Antibiotics: Mechanisms, Effects, and Antimicrobial Resistance

Introduction to Antibiotics and Basic Definitions

• Definition of Antibiotics: An antibiotic is a type of antimicrobial substance intended for killing or preventing the growth of bacteria. While commonly used in medicine and livestock feed, the term is occasionally used broadly to refer to any substance used against microbes.

• Origin of Antibiotics:

  • Natural: Produced by one organism to fight another (e.g., Penicillin).

  • Synthetic: Fully man-made substances (e.g., Sulfonamides and certain antiseptics).

• Consumption Trends: Antibiotic consumption rates are not decreasing on a worldwide scale.

  • Significant increases are noted in low-income and middle-income countries.

  • High-income countries show relatively stable or slightly decreasing rates but remain major consumers.

  • High-consumption regions include Southeast Asia, East Asia, Oceania, Central Europe, Eastern Europe, Central Asia, and Latin America.

• European Consumption (2012 Data):

  • Penicillins: The most widely used class at $2111$ tons ($62\%$).

  • Cephalosporins: $279$ tons ($8\%$).

  • Macrolides: $252$ tons ($7\%$).

  • Fluoroquinolones: $228$ tons ($7\%$).

  • Sulfonamides: $122$ tons ($4\%$).

  • Tetracyclines: $106$ tons ($3\%$).

  • Lincosamides: $59$ tons ($2\%$).

  • Trimethoprim: $40$ tons ($1\%$).

  • Other: $203$ tons ($6\%$).

Main Antibiotic Classes and Action Mechanisms

• Mechanisms Targeting Bacterial Cell Wall:

  • Action: Inhibition of bacterial cell wall synthesis and repair (specifically inhibiting peptidoglycan synthesis). This is a bactericidal action.

  • Representative Classes: $\beta$-lactams (Penicillins, Cephalosporins, Carbapenems, Monobactams), Vancomycin, Fosfomycin, Bacitracin.

  • Specific Compound: Amoxicillin.

• Mechanisms Inhibiting Protein Synthesis:

  • Targeting 50S Ribosomal RNA: Macrolides (Azithromycin, Erythromycin), Chloramphenicol (inhibits peptide bond formation).

  • Targeting 30S Ribosomal RNA: Tetracyclines (prevent tRNA from attaching to the ribosome), Aminoglycosides (Streptomycin), Nitrofurans, Spectinomycin.

  • Other: Fusidic acid (prevents translocation of Elongation Factor G from the ribosome).

• Mechanisms Inhibiting Nucleic Acid Synthesis:

  • DNA Replication: Quinolones and Fluoroquinolones (e.g., Ciprofloxacin) inhibit DNA gyrases or topoisomerases. Metabolic cytotoxic byproducts may also disrupt DNA.

  • RNA Transcription: Rifamycin (binding to RNA polymerase).

• Competitive Antagonism and Antimetabolite Activity:

  • Folic Acid Synthesis: Sulfonamides (e.g., Sulfamethoxazole) compete with para-aminobenzoic acid to inhibit bacterial dihydrofolic acid synthesis (part of C1 metabolism).

  • Trimethoprim: Also interferes with C1 metabolism.

• Alteration of Cell Membranes: Polymyxin B and Polymyxin E.

Regulatory Frameworks and Guidelines

• European Guideline on Environmental Risk Assessment: Covers medicinal products for human use.

• EU Water Framework Directive 208/105/EC: Antibiotics are currently not included in the list of priority pollutants. However, a "Watch list" for Union-wide monitoring (August 2022) includes Sulfamethoxazole and Trimethoprim.

• European Medicines Agency (EMA): Supports the ‑One Health‑ approach, promoting integrated cooperation between human medicine and veterinary fields to tackle antimicrobial resistance.

• Collaborative Action: The European Commission, European Centre for Disease Prevention and Control (ECDC), and the European Food Safety Authority (EFSA) have developed an action plan against the rising threat of antimicrobial resistance.

Environmental Pathways and Occurrence

• Sources of Antibiotic Pollution:

  • Pharmaceutical factories.

  • Hospitals and households (human medicine).

  • Livestock farms and aquaculture (veterinary medicine).

• Pathways: Antibiotics travel through sewage, surface runoff, and soil. They ultimately arrive at Waste-water Treatment Plants (WWTPs).

• Removal Efficiency in WWTPs: Antibiotic residues are not completely removed. Efficiency varies significantly:

  • Uppsala WWTPs: Removal ranges from $0\%$ to $50\%$

  • Azithromycin and Clarithromycin: Roughly $50\%$ removal.

  • Sulfamethoxazole and Trimethoprim: Low removal ($0\% - 10\%$). Microbial biodegradation is the main mechanism for eliminating these low-absorptivity compounds.

• Occurrence in Sweden (WWTP Effluents 2005‑2018):

  • Ciprofloxacin: Found in concentrations up to $200\,ng/L$.

  • Tetracycline: Found up to $125\,ng/L$.

  • Sparfloxacin: Notable peaks around August/September 2018 reaching up to $1500\,ng/L$.

  • Sulfamethoxazole and Trimethoprim: Often detected above the Limit of Quantification (LOQ) of $10\,ng/L$.

• Occurrence in European Surface Waters:

  • Poland (Six rivers and three lakes): Aminoglycosides (< $1000 - 10,000\,ng/L$), Macrolides (< $50 - 5000\,ng/L$).

  • Portugal (Montego, Tagus, Tejo, Douro Rivers): Ciprofloxacin ($0.42 - 44.70\,ng/L$), Sulfamethoxazole (avg. $12.94\,ng/L$).

