Physiopathological Bases and Resistance Mechanisms

Mechanism

Biological Targets of Antimicrobials

The selectivity of antimicrobials is based on the exploitation of fundamental structural and metabolic differences between prokaryotic cells (bacteria), fungal eukaryotes and host animal cells:

🧱 Cell Wall and Membrane Synthesis

The peptidoglycan wall protects the bacteria from osmotic lysis. Its synthesis consists of three stages:

• Cytoplasmic phase: Synthesis of UDP-N-acetylmuramyl-pentapeptide precursors (inhibited by fosfomycin at the MurA step).
• Membrane transport: The carrier lipid bactoprenol transports wall monomers (inhibited by bacitracin).
• External polymerization: Transglycosylation and transpeptidation by Penicillin Binding Proteins (PBPs). Beta-lactams block these PBPs covalently, while glycopeptides bind to the D-Ala-D-Ala residue of the precursor, physically preventing their union.

🧬 Replication and Translation Machinery

The expression and conservation of bacterial genetic information uses specific protein and nucleic acid structures:

• Ribosomal inhibition (Translation): Bacterial type 70S ribosomes are composed of the 30S and 50S subunits. Drugs such as aminoglycosides and tetracyclines bind to the 30S subunit, inducing misreading of the mRNA or blocking tRNA binding. Macrolides, lincosamides and oxazolidinones block the 50S subunit, inhibiting transpeptidation or translocation of the nascent peptide.
• Topoisomerases (Replication): Fluoroquinolones block DNA gyrase (topoisomerase II) and topoisomerase IV, preventing the resolution of DNA supercoils and lethally blocking cellular replication.

Genetic Mechanisms of Antimicrobial Resistance

Bacterial resistance is a dynamic adaptive phenomenon chromosomally encoded or acquired by horizontal gene transfer through plasmids, transposons or integrons. The main mechanisms are grouped into four molecular categories:

Molecular Mechanisms of Drug Evasion

Bacteria have developed highly effective biophysical tools to evade the selective pressure of antibiotics:

1. Enzymatic Destruction (Beta-lactamases):
   • Ambler Class A (Serino-beta-lactamases): Includes classical penicillinases (TEM, SHV), extended spectrum beta-lactamases (ESBL such as CTX-M) and KPC-type carbapenemase. They use an active serine residue to hydrolyze the beta-lactam ring.
   • Class B (Metallo-beta-lactamase - MBL): Like NDM-1, VIM and IMP. They require zinc cations (Zn²⁺) in their catalytic site to cleave the beta-lactam ring. They are resistant to all serine-beta-lactamase inhibitors and carbapenems, being sensitive only to aztreonam.
   • Class C (AmpC cephalosporinases): Expressed in an inducible or constitutively derepressed manner in Enterobacteriaceae of the Space/Escape group. They hydrolyze 1st, 2nd and 3rd generation cephalosporins.
   • Class D (Oxacillinases - OXA): Like OXA-48 in Klebsiella pneumoniae or OXA-23 in Acinetobacter baumannii. They confer variable resistance to carbapenems.
2. Modification of the Cellular Target:
   • The mecA gene in Staphylococcus aureus encodes a modified PBP (PBP2a) with low affinity for almost all beta-lactams (origin of MRSA).
   • The genes vanA and vanB modify the end of the peptidoglycan from D-Ala-D-Ala to D-Ala-D-Lactate, eliminating the molecular target of vancomycin in enterococci (ERV).
3. Active Efflux Pumps and Porin Loss: Multidrug efflux pumps (such as those of the RND superfamily in Gram-negatives, e.g., MexAB-OprM in Pseudomonas aeruginosa) actively efflux beta-lactams, quinolones and tetracyclines. Loss of selective porins (such as the dural porin OprD in P. aeruginosa) blocks the passage of carbapenems (especially imipenem) to the periplasmic space.

Pharmacokinetics

Conceptual Bases of the PK/PD Relationship

The clinical efficacy of an antibiotic does not depend solely on the minimum inhibitory concentration (MIC) in vitro, but on the kinetics of its free tissue concentration (f) in relation to the MIC over time. Three cardinal PK/PD profiles are defined:

⏳ Time-Dependency (fT > CIM)

Efficacy depends on the cumulative time that the free fraction of the drug remains above the MIC at the site of infection. Characteristic of Beta-lactams. The clinical objective is to maximize the time interval of exposure (%T > MIC of 40-70% in stable patients, and ideally 100% of the free fraction in critically ill or immunocompromised patients).

⚡ Concentration-Dependency (fC_max/CIM)

Optimal bactericidal efficacy correlates with the maximum concentration achieved in a single pulse. Characteristic of Aminoglycosides. The objective is to reach a plasma peak of 8 to 10 times the MIC of the pathogen, also minimizing the development of adaptive resistance due to uptake saturation.

📊 Cumulative Exposure (fAUC₂₄/CIM)

The main determinant of bacterial clearance is the total amount of drug exposed in a 24-hour period in relation to the resistance of the germ. Typical of Glycopeptides, Fluoroquinolones, Tetracyclines and Linezolid. In the case of vancomycin, a daily area under the curve target of 400 - 600 is pursued to maximize healing while limiting nephrotoxicity.