Microorganisms are ubiquitous. Although many microorganisms are beneficial and necessary for human well-being, microbial activities may have undesirable consequences, such as food spoilage and disease. Therefore it is essential to be able to kill such microorganisms or inhibit their growth to minimize their destructive effects. The goal is to destroy pathogens and prevent their transmission, and to reduce or eliminate microorganisms responsible for the contamination of water, food, and other substances. This is necessary for preventing contamination by microbes in surgery for maintaining asepsis, in food and drug manufacture, etc.
Control of microorganisms can be achieved by physical
or chemical agents.
Sterilization
is the process by which all living cells, viable spores, viruses, and viroids
are either destroyed or removed from an object or habitat. A sterile object is totally free of
viable microorganisms, spores, and other infectious agents. When sterilization
is achieved by a chemical agent, the chemical is called a sterilant.
Disinfection
is the killing, inhibition, or removal of microorganisms that may cause
disease. The primary goal is to destroy potential pathogens, but disinfection
also substantially reduces the total microbial population. Disinfectants are agents, usually chemical, used to carry out
disinfection and are normally used only on inanimate objects. A disinfectant
does not necessarily sterilize an object because viable spores and a few
microorganisms may remain.
Sanitization
is closely related to disinfection. In sanitization, the microbial population
is reduced to levels that are considered safe by public health standards. The
inanimate object is usually cleaned as well as partially disinfected. For
example, sanitizers are used to clean eating utensils in restaurants.
Antisepsis
is the prevention of infection or sepsis and is accomplished on living tissue
with chemical agents known as antiseptics.
These are chemical agents applied to tissue to prevent infection by killing or
inhibiting pathogen growth and they also reduce the total microbial population.
Decontamination
is the process of making an article or area free of contaminants including
microbial, chemical, radioactive or other hazards.
Substances that kill
organisms often have the suffix –cide, a germicide kills pathogens. A disinfectant or antiseptic can be
particularly effective against a specific group, in which case it may be called
a bactericide, fungicide, algicide, or viricide.
Name of Agents that
inhibit the growth of microorganisms end in -static and when they are removed,
growth will resume. For example,
bacteriostatic and fungistatic.
Conditions
Influencing the Effectiveness of Antimicrobial Agent Activity
The efficiency of an antimicrobial agent (an agent
that kills microorganisms or inhibits their growth) is affected by at least six
factors.
1. Population
size. Because an equal fraction of a microbial population is killed during
each interval, a larger population requires a longer time to die than a smaller
one.
2. Population
composition. The effectiveness of an agent varies greatly with the nature
of the organisms being treated because microorganisms differ markedly in
susceptibility. Bacterial endospores are much more resistant to most
antimicrobial agents than are vegetative forms, and younger cells are usually
more readily destroyed than mature organisms. Some species are able to
withstand adverse conditions better than others. For example Mycobacterium tuberculosis, which causes tuberculosis, is much more
resistant to antimicrobial agents.
3. Concentration or intensity of an antimicrobial agent.Often, the
more concentrated a chemical agent or intense a physical agent, the more
rapidly microorganisms are destroyed. However, agent effectiveness usually is
not directly related toconcentration or intensity. Sometimes an agent is more effective at lower
concentrations. For example, 70% ethanol is more effective than 95% ethanol
because its activity is enhanced by the presence of water.
4. Duration of exposure. The longer a population is exposed to a
microbicidal agent, the more organisms are killed. To achieve sterilization, exposure
duration sufficient to reduce the probability of survival to 10–6 or
less should be used.
5. Temperature. An increase in the temperature at which a chemical
acts often enhances its activity. Frequently a lower concentration of disinfectant
or sterilizing agent can be used at a higher temperature.
6. Local environment. The population to be controlled is surrounded by
environmental factors that may either offer protection or aid in its
destruction. For example, since heat kills more readily at an acid pH, acid
foods and beverages such as fruits and tomatoes are easier to pasteurize than
foods with higher pHs like milk. A second important environmental factor is
organic matter that can protect microorganisms against heating and chemical disinfectants.
For example, the organicmatter in a surface biofilm will protect the biofilm’s microorganisms.
Syringes and medical or
dental equipment should be cleaned before sterilization because the presence of
too much organic matter could protect pathogens and increase the risk of infection.
