7.2 Antibiotic, Antiviral/Anti–COVID-19, and Antifungal Drugs

Overview of Anti-infective Medications

Anti-infective drugs target the three major categories of pathogens: bacteria, viruses, and fungi. Each drug class exploits specific vulnerabilities in how these organisms survive and replicate. For nurses, knowing the differences between these classes matters because it shapes everything from patient education to monitoring for adverse effects.

Antibiotics

Antibiotics treat bacterial infections in one of two ways:

• Bactericidal agents directly kill bacteria (e.g., penicillins, aminoglycosides)
• Bacteriostatic agents slow or stop bacterial growth, letting the immune system finish the job (e.g., tetracyclines, macrolides)

Antibiotics are also classified by how many bacterial species they cover:

• Broad-spectrum antibiotics target a wide range of gram-positive and gram-negative bacteria. They're useful when the causative organism is unknown, but they carry a higher risk of disrupting normal flora and causing superinfection.
• Narrow-spectrum antibiotics target a limited group of bacteria. They're preferred when culture and sensitivity results identify the specific pathogen, because they cause less collateral damage to normal flora.

Common antibiotic classes you should know:

Antivirals

Antivirals don't "kill" viruses the way antibiotics kill bacteria. Instead, they interfere with specific steps in the viral life cycle, such as entry into host cells, replication of viral genetic material, or release of new viral particles.

Key antivirals to know:

• Oseltamivir (Tamiflu) inhibits neuraminidase in influenza, preventing viral release from infected cells. Most effective when started within 48 hours of symptom onset.
• Acyclovir targets herpes simplex and varicella-zoster viruses by inhibiting viral DNA polymerase.
• Remdesivir is a nucleotide analog used for COVID-19 that interferes with viral RNA replication. It's given IV, typically in hospitalized patients.
• Antiretrovirals for HIV (e.g., tenofovir, dolutegravir) target different stages of the HIV life cycle and are used in combination therapy to prevent resistance.

Antifungals

Antifungal drugs can be fungicidal (kill fungi) or fungistatic (inhibit fungal growth), depending on the drug and the dose. Fungal infections range from superficial (athlete's foot) to life-threatening systemic infections (invasive aspergillosis).

• Fluconazole inhibits ergosterol synthesis in the fungal cell membrane. Commonly used for vaginal yeast infections and oral thrush.
• Itraconazole covers a broader range of fungi, including Aspergillus species.
• Amphotericin B binds directly to ergosterol in fungal cell membranes, creating pores that cause cell death. It's reserved for serious systemic infections because of significant toxicity (especially nephrotoxicity). Sometimes called "ampho-terrible" for its side effect profile.
• Terbinafine inhibits a different enzyme in ergosterol synthesis and is used primarily for dermatophyte infections like onychomycosis (nail fungus) and tinea pedis (athlete's foot).

Antibiotic Resistance and Mechanisms of Action

How Antibiotic Resistance Develops

Antibiotic resistance happens when bacteria survive exposure to an antibiotic and pass that survival advantage to future generations. This occurs through two main routes:

1. Spontaneous mutations in bacterial DNA that happen to confer resistance (e.g., altering the drug's target site)
2. Horizontal gene transfer, where bacteria acquire resistance genes from other bacteria via plasmids, transposons, or other mobile genetic elements

Every time antibiotics are used, susceptible bacteria are killed while resistant ones survive and multiply. This is natural selection in action. The more antibiotics are used (and especially misused), the faster resistance spreads.

Why this matters clinically:

• Resistant infections mean longer hospital stays, more expensive treatments, and higher mortality
• MRSA, VRE, and multidrug-resistant gram-negative bacteria are already common in healthcare settings
• The CDC considers antimicrobial resistance one of the most urgent public health threats globally

Mechanisms of Action for Anti-infective Drugs

Understanding how these drugs work helps you anticipate which infections they treat and what side effects to watch for.

Cell wall synthesis inhibitors (penicillins, cephalosporins, vancomycin) block enzymes bacteria need to build their cell walls. Without an intact wall, the bacterium swells and lyses. Human cells lack cell walls, which is why these drugs tend to have a favorable safety profile.

Protein synthesis inhibitors bind to bacterial ribosomes (which differ from human ribosomes) and block protein production:

• Aminoglycosides and tetracyclines bind the 30S ribosomal subunit
• Macrolides bind the 50S ribosomal subunit

Nucleic acid synthesis inhibitors prevent bacteria or viruses from copying their DNA or RNA. Fluoroquinolones (e.g., ciprofloxacin) inhibit bacterial DNA gyrase and topoisomerase IV. Many antivirals also fall into this category by targeting viral polymerases.

Antimetabolites (sulfonamides, trimethoprim) block folic acid synthesis. Bacteria must make their own folic acid, while humans get it from food. This selective toxicity makes these drugs relatively safe for human cells. Sulfamethoxazole and trimethoprim are often combined (Bactrim/Septra) because they block two sequential steps in the folic acid pathway, producing a synergistic effect.

