DISINFECTION--STERILIZATION--DISINFECTANTS.
The discussion of the physiology of bacteria in the preceding chapters
has shown that a number of environmental factors must be properly
correlated in order that a given organism may thrive. Conversely, it
can be stated that any one of these environmental factors may be so
varied that the organism will be more or less injured, may even be
destroyed by such variation. It has been the thorough study of the
above-mentioned relationships which has led to practical methods for
destroying bacteria, for removing them or preventing their growth when
such procedures become necessary.
The process of killing all the living organisms or of removing them
completely is spoken of as _disinfection_ or as _sterilization_,
according to circumstances. Thus the latter term is applied largely in
the laboratory, while the former more generally in practice outside the
laboratory. So also disinfection is most commonly done with chemical
agents and sterilization by physical means, though exceptions are
numerous. The original idea of disinfection was the destruction of
“infective” organisms, that is, organisms producing disease in man or
animals. A wider knowledge of bacteriology has led to the application
of the term to the destruction of other organisms as well. Thus the
cheese-maker “disinfects” his curing rooms to prevent abnormal ripening
of cheese, and the dairy-worker “disinfects” his premises to avoid bad
flavors, abnormal changes in the butter or milk. _Sterilization_ is
more commonly applied to relatively small objects and _disinfection_ to
larger ones. Thus in the laboratory, instruments, glassware, apparatus,
etc., are “sterilized” while desks, walls and floors are “disinfected.”
The surgeon “sterilizes” his instruments, but “disinfects” his
operating table and room. The dairy-workers mentioned above sterilize
their apparatus, pails, milk bottles, etc. Evidently the object of the
two processes is the same, removing or destroying living organisms, the
name to be applied is largely a question of usage and circumstances.
Any agent which is used to destroy microörganisms is called a
“disinfectant.” Material freed from _living_ organisms is “sterile.”
The process of _preventing the growth_ of organisms without reference
to whether they are killed or removed is spoken of as “_antisepsis_,”
and the agent as an _antiseptic_. Hence a mildly applied “disinfectant”
becomes an “antiseptic,” though it does not necessarily follow that
an “antiseptic” may become a disinfectant when used abundantly. Thus
strong sugar solutions prevent the development of many organisms,
though they do not necessarily kill them.
_Asepsis_ is a term which is restricted almost entirely to surgical
operations and implies the taking of such precautions that foreign
organisms are _kept out_ of the field of operation. Such an operation
is an _aseptic_ one, or performed _aseptically_.
A “deodorant or deodorizer” is used to destroy or remove an odor and
does not necessarily have either antiseptic or disinfectant properties.
The agents which are used for the above-described processes may be
conveniently divided into _physical agents_ and _chemical agents_.
PHYSICAL AGENTS.
=1. Drying.=--This is doubtless the oldest method for _preventing the
growth_ of organisms, and the one which is used on the greatest amount
of material at the present time. A very large percentage of commercial
products is preserved and transported intact because the substances
are kept free from moisture. In the laboratory many materials which
are used as food for bacteria (see Chapter XVI) “keep” because they
are dry. Nevertheless, drying should be considered as an _antiseptic_
rather than as a _disinfectant_ process. While it is true that the
_complete_ removal of water would result in the death of all organisms
this necessitates a high temperature, in itself destructive, and does
not occur in practice. Further, though many pathogenic bacteria are
killed by drying, many more, including the spore formers, are not.
Hence drying alone is not a practical method of _disinfecting_.
[Illustration: FIG. 77.--A small laboratory hot-air sterilizer.]
=2. Heat.=--The use of heat in some form is one of the very best
means for destroying bacteria. It may be made use of by combustion,
or burning, as direct exposure to the open flame, as dry heat (hot
air), or as moist heat (boiling water or steam). Very frequently in
veterinary practice, especially in the country, occasionally under
other conditions, the infected material is best burned. This method is
thoroughly effective and frequently the cheapest in the end. Wherever
there are no valid objections it should be used. Exposure to the open
flame is largely a laboratory procedure to sterilize small metallic
instruments and even small pieces of glassware. It is an excellent
procedure in postmortem examinations to burn off the surface of the
body or of an organ when it is desired to obtain bacteria from the
interior free from contamination with surface organisms.
_Dry Heat._--Dry heat is not nearly so effective as moist heat as a
sterilizing agent. The temperature must be higher and continued longer
to accomplish the same result. Thus a dry heat of 150° for thirty
minutes is no more efficient than steam under pressure at 115° for
fifteen minutes. Various forms of hot-air sterilizers are made for
laboratory purposes (Fig. 77). On account of the greater length of
time required for sterilization their use is more and more restricted
to objects which must be used dry, as in blood and serum work, for
example. In practice the use of hot air in disinfecting plants is now
largely restricted to objects which might be injured by steam, as
leather goods, furs, and certain articles of furniture, but even here
chemical agents are more frequently used.
