Spectrum of action

This refers to how wide the range of bacteria that the antibiotic affects. Ideally, if one knows what bacteria is causing the problem, a narrow spectrum drug should be used, since it will not disturb the natural commensals of the body, whereas a broad spectrum drug will knock out all the normal flora of the body.

Broad spectrum

1. If someone has meningitis, you do not have time to find the specific bacterial cause, so you pump them full of 'BenPen' (benzylpenicillin).
2. If the bacteria is resistant to the narrow spectrum antibiotic, use a broad spectrum; eg. methicillin resistant staphylococcus aureus is resistant to penicillins, so you give them Levofloxacin, a fluoroquinolone.

Narrow spectrum

1. Penicillin is a narrow spectrum, as were early cephalosporins
2. Most aminoglycosides are narrow spectrum also, effective against aerobic, gram-negative bacteria, such as Pseudomonas