The same care must be taken when pathogens are destroyed during the preparation
of drinking water. When a city’s water supply has a high content of organic
material, more chlorine must be added to disinfect it.
The
Use of Physical Methods in Control
1. Sunlight
2. Drying
3. Dry
heat
4. Moist
heat
5. Filtration
6. Radiation
7. Ultrasonic
vibrations
Sunlight-
Sunlight possesses bactericidal activity due to ultraviolet rays and heat
rays. Bacteria suspended in water are
readily destroyed by exposure to sunlight.
Drying
– Drying destroys bacteria since moisture is essential for the growth of
bacteria
Heat –
Fire and boiling water has been used for sterilization and disinfection since
the ancient times. Either dry heat or
moist heat could be used. Heat is the most reliable method of
sterilization.
Thermal death point
(TDP) is the lowest temperature at which a microbial suspension is killed in 10
minutes.
Thermal death time
(TDT) is the shortest time needed to kill all organisms in a microbial
suspension at a specific temperature and under defined conditions.
The
decimal reduction time (D) or D
value is the time required to kill 90% of the microorganisms or spores in a
sample at a specified temperature. In a semilogarithmic plot of the population
remaining versus the time of heating, the D value is the time required for the
line to drop by one log cycle or tenfold.
D
values are used to estimate the relative resistance of a microorganism to
different temperatures through calculation of the z value. The z value is the increase in temperature required to
reduce D to 1/10 its value or to reduce it by one log cycle when log D is plotted
against temperature.
The
F value is the time in minutes at a
specific temperature (usually 250°F or 121.1°C) needed to kill a population of
cells or spores.
Factors
influencing sterilization by heat are
1. Nature
of heat – dry heat or moist heat
2.
Temperature
and time
3.
Number
of microorganisms present
4.
Characteristics
of the organisms
5.
Type
of material to be sterilized
Killing
effect of drying heat is due to protein denaturation, oxidative damage, and
high level of electrolytes.
Moist
heat kills by denaturing and coagulating protein. Steam liberates latent heat when it condenses
on a surface and thereby raises the temperature of that surface. When steam condense on spore surface, its
water content will increase and cause hydrolysis and breakdown of
proteins.
Dry heat
Flaming – inoculating loop
or wire, forceps tip, spatula, etc are held in Bunsen burner flame till it is
red hot.
Incineration – contaminated
cloth, animal carcasses etc are burned to destroy microorganisms present in
it.
Hot air oven – this is a dry heat
sterilization method. The items to be
sterilized are paced in an oven at 160- 180oC for 1-2 hours. Dry heat takes more time to kill
microorganisms than moist heat, but it has certain advantages such as it does
not corrode glassware and metal instruments, and can be used to sterilize
powders, oils, liquid paraffin, fats, grease, etc. Dry heat sterilization is slow and is not
useful for heat sensitive materials such as plastic and rubber items.
The
hot air oven is usually heated by electricity using heating elements in the
wall of chamber. Oven will be fitted
with a fan to ensure even distribution of hot air since hot air is a bad
conductor of heat and have low penetrating power. The materials to be sterilized should be dry
and should be arranged in oven such that free circulation of air is
ensured. For sterilization a holding
time of 1 hour at 180oC or 2 hour at 160oC is
required. The oven must be allowed to
cool down before the door is opened, since otherwise glassware may crack due to
sudden or uneven cooling.
Spores
of nontoxigenic strain of Clostridium
tetaniare used a microbiological test of dry heat efficiency. Paper strips impregnated with 106spores
are used, and after sterilization the spores are inoculated into appropriate
media (thioglycollate or cooked meat media) and incubated to test for growth.
Moist Heat
1. Temperature below 1000C
2. Temperature at 1000C
3. Temperature at atmospheric pressure(1000C)
4. Temperature under pressure
Temperature below 1000C–
Pasteurization of milk – Example for this is the Pasteurization of milk. Milk can be pasteurized in different ways. In the holder method the milk is held at 63°C for 30 minutes. In flash pasteurization or high-temperature short-term (HTST) pasteurization there is quick heating to about 72°C for 15 seconds, followed by rapid cooling. Next method is ultrahigh-temperature (UHT) sterilization, where milk is heated at 140 to 150°C for 1 to 3 seconds. By pasteurization all non sporing bacteria such as mycobacteria, brucellae and salmonellae are destroyed. Coxiella burnetii is heat resistant and may survive holder method.
