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Answer is (e)- Obviously the fact that you will remember about tetracylines is that they can discolor teeth in the fetus when taken by the mother during pregnancy. But don't circle that answer because (a) is also characteristic of tetracyclines (they are the most likely of all the antibiotics to cause superinfection), and is an annoying side effect in adults resulting from alteration of the oral, gastric and intestinal flora. The real answer is (e). Tetracyclines are not the drug of choice for prophylaxis against infective endocarditis. This is due to
streptococcal infection. 15-20% of group A streptococci are resistant to tetracyclines, but none are resistant to penicillin or
erythromycin. Recently a non-streptococcal induced subacute bacterial endocarditis has been identified, especially in juvenile
periodontitis patients. The causative bacterium is not susceptible to penicillin or erythromycin. It may be necessary to treat predisposed patients with tetracycline for a few weeks, and then follow this with a course of penicillin or erythromycin.
Remember that these drugs are antagonistic to each other and thus can't be used concurrently. Penicillin is a bactericidal drug which kills or destroys microorganisms by interfering with the
synthesis or function of the cell wall, cell membrane or both. Thus it is most effective against bacteria that are multiplying. Tetracycline is a bacteriostatic antibiotic that acts by inhibiting the growth and multiplication of organisms by inhibiting protein synthesis by binding reversibly to the 30 S subunit of the bacterial ribosome. When the two types are given together, their
effectiveness is negated or reduced.