Chapter 7, Problem 1R

The thermal death time for a suspension of Bacillus subtilis endospores is 30 minutes in dry heat and less than 10 minutes in an autoclave. Which type of heat is more effective? Why?

Summary Introduction

To review:

Which type of heat is more effective based on the thermal death time for a suspension of Bacillus subtilis endospores.

Introduction:

Bacillus subtilis is a rod-shaped, Gram-positive bacterium that are naturally found in soil and vegetation. B. subtilis grow in the mesophilic temperature range. The optimal temperature for the growth of this organism is 25ºC -35ºC.

Explanation of Solution

The cell wall is a rigid structure present outside the cell. Bacillus species are generally isolated by soil sprinkle technique and are used in the production of antibiotics. Ethanol, or strong acid or alkali are not effective in destroying the spores of B. subtilis. The use of chemicals generally does not kill the spores, and it only controls their growth and kills the living cells.

The effective way of destroying the endospores is by passing heat. Dry heat will take more time to penetrate through the cell. As moist heat easily penetrates through the cell, it is the effective way for the destruction of the endospores.

Based on the thermal death time (10 minutes) for B. subtilis endospores in an autoclave, moist heat is the effective method to kill B. subtilis endospores. In a sterilizing chamber, autoclave employs high temperature under pressure. It is used in the sterilization for surgical equipment and medical instruments. It is also used in pharmaceutical and food industries. The elevated temperature and pressure with steam creates an environment that is too high for any microorganisms or spores to live.

Conclusion

Moist heat penetrates into the bacterial cells easily and effectively kills vegetative bacteria as well as their spores. Compared to dry heat (thermal death time for B. subtilis endospores: 30 minutes), B. subtilis endospores are more effectively killed by an autoclave (moist heat).