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Chapter 10 10.1 Fundamentals of Antimicrobial Chemotherapy When selecting an antimicrobial drug, key factors include whether it is bacteriostatic or bactericidal, its spectrum of activity, dosage, administration route, potential side effects, and drug interactions. Bacteriostatic vs. Bactericidal: Bacteriostatic drugs inhibit bacterial growth reversibly, while bactericidal drugs kill bacteria. The choice depends on the infection type and the patient’s immune status. Immunocompromised patients and those with life-threatening infections (e.g., acute endocarditis) require bactericidal drugs. Spectrum of Activity: o Narrow-spectrum drugs target specific bacteria (either gram-positive or gram- negative) and are preferred when the pathogen is identified to minimize disruption to normal microbiota. o Broad-spectrum drugs act against a wide range of bacteria and are used for empiric therapy, polymicrobial infections, surgical prophylaxis, or when resistance develops. However, they increase the risk of superinfections , where the normal microbiota is disrupted, allowing resistant pathogens to cause secondary infections. 10.2 Mechanisms of Antibacterial Drugs Selective toxicity is a key feature of antimicrobial drugs, meaning they target harmful microbes while causing little to no harm to the host. Most available antimicrobial drugs are antibacterial because prokaryotic cells have unique structures that make them easier to target compared to fungi, parasites, and viruses. Each class of antibacterial drugs works in a specific way to disrupt microbial growth. Cell Wall: Blocks production of peptidoglycan, inhibiting cell wall biosynthesis. DNA synthesis : Inhibits DNA synthesis, blocking cell replication. RNA Synthesis Blocks transcription. Plasma membrane : Interferes with bacterial cell membrane or LPN in gram negative outer membrane. Ribosomes: Binds to 70s ribosome, blocking protein synthesis. Metabolic pathways : Compete with bacterial metabolic enzymes, stops synthesis of product. Inhibitors of Cell Wall Biosynthesis: Antibacterial drugs that inhibit cell wall biosynthesis weakens bacterial cell walls, making them more vulnerable to osmotic lysis. This makes them bactericidal and a great example of selective toxicity since human cells don’t have peptidoglycan. Common antibiotics in this category include penicillin (e.g., amoxicillin, methicillin), cephalosporins, vancomycin, and bacitracin. While bacitracin can be taken orally or by injection, it is mainly used in topical ointments like Neosporin due to its potential kidney toxicity. Some of these antibiotics are naturally produced by fungi or bacteria, while others are semi-synthetic, modified in labs for improved effectiveness. Inhibitors of Protein Biosynthesis The cytoplasmic ribosomes found in animal cells (80S) are structurally distinct from those found in bacterial cells (70S), making protein biosynthesis a good selective target for antibacterial drugs. Major classes of protein synthesis-inhibiting antibacterials: Chloramphenicol, macrolides, and lincosamides: -Binds to the 50s ribosomal subunit.
-Prevent peptide bond formation. -Stop protein synthesis. Aminoglycosides: -Binds to the 30s ribosomal subunit. -Impair proofreading, resulting n production of faulty proteins. Tetracyclines: -Binds to the 30s ribosomal subunit. -Blocks the binding of tRNAs, thereby inhibiting protein synthesis. Protein Synthesis Inhibitors That Bind the 30S Subunit: Aminoglycosides and tetracyclines are antibacterial drugs that target the 30S ribosomal subunit, disrupting bacterial protein synthesis. Aminoglycosides, such as streptomycin and gentamicin, impair ribosomal proofreading, making them potent broad-spectrum bactericidal drugs, but they can cause kidney, nerve, and ear toxicity. Tetracyclines, on the other hand, are bacteriostatic, blocking tRNA binding during translation. While effective against many bacteria, their side effects include phototoxicity, permanent tooth discoloration, and potential liver toxicity, especially in patients with kidney issues. Protein Synthesis Inhibitors That Bind the 50S Subunit: Antibacterial drugs that bind to the 50S ribosomal subunit inhibit bacterial protein synthesis. Examples include erythromycin, azithromycin, and chloramphenicol. Erythromycin, discovered in 1952, was the first in this category, while azithromycin, with a broader spectrum, fewer side effects, and a longer half-life, allows for shorter treatment durations like the Z-Pak. Chloramphenicol, discovered in 1947, was the first FDA-approved broad-spectrum antibiotic and was widely used due to its effectiveness. However, severe side effects, including gray baby syndrome and bone marrow suppression, have led to its rare use in humans today, though it is still used in veterinary medicine. Inhibitors of Membrane Function: A small group of antibacterial drugs target bacterial membranes, including polymyxins and daptomycin. Polymyxins (B and E/colistin), discovered in 1947, disrupt the outer and inner membranes of gram-negative bacteria but also damage human kidney and nerve cells, limiting their systemic use. Due to these toxic effects, polymyxin B is mainly used in topical ointments like Neosporin, while oral colistin was historically used for bowel decontamination. Daptomycin, produced by Streptomyces roseosporus , works similarly but specifically targets gram-positive bacteria. It is given intravenously and is generally well