_Moist Heat._--Moist heat may be applied either by boiling in water
or by the use of steam at air pressure, or, for rapid work and on
substances that would not be injured, by steam under pressure.
Boiling is perhaps the best household method for disinfecting all
material which can be so treated. The method is simple, can always
be made use of, and is universally understood. It must be remembered
that all pathogenic organisms, even their spores, are destroyed by a
few minutes’ boiling. The process may be applied to more resistant
organisms, such as are met with in canning vegetables, though the
boiling must be continued for several hours, or what is better,
repeated on several different days. This latter process, known as
“_discontinuous sterilization_,” or “_tyndallization_,” must also be
applied to substances which would be injured or changed in composition
by too long-continued heating, such as gelatin, milk, and certain
sugars. In the laboratory such materials are boiled or subjected to
steaming steam for half an hour on each of three successive days. In
canning vegetables the boiling should be from one to two hours each
day. The principle involved is that the first boiling destroys the
growing cells, but not all spores. Some of the latter germinate by the
next day and are then killed by the second boiling and the remainder
develop and are killed on the third day. Occasionally a fourth boiling
is necessary. It is also true that repeated heating and cooling is more
destructive to bacteria than continuous heating for the same length of
time, but the development of the spores is the more important factor.
Discontinuous heating may also be used at temperatures below the
boiling-point for the sterilization of fluids like blood serum which
would be coagulated by boiling. In this case the material is heated at
55° to 56° for one hour, but on each of seven to ten successive days.
The intermittent heating and cooling is of the same importance as the
development of the spores in this case. (Better results are secured
with such substances by collecting them aseptically in the first place.)
[Illustration: FIG. 78.--The Arnold steam sterilizer for laboratory
use.]
[Illustration: FIG. 79.--Vertical gas-heated laboratory autoclave.]
[Illustration: FIG. 80.--Horizontal gas-heated laboratory autoclave.]
_Steam._--Steam is one of the most commonly employed agents for
sterilization and disinfection. It is used either as “streaming steam”
at air pressure or confined under pressure so that the temperature is
raised. For almost all purposes where boiling is applicable streaming
steam may be substituted. It is just as efficient and frequently
more easily applied. The principle of the numerous forms of “steam
sterilizers” (Fig. 78) is essentially the same. There is a receptacle
for a relatively small quantity of water and means for conducting the
steam generated by boiling this water to the objects to be treated,
which are usually placed immediately above the water. Surgical
instruments may be most conveniently sterilized by boiling or by
steaming in especially constructed instrument sterilizers. If boiled,
the addition of carbonate of soda, about 1 per cent., usually prevents
injury.
[Illustration: FIG. 81.--A battery of two horizontal autoclaves in one
of the author’s student laboratories. Steam is furnished direct from
the University central heating plant.]
_Steam under pressure_ affords a much more rapid and certain method of
destroying organisms. Fifteen to twenty pounds pressure corresponding
to temperatures of 121° to 125° is commonly used. Variations depend on
the bulk and nature of the material. Apparatus for this purpose may
now be obtained from sizes as small as one or two gallons up to huge
structures which will take one or two truckloads of material (Figs.
79-91). The latter type is in common use in canning factories, dairy
plants, hospitals, public institutions, municipal and governmental
disinfecting stations. Very frequently there is an apparatus attached
for producing a vacuum, both to exhaust the air before sterilizing,
so that the steam penetrates much more quickly and thoroughly and for
removing the vapor after sterilizing, thus hastening the drying out of
the material disinfected.
[Illustration: FIG. 82.--A “process kettle” (steam-pressure sterilizer)
used in canning. Diameter, 40 inches; height, 72 inches.]
The smaller types of pressure sterilizers are called “autoclaves”
and have become indispensable in laboratory work. Fifteen pounds
pressure maintained for fifteen minutes is commonly sufficient for a
few small objects. For larger masses much longer time is needed. The
author found that in an autoclave of the type shown in Fig. 81 it
required ten minutes for 500 cc. of water at 15 pounds pressure to
reach a temperature of 100°, starting at room temperatures, 20° to
25°. Autoclaves may be used as simple steam sterilizers by leaving the
escape valves open so that the steam is not confined, hence they have
largely replaced the latter.[18]
[Illustration: FIG. 83.--Horizontal steam chest used in canning.
Height, 32 inches; width, 28 inches; length, 10 feet.]
[Illustration: FIG. 84.--A battery of horizontal rectangular steam
chests in actual use in a canning factory.]
