The document discusses the six critical factors influencing the efficacy of antimicrobial agents, including population size, population composition, concentration, duration of exposure, temperature, and local environment. Each factor plays a unique role, such as varied effectiveness depending on microorganism types and growth stages, and the impact of temperature on antimicrobial activity. Understanding these factors is essential for effective sterilization and antimicrobial treatment.

1. Pharmaceutical
Microbiology
Lecture 10
04.04.2024

2. FACTORS INFLUENCING THE EFFICACY OF
ANTIMICROBIAL AGENTS
It has been well documented that the killing of both pathogenic & non-
pathogenic microorganisms is a complex process.
The efficacy of an antimicrobial agent can be affected by the following six
critical factors:
1. Population Size
2. Population Composition
3. Concentration of Antimicrobial Agent
4. Duration of Exposure
5. Temperature
6. Local Environment

3. 1. Population Size
It is observed that equal fractions of population of
microorganisms are killed during a designated period of
time (interval).
Accordingly, a larger population will certainly needs a
relatively longer duration to be killed than do a smaller one.
Actually, this principle is seriously important when using
chemical antimicrobial agents.

4. 2. Population Composition
 The effectiveness of an antimicrobial agent is also affected by the
types of targeted microorganisms because the sensitivity of each type
of microorganism differs distinctly from another one.
 Additionally, the stage of growth of a microorganism can essentially
influence its sensitivity to antimicrobial agents.

5.  Examples:
1. Microbial endospores are more resistant to a large number of the
antimicrobial agents in comparison to the vegetative (growing &
reproducing) form.
2. Younger cells can be killed more rapidly than do the corresponding
mature ones.
3. Certain species may withstand antimicrobials better than others; like
Mycobacterium tuberculosis (the causative organism for tuberculosis) is
more resistant to antimicrobial agents than do other microorganisms.

6. 3. Concentration of Antimicrobial Agent
One may observe quite often that the more concentrated a ‘chemical
agent’ or ‘intense a physical agent’—the more quickly the
microorganisms get destroyed.

7.  Actually, a small increase in the concentration or an intense spreading
of an antimicrobial agent over the infected surfaces will leads to a
drastic improvement in its effectiveness; however, beyond a certain
point, one may not observe more increase in the rate of killing.
 Furthermore, the ‘Effectiveness of an agent’ is not normally
associated with either concentration or intensity directly. Example :
Ethanol 70% (v/v) is more effective in comparison to 95% (v/v), this is
because (EtOH) activity is markedly enhanced by the presence of
water.

8. 4. Duration of Exposure
 The longer a particular population of microbes is exposed to a -cidal
agent (bactericidal, fungicidal , …), the more number of
microorganisms would be killed.
 Accordingly, an adequate exposure time should used to accomplish a
perfect sterilization.
 Usually, an exposure duration sufficient to decrease the surviving
microbial fraction to 10–6
or less should be used.

9. 5. Temperature
• It has been noticed that an increase in the temperature together with
application of a chemical agent can often results in favorable
improvement in antimicrobial activity.
• Furthermore, a lower concentration of either a sterilizing agent or
disinfectant may be suitably employed at a higher temperature
effectively.

10. 6. Local Environment
• It is, important to note that the microbial population to be controlled is
not isolated; rather surrounded by its environment.
• Several environmental factors can:
 Offer protection, and
 Prevent killing.
• Example:
• As heat kills more rapidly at an acidic pH, hence the acidic beverages and
food products like tomatoes and fruits are more convenient and easier to
be sterilized in comparison to foods having higher pHs e.g., milk.

11. Thanks for Listening
Any question?