Antibiotics spectrum of activity
I have attached a table showing the spectrum of activity of most standard antibiotics. The table is for learning purposes only and should be cross referenced with local prescribing policies if used for prescription screening purposes.
The PDF can be downloaded.
My quick (and rough) way of remembering which bacteria go into which classification:
- The defining gram-positives are the Staphylococci and Streptococci. You need to know this.
- Other gram positives are Enterococcus, Listeria and Clostridium species. Tip: Apart from C diff these are not really mentioned much!
- Anaerobes are Clostridium and Bacteroids
- Atypicals are Legionella, Mycoplasma and Chlamydia species.
- Everything else falls into the gram-negative category
Note Acinetobacter baumanii is not on the above chart. It is a gram negative bacteria, and is an opportunistic pathogen. IMPACT drug of choice is IV ampicillin-sulbactam plus an aminoglycoside. Sulbactam is highly active against Acinetobacter, but it can't be given on its own because bacterial resistance against it develops easily.
Where possible narrow spectrum antibiotics are preferred over broad spectrum ones.
Staphs have developed ways of escaping the killing mechanism of penicillins to varying degrees, note how none of the penicillins are able to kill MRSA.
Conversely, Streptococci are still penicillin sensitive, and should always be treated with penicillin rather than anything else more broad spectrum once you know it is the only causative pathogen.
Tazocin is a 'big gun' broad spectrum penicillin antibiotic, but whilst it has anti-pseudomonas activity, it is unable to deal with MRSA.
The cephalosporins shown in this table are of different generations. The ones at the bottom have more anti-gram negative activity than the first generation cephs. Ceftazidime has anti-pseudomonas activity. Cephalosporins are associated with C difficile infection.
Carbapenems meropenem and imipenem are super broad spectrum big guns, but note these are also defeated by MRSA. So far down to this section of the chart the antibiotics have no activity against atypicals.
Macrolides such as clarithromycin are first line for atypical bacteria, which are often seen in community acquired pneumonia. They additionally have activity against Streptococcus pneumoniae. Note chlamydia is the same family as chlamydia which causes the STD, which can also be treated with a macrolide (or doxycycline).
Aminoglycosides such as the prototype gentamicin are useful in that they are narrow spectrum and the bacteria they target are notable - including MRSA and pseudomonas. However gentamicin's drawback is that it is high risk for bacterial resistance, and require careful dosage calculation particularly in renal patients. To reduce risk of bacterial resistance it is often co-prescribed with another antibiotic where it is also synergistic.
Quinolones are noted for their activity against gram negatives including pseudomonas, and atypicals. Ciprofloxacin has the advantage over the other anti-pseudomonas drugs of being an oral drug. Quinolones do however have various weird side effects and drug interactions, and are associated with C difficile infection.
As can be seen in the chart IV vancomycin and teicoplanin can target the full suite of gram positives, including MRSA. Vancomycin is the drug of choice for MRSA. Oral vancomycin can not be absorbed and is for C diff infection in the GIT only.
Metronidazole is good for anaerobes and protozoa (not shown in the table - such as trichomoniasis, giardiasis, etc) and is first line for C diff infection.
Linezolid like vancomycin also covers the full suite of gram positives, plus some anaerobes. It may be used as an anti-MRSA back up if vancomycin fails, but is much more expensive.
Doxycycline is similar to clarithromycin in that it also targets atypicals and most of the other common pneumonia causing bacteria, including Haemophilus influenzae. However Augmentin and clarithromycin are standard first line antibiotics for community acquired pneumonia, as these are superior with less side effects.