Introduction
Antimicrobials are drugs that kill or slow the growth of microorganisms. Antibacterials act on bacteria and antifungals act on fungi and yeasts. Veterinary technicians prepare, give and dispense these drugs every day. They also flush the doxycycline tablet, collect the culture sample before the first dose, check the urine of a dog on gentamicin and explain to the owner why the full course matters.
This guide groups antimicrobials by where they act on the bacterial cell, then links each class to the adverse effects and species warnings that the VTNE likes to test. It ends with culture and susceptibility testing and the principles of stewardship, the careful use of antimicrobials to slow resistance.
Key facts
- Beta-lactams (penicillins and cephalosporins) block the cross-linking of peptidoglycan in the bacterial cell wall.3
- Aminoglycosides bind the 30S ribosomal subunit. Their main adverse effects are nephrotoxicity, ototoxicity and neuromuscular blockade.5
- In cats, retinal degeneration risk is greatest with enrofloxacin at 5 mg/kg or higher.6
- Dry-pilling doxycycline has caused esophageal erosion and strictures in cats. Follow each dose with about 5 mL of fluid.7
- Keratoconjunctivitis sicca (KCS, dry eye) is one of the most common severe adverse events of trimethoprim-sulfonamides in dogs.9
- Chloramphenicol can cause irreversible aplastic anemia in people. Wear gloves to handle it, and never use it in food animals in the US.11
- Identify the pathogen before the first dose whenever feasible: collect culture samples first.13
How the VTNE tests this
- Recognize drug classes, their mechanisms and clinically relevant side effects (Domain 1 Task F).1
- Classify a named drug (K8, drug classification) and match it to its indications, contraindications, side effects and interactions (K12).1
- Expect scenario items: a cat that vomits after a dry tablet, a dog on gentamicin with dilute urine, a puppy that should not get a fluoroquinolone.
- Expect a dose or volume calculation for an antimicrobial order (Domain 1 Task C).1
Core concepts
Bactericidal or bacteriostatic
A bactericidal drug kills bacteria. A bacteriostatic drug stops them multiplying, so the patient's immune system must clear the infection. The mechanism of action decides which effect a drug is likely to have.2 Tetracyclines are generally bacteriostatic, and a working host defense is essential for them to succeed.7 The label is not fixed: at the high concentrations reached in urine, tetracyclines and sulfonamides can become bactericidal.78
Time-dependent or concentration-dependent
The MIC (minimum inhibitory concentration) is the lowest drug concentration that stops visible bacterial growth. Time-dependent drugs, such as the beta-lactams, work best when blood levels stay above the MIC for most of the dosing interval.23 That is why on-time dosing matters. Concentration-dependent drugs, such as aminoglycosides, work best with a high peak. Aminoglycosides are given once daily at the same total daily dose to reduce the risk of kidney injury.5
Where the drugs act
Cell wall. Beta-lactams interfere with the transpeptidase enzymes (penicillin-binding proteins) that cross-link peptidoglycan strands.3 Some bacteria make beta-lactamase enzymes that destroy the beta-lactam ring. Cephalosporins are generally more resistant to beta-lactamase than penicillins.3
Ribosome. Aminoglycosides bind irreversibly, mainly at the 30S subunit, and are mostly bactericidal.5 Tetracyclines bind the 30S subunit reversibly.7 Phenicols bind the 50S subunit and stop peptide chains from growing; the effect is usually bacteriostatic.11
DNA. Fluoroquinolones inhibit DNA gyrase (topoisomerase II) and topoisomerase IV and are usually bactericidal.6 Metronidazole is reduced inside anaerobic bacteria to toxic products that break DNA strands. Aerobic bacteria lack that pathway, so metronidazole only works against obligate anaerobes (and some protozoa such as Giardia).10
Folate. Sulfonamides mimic PABA (para-aminobenzoic acid) and block dihydropteroate synthetase. Trimethoprim blocks dihydrofolate reductase, a later step in the same pathway.8
Fungal membrane. Azole antifungals (ketoconazole and relatives) block ergosterol synthesis through a fungal cytochrome P450 enzyme. They have a lag time before they work.12
Cephalosporin generations
First-generation cephalosporins (cefazolin, cephalexin, cefadroxil) are usually quite active against many gram-positive bacteria and only moderately active against gram-negative organisms.4 Second-generation drugs (cefoxitin, cefuroxime) add gram-negative activity. Third-generation drugs include ceftiofur, cefpodoxime (approved for dogs) and cefovecin (approved for dogs and cats). Cefepime is fourth generation.4
| Class | Mechanism | Examples | Key adverse effect or warning |
|---|---|---|---|
