Antimicrobial Drugs
Few discoveries have saved more lives than the antibiotic. Inside a single lifetime, infections that once emptied hospital wards became curable. Pneumonia. Sepsis. A wound gone bad. That victory rests on one neat idea. A bacterium is built differently from you. A well-chosen drug can use that difference to kill the microbe and leave the sick person unharmed. This chapter shows how antimicrobials work. It shows how the right one gets picked. And it shows why bacteria are fighting back. Along the way you will see that the drugs only keep working if people handle them well. When cultures get drawn, whether a dose lands on time, how long a course runs: these small choices decide whether the whole enterprise still works for the next person who needs it.
By the end of this chapter you can
- Explain selective toxicity and why it makes antimicrobial therapy possible.
- Define the therapeutic index. Then link a narrow index to the need for drug-level monitoring.
- Name the five bacterial targets that antibacterial drugs attack, and give a drug class for each.
- Tell bactericidal from bacteriostatic, and narrow- from broad-spectrum drugs.
- Describe how susceptibility testing guides drug choice. That means the MIC, Kirby–Bauer, and the Etest.
- Sum up the four main resistance mechanisms, and say how resistance genes spread.
- Explain what MRSA is and why it matters at the bedside.
- State the goals of antibiotic stewardship and the bedside actions that carry it out.
Section 1What makes a good antimicrobial
Selective toxicity: hurt the microbe, spare the host
A drug that kills bacteria is a poison. We put it into a sick person and call it medicine. One idea is what makes that safe. It is called selective toxicity. A good antimicrobial harms the microbe far more than it harms the host. This works because a bacterium is a prokaryote. It is built from parts your own cells simply do not have. It wraps itself in a stiff cell wall made of peptidoglycan. Your cells have no wall at all. Its ribosomes are the machines that build proteins. They are a different size and shape from yours. It also makes its own folic acid from scratch, while you get folate from food. Each of those gaps is a handle a drug can grab. Aim at a part the microbe has and the host lacks. You get a cure with little damage on the side.
Selective toxicity is a matter of degree. It is never perfect. The widest safety margins belong to the drugs that attack the cell wall. Those are the penicillins and the cephalosporins. Human cells own no wall for them to injure. Drugs that hit something bacteria and humans share are riskier. An aminoglycoside can damage the kidney and the inner ear. An antifungal that aims at the fungal cell membrane can disturb human membranes too. Both kinds of membrane are built around sterols. So the closer a microbe sits to us in biology, the harder selective toxicity gets. A fungus is close. A protozoan is close. A human cell hijacked by a virus is closer still. That is exactly why antibacterial drugs are gentler than most antifungals and antivirals.
The therapeutic index: how much room for error
Every drug has a dose that helps. It also has a higher dose that harms. The therapeutic index puts a number on the gap between them. Roughly, it is the toxic dose divided by the effective dose. A large index means a comfortable margin. You would have to overshoot badly before anyone got hurt. That is why penicillin can be given in generous doses with little worry. A small index is called a narrow therapeutic index. Now the helpful dose and the harmful dose sit uncomfortably close. Such a drug has to be dosed with care. Often its level is measured right in the blood.
The graph above is where that idea becomes a bedside tool. The goal is to hold the drug inside the therapeutic window. High enough to kill bacteria. Low enough to spare the host. Two antimicrobials sit at the narrow end of the scale. One is vancomycin. The others are the aminoglycosides, such as gentamicin and tobramycin. With these drugs we do not guess. We draw blood levels. A trough is drawn just before the next dose. It proves the drug has not piled up to a toxic level. Sometimes a peak is drawn as well. It proves the level climbed high enough to work.
| Drug | Therapeutic index | What that means in care |
|---|---|---|
| Penicillins | Wide | Large safety margin; routine levels not needed |
| Cephalosporins | Wide | Well tolerated; watch for allergy, not toxicity |
| Vancomycin | Narrow | Monitor trough levels; watch kidney function |
| Aminoglycosides | Narrow | Monitor peak & trough; risk of kidney & ear toxicity |
- Selective toxicity means hitting a target the microbe has and the host lacks. It is what makes antimicrobial therapy possible.
