10.2 Antibacterial drugs
4 min read•august 16, 2024
Antibacterial drugs are crucial weapons against bacterial infections. They work in various ways, targeting cell walls, protein synthesis, DNA replication, and other vital bacterial processes. Understanding their mechanisms and spectra of activity helps doctors choose the right drug for each infection.
Pharmacokinetics and pharmacodynamics play key roles in antibacterial therapy. How drugs are absorbed, distributed, metabolized, and excreted affects their effectiveness. Factors like drug concentration and bacterial susceptibility determine how well an antibiotic will work against an infection.
Antibacterial Drug Classification
Mechanisms of Action
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- Antibacterial drugs categorized into five main classes based on mechanism of action
- Cell wall synthesis inhibitors
- Protein synthesis inhibitors
- Nucleic acid synthesis inhibitors
- Metabolic pathway inhibitors
- Cell membrane disruptors
- Cell wall synthesis inhibitors interfere with peptidoglycan synthesis in bacterial cell walls
- β-lactams (, )
- (vancomycin)
- Protein synthesis inhibitors target bacterial ribosomes to prevent protein production
- Nucleic acid synthesis inhibitors disrupt DNA replication or RNA transcription in bacteria
- Metabolic pathway inhibitors interrupt essential bacterial metabolic processes
Spectrum of Activity
- Antibacterial drugs classified as or
- Narrow-spectrum effective against specific types of bacteria
- Broad-spectrum effective against wide range of bacteria
- Gram staining characteristics crucial for selecting appropriate antibacterial agents
- Gram-positive bacteria (retain crystal violet dye)
- Gram-negative bacteria (do not retain crystal violet dye)
Pharmacokinetics and Pharmacodynamics of Antibacterial Agents
Pharmacokinetic Processes
- Absorption, distribution, metabolism, and excretion (ADME) vary among drug classes
- Oral antibiotic absorption affected by several factors
- Food intake
- Gastric pH
- Presence of divalent cations (calcium, magnesium)
- Parenteral administration typically ensures complete
- Distribution influenced by drug properties
- Lipophilicity
- Protein binding
- Ability to cross biological barriers (blood-brain barrier, placenta)
- Metabolism primarily occurs in liver through enzymatic processes
- Some drugs undergo first-pass metabolism
- Excretion mainly occurs through renal elimination
- Dose adjustments often required for impaired kidney function
Pharmacodynamic Principles
- Focuses on relationship between drug concentration and antimicrobial effect
- Characterized by time-dependent or concentration-dependent killing
- (MIC) measures lowest concentration inhibiting visible bacterial growth
- (MBC) measures lowest concentration killing 99.9% of bacteria
- MIC and MBC guide dosing strategies and assess antibacterial efficacy
Antibacterial Drug Indications and Adverse Effects
β-lactams and Aminoglycosides
- β-lactams indicated for wide range of bacterial infections
- Contraindicated in patients with penicillin allergies
- Common adverse effects
- (rash, anaphylaxis)
- (diarrhea, nausea)
- Secondary infections (Clostridium difficile)
- Aminoglycosides used for severe
- Contraindicated in patients with renal impairment
- Major adverse effects
- (hearing loss, vestibular dysfunction)
- (acute kidney injury)
Tetracyclines and Macrolides
- Tetracyclines indicated for various infections
- Atypical pneumonia
- Acne
- Contraindicated in pregnancy and young children
- Adverse effects
- (increased sensitivity to sunlight)
- Tooth discoloration in developing teeth
- Macrolides used for respiratory tract infections and atypical pneumonia
- May be contraindicated in patients with certain cardiac conditions
- Common adverse effects
- Gastrointestinal disturbances (nausea, vomiting)
- (risk of arrhythmias)
Fluoroquinolones and Patient Considerations
- Fluoroquinolones have broad spectrum of activity
- Contraindicated in patients with history of tendon disorders
- Adverse effects
- (tendon inflammation or rupture)
- (dizziness, confusion)
- Risk of aortic aneurysm
- Patient factors crucial for selecting appropriate antibacterial agents
- Age
- Pregnancy status
- Comorbidities (renal function, liver disease)
Combination Therapy for Bacterial Infections
Principles and Applications
- Combination therapy uses two or more antibacterial agents simultaneously
- Goals include achieving synergistic effects and preventing resistance development
- Synergism occurs when combined effect greater than sum of individual effects
- Allows for lower doses and reduced toxicity
- Often employed in severe infections
- Sepsis
- Endocarditis
- Infections caused by multidrug-resistant organisms
- Selection considers multiple factors
- Mechanisms of action
- Spectra of activity
- Potential for drug interactions or antagonistic effects
Approaches and Considerations
- Common combination therapy approaches
- Cell wall synthesis inhibitor with protein synthesis inhibitor (β-lactam plus aminoglycoside)
- Drugs targeting different steps in same metabolic pathway (trimethoprim-sulfamethoxazole)
- Risks of combination therapy
- Increased toxicity
- Drug interactions
- Potential for promoting resistance to multiple antibiotics
- Monitoring effectiveness essential for optimizing outcomes
- Clinical response
- Microbiological cultures
- Drug levels (when applicable)
Key Terms to Review (36)
Allergic reactions: Allergic reactions are the body's exaggerated immune response to a substance (allergen) that is usually harmless to most people. This can include a wide range of symptoms, from mild reactions like itching and rashes to severe responses such as anaphylaxis. Understanding these reactions is crucial when considering the use of certain medications and their potential side effects.
