Facets of Steam Sterilization Monitoring

June 1, 2000

Facets of Steam Sterilization Monitoring

Joy T. Kunjappu, PhD, DSc

and sterilization have been favorite recurring themes in infection control. This
repetition stems from the importance of the significant "life and death"
scenario it embraces. Consequently, these topics play a pivotal role in the management of
surgical and allied areas.

Ever since Pasteur promulgated the microbiological implications of infection caused by
microorganisms, many materials and procedures involving germicides, antiseptics, and
bacteriostats have been in vogue to control or arrest their growth. However, these
procedures only served the purpose of eliminating or terminating the microorganisms in a
limited sense and were far away from the desirable objective of their total destruction.
Thus, the term "sterilization" became popular, and it encompasses "...any
process by means of which all forms of microbial life (bacteria, spores, fungi, and
viruses), contained in liquids, on instruments and utensils, or within various substances,
are completely destroyed."1 In modern times, the inclusion of other
infections, such as those caused by prions, falls under this definition.2-3

"Sterilization" is a buzzword in Central Service and Infection Control
departments in any healthcare facility as well as large-scale industrial plants. Various
sterilization processes are in practice today that make use of sterilants such as steam;
ethylene oxide; *-ray and electron beam radiations; ultra-violet (UV) light, chemicals
like formaldehyde, glutaraldehyde, and peracetic acid; and low-temperature hydrogen
peroxide plasma. Sometimes, filtration through special filters physically removes all
microbial life.4 These methods have one goal in common: the total removal or
killing of microorganisms present in the material to be sterilized.

Steam sterilization has acquired a special status as a time-tested sterilization
method. In spite of its inadequacy in sterilizing thermally labile materials, steam
sterilization is still regarded as an ideal choice, primarily due to the non-toxic nature
of steam, its low cost, and the highly efficient sterilizing power it embodies. Other
sterilization methods described above satisfy the need for overcoming the limitations of
high temperature and high pressure steam.

Monitoring the
successful sterilization of materials and equipment used in surgical and related
procedures is of prime concern since even last traces of microorganisms left alive can
inflict infection to a patient. An ideal monitoring protocol will be the one that makes
certain that all the microorganisms present in the system are killed (primary monitoring).
Such a utopian condition obviously cannot be attained due to practical reasons. Hence,
secondary monitoring techniques have been developed where arguments lying in the realm of
inductive and deductive logistics are used to justify the efficacy of the process.

Monitoring steam and other sterilization methods basically are achieved by two general
ways: biological monitoring and chemical monitoring. In biological monitoring, killing
externally added microorganisms (resistant bacterial spore preparations) during a
sterilization cycle is monitored; while in chemical monitoring, the physico-chemical
reactions during a sterilization cycle are monitored. Biological monitoring was considered
to be more realistic than chemical monitoring because, in the former, the efficacy of
sterilization was followed directly by overseeing the system for any traces of added
microorganisms left after sterilization.

In biological monitoring of steam sterilization, bacterial spores, deemed to be most
resistant to moist heat, are incorporated into the detection system called indicators.
More specifically, a spore-bearing organism like Bacillus stearothermophilus (NCA
Strain 1518), which is known to be extremely resistant to moist heat, is used for this
purpose. Total destruction of such a strain is known to occur under steam sterilization
conditions in ~9 minutes at 250º F, and in ~1 minute at 272º F at 100,000 population of
dried spores. If the system is free from these spores subsequent to a sterilization cycle,
it may be inferred that the microorganisms contaminating the sterilized materials will be
totally destroyed well before this period of time. And, indeed, a safety margin is
inducted into the upper limits of these sterilization cycles to ensure the effective
elimination of microbial life with a high probability (1/1,000,000).

Biological monitoring, though convincing and reassuring of the absence of live
microorganisms, is looked at pessimistically due to the inordinate time frames involved in
a typical monitoring experiment. To circumvent such a serious limitation, faster and
convenient methods were sought, and the chemical detection systems were the answer.

In a typical chemical monitoring method for steam sterilization, physico-chemical
transformation of a chemical system on exposure to moist heat is made useful. An ideal
chemical indicator will indicate the temporal sequences of exposure to steam or other
sterilants by distinct color change/s. Time-temperature effects trigger these well
discernible color changes at a stipulated time period.

Before proceeding further to evaluate the potential of similar chemical monitoring
devices to replace the biological monitors, let's examine the birth and evolution of
biological indicators in the context of steam sterilization.5 The first
generation of biological indicators was the Spore Strip. Strips, are made typically of
filter paper, impregnated with known population of bacterial spores and inserted in a
sterilizable glassine envelope. After sterilization, the spore-strip is sent to a
microbiology laboratory for culture test to detect any surviving microorganism. Such a
test usually requires about seven days of incubation and is not generally performed at the
sterilization center. The long wait in getting the test results delays the release of
sterilized packs for their purported function. Thus, "fast with the test" was
the motivating force that led to the invention of biological indicators.

