Understanding Antibiotics:
A Quick-Reference Guide


With so many antibiotics and ever-changing prescribing guidelines, understanding the difference between types of antibiotics and when to use them can be extremely confusing. Not just for medical students, but junior doctors, nurses, paramedics, EMTs, and anyone involved in administering or prescribing antibiotics.

The best way to gain a deeper overview of antibiotics is not by memorising indications, but by  understanding the pharmacological differences between types and how they work on bacteria. This guide offers a quick easy-to-understand and no-BS overview to understanding antibiotics.

Whether you’re a seasoned prescriber or newly graduated, you’ll find this quick reference guide a handy tool for antibiotic prescribing.

Before we get started – a few important terms to familiarise ourselves with:

Important Terms

Whether an antibiotic is bactericidal or bacteristatic depends on the organism and antibiotic concentration.

Antimicrobials are synergistic if their combined activity is greater than the sum of the individual activities. E.g. 1+2 = 4
They are antagonistic if the activity of one drug is compromised by the other. E.g. 2+2 <4

Therapeutic index

Therapuetic index indicates the ‘gap’ between when the blood concentration at which a drug becomes toxic and the concentration at which the drug is effective. The aim is to always administer the minimum effective dose at which there are little or no side effects or toxicity.

With antibiotics with high therapeutic index e.g. beta lactamase, the exact amount given matters less.
With antibiotics with low therapeutic index e.g. amino glycosides, non functional doses may sill have low levels of toxicity

types of antibiotic activity



A beta-lactam, inhibits formation of peptidoglycan cross links in bacterial, cell wall, but have no direct effect on cell wall degradation. Accumulation of peptidoglycan precursors leads to activation of bacterial cell wall hydrolases and autolysins, which further digest existing cell. Safe in pregnancy and children. Drug of choice for streptococcal and meningococcal.

Side-effects: diarrhea, hypersensitivity, nausea, rash, neurotoxicity, urticaria, superinfection.

Fusidic Acid

Bacteriostatic antibiotic often used topically in creams and eyedrops, but can also be given oral or IV. Protein synthesis inhibitor.esp effective on staphylococcus, streptococcus, and corynebacterium spp. Useful against MRSA.

Side-effects: Teratogenic in pregnancy, jaundice, dark urine, light faeces.


Protein synthesis inhibitors. Bind to P site on 50s ribosome. Side-effects: diarrhea, nausea, abdo pain, vomiting


Lincosamide abx, used for anaerobic bacteria & protozoal disease. inhibits protein
synthesis via ribosomal translocation (similar to macroglides).

Side-effects:  c.diff associated diarrhoea (causing pseudo membranous colitis), nausea, abdo pain/
cramps, rash/itch. (stay elevated for 30min after taking)


Inhibit peptidoglycan for cell wall synthesis. Toxic, so used for critically ill and this
with hypersensitivity to beta lactams or have resistant species.

Side-effects: pulmonary fibrosis and impaired lung function by inducing sensitivity to oxygen
toxicity, fever, rash, urticaria, hyper pigmentation, alopecia, raynauds phenomenon
(discoloration of fingers and toes)


Protein synthesis inhibitor. Activity against all gram positive bacteria. Effective
against MRSA.



anti gram negative antimicrobial action

Interact with bacteria membrane phospholipids that exist in gram neg outer
membranes. Given IV. Neurotoxic and nephrotixic, so used as last resort.


Bacteriostatic antibiotic, mainly prophylaxis for UTIs. Prevents bacteria from taking
up folic acid necessary for thymine.


Not orally active. Side-effects: ototoxic, nephrotoxic, neuromuscular paralysis.


Single ring beta lactams


Beta lactamase resistant penicillin, used for multiresistant gram negative. Primarily used against enteribacteriaceae. Side effects same as other beta lactams.



To overcome resistance by beta lactamase, often given with beta lactamase inhibitors e.g. Clavulanic
acid. Beta Lactams Cell wall inhibitors include Penicillin derivatives Cephalosporins, Monobactams, Carbapenems


Beta lactams that are highly resistant to beta lactamases. Broadest spectrum antibiotic group



Bacteriostatic antimicrobial. Cheap and widely available.


Protein synthesis inhibitor. Commonly used for acne.


Inhibits cell wall synthesis, gram positive and negative, no anti anaerobic action. Given both oral and IV, usually 3 times a day. Drug of choice of streptococcal disease except when used empirically for a sore throat, listeria


Classification of Penicillins and Carbapenems
  • Beta lactamase susceptible narrow spectrum: penicillin V, penicillin G
  • Beta lactamase resistant: flucloxacillin, nafcilin, oxacilin*, methicillin*. (*not used)
  • Beta lactamase susceptible broad spectrum penicillins: ampicillin, amoxicillin
  • Penicillins with beta lactamase inhibitors: amoxicillin + clavulanic acid, pipracillin + tazobactam
  • Carbapenems broad spec active against most beta lactamases: meropenem, impenem.

In order of increasing gram negative activity (incl pseudomonas).
Gram positive activity remains.

Co-amoxiclav (amoxicillin + clavulanic acid)

With added beta lactamase inhibitor. Administered Oral and IV tds.
Resistance: MRSA, hospital acquired gram neg organisms incl pseudomonas. Resistance develops via ESBLs (extended spectrum B lactamases), impermeability, efflux, alteredPBPs.
Used for poly microbial infections, beta lactamase producing gram pos and neg infections

Piptazobactam (piperacillin + tazobactam)



Classification of Cephalosporins
In order of increasing activity against gram negative bacteria.

  • First generation: cephalexin, cephalothin, cefadroxil
  • Second generation: cefuroxime, cefotetan, cefomandole
  • Third generation: Cefotaxime, ceftriaxone, ceftazidime
  • Fourth generation: cefipime (anti pseudomonas aeruginosa)

No cephalosporins are effective against enterococci.
Cephalosporins heavily implicated in causing abx associated diarrhoea.


Carbapenems: meropenem

Glycopeptides: vancomycin/teicoplanin

Agents which inhibit protein synthesis:
Aminoglycosides (also bactericidal)
Macroglides e.g. Erythromycin, clarithromycin. (bactericidal in high concentrations)
Fusidic acid
Drugs affecting nucleic acid metabolism
Nucleic synthesis and polymerisation is similar in bacteria and mammals, thus selective toxicity is poor. Exceptions:

  • Quinolones e.g. Ciprofloxacin, moxifloxacin, levofloxacin. Oral anti pseudomonal agents
  • Rifampicin
  • Metronidazole (active against most anaerobes) – cannot drink alcohol.

Gentamicin (aminoglycoside)



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