Aminoglycosides: Aminoglycosides are a class of antibiotics that are primarily effective against aerobic gram-negative bacteria. They work by inhibiting protein synthesis in bacterial cells, leading to cell death. This mechanism makes them a crucial option for treating serious infections caused by specific resistant bacteria.
Antibiotic stewardship: Antibiotic stewardship refers to a set of coordinated strategies designed to improve the use of antibiotic medications, ensuring that patients receive the right drug, at the right dose, for the right duration, and only when necessary. This practice aims to combat antibiotic resistance, reduce adverse effects, and preserve the effectiveness of existing antibiotics while promoting optimal patient outcomes. A strong antibiotic stewardship program is crucial in healthcare settings to manage antibacterial drugs responsibly.
Beta-lactam mechanism: The beta-lactam mechanism refers to the action of beta-lactam antibiotics, which inhibit bacterial cell wall synthesis by binding to and inactivating specific penicillin-binding proteins (PBPs). This disruption leads to weakened cell walls, causing bacterial lysis and ultimately resulting in the death of susceptible bacteria. Understanding this mechanism is crucial for the development and use of antibacterial drugs in treating infections.
Bioavailability: Bioavailability refers to the proportion of a drug that enters the systemic circulation when introduced into the body and is available for therapeutic effect. It is influenced by factors such as the route of administration, formulation of the drug, and individual patient characteristics, making it a crucial aspect of pharmacology, drug development, and therapeutic effectiveness.
Broad-spectrum: Broad-spectrum refers to antimicrobial agents that are effective against a wide variety of pathogens, including both gram-positive and gram-negative bacteria. These agents play a crucial role in treating infections caused by multiple types of bacteria, making them valuable in scenarios where the specific pathogen is unknown or when polymicrobial infections are present.
Broth microdilution: Broth microdilution is a laboratory technique used to determine the minimum inhibitory concentration (MIC) of antimicrobial agents, particularly antibacterial drugs, against specific microorganisms. This method involves diluting a test compound in a liquid growth medium and inoculating it with bacteria to assess the lowest concentration that prevents visible growth, thus providing crucial information for effective treatment options.
Cell Membrane Disruption: Cell membrane disruption refers to the process by which the integrity of a cell's plasma membrane is compromised, leading to potential cell death or dysfunction. This disruption can occur through various mechanisms, including the action of antibacterial drugs that target bacterial membranes, ultimately affecting their survival and replication. The ability of certain drugs to induce cell membrane disruption is crucial for their effectiveness in treating bacterial infections.
Cephalosporins: Cephalosporins are a class of antibiotics derived from the fungus Acremonium, used to treat a variety of bacterial infections. They are similar to penicillins in their mechanism of action, targeting bacterial cell wall synthesis, making them effective against a broad spectrum of gram-positive and gram-negative bacteria.
Cns effects: CNS effects refer to the impacts that drugs or substances have on the central nervous system, which includes the brain and spinal cord. These effects can manifest in various ways, such as changes in mood, cognition, perception, and motor skills. In the context of antibacterial drugs, understanding CNS effects is crucial, as certain antibiotics may lead to neurological side effects that can influence a patient's overall treatment experience and outcomes.
Discovery of penicillin: The discovery of penicillin marked a groundbreaking moment in medicine, as it was the first true antibiotic that could effectively combat bacterial infections. This discovery revolutionized the field of pharmacology and laid the groundwork for the development of antibacterial drugs, significantly reducing mortality rates from infections that were previously untreatable.