[Illustration: FIG. 85.--A battery of cylindrical process kettles in
actual use in a canning factory.]
A process closely akin to sterilization by heat is _pasteurization_.
This means the heating of material at a temperature and for a time
which will destroy the actively growing bacteria but not the spores.
The methods for doing this vary but are essentially two in principle.
1. The material in small quantities in suitable containers (bottles) is
placed in the apparatus; the temperature is raised to 60° to 65° and
maintained for twenty to thirty minutes and then the whole is cooled
(beer, wine, grape juice, bottled milk) (Figs. 92, 93 and 94).
[Illustration: FIG. 86.--A steam chamber used in government
disinfection work. Size, 4 feet 4 inches × 5 feet 4 inches × 9 feet.]
[Illustration: FIG. 87.--Circular steam chamber used in government
disinfection work, 54 inches in diameter.]
[Illustration: FIG. 88.--Portable steam chamber used in government
disinfection work.]
[Illustration: FIG. 89.--Steam chambers on deck of the U. S. quarantine
station barge “Defender.”]
[Illustration: FIG. 90.--Steam chambers in hold of U. S. quarantine
station barge “Protector.” Disinfected space.]
[Illustration: FIG. 91.--Municipal disinfecting station, Washington,
D. C.]
[Illustration: FIG. 92.--A pasteurizer for milk in bottles.]
[Illustration: FIG. 93.--A pasteurizer for grape juice, cider, etc., in
bottles.]
2. Pasteurizing machines are used and the fluid flows through
continuously. In one type the temperature is raised to 60° and by
“retarders” is kept at this temperature for twenty to thirty minutes
(Figs. 95 to 98). In another type the temperature is raised to as
high as 85° for a few seconds only, “flash process” (Fig. 99), and then
the material is rapidly cooled. It is certain that all pathogenic
microörganisms, except the very few spore formers in that stage, are
killed by proper pasteurization. The process is largely employed in the
fermentation and dairy industries.
[Illustration: FIG. 94.--A pasteurizer for beer in bottles.]
[Illustration: FIG. 95.--A continuous milk pasteurizer.]
[Illustration: FIG. 96.--A pasteurizer for cream to be used in making
ice-cream.]
[Illustration: FIG. 97.--A continuous milk pasteurizer with holder;
capacity 1500 pounds per hour. _A_, pasteurizer--the milk flows in
tubes inside of a jacket of water heated to the proper temperature;
_B_, holder; _C_, water cooler; _D_, brine cooler.]
[Illustration: FIG. 98.--A continuous pasteurizing plant in operation.
Similar to Fig. 97 but larger. Capacity, 12,000 pounds per hour. _A_,
pasteurizer; _B_, seven compartment holder; _C_, _D_, coolers.]
=3. Cold.=--That _cold_ is an excellent _antiseptic_ is illustrated
by the general use of refrigerators and “cold storage.” Numerous
experiments have shown that although many pathogenic organisms of a
given kind are killed by temperatures below freezing, not all of the
same kind are, and many kinds are only slightly affected. Hence cold
cannot be considered a practical means for _disinfection_.
[Illustration: FIG. 99.--A “flash process” pasteurizing outfit, with
holder. _A_, flash pasteurizer; _B_, holder; _C_, cooler.]
=4. Light.=--It has been stated (p. 75) that light is destructive to
bacteria, and the advisability of having well-lighted habitations
for men and animals has been mentioned. The practice of “sunning”
bedclothing, hangings and other large articles which can scarcely be
disinfected in a more convenient way is the usual method of employing
this agent. Drying and the action of the oxygen of the air assist
the process to some extent. Undoubtedly large numbers of pathogenic
organisms are destroyed under natural conditions by the combined
effects of drying, direct sunlight and oxidation, but it should not
be forgotten that a very slight protection will prevent the action of
light (Figs. 100 and 101).
[Illustration: FIG. 100.--Effect of light on bacteria. × 7/10. The
plate was inoculated in the usual way. A letter _H_ of black paper was
pasted on the bottom. The plate was then exposed for four hours to the
sun in January outside the window and then incubated. The black paper
protected the bacteria. Outside of it they were killed except where
they happened to be in large masses. Hence the letter shows distinctly.
(Student preparation.)]
=5. Osmotic Pressure.=--Increase in the concentration of substances
in solution is in practical use as an _antiseptic_ procedure.
