Buy Bacterial Protein Synthesis Inhibitors
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Learning objective: Explain the mechanism of action of gentamicin.
6. E Gentam icin is an am inoglycoside antibiotic. These drugs
bind irreversibly to speci c 30S ribosom al subunits and inhibit bacterial protein synthesis in at least three ways:
• Blockade of the “initiation com plex,” the com plex form ed
for initiation of translation that consists of the 30S m icrosom al subunit, m essenger RNA (m RNA), transfer RNA
(tRNA), and som e initiation factors. This blockade leads to
an m RNA chain w ith only a single ribosom e on it, the socalled m onosom e.
• Misreading of m RNA tem plates, w hich leads to the production of aberrant proteins. These proteins m ay be inserted
into cell m em branes, altering perm eability and further
stim ulating am inoglycoside transport (energy-dependent
phase II transport).
• Blockade of translocation (i.e., the ribosom e advancem ent
of three nucleotides is blocked)
For external otitis, the drug would be given topically as ear
drops. Gentam icin can be ototoxic, especially if given system ically.
A−D All of these antibiotics are active against Pseudom onas
aeruginosa, but they do not cause m isreading of the bacterial
m RNA tem plate.
Learning objective: Explain concentration-dependent killing and
the postantibiotic e ect.
7. D The am inoglycoside bactericidal e ect is concentrationdependent; that is, increasing concentrations kill an increasing proportion of bacteria and at a m ore rapid rate. They also
have a long postantibiotic e ect, that is, persistent suppression of bacterial grow th for several hours beyond the tim e
that m easurable drug is present. Most antibiotics exhibit this
e ect against gram -positive cocci. Antibiotics that possess
this e ect against gram -negative bacilli are am inoglycosides,
uoroquinolones, and tetracyclines.
Because of these two properties, a given total am ount of
am inoglycoside m ay have better e cacy and lower toxicity
w hen adm inistered as a higher single daily dose than w hen
given as a lower dose two or three tim es daily.
A, E The bactericidal e ect of am inoglycosides is not tim edependent.
B Protein binding is totally unrelated to the duration of the
antibiotic e ect.
C Am inoglycosides are bactericidal, not bacteriostatic.
Learning objective: Identify the drug used for prophylaxis of
8. C Erythromycin and azithromycin are agents of choice for
w hooping cough. If they are given during the catarrhal stage,
they can am eliorate the disease. After paroxysm s are established, these drugs have no clinical e ect, but they decrease
the duration of infectiousness, thus preventing spread. Contacts of all ages, w hether vaccinated or not, should receive a
10-day course of erythromycin.
A, B, D These drug are active against Bordetella pertussis
but are contraindicated in children.
E, F These drugs are not active against B. pertussis.
Learning objective: Identify the antibiotic used for antimicrobial
prophylaxis in colorectal surgery.
9. D Antibiotic prophylaxis is w idely used for surgical procedures to decrease the degree of bacterial contam ination
during surgery. Prophylaxis is especially advisable for colorectal surgery, ow ing to the large num ber of bacteria com –
prising the intestinal ora. Oral nonadsorbable antibiotics
function e ectively as intestinal decontam inants because
high intralum inal drug concentrations can be easily achieved.
An oral antim icrobial regim en used for colorectal surgical
prophylaxis is neomycin (for aerobic enterobacteriaceae) plus
erythromycin (for anaerobes). Neomycin is preferred over
other am inoglycosides because of its w ider spectrum of antibacterial activity.
A See correct answer explanation.
B Penicillin G given orally is rapidly destroyed by gastric
C, E, F These antibiotics are not active against enterobacteriaceae or anaerobes.
Learning objective: Identify the antibiotic used to treat psittacosis.
10. E The patient’s history suggests that he was a ected by
psittacosis, a pneum onia caused by Chlam ydia psittaci. The
organism is found m ainly in psittacine birds, but other birds
can also be a ected. Hum an infection usually occurs by inhalation of dust from feathers of infected birds. Chlamydiae
are sensitive to tetracyclines, m acrolides, and quinolones.
Doxycycline is often the preferred drug.
A−D All of the listed bacteria can cause pneum onia, but
none of them are sensitive to the paired drug.
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