Disk diffusion test: The disk diffusion test, also known as the Kirby-Bauer test, is a method used to determine the sensitivity of bacteria to antibiotics. In this test, antibiotic-impregnated disks are placed on an agar plate inoculated with the bacteria in question, and the effectiveness of the antibiotics is assessed by measuring the zones of inhibition around the disks. This technique is crucial for selecting appropriate antibacterial drugs in clinical settings.
Empirical Therapy: Empirical therapy refers to the initiation of treatment based on clinical experience and the most likely cause of a condition, rather than specific laboratory results or definitive diagnostic tests. This approach is especially relevant in situations where immediate treatment is crucial, such as bacterial infections, where delaying therapy could lead to worsening outcomes for the patient. The selection of empirical therapy usually relies on knowledge of common pathogens and their susceptibility patterns, guiding clinicians in choosing the appropriate antibacterial drugs.
Fluoroquinolones: Fluoroquinolones are a class of broad-spectrum antibiotics that are effective against a wide variety of bacterial infections. They work by inhibiting bacterial DNA synthesis through targeting DNA gyrase and topoisomerase IV, essential enzymes for bacterial replication and survival. Their potency against both gram-negative and gram-positive bacteria makes them a valuable choice in treating various infections.
Gastrointestinal disturbances: Gastrointestinal disturbances refer to a range of symptoms that affect the digestive system, including nausea, vomiting, diarrhea, constipation, and abdominal pain. These disturbances can be caused by various factors, including infections, medications, and underlying health conditions. They are particularly relevant in the context of certain drug treatments, where they may indicate adverse effects or drug interactions.
Glycopeptides: Glycopeptides are a class of antibiotics that are primarily used to treat serious infections caused by Gram-positive bacteria. These antibiotics work by inhibiting cell wall synthesis, specifically targeting the peptide components of the bacterial cell wall, which is crucial for bacterial survival and integrity. Glycopeptides are especially effective against resistant strains of bacteria, such as Methicillin-resistant Staphylococcus aureus (MRSA), making them an essential tool in the fight against antibiotic resistance.
Gram-negative infections: Gram-negative infections are caused by bacteria that have a thin peptidoglycan layer and an outer membrane, which makes them resistant to many antibiotics. This structural difference is significant because it affects how these bacteria react to antibacterial treatments and contributes to their ability to cause severe illness. Gram-negative bacteria are known for their role in serious infections, including bloodstream infections and pneumonia, often leading to complications that require specialized treatment strategies.
Gram-positive infections: Gram-positive infections are caused by bacteria that have a thick peptidoglycan cell wall, which retains the crystal violet stain used in the Gram staining procedure, appearing purple under a microscope. These types of bacteria are often responsible for common infections and can be effectively targeted by specific antibacterial drugs, making the understanding of their characteristics essential for effective treatment and management.
Introduction of Cephalosporins: Cephalosporins are a class of broad-spectrum antibiotics derived from the fungus Acremonium, which are effective against a wide range of bacterial infections. They belong to the beta-lactam family of antibiotics and work by disrupting the synthesis of bacterial cell walls, making them a vital option in treating various infections, especially those resistant to penicillin.
Macrolides: Macrolides are a class of antibiotics that are effective against a variety of bacterial infections by inhibiting protein synthesis in bacteria. They work primarily by binding to the 50S subunit of the bacterial ribosome, thus preventing the growth and replication of bacteria. Macrolides are commonly used to treat respiratory tract infections, skin infections, and certain sexually transmitted diseases.
Metabolic pathway inhibition: Metabolic pathway inhibition refers to the process by which a substance, often a drug or a metabolite, reduces or halts the activity of specific biochemical pathways within an organism. This inhibition can disrupt normal cellular functions, leading to the death of bacteria when antibacterial drugs target essential metabolic processes, making it a crucial strategy in combating bacterial infections.
Minimum bactericidal concentration: Minimum bactericidal concentration (MBC) is the lowest concentration of an antibacterial agent required to kill a particular bacterium. This term is essential in understanding the efficacy of antibacterial drugs, as it helps to determine the dosage needed for effective treatment against specific bacterial infections.
Minimum inhibitory concentration: Minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that prevents the visible growth of a microorganism after a specified period of incubation. Understanding MIC is crucial for determining the effectiveness of antibacterial and antiparasitic drugs, guiding treatment decisions, and ensuring appropriate dosing in antimicrobial therapy.
