1-5 Beta-Lactam and Other Cell Wall- & Membrane-Active Antibiotics

Comprehensive flashcards covering bacterial cell wall synthesis inhibitors, including penicillins, cephalosporins, monobactams, carbapenems, glycopeptides, lipopeptides, and miscellaneous agents.

Who discovered penicillin in 1928, and which contaminated bacterial culture led to this observation?

Alexander Fleming in London; he observed lysis of Staphylococcus variant cultures caused by contaminating mold of the genus Penicillium.

Which researchers demonstrated the in vivo therapeutic efficacy of penicillin in 1940 at Oxford University?

Florey, Chain, and Abraham demonstrated therapeutic effects parenterally in mice with streptococcal infections.

In the chemical structure of penicillins, what enzymes act at cleavage sites 1 and 2, and what are their respective cleavage products?

Site 1 (the $\beta$-lactam ring) is cleaved by penicillinase (a $\beta$-lactamase) to form penicilloic acid; Site 2 is cleaved by amidase to form $6$-aminopenicillanic acid.

Based on the structural comparison of $\beta$-lactam antibiotics, which sulfur-containing rings are fused to the $\beta$-lactam ring in penicillins versus cephalosporins?

Penicillins have a $5$-membered thiazolidine ring fused to the $\beta$-lactam ring, whereas cephalosporins have a $6$-membered dihydrothiazine ring.

How does cell wall peptidoglycan thickness compare between Gram-positive and Gram-negative bacteria?

In Gram-positive bacteria, the cell wall is $50\text{--}100$ molecules thick, whereas in Gram-negative bacteria, it is only $1$ or $2$ molecules thick.

What is the primary biochemical mechanism of action of $\beta$-lactam antibiotics?

They bind to penicillin-binding proteins (PBPs) and irreversibly inhibit transpeptidase, blocking the cross-linking of peptidoglycan chains and halting bacterial cell wall synthesis.

What is the unit conversion standard for Penicillin G activity?

$1\,\text{unit} = 0.6\,\mu\text{g}$, and $1\,\text{million units} = 0.6\,\text{g}$.

Why is Penicillin V preferred over Penicillin G for oral administration?

Penicillin V (phenoxymethylpenicillin) is acid-resistant and stable in gastric acid, whereas Penicillin G is acid-labile.

What clinical regimen of Benzathine Penicillin G is used for rheumatic fever recurrence prevention?

$1.2\,\text{million units}$ given intramuscularly once every $3\text{--}4\,\text{weeks}$.

Which specific penicillins are classified as penicillinase-resistant (antistaphylococcal) penicillins?

Methicillin, nafcillin, and isoxazolyl penicillins (oxacillin, cloxacillin, dicloxacillin, and flucloxacillin).

Which organisms are typically covered by the extended spectrum of aminopenicillins (ampicillin and amoxicillin)?

Haemophilus influenzae, Escherichia coli, Salmonella, Shigella, Proteus mirabilis, and Helicobacter pylori.

How does oral bioavailability differ between ampicillin and amoxicillin in the presence of food?

Ampicillin has about $40\%$ absorption and must be taken $30\,\text{min}$ before or $2\,\text{h}$ after meals, whereas amoxicillin has about $90\%$ absorption and food does not interfere.

Which antipseudomonal penicillins belong to the carboxypenicillin and acylureidopenicillin classes?

Carboxypenicillins include carbenicillin and ticarcillin; Acylureidopenicillins include azlocillin, mezlocillin, and piperacillin.

What bacterial resistance mechanism is illustrated in this diagram across the cell envelope of a Gram-negative bacterium?

An active antibiotic efflux pump consisting of an efflux transporter, accessory protein, and outer membrane channel.

Which penicillins are eliminated predominantly by hepatic metabolism rather than renal excretion?

Nafcillin, oxacillin, and Penicillin V.

How does probenecid affect the pharmacokinetic elimination of penicillins?

