DISINFECTION AND STERILIZATION (CONTINUED).
CHEMICAL AGENTS.
A very large number of chemical substances might be used for destroying
bacteria or preventing their growth either through direct injurious
action or by the effect of concentration. Those which are practically
useful are relatively few, though this is one of the commonest methods
of disinfecting and the word “disinfectant” is frequently wrongly
restricted to chemical agents.
Chemical agents act on bacteria in a variety of ways. Most commonly
there is direct union of the chemical with the protoplasm of the cell
and consequent injury. Some times the chemical is first precipitated
on the surface of the cell without penetrating at once. If removed
soon enough, the organism is not destroyed. This is true of bichloride
of mercury and formaldehyde. If bacteria treated with these agents in
injurious strength be washed with ammonia or ammonium sulphate, even
after a time which would otherwise result in their failure to grow,
they will develop. Some chemicals change the reaction of the material
in a direction unfavorable to growth, and if the change is enough,
may even kill the bacteria. Some agents remove a chemical substance
necessary to the growth of the organism and hence inhibit it. Such
actions are mainly preventive (antiseptic) and become disinfectant only
after a long time.
ELEMENTS.
=Oxygen.=--Oxygen as it occurs in the air is probably not injurious
to living bacteria but aids them with the exception of the anaërobes.
In the nascent state especially as liberated from ozone (O₃)
hydrogen peroxide (H₂O₂) and hypochlorites (Ca(ClO)₂) it is strongly
bactericidal.
=Chlorine.=--Chlorine is actively disinfectant and is coming into use
for sterilizing water on a large scale in municipal plants (Fig. 108).
[Illustration: FIG. 108.--Apparatus for sterilizing water with liquid
chlorine.]
_Iodine_ finds extended use in aseptic surgical operations and
antiseptic dressings. Bromine, mercury, silver, gold, nickel, zinc
and copper are markedly germicidal in the elemental state but are not
practical.
COMPOUNDS.
=Calcium Oxide.=--Calcium oxide (CaO), _quick lime_, is an excellent
disinfectant for stables, yards, outhouses, etc., where it is used
in the freshly slaked condition as “white wash;” also to disinfect
carcasses to be buried. It is very efficient against the typhoid
bacillus in water, where it is much used to assist in the softening.
=Chloride of Lime.=--Chloride of lime, _bleaching powder_, which
consists of calcium hypochlorite, the active agent, and chloride and
some unchanged quicklime is one of the most useful disinfectants. It
is employed to sterilize water for drinking purposes on a large scale
and to disinfect sewage plant effluents. A 5 per cent. solution is the
proper strength for ordinary disinfection. Only a supply which is fresh
or has been kept in air-tight containers should be used, as it rapidly
loses strength on exposure to the air. The active agent is nascent
oxygen liberated from the decomposition of the hypochlorite.
=Sodium Hypochlorite.=--Sodium hypochlorite prepared by the
electrolysis of common salt has been used to some extent.
=Bichloride of Mercury.=--Bichloride of mercury, _mercuric chloride,
corrosive sublimate_ (HgCl₂), is the strongest of all disinfectants
under proper conditions. It is also extremely poisonous to men and
animals and great care is necessary in its use. It is precipitated by
albuminous substances and attacks metallic objects, hence should not be
used in the presence of these classes of substances.
It is used in a strength of one part HgCl₂ to 1000 of water for general
disinfection. Ammonium chloride or sodium chloride, common salt, in
quantities equal to the bichloride, or citric acid in one-half of the
amount should be added in making large quantities of solution or for
use with albuminous fluids to prevent precipitation of the mercury
(Fig. 109).
None of the other metallic salts are of value as practical
disinfectants aside from their use in surgical practice. In this
latter class come boric acid, silver nitrate, potassium permanganate.
The strong mineral acids and alkalies are, of course, destructive to
bacteria, but their corrosive effect excludes them from practical use,
except that “lye washes” are of value in cleaning floors and rough
wood-work, but even here better _disinfection_ can be done more easily
and safely.
[Illustration: FIG. 109.--Tanks for bichloride of mercury, government
quarantine disinfecting plant.]
ORGANIC COMPOUNDS.
=Carbolic Acid or Phenol.=--Carbolic acid or phenol (C₆H₅ OH) is one
of the commonest agents in this class. It is used mostly in 5 per
cent. solution as a disinfectant and in 0.5 per cent. solution as an
antiseptic. For use in large quantities the crude is much cheaper and,
according to some experimenters, even more active than the pure acid,
owing to the cresols it contains. The crude acid is commonly mixed with
an equal volume of commercial sulphuric acid and the mixture is added
to enough water to make a 5 per cent. dilution, which is stronger than
either of the ingredients alone in 5 per cent. solution.
=Cresols.=--The cresols (C₆H₄CH₃OH, ortho, meta and para), coal-tar
derivatives, as phenol, are apparently more powerful disinfectants.
A great number of preparations containing them have been put on the
market. _Creolin_ is one which is very much used in veterinary practice
and forms a milky fluid with water, while _lysol_ forms a clear frothy
liquid owing to the presence of soap. Both of these appear to be more
active than carbolic acid and are less poisonous and more agreeable to
use. They are used in 2 to 5 per cent. solution.
