Premed · Premed · Microbiology
Lecture 21: Antimicrobial Agents and Mechanisms
Microbiology
Learning Objectives
By the end of this lecture, students will be able to:
- Define selective toxicity and therapeutic index as they relate to antimicrobial agents
- Classify antimicrobial agents by their mechanism of action
- Distinguish between bactericidal and bacteriostatic drugs and explain when each is preferred
- Describe the five major targets of antibacterial agents: cell wall, protein synthesis, nucleic acid synthesis, metabolic pathways, and cell membrane
- Explain the mechanisms of action of major antifungal, antiviral, and antiparasitic drugs
- Describe methods for determining antimicrobial susceptibility (MIC, disk diffusion)
Lecture Content
I. Fundamental Concepts
An antimicrobial agent is any substance that kills or inhibits the growth of microorganisms. The term antibiotic was originally defined by Waksman as a naturally produced antimicrobial substance, but it is now used broadly to include synthetic and semisynthetic agents as well. The central principle underlying antimicrobial therapy is selective toxicity, which means that the drug harms the microbe but causes no harm, or only minimal harm, to the host. This selectivity is based on exploiting structural and metabolic differences between the microbe and the host. The therapeutic index, calculated as the ratio of the toxic dose to the therapeutic dose, provides a quantitative measure of drug safety: the higher the therapeutic index, the safer the drug.
Antimicrobial agents can be classified by their effect on bacteria. Bactericidal agents kill bacteria outright, and examples include the penicillins, fluoroquinolones, and aminoglycosides. Bacteriostatic agents, by contrast, merely inhibit bacterial growth and rely on the host immune system to clear the infection; examples include tetracyclines, chloramphenicol, and macrolides at typical doses. It is important to recognize that some drugs can be bactericidal at high concentrations but only bacteriostatic at low concentrations. Bactericidal agents are preferred in immunocompromised patients and in infections such as endocarditis and meningitis where the host immune response alone cannot be relied upon.
The spectrum of activity describes the range of organisms affected by a given agent. Narrow-spectrum agents are effective against a limited range of organisms, as exemplified by isoniazid, which targets only Mycobacterium species. Broad-spectrum agents are effective against many Gram-positive and Gram-negative species, as seen with carbapenems and tetracyclines. However, broad-spectrum agents increase the risk of superinfection and dysbiosis, including Clostridioides difficile colitis.
II. Targets of Antibacterial Agents
A. Cell Wall Synthesis Inhibitors
Cell wall synthesis inhibitors exploit the fact that mammalian cells lack a cell wall entirely, providing excellent selective toxicity. The beta-lactam antibiotics represent the largest and most clinically important group within this category, and they include the penicillins, cephalosporins, carbapenems, and monobactams. These drugs all contain a beta-lactam ring that mimics the D-Ala-D-Ala terminus of peptidoglycan precursors. By binding and inhibiting penicillin-binding proteins (PBPs), which are the transpeptidases responsible for catalyzing cross-linking of peptidoglycan, beta-lactams weaken the cell wall and ultimately lead to osmotic lysis, making them bactericidal.
Among the penicillins, penicillin G and penicillin V are narrow-spectrum agents primarily active against Gram-positive organisms. The aminopenicillins ampicillin and amoxicillin have broader coverage, while piperacillin extends activity to include Pseudomonas aeruginosa. The cephalosporins are organized by generation: first-generation agents such as cefazolin provide good Gram-positive coverage; third-generation agents such as ceftriaxone offer enhanced Gram-negative activity along with CNS penetration; and fifth-generation agents such as ceftaroline even possess activity against MRSA. The carbapenems, including imipenem and meropenem, represent the broadest-spectrum beta-lactams and are reserved for serious multidrug-resistant infections. Monobactams such as aztreonam target Gram-negative organisms exclusively and are safe to use in penicillin-allergic patients because of their different ring structure.
Because many bacteria produce beta-lactamase enzymes that destroy the beta-lactam ring, beta-lactamase inhibitors such as clavulanic acid, sulbactam, tazobactam, and avibactam are combined with beta-lactam antibiotics to overcome this enzymatic resistance. The combination of amoxicillin with clavulanate is a familiar example.
The glycopeptides vancomycin and teicoplanin work by a different mechanism: they bind directly to the D-Ala-D-Ala terminus of peptidoglycan precursors, physically blocking both transglycosylation and transpeptidation. Because these molecules are too large to penetrate the Gram-negative outer membrane, they are used only for Gram-positive infections, most notably MRSA and C. difficile. Vancomycin is bactericidal and typically administered intravenously because of poor oral absorption, although oral vancomycin is used for C. difficile because it acts locally within the gut lumen.