Probenecid blocks renal tubular secretion of penicillins, thereby increasing their serum concentrations and prolonging their half-life.

What is the major antigenic determinant responsible for penicillin hypersensitivity reactions?

Penicilloic acid (formed by the opening of the $\beta$-lactam ring), which covalently binds to host tissue proteins.

What are the characteristics and incidence of Type I allergic reactions to penicillin?

They are IgE-mediated immediate anaphylactic reactions occurring in $0.015\text{--}0.05\%$ of patients, with a fatality rate of approximately $10\%$.

Which adverse effects are specifically associated with methicillin, oxacillin, and ampicillin?

Methicillin causes interstitial nephritis; oxacillin causes hepatitis; ampicillin causes pseudomembranous colitis and diarrhea.

What specific reagent is used in skin scratch and intradermal testing for penicillin allergy?

Benzylpenicilloyl-polylysine (PPL) or Penicillin G ($10\,000\,\text{IU/mL}$).

What is the basic nucleus of all cephalosporins, and what determines their antibacterial activity versus pharmacokinetic properties?

The nucleus is $7$-aminocephalosporanic acid (cephem nucleus); the C7 radical governs antibacterial activity, while the C3 radical governs metabolism and pharmacokinetics.

Which first-generation cephalosporin is administered parenterally for surgical prophylaxis against Gram-positive cocci?

Cefazolin (IV).

Which second-generation cephamycins have unique activity against anaerobic bacteria such as Bacteroides fragilis?

Cefoxitin and cefotetan.

Which third-generation cephalosporins have marked antimicrobial activity against Pseudomonas aeruginosa?

Ceftazidime and cefoperazone.

What is the clinical role and antibacterial spectrum of fifth-generation cephalosporins like ceftaroline fosamil?

They are active against methicillin-resistant Staphylococcus aureus (MRSA) and enterococci in complicated skin, soft tissue, and pulmonary infections.

What are the common adverse effects associated with cephalosporins?

Allergic reactions ($5\%$ overall; $5\text{--}10\%$ cross-reactivity with penicillin), local irritation, nephrotoxicity, hypoprothrombinemia (vitamin K deficiency), and disulfiram-like reactions.

What is the antibacterial spectrum and clinical utility of the monobactam aztreonam?

It is active exclusively against aerobic Gram-negative bacilli (including P. aeruginosa), lacks activity against Gram-positive cocci and anaerobes, and is safe in penicillin-allergic patients.

Why is imipenem co-formulated with cilastatin?

Cilastatin inhibits renal dehydropeptidase-I, preventing the enzymatic degradation of imipenem into nephrotoxic metabolites and maintaining active drug levels.

What is the mechanism of action of vancomycin?

It binds directly to the D-Ala-D-Ala terminus of peptidoglycan pentapeptides, inhibiting transglycosylase and blocking peptidoglycan elongation.

What causes 'red man syndrome' during vancomycin therapy, and how can it be prevented?

Infusion-related histamine release caused by rapid intravenous administration; it is prevented by slowing the infusion rate and pretreating with an antihistamine such as diphenhydramine.

What is the mechanism of action and primary toxicity of daptomycin?

It binds to the bacterial cell membrane and causes rapid depolarization with potassium ($K^+$) efflux, leading to cell death; its primary adverse effect is myopathy.

What is the indication, mechanism, and recommended dosage of fosfomycin?

It inhibits enolpyruvate transferase (MurA) in early bacterial cell wall synthesis; it is given as a single $3\,\text{g}$ oral dose for uncomplicated lower urinary tract infections in women and is safe in pregnancy.

Why is bacitracin restricted to topical administration?

Bacitracin causes severe nephrotoxicity when given systemically.

What are the clinical use and adverse effects of cycloserine?

It is a second-line agent for drug-resistant Mycobacterium tuberculosis; its use is limited by severe central nervous system toxicity, including headaches, tremors, acute psychosis, and convulsions.