=Alcohol.=--Ordinary (ethyl) alcohol (C₂H₅OH) is largely used as a
_preservative_, also as a disinfectant for the body surface, hands, and
arms. Experiments show that alcohol of 70 per cent. strength is most
strongly bactericidal and that absolute alcohol is very slightly so.
=Soap.=--Experimenters have obtained many conflicting results with
soaps when tested on different organisms, as is to be expected from
the great variations in this article. Miss Vera McCoy in the author’s
laboratory carried out experiments with nine commercial soaps--Ivory,
Naphtha, Packer’s Tar, Grandpa’s Tar, Balsam Peru, A. D. S. Carbolic,
German Green, Dutch Cleanser, Sapolio--and obtained abundant growth
from spores of _Bacillus anthracis_, from _Bacterium coli_ and from
_Staphylococcus pyogenes aureus_ in all cases even when the organisms
had been exposed twenty-four hours in 5 per cent. solutions. From
these results and from the wide variations reported in the literature
it is clear that _soap solutions alone cannot be depended on_ as
disinfectants. Medicated soaps do not appear to offer any advantages in
this respect. The amount of the disinfectant which goes into solution
when the soap is dissolved is too small to have any effect.
=Formaldehyde.=--Formaldehyde (HCHO) is perhaps the most largely used
chemical disinfectant at the present time. The substance is a gas
but occurs most commonly in commerce as a watery solution containing
approximately 40 per cent. of the gas. This solution is variously known
as formalin, formol, and formaldehyde solution. The first two names
are patented and the substance under these names usually costs more.
It is used in the gaseous form for disinfecting closed spaces of all
kinds to the exclusion of most other means today. A great many types
of formalin generators have been devised. The gas has little power of
penetration and all material to be reached should be exposed as much as
possible. The dry gas is almost ineffective, so that the objects must
be moistened or vapor generated along with the gas. A common method
in use is to avoid expensive generators by pouring the formaldehyde
solution on permanganate of potash crystals placed in a vessel removed
from inflammable objects on account of the heat developed which
occasionally sets the gas on fire. The formalin is used in amounts
varying from 20 to 32 ounces to 8-1/2 to 13 ounces of permanganate to
each 1000 cubic feet of space. This method is expensive since one pint
(16 ounces) of formalin is sufficient for each 1000 cubic feet, and
since the permanganate is an added expense. Dr. Dixon, Commissioner of
Health of Pennsylvania, recommends the following mixture to replace the
permanganate, claiming that it works more rapidly and is less expensive
and just as efficient:
1. Sodium bichromate, ten ounces.
2. Saturated solution of formaldehyde, sixteen ounces.
3. Common sulphuric acid, one and a half ounces.
Two and three are mixed together and when cool are poured on the
bichromate which is placed in an earthenware jar of a volume about ten
times the quantity of fluid used. The quantities given are for each
1000 cubic feet of space.
A very simple method is to cause the formalin, diluted about twice
with water to furnish moisture enough, to drop by means of a regulated
“separator funnel” on a heated iron plate. The dropping should be so
regulated that each drop is vaporized as it falls. The plate must
have raised edges, pan-shaped, to prevent the drops rolling off when
they first strike the plate. Formaldehyde has no corrosive (except on
iron) or bleaching action, and is the most nearly ideal closed space
disinfectant today. In disinfecting stations it is made use of in
closed sterilizers such as were described under steam disinfection
particularly in connection with vacuum apparatus. It is also used
in solution as a preservative and as a disinfectant. The commonest
strength is 2 or 3 per cent. of formalin or 0.8 to 1.2 per cent. of
the formaldehyde gas. As an _antiseptic_ it is efficient in dilutions
as high as 1 to 2000 of the gas. It is very irritant to mucous
membranes of most individuals.
=Anilin Dyes.=--Some of the anilin dyes show remarkable selective
disinfectant and antiseptic action on certain kinds of bacteria with
little effect on others. This has been well shown by Churchman in his
work on Gentian Violet. This dye inhibits the growth of _Gram positive_
organisms up to a dilution of one part in 300,000 while for _Gram
negative_ organisms it is without effect even in saturated solution.
This is nicely shown in the accompanying illustration. This inhibiting
effect of anilin dyes is taken advantage of in several methods of
isolating bacteria (Chapter XVIII).
[Illustration: FIG. 110.--The lower half of the plate is plain agar
medium, the upper half the same medium plus gentian violet to make one
part in 300,000. The Gram positive organism is on the right and the
Gram negative on the left. Streak inoculations were made across both
media.]
In addition to the above-discussed disinfectants a large number of
substances, particularly organic, are used in medicine, surgery,
dentistry, etc., as more or less strong antiseptics, and the list is a
constantly lengthening one.
In the laboratory chloroform, H₂O₂, ether and other volatile or easily
decomposable substances have been used to sterilize liquids which could
not be treated by heat or by filtration. The agent is removed either
by slow evaporation or by exhausting the fluid with an air pump. The
method is not very satisfactory, nor is absolute sterilization easily
accomplished. It is much better to secure such liquids aseptically
where possible.