- The beta-lactams, meaning penicillins and cephalosporins, are the safest drugs. Human cells have no wall to injure. But vancomycin also aims at the wall, and it is still hard on the kidney and narrow-index. Target alone does not promise safety.
- The therapeutic index is the gap between the helpful dose and the harmful one.
- Narrow-index drugs need blood-level monitoring. That means vancomycin and the aminoglycosides. The timing of the draw is everything.
Section 2How the drugs work
Five ways to kill a bacterium
Antibacterial drugs come in great variety. Yet they attack a bacterium in only a handful of places. Learn these five targets and almost any antibiotic you meet drops into a mental map. Better still, the map predicts two things. It predicts how selective the drug is. It also predicts the side effects you should watch for.
1. Cell wall synthesis. The peptidoglycan wall is the pressure vessel of the cell. Without it the cell bursts. Penicillins, cephalosporins, and vancomycin block the building of that wall. Growing bacteria then split open and die. Human cells have no wall, so this is the most selective target of all. The penicillin and cephalosporin families share a chemical ring called a beta-lactam. That shared ring is why they are grouped as beta-lactam antibiotics.
2. Protein synthesis. The bacterial ribosome is a 70S machine. It is built from a 30S part and a 50S part. The human ribosome is 80S, built from a 40S part and a 60S part. The sizes differ, so a drug can jam the bacterial machine and leave yours running. Aminoglycosides and tetracyclines hit the 30S part. Macrolides such as azithromycin and erythromycin hit the 50S part, and so does clindamycin. The match is not perfect, though. Your mitochondria carry ribosomes that look bacterial. That overlap helps explain some of the toxicity these drugs cause.
3. Nucleic acid synthesis. Some drugs block the enzymes bacteria use to copy and unwind DNA. The fluoroquinolones work this way. Ciprofloxacin and levofloxacin are the common ones. They shut down bacterial DNA gyrase and topoisomerase. Those enzymes are shaped differently from the human versions.
4. Metabolic (folate) pathways. Bacteria have to build their own folic acid. You eat yours. Sulfonamides and trimethoprim starve the bacterium of folate. They block two steps of that pathway, one after the other. It is a target you simply do not possess.
5. Cell membrane. A few drugs punch holes in the bacterial membrane. The contents of the cell then leak out. These drugs tend to be harsher, because your cells have membranes too. They are held back for stubborn infections. Polymyxins such as colistin attack the outer membrane of gram-negative rods. Daptomycin works the other way around. It wrecks the membrane of gram-positives such as MRSA and VRE, and it cannot touch a gram-negative at all.
Kill it or just stop it: cidal against static
Drugs also differ in how thoroughly they act. A bactericidal drug kills the bacterium outright. Penicillins do that, and so do aminoglycosides and fluoroquinolones. A bacteriostatic drug only halts growth. Tetracyclines, macrolides, and sulfonamides work that way. They hold the population still and leave the finishing blow to the immune system. In a healthy person, static is often enough. It is not always enough. Think of someone whose defenses are down, such as a person on chemotherapy with a low neutrophil count. Think of endocarditis or meningitis, where few immune cells reach the site. In those cases the team reaches for a bactericidal drug. There is no reliable immune backup to mop up the survivors.
| Target | Example drug classes | Why it is selective |
|---|---|---|
| Cell wall synthesis | Penicillins, cephalosporins, vancomycin | Human cells have no cell wall |
| Protein synthesis (ribosome) | Aminoglycosides, tetracyclines, macrolides | Bacterial ribosome (70S) differs from human (80S) |
| Nucleic acid synthesis | Fluoroquinolones | Bacterial DNA enzymes differ from human ones |
| Folate metabolism | Sulfonamides, trimethoprim | Humans get folate from diet; bacteria make it |
| Cell membrane | Polymyxins, daptomycin | Least selective — reserved for resistant cases |
- Antibacterials hit five targets. They are the cell wall, the ribosome, nucleic acids, folate metabolism, and the cell membrane.
- Beta-lactams attack the cell wall. That is the safest and most selective target. Penicillins and cephalosporins are the examples.