| Penicillins | Cell wall, bactericidal, time-dependent | Amoxicillin, ampicillin, penicillin G | Do not mix with aminoglycosides in the same syringe or bag |
| Cephalosporins | Cell wall, bactericidal | Cefazolin, cephalexin, cefpodoxime, cefovecin | Generation predicts spectrum |
| Aminoglycosides | 30S ribosome, bactericidal, concentration-dependent | Gentamicin, amikacin | Nephrotoxicity, ototoxicity, neuromuscular blockade |
| Fluoroquinolones | DNA gyrase and topoisomerase IV, bactericidal | Enrofloxacin, marbofloxacin, pradofloxacin | Cartilage damage in growing animals; retinal degeneration in cats |
| Tetracyclines | 30S ribosome, bacteriostatic | Doxycycline, oxytetracycline, minocycline | Esophageal stricture in cats; tooth and bone deposits in the young |
| Potentiated sulfonamides | Folate pathway | Trimethoprim-sulfadiazine, trimethoprim-sulfamethoxazole | KCS, polyarthritis (Doberman Pinschers), crystalluria |
| Nitroimidazoles | DNA damage, anaerobes only | Metronidazole | Neurotoxicity at high doses; banned in food animals |
| Phenicols | 50S ribosome, bacteriostatic | Chloramphenicol, florfenicol | Aplastic anemia in people; banned in US food animals |
| Azole antifungals | Ergosterol synthesis | Ketoconazole, itraconazole, fluconazole | Liver injury (ketoconazole, especially cats); drug interactions |
Sources: Merck Veterinary Manual pharmacology chapters (references 3 to 8 and 10 to 12); Ekstrand and colleagues 2026 (reference 9).
Procedure or technique
Starting an antimicrobial safely: the technician's role
- Collect samples first. If the veterinarian wants a culture, collect it before the first dose. The pathogen should be identified before treatment starts whenever feasible.13 A cystocentesis urine sample, a swab of a deep wound or a tracheal wash sample taken after antibiotics have started may grow nothing.
- Confirm the order. Check drug, dose in mg/kg, route, frequency and duration against the current weight. Flag a fluoroquinolone ordered for a growing puppy or an enrofloxacin dose of 5 mg/kg or more in a cat for the veterinarian to confirm.6
- Prepare correctly. Reconstitute powders with the stated diluent and label the vial with date, time and concentration. Beta-lactams and aminoglycosides interact chemically and should not be mixed in vitro.14
- Give it on time. Time-dependent drugs lose effect when doses are late.2
- Pill cats with a chaser. After a doxycycline tablet or capsule, give about 5 mL of water by syringe.7
- Monitor. For aminoglycosides, watch urine output and specific gravity and report new polyuria, protein or casts in the sediment.5 For sulfonamides, watch tear production and joints. For metronidazole, watch for wobbliness or tremors.
- Record and report. Note any adverse event in the medical record and tell the veterinarian.
Culture and susceptibility
Culture grows and identifies the organism. Susceptibility testing reports how the isolate responds to each drug, for example as susceptible or resistant, often with an MIC value compared with a CLSI (Clinical and Laboratory Standards Institute) breakpoint.2 An antibiogram is a summary of local susceptibility patterns. It does not give MICs for one patient's isolate and is best used to guide empiric treatment while culture results are pending.13
Empiric treatment means starting a drug based on the likely organism before results arrive. Severely ill patients may need it. If the report later shows resistance but the patient has responded, treatment should not change. If the patient has not responded, the drug should be changed to match the culture result.13
Flowchart
Species differences
| Species or group | Drug | Warning | What the technician does |
|---|---|---|---|
| Cat | Enrofloxacin | Acute retinal degeneration and blindness; risk greatest at 5 mg/kg or more | Double-check the dose; report dilated pupils or bumping into objects |
| Cat | Doxycycline tablets or capsules | Esophageal erosion and stricture after dry-pilling | Follow with about 5 mL of water |
| Cat | Ketoconazole | Hepatotoxicity | Watch appetite and report vomiting |
| Growing dogs and foals | Fluoroquinolones | Cartilage erosion, possible permanent lameness | Flag the order in an immature patient |
| Young animals | Tetracyclines | Deposited in growing bone and unerupted teeth | Flag use in pregnant or very young patients |
| Dog, especially Doberman Pinscher | Trimethoprim-sulfonamides | KCS; polyarthritis overrepresented in Dobermans | Ask about eye discharge and lameness at rechecks |
| Dog | Metronidazole, high dose | Tremors, ataxia, convulsions | Report new wobbliness or tremors |
| Food animals (US) | Chloramphenicol; extralabel nitroimidazoles | Prohibited | Never dispense for a food-producing animal |
Sources: Merck Veterinary Manual (references 5 to 7 and 10 to 12); Ekstrand and colleagues 2026 (reference 9).
