Bacterial drug resus

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antimicrobial resistance mechanisms bacteria

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Bacterial Drug Resistance

Bacterial drug resistance (antimicrobial resistance, AMR) refers to the ability of bacteria to survive and multiply in the presence of antimicrobial agents that would normally inhibit or kill them. It is one of the most pressing public health challenges globally.

1. Core Concepts

Minimum Inhibitory Concentration (MIC) - the lowest antibiotic concentration that inhibits visible bacterial growth in vitro. A bacterium is called:

Susceptible: MIC is below achievable, non-toxic blood/tissue levels
Resistant: MIC exceeds normally attainable levels
Intermediate: MIC falls in between; the drug may still be used at higher doses or in concentrated sites (e.g., urine for fluoroquinolones)
Sherris & Ryan's Medical Microbiology, 8th Ed.

2. Genetic Basis of Resistance

Resistance arises through two broad genetic routes:

A. Mutation Under Antibiotic Pressure

Spontaneous mutations in chromosomal genes can alter drug targets, reduce uptake, or increase efflux. Under antibiotic pressure, sensitive bacteria are killed and resistant mutants are selected. Single-step resistance is more likely when a drug has a single binding target (e.g., early quinolone nalidixic acid binding one topoisomerase subunit; early aminoglycoside streptomycin binding one ribosomal site). Newer agents in each class bind multiple targets to reduce this risk.

Sherris & Ryan's Medical Microbiology, 8th Ed.

B. Horizontal Gene Transfer (HGT)

Resistance genes spread far more rapidly through HGT than through mutation alone. Three mechanisms:

Mechanism	How it works
Conjugation	Direct cell-to-cell DNA transfer via pili (the main route for R plasmids)
Transformation	Uptake of naked DNA from dead bacteria
Transduction	Bacteriophage-mediated gene transfer

R (resistance) plasmids are especially dangerous - they often carry multiple resistance genes simultaneously, can transfer across species, and their acquisition genes (transposons, integrons) allow resistance gene capture and spread. Gram-negative bacteria readily transfer R plasmids across genus boundaries.

Sherris & Ryan's Medical Microbiology, 8th Ed.

3. Mechanisms of Resistance

There are four main categories:

A. Drug Inactivation / Enzymatic Destruction

The most clinically important mechanism. Bacteria produce enzymes that chemically modify or destroy the antibiotic:

Beta-lactamases: Hydrolyze the beta-lactam ring of penicillins and cephalosporins, making them inactive. Hundreds of variants exist:

Extended-spectrum beta-lactamases (ESBLs): Inactivate most penicillins AND cephalosporins
Carbapenemases (e.g., KPC, NDM, OXA-48): Destroy carbapenems - the "last resort" drugs
Beta-lactamase inhibitors (clavulanate, tazobactam, avibactam) block these enzymes and are co-formulated with penicillins to restore activity

Aminoglycoside-modifying enzymes: Transferase enzymes (acetyltransferases, adenylyltransferases, phosphotransferases) add chemical groups to aminoglycosides (gentamicin, tobramycin), reducing their ribosomal binding affinity. These are plasmid-mediated and common in gram-negative bacteria.

Medical Microbiology 9e; Harrison's Principles of Internal Medicine 22e

B. Altered Target Site

The antibiotic's binding target is modified so the drug can no longer attach effectively:

Altered Penicillin-Binding Proteins (PBPs): The most clinically significant example is MRSA (methicillin-resistant S. aureus), which acquires the mecA gene encoding PBP-2A - a novel PBP with very low affinity for all beta-lactams (except ceftaroline). Similarly, penicillin-resistant pneumococci have mutated PBPs.
Vancomycin resistance (VRE): Normally vancomycin binds the terminal D-alanine-D-alanine (D-Ala-D-Ala) residues of peptidoglycan precursors. VRE (mainly E. faecium) encode the van operon (VanA, VanB, VanC) that substitutes D-Ala-D-Ala with:
- D-lactate (VanA, VanB): ~1000-fold reduction in vancomycin binding affinity - high-level resistance
- D-serine (VanC): Lower-level resistance
- Vancomycin-resistant organisms also produce enzymes that destroy D-Ala-D-Ala ending precursors, eliminating additional binding sites
Ribosomal target modification: Methylation of 23S rRNA (by erm genes) blocks macrolide, lincosamide (clindamycin), and streptogramin B binding - the MLS_B resistance phenotype. 16S rRNA methylation blocks all aminoglycosides, including newer agents like plazomicin.
Topoisomerase mutations: Fluoroquinolone resistance commonly arises from mutations in DNA gyrase (GyrA/GyrB) or topoisomerase IV (ParC/ParE), reducing quinolone binding.
Sulfonamide/trimethoprim resistance: Acquisition of alternate DHPS/DHFR enzymes that carry out folate synthesis but have low affinity for the drugs.
Harrison's Principles of Internal Medicine 22e; Sherris & Ryan's Medical Microbiology, 8th Ed.

