11.1 Antibacterial Classes, Mechanisms & Resistances

11.1 Antibacterial Classes, Mechanisms & Resistances

Independent Study Guide Notice: Independent study guide by OpenExamPrep. This educational resource is developed independently by OpenExamPrep and is not sponsored, endorsed, or affiliated with the National Board of Podiatric Medical Examiners (NBPME) or Meazure Learning.

---

Introduction to Antibacterial Pharmacotherapy in Podiatry

Antimicrobial selection in podiatric practice demands an exacting comprehension of bacterial cell biology, pharmacokinetic distribution into soft tissue and cortical bone, and specific mechanisms of acquired resistance. Lower extremity pathology frequently manifests as polymicrobial diabetic foot infections (DFIs), acute paronychia, deep fascial space phlegmon, necrotizing fasciitis, and chronic osteomyelitis. Successful clinical eradication requires matching the pathogen's susceptibility profile with the appropriate bactericidal or bacteriostatic agent while mitigating severe organ toxicities.

$\text{Antimicrobial Efficacy} = f\left(\text{Target Tissue Penetration}, \frac{C_{\max}}{\text{MIC}}, \frac{\text{AUC}}{\text{MIC}}, \text{Time} > \text{MIC}\right)$

---

Cell Wall Synthesis Inhibitors: Beta-Lactams

Bacterial cell walls consist of a rigid peptidoglycan meshwork composed of alternating glycan chains—N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)—cross-linked by peptide side chains. Peptidoglycan cross-linking is catalyzed by transpeptidase enzymes, collectively designated Penicillin-Binding Proteins (PBPs).

Mechanism of Action & Bactericidal Cascade

All beta-lactam antibiotics contain a four-membered beta-lactam ring that structurally mimics the D-alanyl-D-alanine (D-Ala-D-Ala) terminus of normal peptidoglycan precursor pentapeptides. Beta-lactams covalently acylate the active-site serine residue of PBPs, irreversibly inhibiting their transpeptidase activity. Deprived of structural cross-linking, the cell wall weakens, while bacterial autolysins (murein hydrolases) continue uninhibited, precipitating cell wall rupture, osmotic lysis, and rapid bactericidal death.

1. Penicillins

2. Cephalosporins

Cephalosporins are beta-lactams containing a 6-membered dihydrothiazine ring instead of the 5-membered thiazolidine ring of penicillins, rendering them inherently more resistant to standard staphylococcal penicillinases. As cephalosporin generations advance from 1st to 4th, Gram-negative coverage and central nervous system (CNS) penetration progressively expand, while Gram-positive staphylococcal activity is generally highest in 1st generation agents.

                         Evolution of Cephalosporin Generations
    Generation      Exemplar Agents          Gram-Positive         Gram-Negative / Special
    ──────────      ───────────────          ─────────────         ───────────────────────
    1st Gen         Cefazolin, Cephalexin    ++++ (MSSA, Strep)    + (PEcK: Proteus, E. coli, Klebsiella)
    2nd Gen         Cefoxitin, Cefuroxime    +++                   ++ (HEN PEcK; Cefoxitin = Anaerobes)
    3rd Gen         Ceftriaxone, Ceftazidime ++                    ++++ (Enterics; Ceftazidime = Pseudomonas)
    4th Gen         Cefepime                 ++++ (MSSA, Strep)    +++++ (Broad enterics + Pseudomonas)
    5th Gen         Ceftaroline              +++++ (MRSA! PBP2a)   ++++ (Broad enterics, NOT Pseudomonas)

3. Carbapenems: Imipenem, Meropenem & Ertapenem

Carbapenems possess the broadest antimicrobial spectrum of all beta-lactams, featuring exceptional stability against standard beta-lactamases, cephalosporinases, and Extended-Spectrum Beta-Lactamases (ESBLs) produced by Klebsiella and E. coli.

