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All scientists
and technicians who work in microbiology or related areas,
including health care and sterile services, understand the
need for sterilisation. Laboratory equipment needs to be sterile
to prevent contamination, surgical instruments and dressings
need to be sterile to prevent infection, growth media must
be sterile to prevent ambiguous results and discards must
be sterile to ensure that there is no danger of pathogens.
What is rather more difficult to understand is exactly what
is meant by sterilisation. Probably the best definition of
sterilisation is the statistically complete destruction of
all microorganisms including the most resistant bacteria and
spores. This is a condition that is difficult to achieve and
even harder to prove.
Whilst there
are many chemicals, both inorganic and organic, that will
kill micro-organisms, they may not be totally effective and
frequently leave undesirable or even toxic residues. Ultraviolet
and ionising irradiations are also effective biocides. They
disrupt or modify the DNA in the cell to prevent it replicating.
This means that although the cell will be non-viable, it may
still be living. In any case, these methods will only achieve
logarithmic removal not 100% compliance. So if sterility is
an absolute requirement, today's laboratory scientists turn,
as did their predecessors, to steam heat.
Microorganisms
tend to become more active as the temperature of their surroundings
rises but, at around 80oC most, but not all, of them die.
At above 120oC (the temperature of dry saturated steam at
1barg) you can guarantee that there are no living micro-organisms.
The most widely used piece of heat sterilising equipment is
the steam steriliser or autoclave, in which the load to be
sterilised is exposed directly to high temperature steam.
When steam condenses on cooler surfaces, its latent heat is
transferred rapidly and efficiently, so the surface temperature
rises quickly to that at which any microorganisms on it are
destroyed. Other methods of heating, using hot gases like
air or nitrogen, suffer from much lower heat transfer rates
and boundary layer effects, which can insulate and protect
the microorganisms.
The time and
the amount of steam needed for the load to reach sterilisation
temperature will vary with the nature and thermal capacity
of the load. Most autoclaves are user-programmable, allowing
the sterilisation temperature and time and the rate of cooling
to be set to suit the requirements of the load. Although the
temperature of steam is a function of its pressure, simply
achieving pressure in an autoclave does not necessarily mean
that the corresponding temperature has been reached, so temperature
is the critical parameter.
Achieving sterilisation
of the load is only part of the story. Just as critical is
compliance with GLP, UKAS and similar Quality Assurance procedures,
which require proof that sterilisation has been achieved.
For complete sterilisation, the temperature has to be reached
in all parts of the autoclave and its load. This is validated
by automatic monitoring and recording of the temperature inside
the autoclave and the time for which it is held during the
sterilisation procedure.
Astell Scientific,
one of the UK's leading autoclave manufacturers with over
seventy years experience in the business, points out that
it is false economy to buy a cheap autoclave. With several
loads a day, a typical autoclave will perform around 10,000
sterilisation cycles in a ten year period, so reliability
is essential. Not that ten years is old for an autoclave;
there are many examples of twenty-five year old Astell autoclaves
still in daily use. As a UKAS accredited autoclave validation
laboratory, Astell provides pre-delivery validation certification
and annual on-site re-validation. |
