# The Kinetics of Pathogen Killing

Killing microorganisms with chemical agents or by physical means involves a first-order reaction. This implies that no pathogen-killing method kills off all the microorganisms in the target population all at once and instantaneously. Plotting the killing rate against exposure time in a semilog coordinate system results in a straight-line curve (Fig. 1.7).

Sigmoid and asymptotic killing curves are exceptions to the rule of exponential killing rates. The steepness of the killing curves depends on the sensitivity of the microorganisms to the agent as well as on the latter's effectiveness. The survivor/exposure curve drops at a steeper angle when heat is applied, and at a flatter angle with ionizing radiation or chemical disinfectants. Another contributing factor is the number of microorganisms contaminating a product (i.e., its bioburden): when applied to higher organism concentrations, an antimicrobial agent will require a longer exposure time to achieve the same killing effect.

— Bacterial Death Kinetics -

Fig.1.7 The death rate varies

\$ among bacterial species. The

§ - higher the initial concentration j? of a bacterial culture, the longer

.E - an applied antimicrobial agent

§ will require to achieve the same

ID u

Time exposed to antimicrobial agent

Time exposed to antimicrobial agent

Standard sterilization methods extend beyond killing all microorganisms on the target objects to project a theoretical reduction of risk, i.e., the number of organisms per sterilized unit should be equal to or less than 10-6.

The D value (decimal reduction time), which expresses the time required to reduce the organism count by 90%, is a handy index for killing effectiveness.

The concentration (c) of chemical agents plays a significant role in pathogen-killing kinetics. The relation between exposure time (t) and c is called the dilution coefficient (n): t • cn = constant. Each agent has a characteristic coefficient n, for instance five for phenol, which means when c is halved the exposure time must be increased by a factor of 32 to achieve the same effect.

The temperature coefficient describes the influence of temperature on the effectiveness of chemical agents. The higher the temperature, the stronger the effect, i.e., the exposure time required to achieve the same effect is reduced. The coefficient of temperature must be determined experimentally for each combination of antimicrobial agent and pathogen species.