Several additional agents target cell wall synthesis through other mechanisms. Bacitracin inhibits the recycling of bactoprenol, the lipid carrier for peptidoglycan precursors, but its nephrotoxicity limits it to topical use. Fosfomycin inhibits MurA, the enzyme catalyzing the very first step of peptidoglycan synthesis, and is used for uncomplicated urinary tract infections. Cycloserine inhibits D-Ala-D-Ala ligase and serves as a second-line agent for multidrug-resistant tuberculosis. Finally, daptomycin is a lipopeptide that inserts into the Gram-positive cell membrane, causing rapid depolarization and bactericidal activity; it is used to treat MRSA and VRE bacteremia.
B. Protein Synthesis Inhibitors
The selectivity of protein synthesis inhibitors rests on the structural differences between bacterial ribosomes (70S, composed of 30S and 50S subunits) and mammalian ribosomes (80S, composed of 40S and 60S subunits). Drugs targeting the 30S subunit include the aminoglycosides and the tetracyclines. The aminoglycosides, such as gentamicin, tobramycin, amikacin, and streptomycin, bind to the 16S rRNA of the 30S subunit, causing misreading of mRNA and inhibiting translocation. They are bactericidal with concentration-dependent killing but carry risks of nephrotoxicity and ototoxicity. The tetracyclines, including doxycycline and minocycline, along with the glycylcycline tigecycline, block aminoacyl-tRNA binding to the ribosomal A site. They are bacteriostatic and broad-spectrum but can cause photosensitivity and teeth discoloration in children.
Among the 50S subunit inhibitors, the macrolides erythromycin, azithromycin, and clarithromycin bind to 23S rRNA and block translocation. They are primarily bacteriostatic but can be bactericidal at high concentrations, and they are widely used for respiratory infections and atypical organisms. Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase; it is bacteriostatic and carries a rare but serious risk of aplastic anemia, which has limited its use in developed countries. Lincosamides such as clindamycin also bind the 50S subunit and block translocation, providing excellent anaerobic coverage, though they carry a notable risk of C. difficile colitis. The oxazolidinones linezolid and tedizolid bind 23S rRNA and prevent formation of the 70S initiation complex. They are bacteriostatic but active against MRSA and VRE; prolonged use can cause myelosuppression. The streptogramins quinupristin and dalfopristin act as a synergistic pair that binds the 50S subunit, achieving bactericidal activity against vancomycin-resistant E. faecium.
C. Nucleic Acid Synthesis Inhibitors
The fluoroquinolones, including ciprofloxacin, levofloxacin, and moxifloxacin, inhibit the bacterial enzymes DNA gyrase (topoisomerase II) and topoisomerase IV. By preventing DNA supercoiling and decatenation, these drugs induce double-strand breaks and cell death, making them bactericidal. They are broad-spectrum with good oral bioavailability but carry adverse effects including tendinopathy, QT prolongation, CNS effects, and an increased risk of aortic dissection.
The rifamycins, including rifampin and rifabutin, inhibit bacterial DNA-dependent RNA polymerase at its beta subunit, blocking transcription. They are bactericidal and form the cornerstone of tuberculosis therapy. Because rifampin is a potent inducer of cytochrome P450, it causes many drug interactions. Moreover, resistance develops rapidly when rifampin is used alone, so it is always prescribed in combination.
Metronidazole is a prodrug that is activated through anaerobic reduction to form reactive intermediates that damage DNA. It is bactericidal against anaerobes and microaerophiles, including Bacteroides, Clostridium, Helicobacter pylori, Giardia, and Entamoeba. Because aerobes cannot sufficiently reduce the drug, metronidazole has no activity against aerobic organisms.
D. Folate Pathway Inhibitors (Antimetabolites)
The folate pathway is an attractive target because bacteria must synthesize folate de novo, whereas humans obtain it from the diet, providing inherent selective toxicity. Sulfonamides such as sulfamethoxazole are structural analogs of para-aminobenzoic acid (PABA) that competitively inhibit dihydropteroate synthase (DHPS). Trimethoprim inhibits dihydrofolate reductase (DHFR), selectively binding the bacterial enzyme over its mammalian counterpart. When these two agents are combined as TMP-SMX (co-trimoxazole), they produce synergistic killing through sequential blockade of the folate pathway. This combination is used for urinary tract infections, Pneumocystis jirovecii pneumonia, and MRSA skin infections.
