Penicillins MCQs

Diphtheroid-like Gram-positive rods in the cerebrospinal fluid smear of an elderly patient are indicative of L-monocytogenes. Listeria infections are more common in neonates, elderly patients, and those who have been treated with immunosuppressive agents. Cephalosporins do not cover listeria. ( Mnemonic : “LAME”: listeria, atypicals, MRSA, and enterococcus.) Treatment consists of ampicillin with or without an amino glycoside such as gentamicin. Trimethoprim-sulfamethoxazole can also be used. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 802

Penicillins (and cephalosporins) bind to PBPs acting at the transpeptidation stage of cell wall synthesis (the final step) to inhibit peptidoglycan cross-linking. Synthesis of N-acetylmuramic acid is inhibited by fosfomycin. Vancomycin inhibits transglycosylase, preventing elongation of peptidoglycan chains. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 800

Pneumococcal resistance to penicillins is due to changes in the chemical structures of the target penicillin-binding proteins located in the bacterial cytoplasmic membrane. A similar mechanism underlies the resistance of staphylococci to methicillin (MRSA strains). A structural alteration in the d-Ala-d-Ala component of the pentapeptide side chains of peptidoglycans is the basis for a mechanism of resistance to vancomycin. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 801

Each of the drugs listed has activity against some Gram- negative bacilli. All penicillins should be avoided in patients with a history of allergic reactions to any individual penicillin drug. Cephalosporins should also be avoided in patients who have had anaphylaxis or other severe hypersensitivity reactions after use of a penicillin. There is partial cross reactivity between penicillins and the carbapenems such as imipenem and meropenem, but no cross-reactivity between the penicillins and aztreonam. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 799

First- and second-generation cephalosporins are not effective in meningitis because they do not readily enter the cerebrospinal fluid. Instability of penicillins in gastric acid does limit their oral absorption, renal elimination of amoxicillin is inhibited by probenecid, and ticarcillin has activity against several Gram-negative rods. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 802

In patients who have had a severe reaction to a penicillin, it is inadvisable to administer a cephalosporin or a carbapenem such as meropenem. Aztreonam has no significant activity against Gram-positive cocci, so the logical treatment in this case is vancomycin, often with an aminoglycoside (eg, gentamicin) for synergistic activity against enterococci. Reference: Basic & Clinical Pharmacology, Fourteenth Edition By Bertram G. Katzung Pg no. 808