- Bactericidal drugs kill. Bacteriostatic drugs stall growth and lean on the immune system to finish.
- Someone with weak immune defenses usually needs a bactericidal drug.
Section 3Spectrum and choosing a drug
Narrow against broad spectrum
A drug's spectrum of activity is the range of bacteria it works against. A narrow-spectrum drug hits only a few kinds. A broad-spectrum drug hits many kinds at once. Broad sounds better, and in a crisis it can save a life. Breadth still has a cost. A broad-spectrum antibiotic clear-cuts the normal flora along with the pathogen. That opens the door to a superinfection. Two common ones are Clostridioides difficile colitis and a yeast overgrowth. Every needless exposure also nudges the whole bacterial population toward resistance. So the aim, once the culprit is known, is the narrowest drug that will do the job.
Empiric first, then definitive
Cultures take time. A person with sepsis cannot wait a day or two for the result. So therapy usually happens in two stages. The first stage is empiric therapy, which is an educated guess. The guess rests on three things. Where the infection probably started. What the local resistance patterns look like. How sick the person is right now. The team starts a broad-spectrum drug at once, but only after cultures are drawn. Then the lab reports what grew and what it is susceptible to. Now the team switches to definitive therapy. Ideally that means narrowing down to a focused drug, a move called de-escalation. This is why the order of events at the bedside matters so much. Cultures first. Antibiotics second.
Susceptibility testing: proving the drug will work
How does the lab know which drug to recommend? It tests the isolate from that one person against a panel of antibiotics. The key number is the minimum inhibitory concentration (MIC). That is the lowest drug concentration that stops the bacterium from growing. Several methods get at it. The classic one is the Kirby–Bauer test, also called disk diffusion. Paper disks soaked in different antibiotics sit on a lawn of bacteria. Each drug spreads out into the agar. A clear ring called the zone of inhibition forms where growth was stopped. The bigger the zone, the more susceptible the organism. Note what disk diffusion does and does not give you. It sorts the isolate into susceptible, intermediate, or resistant. It does not read out an exact MIC. The Etest strip in the figure sharpens this into a real MIC value. Automated broth microdilution trays do the same job in tiny wells. Either way the result reaches the chart as a letter. S means susceptible. I means intermediate. R means resistant. That report is worth reading closely whenever someone is not improving on the current drug.
| Narrow-spectrum | Broad-spectrum | |
|---|---|---|
| Bacteria covered | Few, targeted | Many |
| Effect on normal flora | Spares most of it | Wipes out much of it |
| Superinfection risk | Lower | Higher (C. diff, yeast) |
| Resistance pressure | Lower | Higher |
| Best used when | The pathogen is known | Empirically, while awaiting cultures |
- Spectrum is the range of bacteria a drug covers. Narrow is preferred once the pathogen is known.
- Broad-spectrum drugs invite superinfection and resistance. Use them empirically, then narrow down.
- Empiric therapy is the first guess. Definitive therapy follows the culture and de-escalates.
- The MIC is the lowest concentration that stops growth. Results are reported as S / I / R.
Section 4How bacteria fight back
Four ways to survive the drug
Resistance is not the drug wearing off. It is evolution in fast forward. The drug kills the susceptible bacteria. It leaves the rare survivor that happened to carry a defense. That survivor's offspring inherit the trick, and soon they are the whole population. There are four main defenses, and the figure lays them out.
Destroy the drug. The famous example is beta-lactamase. It is an enzyme that snips open the beta-lactam ring before the drug can act. Penicillinase in Staphylococcus aureus was the first one found. Gram-negative rods have taken the idea further. ESBLs chew through the cephalosporins, and carbapenemases such as KPC chew through the carbapenems. This is also why some penicillins come paired with a beta-lactamase inhibitor. Amoxicillin–clavulanate is one such pair. The inhibitor soaks up the enzyme so the antibiotic survives.
Change the target. If the drug can no longer recognize its binding site, it cannot act. Change the lock and the key stops working. This is the trick behind MRSA. The bacterium picks up a gene for an altered penicillin-binding protein that beta-lactams cannot grip. VRE does the same thing to vancomycin. It swaps the end of its wall building block, so the drug has nothing to clamp onto.
