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Sterilization may be defined as the statistically complete destruction of all microorganisms including the most resistant bacteria and spores. This is a condition that is difficult to achieve and hard to prove. Whilst there are many chemicals, inorganic and organic, that kill microorganisms they may not be totally effective and can leave undesirable or toxic residues.
Ultraviolet and Ionising radiations are also effective biocides, disrupting or modifying the DNA to prevent replication, but Ultraviolet will not produce the effective results and easy validation that moist heat (steam) sterilization can provide. If sterility is an absolute requirement then today's scientists turn, as their predecessors did, to steam.
Microorganisms tend to become more active as the temperature of their surroundings rises, - most, but not all, die at above 80oC. In the case of Prions the temperature and time requirements for deactivation are much higher. Steam molecules condense on cooler microorganisms, and transfer 2500 joules per gram of steam, very efficiently heating the microorganisms to the temperature at which they are destroyed. Other methods of heating suffer from the much lower heat transfer of hot dry gases and boundary layer effects, which can insulate and protect the microorganisms.
For maximum effect the Steam must be saturated, and this condition, and the temperature and pressure of the steam are easily monitored, facilitating proof that sterilization has occurred. By employing Steam Sterilization techniques a high level of sterility can be achieved and the most popular piece of equipment used in laboratories and hospitals is the steam sterilizer or autoclave.
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The history of microbiology and the development of associated methods starts with Robert Koch. Today, AtmoSAFE will take you on a journey from temperature control in gas-heated incubators to the invention and patentation of mechanical temperature control by Willi Memmert.
The second half of the 19th century was an exciting time for science. Max von Pettenkofer, Rudolf Virchow, Robert Koch and Louis Pasteur - to just name a few - were all ground-breaking pioneers in the field of etiology and the development of hygiene standards. The Charité in Berlin, the Robert KochInstitute and the Pasteur Institute established their worldwide reputation.
Particularly Robert Koch, who - besides Louis Pasteur - is considered to be the father of microbiology, passionately devoted himself to the development of microbiological methods. He replaced the broth that had previously been used as a breeding medium for germ cultivation by solid and transparent media containing gelatin. Furthermore, his research also paved the way for the perfection of the incubator.
At that time, the Berlin-based company Lautenschläger was the leading manufacturer of laboratory equipment. Incubators were always operated at a constant temperature, since temperature control at different temperatures was not yet possible. The "thermostats" were insulated with asbestos or linoleum, heat transfer to the interior was provided through a water jacket, heating was done using gas and even double doors, the interior ones made of glass, were already a matter of course. Calibration of the temperature controller, however, was significantly more difficult than today. Lautenschläger's "thermo regulator" (fig. 2) consisted of a glas tube [G] closed at the bottom end around which the water of the incubator's water jacket could circulate, a space for regulation [R], an adjustable metal tube [Z] and a glass spiral [S] filled with mercury at the bottom end of the glas tube. The gas was introduced at [B] and discharged at [A]. As mercury expands in direct proportion to the temperature, it rose in the glas tube at increasing temperatures, closed the opening [U] of the inlet tube [Z], so that gas could only be introduced in small amounts through a side tube. If the temperaturefell below the desired set temperature, this caused the mercury level to sink and the the main gas tube was opened for reheating. "Calibration" of thetemperature control was done in advance in a waterbath with a slightly higher temperature than the desired constant temperature in the incubator. The inlet tube [Z] was pushed downwards until the gas flame just began to shrink. The water jacket of the incubator was filled with water at the set temperature. Afterwards, the regulator was introduced into the water in the incubator and connected to the gas supply.
It was an exciting time for Willi Memmert when he delivered his first hot air steriliser in 1947. The development of the "Aeolus" marked the beginning of a new passion for the company founder of Memmert – the very passion that had already motivated Robert Koch still prevails at Memmert today: the quest for uniform temperature distribution in the working chamber of an incubator,hot air steriliser or drying oven. Willi Memmert achieved this with ease and had his ground-breaking invention patented in the early 50s already. Until the early 90s, when Memmert as one of the first companies worldwide developed and deployed a PID process controller for temperature regulation, this principle of his mechanical temperature controller guaranteed unique precision.
The greatest inventions are often brilliantly simple. Willi Memmert used the fact that metal surfaces – and also the interior of an incubator or drying oven- expand due to heat. He installed a heated plate in the back of his appliances, connected to a control rod with a higher heat resistance. If the plate expanded due to the heat, the control rod moved upwards, two contacts installed on a seesaw engaged and a relay was triggered, disconnecting the power supply for the heating unit. This way, the incubator or drying oven was used as a control element itself. The way from heating unit to the controller was minimised, and the disadvantages of control delay or inaccuracy as a result of temperature sensors in the working chamber, common until then, were overcome.
Thanks to a control knob filed for patent by Willi Memmert when adjustableincubators, sterilisers and drying ovens were introduced, it was possible to exactly adjust the temperature with a precision of less than ± 0.5 °C. Competitors' appliances generally used printed or engraved scales. The disadvantages were obvious: Back then, each appliance was subject to manufacturer tolerances and a fixed temperature scale made it impossible to calibrate it later on. It has always been Memmert's company philosophy never to save money at the expense of precision in manufacturing. This way, each Memmert appliance was individually calibrated by hand until the introduction of digital control technology. The temperature in the working chamber was measured and then marked on the knob at the corresponding scale position.
However, temperature control only accounted for half the precision ofMemmert's incubators, hot air sterilizers and drying ovens. The other half can be attributed to the unique heating concept. In the early 50s, the appliance walls had been equipped with hollows for heating element integration already. These hollows enlarged the surface for heat transfer and additionally made it possible to place shelves on them. The third decisive advantage was the uniform heat distribution from all sides of the interior.
The will to continuously increase his knowledge made Willi Memmert a pioneer in temperature control technology and Memmert the technological leader for temperature control appliances. If you are interested in finding more answers on the question why Memmert incubators, hot air sterilisers andovens are still of unparalleled precision today.
Picture credit: Lehrbuch der Mikrobiologie, Friedberger und Pfeiffer, 19; German Museum of Medical History
Author: Memmert GmbH