HOW DOES AN ANTIBIOTIC WORK

There are three factors that determines the workings of an antibiotic

The antimicrobial drug effect depends upon

MINIMUM INHIBITORY CONCENTRATION

It is is the lowest concentration of an antibiotic that prevents visible growth of a bacterium.

OPTIMAL DOSE

Normally, its known as the dose of a drug that will produce the desired effect with minimum undesirable symptoms. But in this case we will define it differently.

It is the dose at which inhibitory concentration of 90 percent is achieved at the site of infection.

From these two definitions above , we conclude that lesser is the MIC and the optimal dose, more will be the antimicrobial drug effect.

ANTIMICROBIAL EFFECT W.R.T CONCENTRATION TIME CURVE

Based on this principle , the antibiotics behave in three different ways.

a. TIME DEPENDENT ANTIBIOTICS

If the plasma concentration of the drug is maintained above the MIC (Minimum Inhibitory Concentration) for the longest part of the day / time then maximum antimicrobial drug effect will be seen. So time is the critical factor here. They follow time dependent killing. They are given in continuous doses or multiple doses to achieve the maximal effect. It has no significant post antibiotic effect

Examples include

Beta lactams such as Penicillins
Vancomycin
Clindamycin
Erythromycin
Flucytosine

b. CONCENTERATION DEPENDENT ANTIBIOTICS

If the plasma concentration peak of a drug A is higher then that of a drug B then drug A will have a better antimicrobial effect than drug B. So in concentration dependent antibiotics , peak plasma concentration matters more. They follow concentration dependent killing.  To achieve maximal clinical effect, its given in single high dose rather than dividing the doses. It has significant and strong post antibiotic effect.

Examples include

Aminoglycosides
Rifampicin

c. CONCENTRATION TIME DEPENDENT ANTIBIOTICS

In contrast to the time and concentration depend antibiotics effect, this class has a different response. Maximal clinical effect is achieved if the area under the curve is covered more by the antibiotic. If a drug is administered , it reaches steady state level then surpasses the MIC and then comes back to baseline so practically it includes both the time and concentration.

The AUC here represents the bioavailability and cumulative dose. This class is independent of dosing i.e it can be given together or in divided doses. It has some significant post antibiotic effect.

Examples include

Fluoroquinolones
Azithromycin
Clarithromycin