Various kinds of “sugar preserves,” salt meats and condensed milk
are illustrations. It must be remembered that a similar increase in
concentration occurs when many substances are dried, and is probably
as valuable in the preservative action as the loss of water. That the
process cannot be depended on to _kill_ even pathogenic organisms is
shown by finding living tubercle bacilli in condensed milk. The placing
of bacteria in water or in salt solution in order to have them die and
disintegrate (greatly aided by vigorous shaking in a shaking machine)
(“autolysis,” p. 126) is a laboratory procedure to obtain cell
constituents. It is not a practical method of disinfection, however.
[Illustration: FIG. 101.--Effect of light on bacteria. × 7/10. This
plate was treated exactly as the plate in Fig. 100, except that the
letter is _L_, and that it was exposed inside the window and wire
screen. The window was plate glass. It is evident that few of the
bacteria were killed, since the letter _L_ is barely outlined. The
exposure was at the same time as the plate in Fig. 100. (Student
preparation.)]
=6. Electricity.=--Electricity, though not in itself injurious to
bacteria, is used as an indirect means for destroying bacteria in a
practical way. This is done by electrical production of some substance
which is destructive to bacteria as in ozone water purification
(Petrograd, Florence, and elsewhere), or the use of ultra-violet rays
for the same purpose (Marseilles, Paris) and for treatment of certain
disease conditions. Electricity might be used as a source of heat for
disinfecting purposes should its cheapness justify it. It has also
been used in the preservation of meats to hasten the _penetration of
the salt_ and thus reduce the time of pickling. Electrolyzed sea water
has been tried as a means of flushing and disinfecting streets, but
it is very doubtful if the added expense is justified by any increased
benefit. A number of electric devices have been put forth for various
sterilizing and disinfecting purposes and doubtless will continue to
be, but everyone should be carefully tested before money is invested in
it.[19]
[Illustration: FIG. 102.--An electric milk purifier (pasteurizer). The
milk flowing from cup to cup completes the circuit when the current is
on. The effect is certainly a heat effect. Sparking occurs at the lips
of the cups.]
[Illustration: FIG. 103.--One of the ten filter beds of the Columbus
water filtration plant with the filtering material removed. Sand is
the filtering material. All of the beds together have a capacity of
30,000,000 gallons daily.]
[Illustration: FIG. 104.--Suction filtration. _A_, Berkefeld filter in
glass cylinder containing the liquid to be filtered; _B_, sterile flask
to receive the filtrate as it is drawn through; _C_, water pump; _D_,
manometer, convenient for detecting leaks as well as showing pressure;
_E_, bottle for reflux water.]
[Illustration: FIG. 105.--Pressure filtration. _A_, cylinder which
contains the filter candle; _B_, cylinder for the liquid to be
filtered; _C_, sterile flask to receive the filtrate; _D_, air pump to
furnish pressure.]
=7. Filtration.=--Filtration is a process for rendering fluids sterile
by passing them through some material which will hold back the
bacteria. It is used on a large scale in the purification of water for
sanitary or manufacturing reasons (Fig. 103). Air is also rendered
“germ free” in some surgical operating rooms, “serum laboratories” and
breweries by filtration. In the laboratory it is a very common method
of sterilizing liquids which would be injured by any other process.
The apparatus consists of a porous cylinder with proper devices for
causing the liquid to pass through either by suction (Fig. 104) where
the pressure will be only one atmosphere (approximately 15 pounds per
square inch), or by the use of compressed air at any desired pressure
(Fig. 105). The two main types of porous cylinders (“filter candles,”
“bougies”) are the Pasteur-Chamberland (Fig. 106) and the Berkefeld.
The former are made of unglazed porcelain of different degrees of
fineness, the latter of diatomaceous earth (Fig. 107) The Mandler
filter of this same material is now manufactured in the United States
and is equal if not superior to the Berkefeld. The designs of complete
apparatus are numerous.
[Illustration: FIG. 106.--Pasteur-Chamberland filter candles about
one-half natural size.]
=8. Burying.=--This is a time-honored method of disposing of infected
material of all kinds and at first thought might not be considered
a means of _disinfection_. As a matter of fact, under favorable
conditions it is an excellent method. The infected material is
removed. Pathogenic organisms tend to die out in the soil owing to an
unfavorable environment as to temperature and food supply, competition
with natural soil organisms for what food there is, and the injurious
effects of the products of these organisms. Care must be taken that
the burial is done in such a way that the _surface_ soil is not
contaminated either directly or by material brought up from below by
digging or burrowing animals, insects, worms, or movement of ground
water to the surface. Also that the underground water supply which is
drawn upon for use by men or animals is not contaminated. Frequently
infected material, carcasses of animals, etc., are treated in some
way so as to aid the natural process of destruction of the organisms
present, especially by the use of certain chemical agents, as quicklime
(see p. 158).
[Illustration: FIG. 107.--Berkefeld filter candles about one-half
natural size.]