Narrow-spectrum: Narrow-spectrum refers to a classification of antimicrobial agents that are effective against a limited range of microorganisms, typically targeting specific types of bacteria. This specificity can help minimize the impact on beneficial flora and reduce the likelihood of developing antibiotic resistance, making these agents particularly useful in targeted therapy.
Nephrotoxicity: Nephrotoxicity refers to the harmful effects of substances on the kidneys, which can lead to acute or chronic kidney injury. This condition is significant in understanding how various drugs and compounds, particularly in the context of infections, viral treatments, and immune suppression, can adversely affect renal function. Awareness of nephrotoxicity is crucial for optimizing treatment regimens and minimizing potential kidney damage during therapy.
Nucleic acid synthesis inhibition: Nucleic acid synthesis inhibition refers to the process by which certain drugs interfere with the synthesis of nucleic acids, such as DNA and RNA, ultimately disrupting the ability of bacteria to replicate and produce proteins. This mechanism is crucial in the development of antibacterial drugs, as it targets the essential functions of bacterial cells, leading to their death or stunted growth. By inhibiting nucleic acid synthesis, these drugs effectively combat bacterial infections and contribute to the treatment of various diseases.
Ototoxicity: Ototoxicity refers to the property of certain substances that cause damage to the inner ear or auditory nerve, leading to hearing loss or balance issues. This phenomenon is particularly relevant in the context of specific medications, where their use can result in auditory dysfunction, impacting a patient's quality of life. Ototoxic agents often include certain diuretics and antibacterial drugs, which may lead to irreversible damage if not monitored carefully.
Penicillins: Penicillins are a group of antibiotics that are derived from Penicillium fungi and are primarily used to treat bacterial infections. They work by inhibiting the synthesis of bacterial cell walls, which leads to the death of susceptible bacteria. This class of antibacterial drugs has played a pivotal role in the treatment of infectious diseases since its discovery in the early 20th century.
Photosensitivity: Photosensitivity refers to an increased sensitivity of the skin to sunlight or artificial light, which can lead to adverse reactions such as rashes or burns. This phenomenon is particularly important in the context of certain antibacterial drugs, as some medications can heighten the skin's response to UV radiation, resulting in a greater risk of sunburn and skin damage when exposed to sunlight.
Protein synthesis inhibition: Protein synthesis inhibition refers to the process by which certain drugs or compounds interfere with the production of proteins in bacterial cells. This mechanism is crucial for combating bacterial infections, as proteins are essential for various cellular functions and the survival of bacteria. By disrupting protein synthesis, these inhibitors can effectively halt bacterial growth and replication, making them vital tools in antibacterial therapy.
QT interval prolongation: QT interval prolongation refers to an extended duration of the QT interval on an electrocardiogram (ECG), which represents the time it takes for the heart's electrical system to reset after each heartbeat. This condition can lead to serious heart rhythm disorders, increasing the risk of life-threatening arrhythmias like Torsades de Pointes. Certain antibacterial and antifungal agents can impact cardiac repolarization, contributing to this prolonged QT interval.
Rifamycins: Rifamycins are a class of antibiotics primarily used to treat infections caused by mycobacteria, such as tuberculosis and leprosy. These antibiotics work by inhibiting bacterial RNA synthesis, making them crucial in the treatment of resistant strains of these infections. Rifamycins not only have broad antibacterial properties but also play a vital role in combination therapy to prevent the development of drug resistance.
Sulfonamides: Sulfonamides are a group of synthetic antimicrobial agents that inhibit the growth of bacteria by interfering with their ability to synthesize folic acid. They were among the first antibiotics used in clinical practice and remain important in the treatment of various bacterial infections.
Tendinopathy: Tendinopathy refers to a condition characterized by the degeneration or inflammation of a tendon, often resulting from repetitive strain or overuse. This condition can lead to pain, swelling, and impaired function of the affected tendon, making it significant in understanding musculoskeletal health and injury management.
Tetracyclines: Tetracyclines are a class of broad-spectrum antibiotics that are effective against a wide variety of bacteria. They work by inhibiting protein synthesis in bacterial cells, making them crucial in treating various infections. Due to their effectiveness and the growing issue of antibiotic resistance, tetracyclines are significant in the context of antibacterial drugs, often being used when other antibiotics fail or are not appropriate.
Trimethoprim: Trimethoprim is an antibacterial medication that works primarily by inhibiting bacterial dihydrofolate reductase, which is crucial for the synthesis of folate and subsequently for nucleic acid production. This drug is often used in combination with sulfamethoxazole, enhancing its effectiveness against various bacterial infections, particularly urinary tract infections and certain respiratory infections.