• Imipenem-Cilastatin: Imipenem is inactivated in the renal proximal tubule by the brush-border enzyme dehydropeptidase I. It is always co-formulated with cilastatin, a specific competitive inhibitor of dehydropeptidase I, preventing renal drug inactivation and toxic tubular metabolite accumulation. High-yield adverse effect: lowers seizure threshold (highest neurotoxicity among carbapenems, particularly in renal failure).
• Meropenem: Stable against dehydropeptidase I (does not require cilastatin); significantly lower neurotoxicity / seizure risk; potent activity against Pseudomonas aeruginosa and anaerobes.
• Ertapenem (Invanz): Unique daily-dosed carbapenem lacking coverage against the "PEA" pathogens: Pseudomonas aeruginosa, Enterococcus, and Acinetobacter. Highly favored for outpatient parenteral antibiotic therapy (OPAT) in ESBL-producing enterobacteriaceae diabetic foot infections not involving Pseudomonas.

4. Monobactams: Aztreonam

Aztreonam is a monocyclic beta-lactam that binds exclusively to PBP3 of aerobic Gram-negative bacteria, including Pseudomonas aeruginosa. It exhibits zero activity against Gram-positive bacteria or anaerobes.

• Cross-Reactivity Pearl: Aztreonam does not share cross-reactivity with penicillin-allergic patients due to its unique monobactam core structure. It can be safely administered to patients with documented severe Type I penicillin anaphylaxis. Sole Exception: It shares an identical chemical side-chain with ceftazidime; cross-allergic hypersensitivity occurs between aztreonam and ceftazidime.

Glycopeptides & Lipopeptides: Vancomycin & Daptomycin

Vancomycin: Mechanism & Kinetics

Vancomycin is a large, tricyclic glycopeptide antibiotic active exclusively against Gram-positive bacteria (cannot penetrate the outer membrane porins of Gram-negatives).

• Mechanism of Action: Vancomycin binds directly with high avidity to the terminal D-alanyl-D-alanine (D-Ala-D-Ala) residues of nascent peptidoglycan pentapeptide chains. This sterically caps the substrate, preventing both transglycosylase (peptidoglycan chain elongation) and transpeptidase (cross-linking) reactions, leading to osmotic cell lysis.
• Clinical Indications: Parenteral vancomycin is the gold-standard intravenous bactericidal agent for MRSA, Staphylococcus epidermidis (prosthetic hardware infections), and Enterococcus faecalis. Oral vancomycin is not absorbed systemically ($F \approx 0\%$) and is used exclusively for intraluminal treatment of severe Clostridioides difficile colitis.
• Adverse Effects:
  • Red Man Syndrome (Vancomycin Flushing Reaction): An immediate, non-IgE-mediated pseudoallergic anaphylactoid reaction caused by direct, non-immune stimulation of mast cell degranulation with massive histamine release. Manifests as intense erythematous flushing, pruritus, and maculopapular rash across the face, neck, and upper torso, occasionally accompanied by hypotension. Prevention/Management: Slow the IV infusion rate (infuse over $\ge 60$ to $120$ minutes) and pretreat with $H_1/H_2$ antihistamines.
  • Nephrotoxicity: Dose-dependent acute tubular necrosis, marked by elevated serum creatinine; amplified dramatically when co-administered with aminoglycosides or piperacillin-tazobactam.
  • Ototoxicity: Cochlear and vestibular damage; often irreversible; exacerbated by loop diuretics (furosemide).
  • Thrombophlebitis: Severe local irritation at peripheral IV insertion sites.
• Mechanism of Vancomycin Resistance (VRE): Mediated by plasmid-borne vanA or vanB operons (predominantly in Enterococcus faecium). The bacterial ligase alters the peptidoglycan synthesis pathway, substituting a terminal D-alanyl-D-lactate (D-Ala-D-Lac) or D-Ala-D-Serine for D-Ala-D-Ala. This single carboxylate-to-ester substitution loses a critical hydrogen bond, reducing vancomycin binding affinity by more than $1,000\text{-fold}$ and producing high-level clinical resistance.

Daptomycin: Lipopeptide Depolarization

Daptomycin is a cyclic lipopeptide active exclusively against Gram-positive organisms, including MRSA, Vancomycin-Intermediate S. aureus (VISA), and Vancomycin-Resistant Enterococci (VRE).

