl
tRNA from the acceptor site to the peptidyl site of the bacterial ribosome. The ultimate effect is
mainly bacteriostatic.