Pump the drug out. Efflux pumps sit in the membrane. They bail the antibiotic back out before it reaches a lethal level inside the cell. Some pumps move several unrelated drug classes, so one pump can cause broad resistance. Tetracycline resistance often works this way.
Keep the drug out. A gram-negative cell gates its outer membrane with channels called porins. Narrow those channels, or lose them, and less drug gets in at all. Paired with a pump, the level inside never gets high enough to kill.
How resistance spreads, and why it spreads so fast
Resistance can start with a plain mutation. Its frightening speed comes from somewhere else. It comes from horizontal gene transfer. Bacteria hand resistance genes to one another, even across species. The genes travel on mobile loops of DNA called plasmids. One plasmid can carry resistance to several drug classes at once. So a single exposure can select for a strain resistant to drugs it never even met. Every needless antibiotic adds to the selective pressure that favors these survivors. That is true in a clinic, on a farm, and in a fish tank.
The named threats: MRSA and friends
MRSA stands for methicillin-resistant Staphylococcus aureus. It resists essentially every beta-lactam antibiotic. That knocks out the safest and most familiar drugs on the shelf. The team is pushed toward vancomycin and its relatives instead. MRSA spreads readily in hospitals and in the community. So people known to carry it are often placed on contact precautions. It does not travel alone. VRE means vancomycin-resistant enterococci. ESBL-producing and carbapenem-resistant gram-negatives sit on the same list. So does multidrug-resistant tuberculosis. The common thread is simple and grim. Our shelf of reliable drugs is shrinking faster than we are refilling it.
| Mechanism | What the bacterium does | Example |
|---|---|---|
| Drug inactivation | Makes an enzyme that destroys the drug | Beta-lactamase vs penicillins |
| Target modification | Alters the drug's binding site | MRSA's altered binding protein |
| Efflux pump | Pumps the drug back out | Tetracycline resistance |
| Reduced uptake | Keeps the drug from entering | Altered porins in gram-negatives |
- Bacteria resist drugs in four ways. They destroy the drug, change the target, pump it out, or keep it out.
- Resistance genes spread fast by horizontal gene transfer on plasmids, driven by selective pressure.
- MRSA resists all beta-lactams by changing the drug's binding site. That forces a switch to vancomycin.
- Contact precautions and hand hygiene hold resistant organisms in place on a unit.
Section 5Stewardship: using the drugs well
What stewardship means
Antibiotic stewardship is the shared effort to use these drugs only when they help, and to use them well. It is often boiled down to the five rights of antimicrobials. The right drug. The right dose. The right route. The right duration. The right person. The purpose runs two ways. One aim is a better outcome for the person in front of you, with fewer side effects, less C. diff, and a shorter stay. The other aim is to protect the drugs themselves for the people who come after. Resistance is a shared resource being spent. So stewardship is a public-health duty, not just a way to hold down costs.
Stewardship at the bedside
Stewardship gets described as work for doctors and pharmacists. In truth it is touched at nearly every point by whoever is at the bedside. That person draws cultures before the first dose. That person gives antibiotics on time, so blood levels stay effective. That person watches for improvement and for toxicity. And that person is placed to ask the questions that trim needless drug use. Can this IV antibiotic switch to a pill, now that the person is eating and getting better? Is this still needed on day five, or is it just still ordered? Are two drugs covering the same bug? One more job belongs here too. It is the last check on the allergy history. Clearing up a vague penicillin allergy keeps someone off a broader, harsher drug for no reason.
Adherence and what you teach
Stewardship reaches past the hospital door. Out there, teaching is the tool. Explain that antibiotics do nothing for colds, flu, and most sore throats. Asking for them just in case carries real risk and no benefit. Teach three habits. Take the drug exactly as prescribed. Never save or share leftover antibiotics. Never use someone else's. The old blanket rule to always finish every pill is being refined, because some infections do better on a shorter course. So the honest, durable message is simpler and stronger. Take it the way it was prescribed. Call before stopping early or doubling up.
| Stewardship goal | What that looks like in practice |
|---|---|
| Right diagnosis | Collect cultures before the first antibiotic dose |
| Right timing | Give doses on schedule to keep levels effective |
| Right duration | Prompt the team to review or de-escalate ongoing orders |
| Right route | Flag when a stable, eating person could switch IV → oral |
| Right person | Verify and clarify the allergy history |
| Prevention | Hand hygiene, timely device removal, teaching |
- Stewardship means the right drug, dose, route, duration, and person. It serves the person now and the people later.
