Mechanisms of Action
Different classes hit the organism at different points. Penicillins interfere with biosynthesis of the pathogen cell wall. Sulfonamides, antimycobacterials, and trimethoprim-sulfamethoxazole block the organism from using substances essential to its growth or development. Aminoglycosides, macrolides, and chloramphenicol interfere with protein synthesis, making cell division nonfunctional. Fluoroquinolones interfere with DNA synthesis, so the cell cannot divide and dies. Antifungals and antiprotozoals alter cell membrane permeability, leaking out essential components and killing the cell.
Therapeutic Action
The goal is to interfere with the normal function of the invading organism so it cannot reproduce, causing cell death. Narrow-spectrum agents are selective and effective against only a few microorganisms. Broad-spectrum agents interfere with biochemical reactions in many kinds of microorganisms. Agents that kill the cell are bactericidal; agents that stop it from reproducing or dividing are bacteriostatic.
Resistance
Over time, invading pathogens adapt to an anti-infective and produce cells the drug no longer affects. Bacteria develop resistance several ways. Through enzyme production: strains once susceptible to penicillin now produce penicillinase, which inactivates penicillins before they can act. Through cell membrane permeability changes, including altered transport systems that keep the drug out of the cell. Through binding site alteration, which prevents the drug from being accepted into the cell. And through chemical production that acts as an antagonist to the drug.
Vancomycin (Vancocin, Vancoled) interferes with cell wall synthesis in susceptible bacteria. It was developed for patients allergic to penicillins and cephalosporins and for staphylococcal infections resistant to those drugs. It is highly toxic and reserved for specific situations, because it can cause renal failure, ototoxicity, superinfections, and red man syndrome, now called vancomycin infusion reaction, a rate-dependent histamine release that causes flushing and erythema of the face, neck, and upper torso when the drug is infused too fast; slowing the infusion lowers the risk (StatPearls).
Prevention of Resistance
Resistance and new strains are a public health concern, and nursing care helps prevent both. On dosing, collaborate with the provider on around-the-clock dosing to flatten the peaks and valleys in drug concentration and hold a constant therapeutic level, since resistant microbes emerge during the low-concentration windows. On duration, drive home that the patient must take the antibiotic exactly as prescribed and never save or share doses, so the microbes are completely eliminated and never get the chance to grow resistant strains (CDC).
Indications
In children, use caution: early exposure can lead to early sensitivity, and treating pediatric ear infections that may actually be viral contributes to resistance. Children are prone to GI and nervous system effects, so monitor hydration and nutrition closely. Adults tend to demand a quick cure, so drug allergies and resistant strains are a real problem in this group, and extra caution applies in pregnant and lactating women because many agents are teratogenic and cross into breast milk. Older adults present differently than younger patients, so culture and sensitivity tests matter for identifying the type and extent of infection; they are prone to severe GI, renal, and neurologic effects, and liver function must always factor into the plan.
Adverse Effects
Kidney damage: drugs like aminoglycosides are directly toxic to fragile kidney cells, causing anything from renal dysfunction to full renal failure, so keep the patient well hydrated to support excretion. GI toxicity: many agents are directly toxic to the cells lining the GI tract, causing nausea, vomiting, stomach upset, and diarrhea, and some are toxic to the liver, causing hepatitis or liver failure. Neurotoxicity: some agents damage nerve tissue where they accumulate in high concentration. Aminoglycosides collect in the 8th cranial nerve and cause dizziness, vertigo, and hearing loss; chloroquine, used for malaria, accumulates in the retina and optic nerve and causes blindness. Hypersensitivity reactions: most agents are protein bound for transport through the cardiovascular system and can induce antibody formation in susceptible people, so the next exposure may trigger an immediate or delayed allergic response. Superinfections: broad-spectrum agents destroy normal flora, letting opportunistic pathogens kept in check by that flora invade tissue, commonly Proteus and Pseudomonas.
Frequently Asked Questions
Why does finishing the prescribed course matter? Stopping early or skipping doses leaves the hardiest microbes alive to multiply and pass on resistance. The CDC advises taking antibiotics exactly as prescribed and never saving or sharing them, because more than 2.8 million resistant infections occur in the United States each year (CDC).
What is the difference between bactericidal and bacteriostatic agents? Bactericidal drugs kill the organism outright, while bacteriostatic drugs stop it from reproducing so the immune system can clear it. The choice depends on the infection and the patient's immune status.
What is the difference between narrow- and broad-spectrum antibiotics? Narrow-spectrum agents target only a few organisms and spare more normal flora, while broad-spectrum agents work against many organisms but carry a higher risk of superinfection. Culture and sensitivity testing helps narrow therapy.
Why is culture and sensitivity testing done before starting antibiotics? It identifies the organism and which drugs will work, so therapy can be targeted rather than guessed. When possible, obtain cultures before the first dose, since antibiotics can mask the organism.
What is red man syndrome and how is it prevented? It is a rate-dependent reaction to vancomycin, now called vancomycin infusion reaction, that releases histamine and causes flushing and erythema of the face, neck, and upper torso. Slowing the infusion and giving an antihistamine prevent or abort most reactions (StatPearls).
Why are some antibiotics nephrotoxic or ototoxic? Certain agents, like aminoglycosides, concentrate in fragile kidney cells and in the eighth cranial nerve, causing renal injury and hearing or balance problems. Keep patients well hydrated and monitor renal function and drug levels.