• Mechanism of Action: Daptomycin inserts its lipophilic decanoyl tail into the bacterial cell membrane in a calcium-dependent manner, oligomerizing to form transmembrane ionic pores. This induces rapid potassium ion ($K^+$) efflux, precipitating membrane depolarization, loss of membrane potential, arrest of macromolecular synthesis, and rapid cell death without whole-cell lysis.
• High-Yield Clinical Restriction: Daptomycin is completely inactivated by pulmonary surfactant. It is strictly contraindicated for pulmonary infections (pneumonia), but represents a premier first-line agent for right-sided endocarditis, bacteremia, complicated diabetic foot soft tissue infections, and osteomyelitis caused by MRSA/VRE.
• High-Yield Adverse Effect: Skeletal muscle myopathy and rhabdomyolysis. Serum creatine phosphokinase (CPK) must be monitored weekly at baseline and during therapy. Concomitant administration of HMG-CoA reductase inhibitors (statins) should be temporarily discontinued due to additive skeletal muscle toxicity.

---

Protein Synthesis Inhibitors: 30S Subunit

Bacterial ribosomes consist of a 70S structure composed of a smaller 30S subunit (16S rRNA + 21 proteins) and a larger 50S subunit (23S and 5S rRNA + 31 proteins). Human eukaryotic cytoplasmic ribosomes consist of an 80S structure (40S + 60S), providing selective antimicrobial targeting (although eukaryotic mitochondrial ribosomes resemble bacterial 70S ribosomes, accounting for certain toxicities).

                     Ribosomal Protein Synthesis Inhibitors
    
          30S Subunit Inhibitors                 50S Subunit Inhibitors
          ──────────────────────                 ──────────────────────
    Aminoglycosides (Bactericidal)         Macrolides (Bacteriostatic)
      - Gentamicin, Tobramycin, Amikacin     - Azithromycin, Clarithromycin
      - Irreversible binding                 - Blocks 23S rRNA translocation
      - Oxygen-dependent transport           - CYP3A4 inhibition, QT prolongation
      - Nephrotoxicity, Ototoxicity
                                           Clindamycin (Lincosamide)
    Tetracyclines (Bacteriostatic)           - Halts peptidyltransferase
      - Doxycycline, Minocycline             - Anti-toxin in Necrotizing Fasciitis
      - Blocks aminoacyl-tRNA to A-site      - High C. difficile risk
      - Chelation with Ca2+ (teeth/bone)
                                           Linezolid (Oxazolidinone)
                                             - Prevents 70S initiation complex
                                             - Bone marrow suppression, MAOI

Aminoglycosides: Gentamicin, Tobramycin, Amikacin

Aminoglycosides are polycationic, highly polar molecules that bind irreversibly to the 16S rRNA of the 30S ribosomal subunit.

• Mechanism of Action:
  1. Blocks formation of the initiation complex.
  2. Induces ribosomal miscoding (misreading) of mRNA codons, inserting aberrant amino acids into growing polypeptide chains (producing toxic non-functional proteins).
  3. Halts translation by premature termination.
  • Aminoglycosides are uniquely bactericidal among protein synthesis inhibitors. Their killing kinetics display concentration-dependent killing and a prolonged post-antibiotic effect (PAE).
• Transport Requirement: Aminoglycoside entry across the bacterial cytoplasmic membrane requires an active, oxygen-dependent electron transport system. Consequently, aminoglycosides are completely ineffective against obligate anaerobes (Bacteroides, Clostridium) and display poor intracellular penetration in hypoxic, acidic, necrotic wound abscesses.
• Synergism: Routinely paired with cell wall agents (ampicillin, penicillin, vancomycin) in enterococcal and streptococcal endocarditis; cell wall damage permits aminoglycoside penetration into the cytoplasm.
• Adverse Effects (The "3 Ns/Os"):
  • Nephrotoxicity: Accumulates within renal proximal tubular epithelial cells, causing acute tubular necrosis (ATN); non-oliguric renal failure.
  • Ototoxicity: Vestibular (vertigo, ataxia) and cochlear (irreversible sensorineural hearing loss) toxicity via hair cell destruction in the organ of Corti.
  • Neuromuscular Blockade: Blocks presynaptic voltage-gated calcium channels, inhibiting acetylcholine release at the motor endplate; can trigger respiratory paralysis in patients with Myasthenia Gravis (antidote: IV calcium gluconate and neostigmine).
  • Teratogenicity: Causes congenital sensorineural deafness (CN VIII toxicity).

