Moist Heat Sterilization Introduction
Of all the options to sterilize (killing or eliminating all microorganisms, which includes the spores of bacterial growth) moist heat that is saturated steam with pressure has been the most frequently utilized and most reliable method. Moisture has more penetrating power than dry heat, and at a certain temperature, causes a greater reduction in the quantity of live microorganisms. Steam sterilization is safe cheap, low-cost, quickly microbicidal, as well as sporicidal. It quickly heats and penetrates into fabrics.
Moist heat sterilization with autoclave is used to sterilization of biohazardous waste as well as heat and moisture-resistant materials, like the aqueous preparation (culture media). This technique is also employed to sterilize surgery dressings as well as medical equipment.
The most popular type of steam sterilizer that is used in the microbiology lab is the gravity displacement one. The other types of autoclaves are vacuum/gravity assisted.
Moist Heat sterilization Principle
The moist heat kills microorganisms through the irreversible degradation of structural proteins and enzyme. The temp at which the denaturation takes place is dependent on the quantity that of water is present. Sterilization with saturated steam therefore requires exact control of temperature, time as well as pressure.
Pressure can be used to attain the temperatures required to kill microorganisms. Certain temperatures are required to ensure that the microbicidal action is maintained. The minimum time for sterilization should be determined from the point when all the items that need to be sterilized are at the required temperature all over.
The suggested time to sterilize using an autoclave should be 15 min at 121°C (200 KPa). The temperature is used to monitor and control the process. The pressure is used mostly to achieve the necessary steam temperature.
Alternative conditions, including various combinations of temperature and timing The following conditions are available.
1 1 atm = 325 Pa
| Temperature (°C) | Approximate corresponding pressure (kPa) | Minimum sterilization time (min) |
| 126-129 | 250 (~2.5 atm) | 10 |
| 134-138 | 300 (~3.0 atm) | 5 |
In certain cases (e.g. thermolabile substances) sterilization is able to be performed at temperatures that are lower than 121°C when the combination of temperature and time is validated.
Monitoring of steam sterilization process
As with other sterilization systems this process is monitored with chemical, mechanical, as well as biological indicators. Steam sterilizers are typically monitored with a printed report (or visually) by taking temperature, the duration at which the temperature is measured as well as pressure.
Chemical indicators are fixed to the outside and integrated inside the package to track the temperature, time and temperatures. Indicates that the autoclave is readily available. The changes in the colour of the tape is a sign of that the sterilization process is in order.
Temperature-monitoring probes should be inserted into representative containers, with additional probes placed in the load at the potentially coolest and least accessible parts of the loaded chamber. The conditions must be within +2 degrees Celsius and +10 10 kPa (+-0.1 atm) of the values required. Each cycle should be documented in a time-temperature graph or any other method suitable to the situation.
Biological Indicators
The efficacy of steam sterilization can be measured by a biomarker using an envelope that is contaminated with spores from Geobacillus Stearothermophilus (formerly Bacillus stearothermophilus, e.g. ATCC 9953 (or CIP 52.81) where the value of D (i.e. 90% reduction of number of microbial species) will be 1.5-2.5 minutes at 121 degrees Celsius and 100 spores per indicator (this is based on the worst-case scenario where an item could include a one hundred spores that have the similar resistance to that of Bacillus Stearothermophilus). After sterilization has been completed, the strip is taken off and placed in tryptone soya broth and incubated for five days. The absence of growth of Geobacillus stearothermophilus suggests proper sterilization.
list of commonly used bilogical indicators (BIs)
| Spores of Bacteria | D Value |
| Geobacillus stearothermophilus (most common) | 1.5-2.5 |
| Bacillus coagulans | 0.3 |
| Clostridium sporogenes | 0.8-1.4 |
| Bacillus atropheus | 0.5 |
Positive results from spore tests are extremely rare and may be the result of the operator’s error, insufficient steam distribution, or even malfunctioning equipment.
Advantages of Steam Sterilization Method
- Non-toxic for staff, patients and the surroundings
- Cycle is easy to manage and keep track of
- Rapidly microbicidal
- The least affected by organic and inorganic soils, among the sterilization methods identified
- Rapid cycle time
- Penetrates medical packing, device lumens
Disadvantages of Steam Sterilization Method
- Deleterious for heat-sensitive instruments
- Instruments of microsurgery damaged by repeated exposure
- It can leave instruments wet which causes them to rust.
- The risk of burning.
