Choosing the right antibiotic requires balancing efficacy, resistance risk, patient safety, and stewardship. Resistance rates vary widely by geography, hospital, and community setting. Always confirm with local microbiology guidance and susceptibility testing (e.g. EUCAST/UKMIC, BNF, NICE).

🌍 Why Sensitivities Matter

Antibiotic resistance is a global health emergency - predicted to cause 10 million deaths per year by 2050 if unchecked.
Empiric antibiotics should cover the likely pathogen but be de-escalated once sensitivities are known.
Broad-spectrum use (e.g. carbapenems, fluoroquinolones) accelerates resistance → use sparingly and document indication.
Local antibiograms (hospital-specific data) guide the best empiric choice.

Gram-positive organisms

These bacteria often cause skin, soft-tissue, bloodstream, and respiratory infections. Resistance to beta-lactams is a major concern (e.g. MRSA, VRE).

Organism	Typical Sensitivities
Enterococcus faecalis	Ampicillin, Vancomycin, Teicoplanin
Enterococcus faecium	Vancomycin, Teicoplanin, Linezolid 💡 Glycopeptide-resistant (VRE): Linezolid, Tigecycline, Daptomycin
MRSA (Methicillin-resistant S. aureus)	Clindamycin, Vancomycin, Rifampicin (⚠️ never alone), Linezolid, Daptomycin, Tetracyclines, Tigecycline, Co-trimoxazole
Staphylococcus aureus (MSSA)	Flucloxacillin (first-line), Clindamycin
Streptococcus pyogenes	Penicillin (no resistance yet reported), Clindamycin, Vancomycin
Streptococcus pneumoniae	Penicillin, Cephalosporins, Levofloxacin, Vancomycin (if resistant)

💡 Clinical pearl: Always treat MRSA bacteraemia in consultation with microbiology. Duration is typically ≥2 weeks IV therapy.

Gram-negative organisms

Gram-negatives often cause urinary tract, intra-abdominal, and hospital-acquired infections. Resistance is rapidly emerging, especially ESBLs and carbapenemase-producers.

Organism	Typical Sensitivities
E. coli / Coliforms	Amoxicillin (if sensitive), Trimethoprim, Cefuroxime, Ciprofloxacin, Co-Amoxiclav
Enterobacter / Citrobacter spp.	Ciprofloxacin, Meropenem, Ertapenem, Aminoglycosides
ESBL-producers	Meropenem (gold standard), Temocillin, Aminoglycosides
CPE (Carbapenemase producers)	Tigecycline, Colistin, Aminoglycosides; combinations may be needed
Haemophilus influenzae	Amoxicillin (resistance common), Co-Amoxiclav, Macrolides, 2nd/3rd-gen Cephalosporins, Ciprofloxacin
Legionella pneumophila	Azithromycin, Levofloxacin, Doxycycline
Neisseria gonorrhoeae	Ceftriaxone (preferred), Cefixime, Spectinomycin
Neisseria meningitidis	Penicillin, Cefotaxime, Ceftriaxone; Chloramphenicol (allergy)
Pseudomonas aeruginosa	Piperacillin-tazobactam, Ceftazidime, Cefepime, Meropenem, Aztreonam, Aminoglycosides, Ciprofloxacin
Salmonella typhi	Ceftriaxone, Azithromycin (for mild disease), Chloramphenicol (⚠️ resistance common)

💡 Clinical pearl: Avoid fluoroquinolones in uncomplicated UTIs unless local resistance rates <10%.

Strict Anaerobes

Important in intra-abdominal infections, abscesses, and dental sepsis. Always cover anaerobes when treating perforations.

Organism	First-line / Alternatives
Bacteroides spp.	Metronidazole, Clindamycin, Co-Amoxiclav, Piperacillin-tazobactam, Meropenem
Clostridioides difficile	Metronidazole (mild), Oral Vancomycin, Fidaxomicin (recurrent)
Clostridium spp. (other)	Penicillin, Metronidazole, Clindamycin
Fusobacterium spp.	Penicillin, Metronidazole, Clindamycin

Other Notable Pathogens

Organism	First-line Treatment
Chlamydia trachomatis	Azithromycin (single dose) or Doxycycline 7 days
Treponema pallidum (Syphilis)	Benzathine Penicillin (gold standard), Doxycycline if allergic
Mycoplasma pneumoniae	Macrolides (Azithromycin, Clarithromycin) or Doxycycline
Mycobacterium tuberculosis	Rifampicin + Isoniazid + Pyrazinamide + Ethambutol (specialist protocols)

Resistance Mechanisms (Exam / Clinical Notes)

Beta-lactamases (ESBLs, AmpC, carbapenemases) → break down penicillins & cephalosporins.
Target modification: MRSA alters penicillin-binding proteins (PBP2a).
Efflux pumps: Pseudomonas, Neisseria resist quinolones and macrolides.
Biofilm production: Coagulase-negative staphylococci, Pseudomonas.

💡 Clinical Stewardship Pearls

Always narrow therapy once culture results are back.
Avoid unnecessary dual cover unless directed by microbiology.
Monitor renal/hepatic function for dose adjustment.
Never use Rifampicin or Fluoroquinolones as monotherapy for serious infections.
Document indication, duration, and review date for every antibiotic prescription.

📚 References

BNF – British National Formulary
NICE Guidelines
EUCAST – European Committee on Antimicrobial Susceptibility Testing
WHO AWaRe Antibiotic Classification (2023 update)

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

Organism and sensitivities

🦠 Organisms and Antibiotic Sensitivities: A Comprehensive Guide