Inspissation – media such as Lowenstein Jensen and Loefflers serum are sterilized by using Inspissator. By heating at 80-850C for 30 minutes on three successive days.
Temperature at 1000C
Boiling – vegetative
bacteria are killed almost immediately at 90-1000C, but sporing
bacteria require prolonged periods of boiling.
Hard water should not be used, sterilization may be promoted by the
addition of 2% sodium bicarbonate.
Temperature at atmospheric pressure (1000C)
–
Free steam is used to sterilize culture media if the media cant withstand high
temperature.
Koch or Arnold Steamer –This is a tinned
copper cabinet with conical lid that enables drainage of condensed steam. Materials to be sterilized is placed in a
perforated tray kept above water level so that they are surrounded by
steam. 90 minutes exposure is needed for
sterilization.
Tyndallisation
or intermittent sterilization – the media is exposed to 1000C
for 20 minutes on three successive days.
Here the first exposure kills all vegetative bacteria. The spores will germinate and will be killed
on the subsequent occasions. This method
fail with spores of certain anaerobes and thermophiles.
Temperature under pressure
Steam
sterilization is carried out with an autoclave, a device like a pressure cooker.
The development of the autoclave by Chamberland in 1884 stimulated the growth
of microbiology. Water is boiled to produce steam, which is released into the
autoclave’s chamber. The air initially present in the chamber is forced out
until the chamber is filled with saturated steam and the outlets are closed.
Hot, saturated steam continues to enter until the chamber reaches the desired
temperature and pressure, usually 121°C and 15 pounds of pressure. At this
temperature saturated steam destroys all vegetative cells and endospores in a
small volume of liquid within 10 to 12 minutes. Treatment is continued for
about 15 minutes to ensure sterility.
The
chamber should not be packed too tightly because the steam needs to circulate
freely and contact everything in the autoclave. Bacterial endospores will be
killed only if they are kept at 121°C for 10 to 12 minutes. When a large volume
of liquid must be sterilized, an extended sterilization time will be needed
because it will take longer for the center of the liquid to reach 121°C.
Moist
heat is thought to kill so effectively by degrading nucleic acids and by
denaturing enzymes and other essential proteins. It also may disrupt cell membranes.
For
determining the efficiency of moist heat sterilization, spores of Bacillus stearothermophilus or Clostridium PA3679 is used as test
organism. The spores of this organism
need an exposure of 12 minutes at 121oC to be killed.
Low Temperatures
Often
the most convenient control technique is to inhibit the growth and reproduction
of microbes by the use of either freezing or refrigeration. This is particularly important in food
microbiology. Freezing at -20°C or lower stops microbial growth because of the
low temperature and the absence of liquid water. Some microorganisms will be
killed by ice crystal disruption of cell membranes, but freezing does not
destroy contaminating microbes.
Refrigeration
slows microbial growth and reproduction, but does not halt it completely. Refrigerated items may be ruined by growth of
psychrophilic and psychrotrophic microorganisms, particularly if water is
present. Thus refrigeration is a good
technique only for shorter-term storage of food and other items.
Filtration
Filtration
is an excellent way to reduce the microbial population in solutions of
heat-sensitive material, and it can be used to sterilize solutions. Rather than
destroying microorganisms, the filter simply removes them.
There
are two types of filters.
Depth filters consist of
fibrous or granular materials that are bonded into a thick layer with twisting
channels of small diameter. The solution containing microorganisms is sucked
through this layer under vacuum, and microbial cells are removed by physical
screening or entrapment and also by adsorption to the surface of the filter
material. Types of Depth filters are
Candle filters are used for
water purification (unglazed ceramic filter such as Chamberland and Doulton
filter and Diatomaceous earth filter such as Berkefield filters and Mandler
filters))
Asbestos filters are disposable
single use discs having high absorbing capacity, alakalinise the filtered
liquid and are carcinogenic. Examples
are Seitz and Sterimat Filters
Sintered glass
filters prepared
by heat fusing finely powdered glass particles.
These are of low absoption property, cleaned easily, but are brittle and
expensive.