E. Cell Membrane Disruptors
The polymyxins, including colistin (polymyxin E) and polymyxin B, are cationic peptides that bind lipopolysaccharide and disrupt the Gram-negative outer membrane, leading to cell lysis. They serve as last-resort agents for carbapenem-resistant Gram-negative infections caused by organisms such as Acinetobacter, Pseudomonas, and carbapenem-resistant Enterobacterales, but their use is limited by nephrotoxicity and neurotoxicity. Daptomycin, as noted above, inserts into the Gram-positive cell membrane and causes depolarization.
<image>A bacterial cell diagram showing the five major targets of antibacterial agents. The cell wall layer is highlighted with labels for beta-lactams (targeting PBPs/transpeptidases), glycopeptides (binding D-Ala-D-Ala), and bacitracin (bactoprenol recycling). Inside the cell, the 70S ribosome is shown with the 30S subunit (aminoglycosides, tetracyclines) and 50S subunit (macrolides, chloramphenicol, linezolid, clindamycin) labeled. The DNA region shows fluoroquinolones targeting DNA gyrase/topoisomerase IV and rifampin targeting RNA polymerase. The metabolic pathway section shows the folate synthesis pathway with sulfonamides inhibiting DHPS and trimethoprim inhibiting DHFR. The cell membrane is labeled with polymyxins (Gram-negative) and daptomycin (Gram-positive). Each drug class is color-coded with bactericidal drugs in red and bacteriostatic drugs in blue.</image>
III. Antifungal Agents
Treating fungal infections is inherently more challenging than treating bacterial infections because fungal cells are eukaryotic, leaving fewer unique targets available and raising the potential for host toxicity. Most antifungal strategies target ergosterol, the principal sterol in the fungal cell membrane that is absent from mammalian cells.
The polyene antifungals, amphotericin B and nystatin, bind directly to ergosterol and form membrane pores, causing cell lysis. Amphotericin B provides broad-spectrum coverage but is limited by nephrotoxicity, which lipid formulations help to reduce. The azole antifungals, including fluconazole, itraconazole, voriconazole, and posaconazole, inhibit lanosterol 14-alpha-demethylase (CYP51), blocking ergosterol synthesis. They are generally fungistatic, though some species are killed at achievable concentrations. Because azoles also inhibit human cytochrome P450 enzymes, they cause many drug interactions. The allylamines, particularly terbinafine, inhibit squalene epoxidase and block an early step in ergosterol biosynthesis; they are used mainly for dermatophyte infections.
A separate category of antifungals targets the fungal cell wall. The echinocandins, including caspofungin, micafungin, and anidulafungin, inhibit beta-1,3-glucan synthase, disrupting cell wall integrity. Because mammalian cells lack beta-glucan, these drugs offer excellent selective toxicity. They are fungicidal against Candida and fungistatic against Aspergillus.
For nucleic acid synthesis, flucytosine (5-FC) is converted within fungal cells to 5-fluorouracil, which inhibits thymidylate synthase and incorporates into RNA. It is used in combination with amphotericin B for Cryptococcus meningitis. Additionally, griseofulvin disrupts microtubule function and is used for dermatophyte infections.
IV. Antiviral Agents
Antiviral drug development is challenging because viruses use host cell machinery for replication, leaving limited virus-specific targets. Most antivirals therefore target virus-encoded enzymes. The nucleos(t)ide analogs represent the largest and most important class. Acyclovir is a guanosine analog that is activated by viral thymidine kinase in cells infected with HSV or VZV. Once activated, it is incorporated into viral DNA by the viral DNA polymerase, causing chain termination. Ganciclovir is activated by CMV UL97 kinase and is used to treat CMV infections. Tenofovir and emtricitabine are nucleotide and nucleoside reverse transcriptase inhibitors used for both HIV and HBV. Sofosbuvir inhibits the NS5B RNA polymerase and forms the backbone of curative HCV regimens. Remdesivir, an adenosine analog, inhibits viral RNA-dependent RNA polymerase and was used for SARS-CoV-2.
The non-nucleoside reverse transcriptase inhibitors (NNRTIs), such as efavirenz and rilpivirine, bind an allosteric site on HIV-1 reverse transcriptase. Protease inhibitors include ritonavir and darunavir for HIV, nirmatrelvir for the SARS-CoV-2 main protease, and direct-acting antivirals like simeprevir and glecaprevir for HCV. The integrase strand transfer inhibitors (INSTIs), dolutegravir and bictegravir, block HIV integrase to prevent proviral DNA integration and form the backbone of modern HIV regimens. Neuraminidase inhibitors such as oseltamivir and zanamivir block influenza neuraminidase to prevent viral release from infected cells. Entry and fusion inhibitors include enfuvirtide (which blocks HIV fusion), maraviroc (a CCR5 antagonist for HIV), and baloxavir (a cap-dependent endonuclease inhibitor for influenza). Pegylated interferon-alpha was historically used for HBV and HCV, inducing a broad antiviral state.