Fluoroquinolone cartilage damage in growing dogs and foals is thought to come from chelation of magnesium in cartilage, which has a poor blood supply.6 Tetracyclines tend to chelate calcium (doxycycline less so), so they are laid down irreversibly in growing bone and in the dentin and enamel of unerupted teeth.7 Aminoglycoside vestibular injury causes nystagmus, incoordination and loss of the righting reflex, and it is often irreversible.5
Calculation
Worked example. The veterinarian orders cefazolin 22 mg/kg IV for a 30 kg dog before surgery. The reconstituted vial is 100 mg/mL. How many mL do you draw up?
Dose: 30 kg x 22 mg/kg = 660 mg (kg cancels).
Volume: 660 mg / 100 mg/mL = 6.6 mL (mg cancels).
Answer: 6.6 mL. Check: 6.6 mL x 100 mg/mL = 660 mg.
The numbers in this example illustrate the method. Always use the dose the veterinarian orders and the concentration on the vial you are holding.
Safety, legal and client education
- Chloramphenicol. In people it can cause aplastic anemia that is not related to dose and may appear after the drug is stopped; the incidence is about 1 in 25,000 to 40,000. Handlers should wear gloves and be trained in safe handling.11 Explain this to any owner who will give it at home.
- Food animals. Chloramphenicol is prohibited in food-producing animals in the US and Canada, and extralabel use of nitroimidazoles is prohibited in food-producing animals.1110
- Urine color. Metronidazole can turn urine reddish brown. Warn the owner so they do not mistake it for blood.10
- Stewardship. Good stewardship means choosing the most appropriate drug, dose, route and duration, and reassessing whether treatment is still needed. It is about better prescribing, not only less prescribing.15 Critically important drugs are kept for severe or multidrug-resistant infections.15 The technician supports this by collecting cultures properly, giving doses on time and explaining to owners why the dose, timing and course must be followed exactly as written.
Common mistakes and exam traps
- Collecting the culture after the first antibiotic dose. The sample comes first.
- Mixing an aminoglycoside and a penicillin or cephalosporin in one syringe. Give them separately.
- Choosing metronidazole for an aerobic infection. It only works on obligate anaerobes and some protozoa.
- Thinking bacteriostatic means weak. It means the immune system finishes the job, so it matters in immunosuppressed patients.
- Assuming a "resistant" report always means the drug must change. If the patient is improving, it usually stays.
- Confusing which drug harms the person giving it. Chloramphenicol is the human health risk; enrofloxacin retinal damage is a cat risk.
Quick review
- Beta-lactams block cell wall cross-linking and are time-dependent.
- First-generation cephalosporins are best against gram-positive bacteria.
- Aminoglycosides: 30S, bactericidal, once daily, watch the kidneys and ears.
- Fluoroquinolones block DNA gyrase; avoid in growing animals; enrofloxacin can blind cats at 5 mg/kg or more.
- Tetracyclines are 30S and bacteriostatic; chase doxycycline with about 5 mL of water in cats.
- Sulfonamides and trimethoprim block two steps of folate synthesis; watch for KCS.
- Metronidazole kills anaerobes and Giardia; high doses cause ataxia and tremors.
- Chloramphenicol binds 50S; gloves for handlers; banned in US food animals.
- Azoles block ergosterol; ketoconazole can injure the cat liver.
- Collect culture samples before the first dose.
- An antibiogram guides empiric treatment until the culture report arrives.
- Stewardship means the right drug, dose, route and duration, with reassessment.