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

• Antimicrobial – active against microbes
• Antibiotic – naturally occurring, active against bacteria. Though many synthetic.
• Bactericidal – actively kills bacteria
• Bacteristatic – prevents bacteria from multiplying. Requires host innate defenses.

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

ANTI-GRAM-POSITIVE ANTIMICROBIALS

Penicillin

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.

• Resistance: Over 80% of staphylococci, s pneumoniae in USA & Spain, gonococcal worldwide
• Penicillin G (benzyl penicillin) – given IV
• Penicillin V (phenoxymethylpenicillin) – given oral. less active against gram neg.
• Procaine penicillin – given IV
• Benzathine penicillin – acts longer, given IV

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.

Macroglides

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

• Azithromycin: Used for otitis media, strep throat, pneumonia, typhoid, sinusitis, and STDs
• Clarithomycin: pharyngitis, tonsillitis, acute maxillary sinusitis, bronchitis, pneumonia, skin
  infections, legionellosis, H pylori, lyme disease
• Erythromycin: often used for penicillin allergics, with similar (slightly wider) antimicrobials
  spectrum to penicillin. Bette coverage of atypical organisms incl mycoplasma & legionellosis. Alters effectiveness of oral contraceptive.
  ‣ Roxithromycin: dived from erythromycin.
• Telithromycin: for community acquired pneumonia of mild – moderate severity.

Clindamycin

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)

Glycopeptides

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)

• Vancomycin: treatment for organisms resistant to penicillin or allergic to beta lactams, pseudo
  membranous colitis. prophylaxis for endocarditis and prosthesis. Empiric for MRSA.
• Teicoplanin: similar spectrum of activity as vancomycin.
• Telavancin: for complicated skin and skin structure infections (cSSSI)
• Bleomycin: induces DNA strand breaks. anticancer for hodgkins lymphoma, squamous cell
  carcinomas, testicular cancer, plantar warts.

Oxazolidinones

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

ANTI-GRAM-NEGATIVE ANTIMICROBIALS

Polymyxin

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

Trimethoprim

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

Aminoglycosides

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

• Streptomycin: protein synthesis inhibitor. For TB. must be given IM.
• Gentamicin: heat-stable and used in orthopedic surgery. NOT used against N gonorrhoae, N meningitidis, L. Pneumophilia, bc of risk of shock from lipid A endotoxin.
• Neomycin: in many topical medications. Not given IV bc particularly nephrotoxic.

Monobactams

Single ring beta lactams

Temocillin

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

BROAD SPECTRUM ANTIMICROBIALS

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

Carbapenems

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

Cephalosporins

Chloramphenicol

Bacteriostatic antimicrobial. Cheap and widely available.

Tetracycline

Protein synthesis inhibitor. Commonly used for acne.

Amoxicillin

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

Flucloxacillin

• Inhibits cell wall synthesis, gram positive. Oral and IV 4x/day
• Drug of choice for all s. aureus infections except MRSA.
• Sometimes used as mono therapy for cellulitis.
• Resistance develops via alteration of penicillin binding proteins (PBPs)

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)

• Cell wall synthesis + B lactamase inhibitor
• Gram pos, neg, anti pseudomonal, but no anti anaerobic.
• Given IV tds, expensive
• Resistance: MRSA, hospital acquired gram neg organisms incl pseudomonas. Developed by ESBLs, impermeability, efflux, altered PBPs
• For poly microbial infections, B lactamase producing gram + and – infections, pseudomonas.

Cefuroxime

• Inhibit cells wall synthesis. Gram pos and neg, not active against pseudomonas spp
• IV tds
• Resistance: MRSA, resistance in gram neg. develops via production of ESBLs, impermeability, efflux, altered PBPs
• Cheapest cephalosporin
• Used for pneumonia, serious intra abdominal (with metronidazole), and UTIs

Ceftriaxone

• Inhibit cells wall synthesis. Gram pos and neg, not active against pseudomonas spp
• IV once a day
• Resistance: MRSA, resistance in gram neg. develops via production of ESBLs, impermeability, efflux, altered PBPs
• Expensive
• Used in meningitis and pneumonia

Ceftazidime

• Cell wall inhibitor
• Gram negative incl pseudomonas
• IV tds, expensive
• Resistance: MRSA, resistance in gram negative organisms
• Used for serious Intra abdominal and UTI esp with pseudomonas involvement,
• Do not use for community acquired pneumonias (little activity against s pneumoniae)

Carbapenems: meropenem

• Gram pos, neg, anti anaerobic, anti pseudomonas
• IV tds, expensive
• Resistance: MRSA, rare in gram neg organisms, via production of metallo-B-lactamases and impermeability, efflux, altered PBPs
• Used in polymicrobial infection, ESBL producing gram neg infections, mostly as third line in its settings for abx resistance

Glycopeptides: vancomycin/teicoplanin

• Active against gran positive only. Slowly bactericidal
• IV 1-2 times a day, expensive
• Large molecule, poor penetration, none into CSF
• Nephrotoxic. Must be given by slow infusion otherwise 50% chance of red mans syndrome. (mass histamine release, brigt red, drop in bp)
• Resistant VRE. Develops via alteration of binding site.
• Used for problem gram positive infections

Gentamicin (aminoglycoside)

• Acts on 30s ribosome
• Gram positive (staph not strep) and gram anti-pseudomonal
• IV once a days tds, or bd, inexpensive
• 100% water soluble, requires O2
• Nephrotoxic and ototoxic
• Acquired resistance in gram neg organism. Develops via impermeability and efflux.
• Used as adjunct to serious systemic sepsis including pseudomonas involvement, • true antimicrobial synergy with Beta lactams for IE, group B and enterococcal sepsis.
• Do not use for meningitis or intracellular infections

Macroglides

• Gram positive (H influenzae)
• Oral and IV (thrombophlebitis), twice a day, inexpensive
• azithromycin excellent intracellular penetration
• Resistance known in staphs and streps. Develops by mutation of the binding site.
• Drug of choice for atypical agents, in penicillin allergy, STIs

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