Cell membrane disruptors (polymyxins, amphotericin B) compromise the integrity of cell membranes. Polymyxins target bacterial membranes; amphotericin B targets ergosterol in fungal membranes. These drugs tend to have more toxicity because human cell membranes share some structural similarities.

Pharmacokinetics and Pharmacodynamics

Pharmacokinetics (what the body does to the drug) covers four processes: absorption, distribution, metabolism, and excretion. For anti-infectives, a few PK points are especially relevant:

• Absorption: Some antibiotics must be taken on an empty stomach (e.g., certain penicillins), while others absorb better with food (e.g., itraconazole capsules). Tetracyclines and fluoroquinolones chelate with calcium, iron, and other multivalent cations, which drastically reduces absorption.
• Distribution: The drug must reach the site of infection at adequate concentrations. Not all antibiotics cross the blood-brain barrier well, which matters for treating meningitis (ceftriaxone does; many others don't).
• Metabolism: Many anti-infectives are metabolized by CYP450 liver enzymes, creating potential for drug interactions.
• Excretion: Aminoglycosides are excreted renally, so renal function must be monitored. Dose adjustments are often needed in patients with kidney impairment.

Pharmacodynamics (what the drug does to the body) describes the drug's mechanism and its relationship between concentration and effect. Two dosing concepts matter here:

• Concentration-dependent killing (aminoglycosides, fluoroquinolones): Higher peak concentrations produce greater bacterial killing. These drugs are often given in larger, less frequent doses.
• Time-dependent killing (penicillins, cephalosporins): Effectiveness depends on how long the drug concentration stays above the minimum inhibitory concentration (MIC). These drugs work best with more frequent dosing or continuous infusions.

Side Effects, Drug Interactions, and Patient Education

Side Effects of Anti-infectives

GI distress is the most common side effect across nearly all anti-infective classes. Nausea, vomiting, and diarrhea occur because these drugs disrupt normal gut flora. Clostridioides difficile (C. diff) infection is a serious complication, particularly with broad-spectrum antibiotics like clindamycin, fluoroquinolones, and cephalosporins.

Allergic reactions range from mild rash to life-threatening anaphylaxis. Penicillin allergy is the most commonly reported drug allergy. Patients with a true penicillin allergy may also cross-react with cephalosporins (risk is low, roughly 1-2%, but higher with first-generation cephalosporins).

Organ-specific toxicities to monitor:

• Nephrotoxicity: Aminoglycosides, amphotericin B, vancomycin. Monitor BUN, creatinine, and urine output. Trough levels are drawn for aminoglycosides and vancomycin to keep concentrations in a safe range.
• Ototoxicity: Aminoglycosides can damage the vestibulocochlear nerve, causing hearing loss or balance problems. Assess for tinnitus, dizziness, or hearing changes.
• Hepatotoxicity: Azole antifungals (especially ketoconazole), isoniazid, and some macrolides. Monitor liver function tests.
• QT prolongation: Fluoroquinolones, macrolides, and some azole antifungals can prolong the QT interval, increasing the risk of dangerous arrhythmias. Avoid combining multiple QT-prolonging drugs.

Superinfection occurs when anti-infectives wipe out normal protective flora, allowing resistant organisms (like C. diff or Candida) to overgrow. Watch for new-onset diarrhea, oral thrush, or vaginal yeast infections during antibiotic therapy.

Key Drug Interactions

• CYP450 interactions: Macrolides (especially erythromycin) and azole antifungals are CYP450 inhibitors. They can raise levels of warfarin, statins, cyclosporine, and many other drugs to dangerous concentrations.
• Chelation: Tetracyclines and fluoroquinolones bind to calcium, magnesium, aluminum, and iron. Patients should separate these antibiotics from antacids, dairy products, and multivitamins by at least 2 hours.
• Methotrexate and trimethoprim: Both are folate antagonists. Using them together increases the risk of bone marrow suppression.

Patient Education for Anti-infectives

1. Complete the full course. Even if you feel better after a few days, stopping early allows surviving bacteria to repopulate and potentially develop resistance.
2. Take as directed. Follow the prescribed dose, frequency, and timing (with or without food). Set reminders if needed.
3. Report side effects promptly. Rash, difficulty breathing, severe diarrhea (especially watery or bloody), or signs of hearing changes all warrant contacting the provider.
4. Never share antibiotics or use leftovers. The wrong antibiotic for the wrong infection won't help and can promote resistance.
5. Disclose all medications. This includes OTC drugs, supplements, and herbal products, since drug interactions with anti-infectives are common.
6. Practice infection prevention. Hand hygiene, staying current on vaccinations, safe food handling, and avoiding close contact with sick individuals all reduce the need for anti-infectives in the first place.