Membrane filters are porous circular membranes, a
little over 0.1 mm thick, made of cellulose acetate, cellulose nitrate,
polycarbonate, polyvinylidene fluoride, or other synthetic materials. Although a wide variety of pore sizes are
available, membranes with pores about 0.2 
m in diameter are used to remove most
vegetative cells from solutions. The membranes are held in special holders. The
solution is pulled or forced through the filter with a vacuum or with pressure
from a syringe or peristaltic pump and collected in previously sterilized containers.
Membrane filters remove microorganisms by screening them out much as a sieve
separates large sand particles from small ones.
They can’t filter viruses.
These
filters are used to sterilize pharmaceuticals, ophthalmic solutions, culture
media, oils, antibiotics, and other heat-sensitive solutions.
Air
also can be sterilized by filtration. Two common examples are surgical masks
and cotton plugs on culture vessels that let air in but keep microorganisms
out.
Laminar flow
biological safety cabinets are one of the most important air filtration
systems. It employs high-efficiency
particulate air (HEPA) filters, which remove 99.97% of 0.3µm particles. Laminar
flow biological safety cabinets force air through HEPA filters and pass a
vertical curtain of sterile air across the cabinet opening. This protects a
worker from microorganisms being handled within the cabinet and prevents
contamination of the room. A person uses these cabinets when working with pathogenic
microorganisms. They are also employed in research labs
and industries for conducting assays, preparing media, examining tissue
cultures, etc.
Laminar Flow Cabinets can be produced as both horizontal and
vertical cabinets.
Horizontal Laminar Flow Cabinets – Here the direction
of air is across the work in a horizontal direction. The constant flow of
filtered air provides material and product protection.
Vertical Laminar Flow Cabinets - Here the laminar air is directed vertically
downwards onto the working area. The air can leave the working area via holes
in the base. Vertical flow cabinets can provide greater operator protection.
Radiation
There are 2 general types of radiation used for
sterilization, ionizing radiation and non-ionizing radiation. Ionizing
radiation is the use of short wavelength, high-intensity radiation to destroy
microorganisms. This radiation can come in the form of gamma or X-rays that
react with DNA and cause cell killing. Non-ionizing radiation uses longer
wavelength and lower energy. As a result, non-ionizing radiation loses the
ability to penetrate substances, and can only be used for sterilizing surfaces.
The most common form of non-ionizing radiation is ultraviolet light.
Ultraviolet (UV) radiation
Ultraviolet
lamps are used to sterilize workspaces and tools used in microbiology
laboratories and health care facilities. UV light at germicidal wavelengths
(two peaks, 185 nm and 265 nm) causes adjacent thymine molecules on DNA to
dimerize, thereby inhibiting DNA replication (even though the organism may not
be killed outright, it will not be able to reproduce). However, since
microorganisms can be shielded from ultraviolet light in fissures, cracks and
shaded areas, and it does not penetrate glass, dirt films, water, and other
substances very effectively, UV lamps should only be used as a supplement to
other sterilization techniques.
UV
lamps are sometimes placed on the ceilings of rooms or in biological safety
cabinets to sterilize the air and any exposed surfaces. Because UV radiation
burns the skin and damages eyes, people working in such areas must be certain
the UV lamps are off when the areas are in use. Commercial UV units are
available for water treatment. Pathogens and other microorganisms are destroyed
when a thin layer of water is passed under the lamps.
Ionizing radiation includes gamma rays, X-rays, and high-speed electron
beams.
This is an excellent sterilizing
agent and penetrates deep into objects. It will destroy bacterial endospores
and vegetative cells, both procaryotic and eucaryotic; but not effective
against viruses. Gamma radiation from a cobalt 60 source is used in the cold
sterilization of antibiotics, hormones, sutures, and plastic disposable
supplies such as syringes. Gamma radiation has also been used to process meat
and other food to eliminate the threat of pathogens as Escherichia coli,
Staphylococcus aureus and Campylobacter jejuni. Both the Food and
Drug Administration and the World Health Organization have approved food
irradiation and declared it safe.
Ultrasonic vibrations
Ultrasonic
vibrations possess antibacterial activity.
Ultrasonic vibrations (i.e. having frequency
greater than 18 kHz) can be used to disrupt cells. The cells are subjected to
ultrasonic vibrations by introducing an ultrasonic vibration emitting tip into
the cell suspension. The ultrasonic vibration could be emitted continuously or
in the form of short pulses. A frequency of 25 kHz is commonly used for cell
disruption. The duration of ultrasound needed depends on the cell type, the
sample size and the cell concentration. But this is not widely used to achieve
sterilization or disinfection.