Tetracyclines: Doxycycline, Minocycline

Tetracyclines reversibly bind to the 30S ribosomal subunit, physically blocking the docking of incoming aminoacyl-tRNA to the ribosomal acceptor (A-site), halting polypeptide chain elongation (bacteriostatic).

• Spectrum & Indications: Broad coverage against community-acquired MRSA (CA-MRSA), atypicals (Mycoplasma, Chlamydia), spirochetes (Borrelia burgdorferi — first-line for Lyme disease), Rickettsia rickettsii (Rocky Mountain Spotted Fever), and cutaneous Vibrio vulnificus.
• Pharmacokinetic Interaction (Chelation): Tetracyclines form insoluble chelation complexes with multivalent cations ($Ca^{2+}, Mg^{2+}, Fe^{2+}, Al^{3+}$). Co-administration with milk/dairy products, antacids, or iron supplements prevents gastrointestinal absorption.
• High-Yield Adverse Effects:
  • Teeth Discoloration & Bone Growth Retardation: Chelates calcium orthophosphate in developing teeth and bones, causing permanent brown-yellow dentin discoloration and enamel hypoplasia; contraindicated in children $<8$ years of age and pregnant women.
  • Photosensitivity: Exaggerated sunburn reactions upon minimal ultraviolet exposure.
  • Pill-Induced Esophagitis: Doxycycline capsules cause direct mucosal ulceration if swallowed without water; patients must take with a full glass of water and remain upright for 30 minutes.
  • Doxycycline Elimination Pearl: Doxycycline is eliminated non-renally via the gastrointestinal tract and biliary excretion; no dose adjustment is required in renal failure.

---

Protein Synthesis Inhibitors: 50S Subunit

Macrolides: Azithromycin, Clarithromycin, Erythromycin

Macrolides contain a large macrocyclic lactone ring that binds reversibly to the 23S rRNA of the 50S ribosomal subunit, blocking the translocation step (peptidyl-tRNA fails to advance from the A-site to the P-site, halting protein synthesis; bacteriostatic).

• Clinical Pearls & Toxicities:
  • CYP3A4 Inhibition: Erythromycin and clarithromycin are potent inhibitors of hepatic CYP3A4, dramatically elevating plasma concentrations of statins (triggering rhabdomyolysis), warfarin (elevating INR/bleeding), and theophylline. Azithromycin does not inhibit CYP3A4 and carries the fewest drug interactions.
  • QTc Prolongation & Torsades de Pointes: Macrolides block cardiac delayed rectifier potassium channels ($I_{Kr}$), predisposing patients to fatal polymorphic ventricular arrhythmias.
  • Gastrointestinal Motility: Erythromycin acts as a direct motilin receptor agonist in the stomach and duodenum, stimulating migrating motor complexes (used clinically for diabetic gastroparesis).

Clindamycin: Lincosamide Antimicrobial

Clindamycin binds to the 50S ribosomal subunit at a site overlapping macrolides, inhibiting peptidyltransferase and peptide chain elongation.

• Spectrum: Potent activity against Gram-positive cocci (MSSA, community-acquired MRSA, Streptococcus) and anaerobes (Bacteroides fragilis, Peptostreptococcus, Fusobacterium). Inactive against aerobic Gram-negative bacilli.
• High-Yield Antitoxin Effect: In severe toxic shock syndrome (Staphylococcus aureus) or necrotizing fasciitis (Streptococcus pyogenes / Group A Strep), bacterial exotoxins (e.g., Streptococcal pyrogenic exotoxins SpeA/SpeC, TSST-1) drive overwhelming cytokine storm. Penicillins kill bacteria via cell wall lysis, which initially releases preformed endotoxins and toxins without stopping toxin production. Clindamycin directly suppresses ribosomal protein synthesis, instantly shutting down bacterial exotoxin and superantigen production. It is co-administered as an obligate antitoxin adjunct to high-dose penicillin or vancomycin in limb- and life-threatening necrotizing soft tissue infections.
• Adverse Effect: Pseudomembranous Colitis (Clostridioides difficile): Clindamycin disrupts normal colonic anaerobic flora, permitting C. difficile overgrowth and elaboration of Toxin A (enterotoxin) and Toxin B (cytotoxin). (Treated with oral vancomycin or fidaxomicin).

