Some bocteria produce highly resistant cells called endospores (Section 4.12). The heat resistonce of vegetative cells ond endospores from the same orgonism differs considerably. For instance, in the outoclave (see later) a temperature of 121 C is normolly reached. Under these conditions, endospores moy require 4-5 minutes for a decimal reduction. whereos vegetative cells may require only o.1-0.5 min ot 65 C. Becouse of this difference, effective heat sterllzation procedures must be designed to destroy endospores.
Endospores can survive heat that would ropidiy kill vegetative cells of the sorne species. A major factor in heat resistance is the omount and state of woter within the endospore. During endospore formation, the protoplosm is reduced to a minimum volume as a result of the accumulation of caz -dipicolinic ocld complexes and smoll acid-soluble spore proteins (SASP), This mixture forms a cytoplasmic gel, and a thick cortex then forms around the developing endospore. Contraction of the cortex results in a shrunken, dehydrated cell containing only 10-30% of the water of a vegetative cell
The Autoclave
The autoclave is o sealed heating device that allows the entrance of steam under pressure. Killing of heat-resistant endospores requires heating at temperatures above 100 C, the boiling point of water at normal atmospheric pressure. This is accomplished by applying steam under pressure (Flgure 27.30). The outoclave uses steam under 1.1 kilograms square centimeter (kg/cm2) It5 pounds square inch (lb/inz) pressure, which yields a temperature of 121 C. At 121 C, the time to achieve sterilization is generally 10-15 minutes