Veterinary Antibiotic Spectrum of Activity Reference
General spectrum-of-activity reference by antibiotic class, for pharmacology education - not a prescribing guide.
A spectrum reference, not a prescribing guide
This is a general spectrum-of-activity reference for pharmacology education, not a prescribing guide. Actual drug selection for a specific infection depends on culture and susceptibility results where available, site of infection, patient factors, and current clinical guidelines - always apply antimicrobial stewardship principles and consult your formulary.
Spectrum of activity by drug class
| Drug class (examples) | Spectrum of activity |
|---|---|
| Penicillins (amoxicillin, ampicillin) Amoxicillin-clavulanate (Clavamox) extends coverage to beta-lactamase-producing organisms and some anaerobes. | Gram-positive predominant, limited gram-negative coverage; ineffective against beta-lactamase-producing staph. |
| 1st-generation cephalosporins (cephalexin) | Strong gram-positive coverage (especially Staphylococcus), modest gram-negative coverage. |
| Fluoroquinolones (enrofloxacin, marbofloxacin) Caution: avoid in young/growing animals due to cartilage effects; use cautiously in cats, since enrofloxacin especially carries a dose-dependent retinal toxicity risk at higher doses. | Strong gram-negative coverage including Pseudomonas, modest gram-positive, generally poor anaerobic and streptococcal coverage. |
| Potentiated sulfonamides (trimethoprim-sulfadiazine/sulfamethoxazole) Caution: associated with keratoconjunctivitis sicca (dry eye) with long-term use in dogs, and hypersensitivity reactions reported more often in Doberman Pinschers. | Broad gram-positive and gram-negative coverage, with good CNS and prostatic tissue penetration. |
| Aminoglycosides (gentamicin, amikacin) Caution: nephrotoxic and ototoxic with systemic use - typically reserved for topical use or short-course systemic therapy with monitoring. | Strong gram-negative coverage including Pseudomonas. |
| Metronidazole Also used for its GI anti-inflammatory/immunomodulatory properties independent of its antimicrobial effect. | Anaerobic bacteria and some protozoa (e.g. Giardia). |
| Clindamycin | Gram-positive organisms and anaerobes, with good penetration into bone and dental tissue. |
| Tetracyclines (doxycycline) | Broad coverage including many tick-borne pathogens (Ehrlichia, Anaplasma, Rickettsia), Mycoplasma, and some gram-positive/gram-negative organisms. |
Spectrum descriptions are general patterns, not guarantees of efficacy against a specific isolate. Local resistance patterns vary - culture and susceptibility testing takes precedence over general spectrum when available.
Frequently asked questions
Why might enrofloxacin be avoided in a growing puppy?
Fluoroquinolones can damage developing cartilage in young, growing animals, so they're generally avoided until skeletal maturity except when no safer alternative exists.
What extends amoxicillin's spectrum when combined with clavulanate?
Clavulanate inhibits bacterial beta-lactamase enzymes, protecting the amoxicillin from being broken down and extending coverage to beta-lactamase-producing bacteria and some anaerobes that amoxicillin alone can't reach.
Why is metronidazole sometimes used even when infection isn't anaerobic?
It also has anti-inflammatory and immunomodulatory effects independent of its antimicrobial spectrum, which is part of why it's used in some chronic GI conditions beyond strict anaerobic infections.
Why do fluoroquinolones need cautious dosing in cats specifically?
Enrofloxacin in particular carries a dose-dependent risk of retinal toxicity and potential blindness in cats, so feline dosing must stay within labeled limits.