The number that decides treatment
The single most useful number in antibacterial medicine is the minimum inhibitory concentration, or MIC: the lowest drug concentration that stops visible bacterial growth in a tube or well. A low MIC means the drug is potent against that organism; a high MIC means you would need a punishing dose, often more than the body can safely reach.
- Prepare a series of tubes with doubling dilutions of the antibiotic (e.g. 0.5, 1, 2, 4, 8 µg/mL).
- Add a standardized number of bacteria to each tube and incubate overnight.
- Read the lowest concentration that stays clear (no growth) — that is the MIC.
- Compare the MIC against published breakpoints to call the strain susceptible or resistant.
Kill or pause?
Stopping growth and actually killing are different feats. A bacteriostatic drug freezes the population so your immune system can mop up; a bactericidal drug kills the cells outright. The related MBC (minimum bactericidal concentration) measures the dose that kills 99.9%. For a patient with a weak immune system or an infection in a hard-to-reach site, the difference can matter a great deal.
Spectrum: who is in range
A drug's antimicrobial spectrum is the list of organisms it reaches at safe doses. A narrow-spectrum agent hits one tribe of bacteria — handy when you know the culprit and want to spare the patient's healthy flora. A broad-spectrum agent covers many tribes — vital when the patient is crashing and you cannot wait for the lab. Gram-positive and Gram-negative bacteria differ in their outer envelope, and that envelope often decides whether a drug can even get in.
MIC and spectrum together turn selective toxicity into numbers. They are also the scoreboard for the next guide: when bacteria fight back, the very first sign is an MIC that quietly climbs out of reach.