<image>A diagram showing antiviral drug targets mapped onto the viral replication cycle. A host cell is depicted with a virus attaching and entering. Step 1 (Entry): entry/fusion inhibitors (enfuvirtide, maraviroc) blocking viral attachment and fusion. Step 2 (Uncoating): amantadine blocking influenza M2 ion channel (historical). Step 3 (Genome replication): nucleoside analogs (acyclovir for HSV DNA polymerase, tenofovir/emtricitabine for HIV reverse transcriptase, sofosbuvir for HCV NS5B, remdesivir for SARS-CoV-2 RdRp); NNRTIs binding HIV RT allosteric site. Step 4 (Integration -- HIV specific): integrase inhibitors (dolutegravir) blocking proviral DNA insertion. Step 5 (Protein processing): protease inhibitors (darunavir for HIV protease, nirmatrelvir for SARS-CoV-2 Mpro) blocking polyprotein cleavage. Step 6 (Assembly and release): neuraminidase inhibitors (oseltamivir) preventing influenza release. Each step is numbered and color-coded with drug names listed alongside.</image>
V. Antiparasitic Agents
Antimalarial therapy is dictated by the species of Plasmodium and patterns of resistance. Chloroquine accumulates in the Plasmodium digestive vacuole and inhibits heme detoxification, but resistance through PfCRT mutations is now widespread. Artemisinin-based combination therapies (ACTs), such as artemether-lumefantrine, are the current standard of care; artemisinins generate free radicals that damage parasite proteins. Atovaquone-proguanil (Malarone) inhibits mitochondrial electron transport at complex III and is used for both prophylaxis and treatment.
Among antiprotozoal agents, metronidazole is effective against Giardia, Entamoeba, and Trichomonas. Nitazoxanide provides broad-spectrum activity against Cryptosporidium and Giardia. Pentamidine and suramin are used for trypanosomiasis, while amphotericin B and miltefosine treat visceral leishmaniasis.
The antihelminthic agents include albendazole and mebendazole, which inhibit beta-tubulin polymerization and serve as broad-spectrum antihelminthics. Ivermectin binds glutamate-gated chloride channels to cause paralysis and is used for Strongyloides, Onchocerca, and ectoparasites. Praziquantel increases membrane permeability to calcium, causing paralysis and tegumental damage; it is the drug of choice for schistosomiasis and cestode infections.
VI. Antimicrobial Susceptibility Testing
The minimum inhibitory concentration (MIC) is defined as the lowest concentration of a drug that inhibits visible bacterial growth after overnight incubation. It can be determined by broth microdilution (the gold standard), Etest gradient strips, or automated systems such as Vitek and MicroScan. The minimum bactericidal concentration (MBC) is the lowest concentration that kills 99.9% of bacteria; an MBC-to-MIC ratio greater than 4 suggests tolerance.
The disk diffusion method, also known as the Kirby-Bauer method, involves placing antibiotic-impregnated disks on an inoculated agar plate and measuring the zone of inhibition diameter after incubation. These diameters are compared to CLSI or EUCAST breakpoints and classified as susceptible, intermediate, or resistant. Automated susceptibility testing systems provide rapid results within 6 to 12 hours and can be integrated with clinical decision support tools. At the institutional level, cumulative susceptibility data are compiled into an antibiogram, which guides empiric therapy choices before culture and sensitivity results become available.
<image>A two-panel figure on antimicrobial susceptibility testing. Panel A: Kirby-Bauer disk diffusion -- a Mueller-Hinton agar plate inoculated with a bacterial lawn; multiple antibiotic disks placed on the surface; after incubation, clear zones of inhibition are visible around most disks with one disk showing no zone (resistant); a ruler measuring the zone diameter; a table comparing the diameter to CLSI breakpoints to classify as S, I, or R. Panel B: Broth microdilution -- a 96-well microplate with serial two-fold dilutions of antibiotic across the rows; wells with bacterial growth appear turbid while wells at and above the MIC are clear; the MIC is identified as the first clear well; an Etest strip on agar showing an elliptical zone of inhibition with the MIC read where the zone intersects the strip.</image>