Linezolid: Oxazolidinone

Linezolid binds to a unique site on the 23S rRNA of the 50S ribosomal subunit, physically preventing the assembly of the functional 70S initiation complex (bacteriostatic against staphylococci and enterococci; bactericidal against streptococci).

• Indications: Reserved for severe infections caused by multi-drug-resistant Gram-positive pathogens, including MRSA, VISA, and Vancomycin-Resistant Enterococci (VRE). Exhibits $100\%$ oral bioavailability ($F = 1.0$), facilitating smooth outpatient oral conversion.
• High-Yield Toxicities:
  • Myelosuppression: Reversible bone marrow suppression—predominantly thrombocytopenia, followed by anemia and leukopenia—occurring characteristically after $>2$ weeks of therapy. Weekly complete blood count (CBC) monitoring is mandatory.
  • Serotonin Syndrome: Linezolid is a weak, non-selective monoamine oxidase inhibitor (MAOI). Co-administration with SSRIs, SNRIs, tricyclic antidepressants, or tramadol triggers lethal Serotonin Syndrome (autonomic instability, hyperreflexia, clonus, hyperthermia).
  • Mitochondrial Toxicity: Inhibits mitochondrial protein synthesis over prolonged courses, causing lactic acidosis, peripheral neuropathy, and irreversible optic neuropathy (blindness).

---

Nucleic Acid & Folate Synthesis Inhibitors

Fluoroquinolones: Ciprofloxacin, Levofloxacin, Moxifloxacin

Fluoroquinolones enter bacteria via porins and inhibit two essential type II topoisomerases:

1. DNA Gyrase (Topoisomerase II): Relieves positive supercoiling ahead of the replicating DNA fork (primary target in Gram-negative bacteria).
2. Topoisomerase IV: Decatenates (separates) intertwined daughter DNA circles following replication (primary target in Gram-positive bacteria). Inhibition induces double-stranded DNA breaks and cellular death (rapidly bactericidal).

Folate Antagonists: Trimethoprim-Sulfamethoxazole (TMP-SMX)

Bacteria cannot absorb exogenous folate; they must synthesize tetrahydrofolic acid ($THF$) de novo from para-aminobenzoic acid (PABA) to synthesize purines, thymidine, and methionine.

$\text{PABA} \xrightarrow[\textbf{Inhibited by Sulfonamides}]{\text{Dihydropteroate Synthase}} \text{Dihydropteroic Acid} \rightarrow \text{Dihydrofolate (DHF)} \xrightarrow[\textbf{Inhibited by Trimethoprim}]{\text{Dihydrofolate Reductase}} \text{Tetrahydrofolate (THF)}$

• Sequential Synergistic Blockade: Sulfamethoxazole (PABA analog) competitively inhibits dihydropteroate synthase. Trimethoprim competitively inhibits dihydrofolate reductase (DHFR). Individually bacteriostatic, the combination is synergistically bactericidal.
• Indications: Excellent oral choice for uncomplicated Community-Acquired MRSA (CA-MRSA) skin and soft tissue infections, Pneumocystis jirovecii pneumonia, and Stenotrophomonas maltophilia.
• High-Yield Adverse Effects:
  • Hypersensitivity Reactions: Type I urticaria/anaphylaxis, Type III serum sickness, and severe life-threatening Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN).
  • Hyperkalemia: Trimethoprim structurally resembles the potassium-sparing diuretic amiloride, blocking epithelial sodium channels (ENaC) in the renal cortical collecting tubule, impairing sodium reabsorption and potassium excretion.
  • Hematologic Toxicity: Megaloblastic anemia, leukopenia, and thrombocytopenia via folate depletion (antidote: leucovorin / folinic acid).
  • Kernicterus: Sulfonamides displace bilirubin from serum albumin; contraindicated in pregnancy (3rd trimester) and neonates.

---

High-Yield Antimicrobial Spectrum & Clinical Selection Guide