  • United Kingdom (Taff River): Fluoroquinolones ($11 - 21\,ng/L$), Trimethoprim ($30 - 120\,ng/L$).

Ecological Impacts on Microbial Communities

• Predicted Non-Effect Concentration (PNEC): The concentration below which no adverse effects on the environment are expected.

  • Environmental PNEC ($PNEC_{env}$) values:

    • Ciprofloxacin: $0.15\,ng/L$

    • Amoxicillin: $0.78\,ng/L$

    • Trimethoprim: $5.8\,ng/L$

    • Azithromycin: $19\,ng/L$

    • Tetracycline: $1000\,ng/L$

• Impacts on Community Structure:

  • Soil/Sediment: Observed decreases in bacterial biomass and abundance. Shifts in community diversity, such as an increase in fungal-to-bacterial Phopholipid Fatty Acid (PLFA) ratios.

  • Diversity Shifts: Quinolones and Fluoroquinolones can lead to higher abundance in Deltaproteobacteria, Clostridia, and Bacilli. Penicillin G has been linked to shifts in Epsilonproteobacteria.

  • Ammonia Oxidation: Sulfadiazine exposure has been shown to increase the Ammonia Oxidant Archaea (AOA) to Ammonia Oxidant Bacteria (AOB) ratio.

• Impacts on Community Function:

  • Respiration: Decrease in microbial catabolic activity.

  • Nitrification: Streptomycin has shown $75\%$ inhibition of nitrifying activity in activated sludge. Oxytetracycline inhibits the conversion of ammonia to nitrite and nitrate.

  • Pollution-Induced Community Tolerance (PICT): Exposure to antibiotics leads to an increase in community tolerance as sensitive species die off and resistant ones survive.

  • Greenhouse Gases: Mixture of antibiotics (Amoxicillin, Ciprofloxacin, Erythromycin, Sulfamethoxazole, Tetracycline) can enhance the production of $CH_4$ and $CO_2$ in river sediments.

Antibiotic Resistance Mechanisms and Genetics

• Definition of Antimicrobial Resistance (AMR): Microbes evolve to become resistant due to genetic material that confers protection. Resistance is exacerbated by the misuse and overuse of antibiotics.

• Cellular Mechanisms of Resistance:

  • Active Efflux: Efflux pumps transport the drug out of the cell.

  • Target Protection: Proteins protect the antibiotic target site.

  • Target Site Modification: Structural change in the target so the antibiotic cannot bind.

  • Target Bypass: Development of a new protein with the same metabolic capacity that is not affected by the antibiotic.

  • Inactivation: Enzymes modify or destroy the antibiotic molecule (e.g., $\beta$-lactamases).

  • Decreased Influx: Downregulation of porins in the outer membrane to prevent antibiotic entry.

• Mobile Genetic Elements (MGE) and Horizontal Gene Transfer (HGT):

  • Conjugation: Transfer of plasmids (circular DNA) between donor and recipient cells via direct contact.

  • Transduction: DNA transfer mediated by a bacteriophage.

  • Transformation: Uptake of free DNA released from dead bacterial cells.

• Multidrug Resistance: Plasmids can contain multiple Antibiotic Resistance Genes (ARGs) as well as resistance to heavy metals/biocides. A bacterium picking up a single plasmid (e.g., pUUH239.2) can become resistant to several drug classes simultaneously.

• Global Health Threat: In 2021, antibiotic resistance caused approximately $700,000$ deaths per year. This is predicted to reach $10$ million deaths by 2050.

Research Case Studies

• Uppsala STPs & Swedish Surface Waters: Comparison of concentrations to environmental toxicity. Chlortetracycline levels detected in surface waters and Sewage Treatment Plants (STPs) frequently exceed concentrations that cause inhibition in limnic bacterial communities.

• Mediterranean River (Impacted by Urban Wastewater):

  • Hot Spots: Hospital effluents and WWTP influents have the highest concentrations of antibiotics (e.g., Ciprofloxacin up to $14,377\,ng/L$ in hospital effluent) and high copies of ARGs.

  • Correlation: High antibiotic concentrations co-occur with higher copies of resistance genes (e.g., blaCTX-M, qnrS, sul1).

• Göta Älv (Sweden) Study:

  • Methodology: Used qPCR and shotgun metagenomics to characterize sediment bacterial communities along a $95\,km$ gradient.

  • Findings: WWTP effluents serve as sources of ARGs. Ciprofloxacin was the only antimicrobial detected above its Predicted No-Effect Concentration for resistance selection ($PNEC_{res}$).

  • Resistome: sul1 and ermB were dominant ARGs. Gene abundances were $5000$-fold lower in river sediments than in effluents, but higher levels were found downstream compared to upstream sites. Efflux pump genes were the most prevalent resistance mechanism in sediments.

Summary of Environmental Risks and Knowledge Gaps

• Inadequate Monitoring: There is a need for more consistent data and lower quantification limits to estimate trends and risks accurately.

• Ecological Threat: Pollution threatens natural microbial biodiversity and ecosystem functions like nitrification.

• Resistance Selection: The concentration required to select for resistance ($PNEC_{res}$) is often much lower than the concentration required for acute environmental toxicity ($PNEC_{env}$).

• Data Gaps:

  • Limited ecotoxicological data for environmental fungi, cyanobacteria, and microbial communities.

  • Lack of experimental data on microbial biodegradation in natural settings.

  • Limited understanding of factors influencing the spread of Antibiotic Resistant Bacteria (ARB) and their long-term ecological effects.