- At the bedside that means cultures first, doses on time, and a close watch for toxicity.
- Speak up to de-escalate, to switch IV to oral, and to clarify allergies.
- Teach adherence. Take as prescribed, never share or save antibiotics, and expect nothing from them for a viral illness.
The chapter in one breath
- Selective toxicity means striking a target only the microbe owns. It is why antimicrobials cure without poisoning the host.
- The therapeutic index is the safety margin. Narrow-index drugs need blood-level monitoring, and that means vancomycin and the aminoglycosides.
- Antibacterials hit five targets. They are the cell wall, the ribosome, nucleic acids, folate metabolism, and the cell membrane.
- Drugs are bactericidal or bacteriostatic. They are also narrow- or broad-spectrum. Narrow and cidal both have their place, and so does empiric breadth.
- Susceptibility testing finds the MIC. Kirby–Bauer, the Etest, and broth microdilution all do it. The report comes back as S / I / R and guides definitive therapy, after cultures are drawn.
- Bacteria resist by destroying, dodging, ejecting, or excluding the drug. Resistance then spreads fast on plasmids. MRSA is the headline example.
- Stewardship protects the person and the drugs at the same time. Cultures first. Doses on time. De-escalation argued for. Adherence taught.
ReferenceWords worth knowing
A quick reference for the terms in this chapter. Skim it now, or circle back whenever a word trips you up.
- Adherence
- Taking a medication exactly as prescribed, with the right dose, timing, and duration.
- Antibiotic stewardship
- The shared effort to use antimicrobials only when needed and to use them well, protecting both the person treated and the drug's future usefulness.
- Bactericidal
- Describes a drug that kills bacteria outright.
- Bacteriostatic
- Describes a drug that halts bacterial growth and leaves the immune system to finish.
- Beta-lactam
- The chemical ring shared by penicillins and cephalosporins. It is the part a beta-lactamase attacks.
- Beta-lactamase
- A bacterial enzyme that destroys beta-lactam antibiotics. A common resistance mechanism.
- Broad-spectrum
- Active against many kinds of bacteria. Handy empirically, but harder on the normal flora.
- De-escalation
- Narrowing therapy from a broad drug to a focused one once cultures name the pathogen.
- Efflux pump
- A membrane transporter that pumps an antibiotic back out of the cell before it can act.
- Empiric therapy
- Best-guess antibiotic treatment started before culture results are back.
- Horizontal gene transfer
- The passing of genes, resistance genes included, between bacteria, often on plasmids.
- Kirby–Bauer test
- A disk-diffusion susceptibility test. The size of the zone of inhibition reflects susceptibility.
- Minimum inhibitory concentration (MIC)
- The lowest drug concentration that prevents visible bacterial growth.
- MRSA
- Methicillin-resistant Staphylococcus aureus. It resists all beta-lactams through an altered binding protein.
- Narrow-spectrum
- Active against only a few kinds of bacteria. Preferred once the pathogen is known.
- Selective toxicity
- Harming the microbe far more than the host, by aiming at a structure the microbe has and the host lacks.
- Spectrum of activity
- The range of bacterial types a given antimicrobial can affect.
- Superinfection
- A new infection that follows a drug's disruption of the normal flora, such as C. diff or yeast.
- Susceptibility testing
- Lab testing of one person's isolate against antibiotics, reported as S, I, or R.
- Therapeutic index
- The safety margin between a drug's effective dose and its toxic dose.
- Trough level
- The drug concentration measured just before the next dose, used to avoid toxic build-up.
- Zone of inhibition
- The clear ring of stopped growth around an antibiotic disk on a culture plate.