14.2 Fundamentals of Antimicrobial Chemotherapy

Antimicrobial Drug Spectrum and Effects

Antimicrobial drugs are the primary tools for treating bacterial infections. They differ in how they act on bacteria, which bacteria they target, and what consequences they carry for the patient's own microbiota. Getting these distinctions right is essential for understanding why certain drugs are chosen over others in clinical settings.

Bacteriostatic vs. Bactericidal Antibiotics

These two categories describe what the drug does to bacteria, and the distinction matters for treatment decisions.

Bacteriostatic antibiotics suppress bacterial growth and reproduction without directly killing the cells. They hold the population in check so the host's immune system can finish the job. Examples include tetracyclines, sulfonamides, and macrolides. These are commonly used for less severe infections or in patients with a competent immune system.

Bactericidal antibiotics directly kill bacteria by disrupting essential cellular processes like cell wall synthesis, protein synthesis, or DNA replication. Examples include beta-lactams, aminoglycosides, and fluoroquinolones. These are preferred for severe infections or in immunocompromised patients who can't rely on their own immune defenses.

Broad-Spectrum vs. Narrow-Spectrum Antimicrobials

• Broad-spectrum antimicrobials target a wide range of both Gram-positive and Gram-negative bacteria (e.g., amoxicillin, ciprofloxacin). They're used empirically when the specific pathogen hasn't been identified yet. The tradeoff: they're more likely to disrupt normal microbiota and promote resistance development.
• Narrow-spectrum antimicrobials target a limited range of bacteria or specific species (e.g., penicillin G, clindamycin). They're preferred when the pathogen is known because they cause less collateral damage to normal flora. The risk is that they may be ineffective if the pathogen is misidentified or if multiple pathogens are involved.

The general principle: use the narrowest spectrum that will effectively treat the infection. This minimizes disruption to the microbiome and slows resistance development.

Superinfections in Antimicrobial Therapy

A superinfection is a new, secondary infection that develops during or after treatment of the original infection. It happens because antimicrobial therapy disrupts the normal microbiota, creating an opening for opportunistic pathogens to overgrow.

Two of the most clinically significant examples:

• Clostridioides difficile colonizes the gut after antibiotics wipe out competing normal flora, causing severe diarrhea and pseudomembranous colitis.
• Candida species (yeast) can overgrow in the mouth, GI tract, or vagina when bacterial competitors are eliminated.

Risk factors include prolonged antibiotic use, broad-spectrum therapy, and immunosuppression. Superinfections are particularly dangerous because the opportunistic organisms involved are often resistant to the original antibiotic, making them harder to treat. They also contribute to further selective pressure on bacterial populations, driving additional resistance.

Antimicrobial Drug Administration and Interactions

Dosage and Administration Route

Getting the dose right is a balancing act. Too little drug leads to treatment failure and can promote resistance (sub-therapeutic concentrations allow partially resistant bacteria to survive and multiply). Too much increases the risk of toxicity and adverse effects.

The minimum inhibitory concentration (MIC) is the lowest concentration of a drug that prevents visible bacterial growth. It's a key lab value used to guide dosing decisions. Doses need to be adjusted based on patient factors like age, weight, and renal function (since the kidneys clear many antibiotics).

Administration route directly affects how quickly and reliably the drug reaches effective concentrations:

• Oral: Most convenient, but bioavailability can be lower and onset is slower (e.g., amoxicillin)
• Intravenous (IV): Rapid onset and 100% bioavailability, but requires medical supervision and carries infection risk at the injection site (e.g., vancomycin)
• Intramuscular (IM): Intermediate onset and bioavailability, useful when oral isn't an option (e.g., ceftriaxone)

Factors Influencing Drug Side Effects

Side effects depend on both patient-specific and drug-related factors:

• Patient factors: Age, comorbidities, genetic variations in drug metabolism (pharmacogenomics), and concurrent medications that might interact
• Drug factors: Dose, frequency, and duration of therapy all influence the likelihood and severity of adverse effects

Strategies to reduce side effects:

1. Adjust dose or dosing interval to minimize peak drug levels and toxicity
2. Switch to an alternative drug with a better safety profile when appropriate
3. Monitor for early signs of adverse effects (e.g., checking kidney function during aminoglycoside therapy)
4. Provide supportive care and manage side effects as they arise
5. Educate patients about what to watch for and when to seek help

Drug Interactions and Clinical Impact

When two drugs are given together, they can interact in ways that help or hurt treatment.

Positive interactions:

• Synergy occurs when two drugs together produce a greater effect than either alone. A classic example: aminoglycosides combined with beta-lactams. The beta-lactam damages the cell wall, allowing the aminoglycoside better access to its ribosomal target inside the cell.
• Reduced toxicity: One drug can offset the harmful effects of another (e.g., leucovorin rescue with high-dose methotrexate).

Negative interactions:

• Antagonism occurs when one drug reduces the effectiveness of another. The bacteriostatic + bactericidal combination mentioned earlier is the textbook example.
• Increased toxicity: One drug can worsen the side effects of another. For instance, combining macrolides with fluoroquinolones can increase the risk of QT prolongation (a dangerous heart rhythm change).

Careful evaluation of potential interactions is critical when designing treatment plans, especially for patients on multiple medications.

Pharmacokinetics and Pharmacodynamics in Antimicrobial Therapy

Pharmacokinetics (PK)

Pharmacokinetics describes how the body processes a drug. Think of it as what the body does to the drug. It covers four stages, often abbreviated as ADME:

1. Absorption: Drug enters the bloodstream (route-dependent)
2. Distribution: Drug travels to tissues and the infection site
3. Metabolism: Drug is chemically modified, primarily in the liver
4. Excretion: Drug is eliminated, primarily through the kidneys

PK determines the drug concentration that actually reaches the site of infection, which directly affects whether the drug will work.

Pharmacodynamics (PD)

Pharmacodynamics describes the relationship between drug concentration and its effect on the target organism. Think of it as what the drug does to the microbe. PD parameters help determine optimal dosing strategies:

• Time-dependent killing (e.g., beta-lactams): Efficacy depends on how long the drug concentration stays above the MIC. Frequent dosing or continuous infusion works best.
• Concentration-dependent killing (e.g., aminoglycosides): Efficacy depends on how high the peak concentration is relative to the MIC. Higher, less frequent doses work best.

Understanding both PK and PD together is how clinicians design dosing regimens that maximize bacterial killing while minimizing toxicity.

Antimicrobial Stewardship

Antimicrobial stewardship refers to coordinated efforts to promote responsible use of antimicrobials. The goal is to preserve drug effectiveness and slow resistance development. Core principles include:

• Selecting the most appropriate agent (narrowest effective spectrum)
• Optimizing dose and route based on PK/PD data
• Minimizing treatment duration to what's clinically necessary
• De-escalating from broad-spectrum to narrow-spectrum therapy once culture results are available

Biofilm Considerations

Biofilms are structured communities of bacteria encased in a self-produced matrix of polysaccharides, proteins, and DNA. They form on surfaces like medical devices (catheters, prosthetic joints) and damaged tissue.

Biofilms pose a serious challenge to antimicrobial therapy for several reasons:

• The matrix acts as a physical barrier that prevents drugs from penetrating to the bacteria inside
• Bacteria within biofilms have altered metabolic states (many are slow-growing or dormant), making them less susceptible to drugs that target active cellular processes
• Biofilm-associated infections often require significantly higher drug doses, longer treatment durations, or even physical removal of the biofilm (e.g., removing an infected device) to achieve cure