You're browsing as an anonymous user. Join the community today to view notes at your university, edit pages, and share knowledge!

Causative Organisms and Antibiotics Guide

From Mediwikis

Antibiotic choice should always be guided by local policies and microbiology advice. This guide tries to explain how such decisions about antibiotic use are made.

The choice of antibiotic will depend on both the causative organisms, site of infection and patient factors such as allergies and co-morbidities.

Causative organisms

The aim of antibiotic therapy is to use the narrowest spectrum drug possible for an infection. This is to avoid resistance developing, and to avoid disruption of host commensal flora, with subsequent risk of Clostridium difficile infection. There are three main ways antibiotics are used in practice:

  • Targeted - To treat a known organism grown from cultures
  • Empirical - To treat a known or suspected infection
  • Prophylaxis - To prevent an infection

The microbiology laboratory will usually provide information on what antibiotics an organism is sensitive to. This information may take longer to obtain, so occasionally an organism will be reported with sensitivities to follow. Generally, the narrowest spectrum antibiotic suitable for the patient should be used.

For empirical and prophylactic therapy consideration should be given to the likely organisms. For example, most celluitis is caused by Staphylococcus aureus or Streptococci spp., so a relatively narrow-spectrum antibacterial can be used first-line. In contrast, an immunocompromised patient can be susceptible to a wide range of organisms, so broad-spectrum cover is used first-line.

Source of infection

It is important to choose an antibiotic that will reach therapeutic levels in the site of infection. This is especially important in urinary tract and gastrointestinal infections.

For example, vancomycin given intravenously is effective against many infections but does not treat C. difficile, as vancomycin is almost entirely renally excreted, so will not achieve therapeutic levels in the gastrointestinal tract. Conversely, oral vancomycin is extremely effective against C. difficile, but is not absorbed from the gut so is ineffective against other infections.

Side-effects and toxicity

Although antibiotics are designed to target bacterial as opposed to mammalian cells they can still cause a variety of adverse effects, ranging from gastrointestinal disturbances to renal or hepatic failure. The risk of adverse effects in an individual patient should be considered when choosing an antibiotic. For example, aminoglycosides are associated with renal toxicity, so should usually be avoided in patients with pre-existing kidney disease.


Many patients report allergic reactions to various antibiotics, though true allergy is uncommon. Often side-effects, such as gastrointestinal disturbances, are reported as allergies. It is important to clarify allergies when taking a history, as while anaphylaxis to a drug is a contra-indication, other reactions are not.


Renal and hepatic impairment can limit antibiotic choice, or affect doses given, depending on how the antibiotic is metabolised. Other co-morbidities can interact with antibiotics, for example quinolones lower the seizure threshold in epilepsy. Other medications can interact with antibiotics, so may need to be adjusted or stopped. For example, simvastatin levels are increased by macrolides, so simvastatin is usually stopped during macrolide therapy.

Route of administration

Categories of antibiotics, see Antibiotics for more information

Ideally, medications should be given orally, and usually this is the only option in primary care. Some antibiotics are not absorbed by the oral route, while some are not available as parenteral preparations.

Pharmacological properties

When learning about a particular antibiotic it may be useful to consider the following:

  • Mechanism and spectrum of action
    • How does it work?
    • Which bacteria does it work on?
  • Routes of administration
    • Oral / IV?
  • Pharmacokinetics
    • Distribution
      • Which body fluids have therapeutic concentrations?
    • Excretion
      • Hepatic metabolism?
      • Renal/ biliary excretion?
  • Adverse effects
    • Especially common or serious side-effects.
    • Do drug levels need to be monitored?