tolerated, though it can cause reversible muscle toxicity. Inhibitors of Nucleic Acid Synthesis: Some antibacterial drugs inhibit nucleic acid synthesis, such as rifampin and fluoroquinolones. Rifampin, a semisynthetic rifamycin, blocks bacterial RNA polymerase, making it effective against tuberculosis, often used in combination with other drugs. However, it can increase liver enzyme activity, leading to liver toxicity and reducing the effectiveness of other medications. Fluoroquinolones, like ciprofloxacin, kill bacteria by blocking DNA replication. Inhibitors of Metabolic Pathways: Some synthetic antibacterial drugs work as antimetabolites, blocking bacterial metabolic pathways. Sulfonamides (sulfa drugs), the oldest synthetic antibacterials, inhibit folic acid synthesis, which is essential for bacterial DNA production. Since
Drug resistance can occur through several mechanisms. One is drug inactivation, where enzymes chemically modify or destroy an antimicrobial. Another is preventing drug uptake or promoting efflux, where bacteria inhibit drug accumulation or actively pump it out. T arget modification occurs when bacteria alter the structure of the drug’s target, preventing drug binding. Additionally, bacteria may overproduce the target enzyme or develop bypass mechanisms to avoid the drug’s effects. These strategies are common in resistance to antibiotics like penicillins, macrolides, tetracyclines, and others. Multidrug-Resistant Microbes and Cross Resistance : MDRs are colloquially known as “superbugs” and carry one or more resistance mechanism(s), making them resistant to Multiple antimicrobials. In cross-resistance, a single resistance mechanism confers resistance to multiple antimicrobial drugs. For example, having an efflux pump that can export multiple antimicrobial drugs is a common way for microbes to be resistant to multiple drugs by using a single resistance mechanism. Methicillin-Resistant Staphylococcus aureus (MRSA ): emerged soon after methicillin was introduced, developing resistance through a low-affinity penicillin-binding protein (PBP), making it resistant to all β-lactam antibiotics. MRSA is a widespread opportunistic pathogen, causing skin infections, pneumonia, and septicemia. Initially a hospital-acquired infection (HA-MRSA), it has now spread to the community (CA-MRSA). Around one-third of people carry S. aureus in their nasal microbiota, with about 6% harboring methicillin-resistant strains. Multidrug-Resistant Mycobacterium tuberculosis: It is resistant to rifampin and isoniazid, the primary tuberculosis treatments. Extensively drug-resistant M. tuberculosis (XDR-TB) is also resistant to fluoroquinolones and at least one second-line drug (amikacin, kanamycin, or capreomycin), severely limiting treatment options. These resistant strains are especially dangerous for immunocompromised individuals, such as those with HIV. Resistance often arises from the misuse of tuberculosis medications, promoting the selection of resistant bacteria. 10.5 Testing the Effectiveness of Antimicrobials The Kirby-Bauer Disk Diffusion Test: is used to determine bacterial susceptibility to antimicrobial drugs. A Mueller-Hinton agar plate is inoculated with a bacterial sample, and drug- impregnated filter paper disks are placed on the surface. As the bacteria grow, the drug diffuses into the agar, creating a zone of inhibition around the disk if the bacteria are susceptible. The diameter of this zone is measured and compared to a standardized chart to determine resistance or susceptibility. However, this test has limitations, as it does not differentiate between bacteriostatic and bactericidal effects or compare drug potencies due to variability in factors like drug diffusion and agar thickness. Dilution Tests: the limitations of the Kirby Bauer disk diffusion test do not allow for a direct comparison of antibacterial potencies to guide selection of the best therapeutic choice. Dilution tests help determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of an antimicrobial drug. The MIC is the lowest drug concentration that inhibits visible bacterial growth, while the MBC is the lowest concentration that kills ≥99.9% of the bacterial inoculum. In a microbroth dilution assay, a drug is serially diluted in test tubes with
bacterial cultures, and the MIC is identified by the lowest concentration that prevents turbidity (cloudiness). To determine the MBC, bacteria from non-turbid tubes are plated on antibiotic-free agar to check for growth. MIC tests can also be conducted in 96-well microdilution trays, allowing for automated testing of multiple drugs and bacteria using visual or spectrophotometric detection methods. 10.6 Current Strategies for Antimicrobial Discovery With the rise of antimicrobial resistance, new strategies for antimicrobial discovery are essential. One approach is developing more effective semisynthetic derivatives , though resistance to them evolves quickly. Scientists use high-throughput screening to test large numbers of soil and microbial products for antimicrobial activity. While soil has been widely studied, marine environments remain an untapped resource for new antimicrobial compounds. Combinatorial chemistry is also used to synthesize and test large sets of related compounds. Other promising strategies include developing inhibitors that block resistance mechanisms, restoring the effectiveness of older drugs, and targeting virulence factors to slow infections and support the immune system.
