Antimicrobial Agents & Resistance Quiz

Antimicrobial Mechanisms of Action and Bacterial Resistance Mechanisms

• Antibacterial mechanisms of action include targeting bacterial cell wall biosynthesis, folate synthesis, DNA replication, DNA transcription, and mRNA translation.
• Antimicrobial agents, including antibacterials, antifungals, antiseptics, antibiotics, preservatives, sterilants, and disinfectants, all have the capacity to kill or suppress microbial growth.
• Bacterial resistance mechanisms include acquired resistance, biofilm formation, enzymatic inactivation, efflux, and target site modification.
• Antimicrobial drug resistance was observed soon after the discovery of antibiotics.
• The World Health Organization estimates at least 10 million deaths per year by 2050, due to infections caused by drug-resistant bacteria.
• Antibiotic stewardship is the appropriate use of antimicrobials to maximize their current effects and ensure their availability for future generations.

Antimicrobial Agents

• Antimicrobials include antibacterials, antifungals, antiseptics, antibiotics, preservatives, sterilants, and disinfectants.

Antimicrobial Drug Resistance

• Resistance is a natural consequence of drug exposure from the use and inappropriate use of antimicrobial agents.
• It is observed soon after the discovery of antibiotics.

Antibiotic Stewardship

• Refers to the appropriate use of antimicrobials to maximize their current effects and improve the chances of their being available for future generations.

Antimicrobial Targets and Mechanisms of Action

• Inhibition of Bacterial Cell Wall Biosynthesis
• Inhibition of Folate Synthesis
• Interference with DNA Replication
• Interference with DNA Transcription
• Interference of mRNA Translation

Inhibition of Bacterial Cell Wall Biosynthesis

• Gram-positive and gram-negative bacteria have a multilayered cell wall structure composed of an inner cytoplasmic membrane, a peptidoglycan layer, and in gram-negative bacteria, an outer membrane.

Inhibition of Bacterial Cell Wall Biosynthesis- Peptidoglycan Structure

• Peptidoglycan structure consists of alternating disaccharides (NAG, NAM) with peptide cross-links.
• The peptide consists of L- and D-amino acids, typically ending in D-alanyl-D-alanine (D-Ala-D-Ala).
• The D-Ala-D-Ala sequence is critical for cross-linking and drug targeting.

Inhibition of Bacterial Cell Wall Biosynthesis Stages

• Synthesis of precursors in the cytoplasm
• Transport of lipid-bound precursors across the cytoplasmic membrane
• Insertion of glycan units into the cell wall
• Transpeptidation linking and maturation.
• D-Cycloserine and bacitracin inhibit the first two steps, respectively.

Most Commonly Used Inhibitors of Cell Wall Biosynthesis

• β-lactams (e.g., penicillin and cephalosporins) act by forming covalent complexes with enzymes that generate the mature peptidoglycan molecule and bind PBPs (penicillin binding proteins). They inhibit cross-linking and can cause lysis.
• Glycopeptides (e.g., vancomycin, dalbavancin, teicoplanin) bind D-Ala-D-Ala, block transpeptidation, and are effective for Gram-positives.

Inhibition of Folate Synthesis

• Mediated by two key enzymes: dihydropteroate synthase and dihydrofolate reductase.
• These enzymes mediate the formation of tetrahydrofolate (THF) from dihydrofolate.
• Examples include Sulfamethoxazole (SMZ) and Trimethoprim (TMP)

Interference with DNA Replication

• Quinolones affect DNA replication by targeting topoisomerases II (DNA gyrase) and IV.
• These enzymes are important in controlling DNA topology, replication, and decatenation at the end of the bacterial DNA replicative cycle.

Interference with DNA Transcription

• Rifampin is used in combination with other antibacterial classes to treat Mycobacterium tuberculosis infection.
• The target in M. tuberculosis is the RNA polymerase β subunit at an allosteric site, blocking RNA chain elongation.
• This aborts RNA transcription at the initiation step.

Interference of mRNA Translation

• Aminoglycosides, tetracyclines, macrolides (erythromycin, clarithromycin, and azithromycin), ketolides, oxazolidinones, glycylcyclines, and streptogramins interfere with mRNA translation.

Antimicrobial Susceptibility Testing

• Performed on bacteria and fungi to determine which antimicrobials are effective.
• Methods include disk diffusion and dilution (minimal inhibitory concentration [MIC]).

Factors to Consider When Determining Whether Testing Is Warranted

• Body site from which the bacterium was isolated
• Presence of other organisms and quality of the specimen from which the organism was grown
• Host's status

Body Site

• Normal microbiota: E. coli (lower GI), viridans group streptococci (throat), coagulase-negative staphylococci (wound), yeasts (vaginal specimens or throat)

Presence of Other Bacteria and Quality of Specimen

• A pure culture of a bacterium (e.g., E. coli, above 10^5 CFU/mL) is more likely to represent a true infection and should be tested. A few K. pneumoniae organisms in the presence of other organisms in a specimen (like oropharyngeal flora in sputum) might not be significant.

Host Status

• Species usually viewed as normal microbiota might be responsible for an infection in immunocompromised patients and may therefore require testing.

Selecting Antimicrobial Agents for Testing and Reporting

• 10 to 15 antimicrobial agents are typically tested routinely against Enterobacteriaceae, Pseudomonas spp., nonfastidious gram-negative bacilli (e.g., Acinetobacter spp., Stenotrophomonas maltophilia, and Burkholderia cepacia), staphylococci, and enterococci.

Reporting of Susceptibility Test Results

• Results are reported showing susceptibility (S) or resistance (R) for each tested drug.
• Tables are used to present these results, group microorganisms based on specific antibiotics and interpret results properly.

Traditional Antimicrobial Susceptibility Test Methods

• Methods, like the Kirby-Bauer test, are used in traditional susceptibility tests to measure the effectiveness of antibiotics by observing zones of inhibition.

Test your knowledge on antimicrobial drug resistance, the role of various agents like β-lactams, glycopeptides, and the mechanisms behind their effectiveness. This quiz covers critical concepts in antibiotic stewardship and the implications of improper use of these drugs.