C. Reduced Drug Entry (Decreased Permeability)

Most relevant in gram-negative bacteria, which have an outer membrane:

Porin mutations: Beta-lactams and other hydrophilic drugs enter gram-negative bacteria through protein channels (porins, e.g., OmpC, OmpF). Mutations that reduce the size, charge, or expression of these porins reduce intracellular drug concentrations.
This mechanism alone produces only low-level resistance but, combined with other mechanisms (especially beta-lactamase production), can produce high-level carbapenem resistance in organisms like K. pneumoniae and Acinetobacter baumannii.
Medical Microbiology 9e

D. Active Efflux Pumps

Multi-drug efflux pumps actively export antibiotics out of the bacterial cell before they can reach their targets. Several families exist (e.g., RND family in gram-negatives):

MexXY-OprM in P. aeruginosa: Overexpression (from regulatory mutations) confers resistance to aminoglycosides, fluoroquinolones, and beta-lactams simultaneously
Tetracycline-specific efflux: Plasmid-mediated pumps specifically export tetracyclines
Broad-spectrum pumps: Export multiple drug classes simultaneously - a key driver of multidrug resistance (MDR)
Harrison's Principles of Internal Medicine 22e; Katzung's Basic and Clinical Pharmacology, 16th Ed.

4. Important Resistant Organisms (ESKAPE Pathogens)

Organism	Key Resistance Mechanism	Drug of Concern
MRSA (S. aureus)	PBP-2A (mecA gene)	All beta-lactams
VRE (Enterococcus)	Van operon (D-Ala-D-Lac)	Vancomycin
ESBL producers (e.g., K. pneumoniae, E. coli)	ESBLs	Penicillins, cephalosporins
CRE (Carbapenem-resistant Enterobacterales)	KPC, NDM, OXA carbapenemases	Carbapenems
CRPA (P. aeruginosa)	MexXY efflux + porin loss	Broad-spectrum
CRAB (A. baumannii)	OXA-23/OXA-24 carbapenemases	Carbapenems

A 2025 systematic review in Clin Microbiol Rev on drug-resistant A. baumannii highlights emerging treatment options for this WHO priority pathogen.

5. VISA/VRSA - A Special Example

VISA (Vancomycin-intermediate S. aureus) arises not through the van operon but through multiple chromosomal mutations causing a thickened, poorly cross-linked cell wall. This thickened wall contains extra D-Ala-D-Ala stem peptides that "mop up" vancomycin molecules away from the cell membrane, preventing drug access to the active synthesis zone. This phenotype is often selected during prolonged vancomycin courses and may revert to susceptibility when selection pressure is removed (due to the metabolic cost of a thickened wall).

Harrison's Principles of Internal Medicine 22e

6. Intrinsic vs. Acquired Resistance

Type	Definition	Example
Intrinsic	Natural property of all members of a species	P. aeruginosa intrinsically resistant to many agents due to low outer-membrane permeability; E. gallinarum has chromosomal VanC
Acquired	Gained through mutation or HGT	MRSA, ESBL producers, VRE

7. Detecting Resistance in the Laboratory

Disk diffusion (Kirby-Bauer): Antibiotic-impregnated disks on agar; zone of inhibition inversely proportional to MIC
Broth microdilution: Gold standard for exact MIC determination
E-test: Gradient strip; reads MIC directly from where growth edge intersects the strip
Molecular testing: PCR/sequencing to detect known resistance genes (e.g., mecA, van, blaKPC, blaNDM) - faster than phenotypic methods but cannot detect novel mechanisms
Sherris & Ryan's Medical Microbiology, 8th Ed.

8. Combating Resistance

Antimicrobial stewardship: Prescribe only when necessary, narrow spectrum, shortest effective course
Combination therapy: Synergy (e.g., beta-lactam + aminoglycoside for serious enterococcal infections) and prevention of resistance emergence
Beta-lactamase inhibitors: Clavulanate, tazobactam, avibactam, relebactam restore beta-lactam activity against ESBL/KPC producers
Novel agents: Ceftaroline (binds PBP-2A - active against MRSA), ceftazidime-avibactam, ceftolozane-tazobactam, plazomicin (not modified by most transferases)
Infection control: Prevent nosocomial spread of resistant organisms

Summary of the 4 core resistance mechanisms:

1. Enzymatic inactivation    → destroy or modify the antibiotic (beta-lactamases, AMEs)
2. Target alteration         → change what the drug binds to (PBP-2A, VanA, ribosomal methylation)
3. Reduced permeability      → block drug entry (porin loss in gram-negatives)
4. Active efflux             → pump drug out before it acts (MexXY, Tet pumps)

All four can occur simultaneously in a single organism, producing high-level multidrug resistance.

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