Question
Tyndallisation.
Answer
Tyndallisation (intermittent sterilization) is a method of sterilizing heat-labile fluids/media that cannot withstand the high temperature of autoclaving, by exploiting the fact that bacterial spores germinate into heat-susceptible vegetative forms if given a period of favourable incubation.
Procedure: the material is subjected to free-flowing steam at 100°C for 20 minutes on three successive days, with the material kept at room temperature (allowing spore germination) in the intervals between heating sessions. On the first day’s steaming, all vegetative organisms are killed, but any resistant spores survive; during the subsequent incubation period, these spores germinate into vegetative cells (which are much more heat-susceptible); the second day’s steaming kills these newly germinated cells, along with any further spores that had germinated; the third cycle ensures elimination of any remaining resistant forms.
Rationale: named after John Tyndall, who devised the method before the principle of bacterial spore dormancy/germination was fully understood; it achieves an effective sterilization outcome using only 100°C (rather than the 121°C under pressure needed for a single autoclave cycle) by relying on repeated cycles rather than a single higher-temperature exposure.
Uses: sterilizing culture media and other fluids containing heat-labile components (e.g., media with sugars or other substances that would be damaged/degraded by standard autoclaving temperatures), where the material must remain intact but complete sterility (including spore elimination) is still required.
Limitation: time-consuming (three days) and requires that spores actually germinate during the incubation intervals, which depends on suitable conditions (nutrients, temperature) being present in the material.