About a quarter century back, a new version of biological indicators came into
existence. They were referred to as "self-contained indicators" because the
growth medium was incorporated in an isolated vial within the outer plastic tube that
contained the spore strip. A pH indicator was used for detection of bacterial growth. This
system responded to bacterial growth in one to two days. Also, it gave the flexibility of
incubating the steam processed biological indicator strip within the department aside from
the short duration needed for the test compared to the classical spore strip. But, even
these reduced time scales were considered to be too long to provide practical feasibility.

Another form
of biological indicator was introduced in the first half of this decade to reduce the
response time using the denaturing of certain enzymes present in the bacterial spore. Such
a procedure improved the time profile for detection and yielded the results in one to
three hours depending on the sterilization cycle. A fluorogenic biochemical reaction
served as the detection system, and a dedicated fluorescence instrument was needed for
this test. A direct correlation between spore killing and enzyme denaturation process
formed the focal point of this method. Moreover, one product was provided with a regular
self-contained spore strip also in the same system for further confirmation of bacterial

A recent addition to this evolutionary saga of biological indicators is an enzyme-based
product with a response time of about 20 to 30 seconds. The indicator system contained a
pure enzyme that underwent denaturing during the sterilization cycle. A reagent added to
the sterilized indicator tube developed an end color in the event of sterilization
failure. This cannot be viewed as a biological indicator since the enzyme itself may be
regarded as a chemical, notwithstanding the fact that its origin may be biological. Some
people consider this type of indicator as a distinct class referred to as "enzyme
indicators." But here, they will be included under chemical indicators, though it
lacks the important characteristic of a chemical indicator, viz. an instant read

Chemical indicators make use of either a physical or chemical transformation of a
chemical under the sterilization conditions leading to color changes. They may be
"process indicators," the type used in autoclave tape or printed on
sterilization bags and pouches, which indicate whether an item is processed or not. Or,
they may be the "tell-tale" type as in a Bowie-Dick pack, which monitors the
progress of the sterilization process manifested in the form of color changes, functioning
as a diagnostic tool. Some chemical indicators are tailor made to detect a particular
sterilization cycle. The chemical indicators are available under different names,6
such as integrators, emulators, multi-parameter indicators, etc. based on the their
function-performance relationship. Some of the terms have their origin in the
classification systems of various enforcement agencies.

Recently, there is a renewed interest in exploring the wide possibilities of chemical
monitoring. It is practically feasible to arrive at an appropriate chemical system that
will respond to all the critical parameters of a specific sterilization cycle for any mode
of sterilization. This is because the immense potential of multimillion chemical reactions
can be adapted, in principle, to suit the requirement of any kind of sterilant.
Furthermore, the abundant variations of color shades that can be generated from many
chemical reactions make it possible to attain any desired color to mark the end point of a
sterilization cycle. Usually, a chemically active system consisting of a thermochromic
material is incorporated in a special ink formulation that is printed on a suitable
substrate to form a chemical indicator. Thermochromism refers to the phenomenon of color
change by the agency of heat.7

Thus, there is a renaissance in the application of chemical indicators in sterilization
technology. A chemical indicator, suitably calibrated, respecting all the critical
parameters of a sterilization cycle can function as an effective substitute of biological
monitoring. A case in point is the chemical indicators belonging to the class of
integrators. These integrating indicator devices for steam respond to the time and
temperature parameters integrating their effects on the performance of the indicator. This
is achieved by regulating the trigger of the color changing chemistry or the physical
process that indicates the end point.

In spite of the inconveniences associated with the use of biological indicators, they
have high status in sterilization technology since recommendations of many professional
bodies make its periodic or frequent use mandatory. For example, the Association of
periOperative Registered Nurses (AORN)8 recommends biological monitoring on a
daily basis; whereas, the Association for the Advancement of Medical Instrumentation
(AAMI)9 stipulates weekly monitoring. However, both the associations recommend
biological monitoring with every load that contains implantable materials.

Some time back, the concept of parametric release was put forward as a means of
sterilization monitoring.10 This is based on the principle that the parameters
such as temperature, pressure, and time related to a (steam) sterilization cycle, if
monitored under controlled conditions for each load, can by itself provide a quality
assurance parameter of successful sterilization and may substitute other modes of
monitoring. Though this concept has obtained respectability in a few European countries,
it has not been acceptable to a vast majority. Steam sterilization monitoring is a
continually evolving topic waiting to absorb novel concepts and technology.

Joy T. Kunjappu, PhD, DSc, is a consultant for Chemicals and Consulting (New York,
NY). He may be reached at (212) 948-4828 and [email protected].
The author would like to thank Dr. Jane Schuman for inoculating useful ideas.

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