Antibiotics- The Need To Know Table

MRSA Gram Positives Gram Negatives Pseudomonas Anaerobes Atypicals
Amoxicillin/ Ampicillin
1st Generation Cephalosporins
2nd Generation Cephalosporins
3rd Generation Cephalosporins
4th Generation Cephalosporins
5th Generation Cephalosporins

Group Drugs Cellular Target Diseases G- Negative G-Positive Anaerobes Spectrum Cautions
Sulphonamides Sulfadiazine Sulfamethoxazole Folate synthesis from PABA IBD- Sulfasalazine +Pyrimethamine for toxoplasmosis - - - - Folate deficiency (megaloblastic anaemia), Teratogenic, N+V
Trimethoprim Tetrahyrofolate synthesis Urinary-tract infections, Exacerbations of Chronic bronchitis; Co-Trimixazole for Pneumocystis pneumonia (common pneumonia with AIDS) - - - - -
β-Lactams Bacterial cell wall synthesis Broad
Penicillins Benzylpenicillin, Flucloxacillin, Amoxicillin, Ampicillin, Piperacillin, Benzylpenicillin- Bacterial Meningitis, Endocarditis, Skin, soft tissue. Flucloxacillin- bone and joint infections (S. aureus or S. pyogenes). Amoxicilllin- Otitis media, Pneumonia, Bronchitis, Gonnorhea Y Y - N+V, Hypersensitivity, Encaphalopathy, Electrolyte imbalance
Cephalosporins Cefuroxime, Cefalexin, Cefotaxime Septicaemia, pneumonia, meningitis, biliary-tract infections, peritonitis, and urinary-tract infections Y Y Y D+V, Hypersensitivity, Nephrotoxicity, Alcohol intolerance
Carbapenems Imipenem Iatrogenic septicaemia, pneumonia, intra-abdominal, skin and soft-tissue , complicated urinary-tract infections but NOT MRSA Y Y Y
Monobactam Aztreonam Pseudomonas, N. meningitidis, H. influenzae Y N N Narrow As above, but less likely to cause hypersensitivity reaction in those allergic to penicillin
Glycopeptides Vancomycin, Teicoplanin Bacterial cell wall synthesis MRSA, Endocarditis, Peritonitis, C. dif N Y - Narrow Fever, rash, oto/nephrotoxicity
Tetracyclines Tetracycline, Doxycycline, Oxytetracycline Protein synthesis inhibitors (tRNA) Chlamydia, Acne, Exacerbations of Chronic Bronchitis, Lyme disease. Can be used in renal impairment N Y - Broad GI Irritation, Vit B deficiency, Dental staining, Teratogenic, Hepatotoxicity, Photosensitivity
Amphenicols Chloramphenicol Inhibits protein synthesis & chain formation Topically for bacterial conjunctivitis, oral for serious infections- Resistant H. influenzae, meningitis in penicillin-intolerant patients Y Y Y Broad
Aminoglycosides Gentamycin, Streptomycin, Neomycin Disrupts protein synthesis (proofreading), disrupts bacterial cell membrane Endocarditis and other serious infections resistant to other antibiotics Y Some N Must be given IV, and calculated by weight.
Macrolides Erythromycin, Clarithromycin, Azithromycin Translocation of tRNA during protein synthesis Many similar uses to penicillins- respiratory & soft tissue infections N, except N. gonnorheae, H. influenzae Y - Broad N+V, Some reports of hypersensitivity
Quinolones Ciprofloxacin, Levofloxacin Inhibits DNA supercoiling (Topoisomerase II) Iatrogenic and resistant infections, including pneumonia, C. difficile, catheter-associated infections. Y Y N Broad Mild- N+V, rashes, headache, dizziness
Group Drugs Cellular Target Diseases G- Negative G-Positive Anaerobes Spectrum Cautions

Bacterial Targets

Staphylococcus- Skin, lung, bones, endocarditis

  • Flucloxacillin, Penicillin, Vancomycin(MRSA)

Streptococcus-Skin, pharynx, lung, bones

  • Benzylpenicillin (Penicillin G), amoxicillin

Mycobacterium- Any organ, frequently lungs

  • Rifampicin & Isoniazid

Clostridium- Bowels

  • Metranidazole

Anaerobes- Sepsis, aspiration pneumonia, abcesses, brain

  • Metranidazole

Neisseria- Gonads, meninges

  • Benzylpenicillin (Penicillin G)

Haemophilus- Meninges, skin, bones, septicaemia

  • Co-amoxiclav, clarithromycin, tetracycline, trimethoprim

Pseudomonas- Lung, ears, burns, bones

  • Aminoglycosides, carbapenams.