Chemical agents
Chemicals are often employed
in disinfection and antisepsis. Many different chemicals are available for use
as disinfectants, and each has its own advantages and disadvantages. Many
factors such as the kind of microorganisms present, the concentration and
nature of the disinfectant to be used, and the length of treatment, etc
influence the treatment. Dirty surfaces must be cleaned before a disinfectant
or antiseptic is applied.
An ideal disinfectant must be
effective against a wide variety of infectious agents (gram-positive
and gram-negative bacteria,
acid-fast bacteria, bacterial endospores, fungi, and viruses) at high dilutions
and in the presence of organic matter. Although the chemical must be toxic for
infectious agents, it should not be toxic to people or corrosive for common
materials. The disinfectant should be stable upon storage, odorless or with a
pleasant odor, soluble in water and lipids for penetration into microorganisms,
and have a low surface tension so that it can enter cracks in surfaces. If
possible the disinfectant should be relatively inexpensive.
Phenolics
Phenol was the first widely
used antiseptic and disinfectant. In 1867 Joseph Lister employed it to reduce
the risk of infection during operations. Today phenol and phenolics (phenol
derivatives) such as cresols, xylenols, and orthophenylphenol are used as
disinfectants in laboratories and hospitals. The commercial disinfectant Lysol
is made of a mixture of phenolics.
Phenolics act by denaturing
proteins and disrupting cell membranes. Phenolics are tuberculocidal, effective
in the presence of organic material, and remain active on surfaces long after
application. However, they do have a disagreeable odor and can cause skin
irritation.
Hexachlorophene has been one
of the most popular antiseptics because it persists on the skin once applied
and reduces skin bacteria for long periods. However, it can cause brain damage.
Alcohols
Alcohols are among the most
widely used disinfectants and antiseptics. They are bactericidal and fungicidal
but not sporicidal; some lipid-containing viruses are also destroyed. The two
most popular alcohol germicides are ethanol and isopropanol, usually used in
about 70 to 80% concentration. They act by denaturing proteins and possibly by
dissolving membrane lipids. A 10 to 15 minute soaking is sufficient to
disinfect thermometers and small instruments.
Halogens
A halogen is any of the five
elements (fluorine, chlorine, bromine, iodine, and astatine) in the periodic
table. They exist as diatomic molecules in the free state and form salt like
compounds with sodium and most other metals. The halogens iodine and chlorine
are important antimicrobial agents.
Iodine is used as a skin
antiseptic and kills by oxidizing cell constituents and iodinating cell
proteins. At higher concentrations, it may even kill some spores. Iodine often
has been applied as tincture of iodine, 2% or more iodine in a water-ethanol
solution of potassium iodide. Although it is an effective antiseptic, the skin
may be damaged, a stain is left, and iodine allergies can result. More recently
iodine has been complexed with an organic carrier to form an iodophor. Iodophors
are water soluble, stable, and nonstaining, and release iodine slowly to
minimize skin burns and irritation. They are used in hospitals for preoperative
skin degerming and in hospitals and laboratories for disinfecting. Some popular
brands are Wescodyne for skin and laboratory disinfection and Betadine for
wounds.
Chlorine is the usual
disinfectant for municipal water supplies and swimming pools and is also
employed in the dairy and food industries. It may be applied as chlorine gas,
sodium hypochlorite, or calcium hypochlorite, all of which yield hypochlorous
acid (HClO) and then atomic oxygen. The result is oxidation of cellular
materials and destruction of vegetative bacteria and fungi, although not spores. Death of almost all microorganisms usually occurs within
30 minutes.
Since organic
material interferes with chlorine action by reacting with chlorine and its
products, an excess of chlorine is added to ensure microbial destruction.
Another problem is that chlorine
reacts with
organic compounds to form carcinogenic trihalomethanes, which must be monitored
in drinking water. Ozone sometimes has been used successfully as an alternative
to chlorination
in Europe and
Canada.
Chlorine is also
an excellent disinfectant for individual use because it is effective,
inexpensive, and easy to employ. Small quantities of drinking water can be
disinfected with halazone tablets. Halazone (parasulfone dichloramidobenzoic
acid) slowly releases chloride when added to water and disinfects it in about a
half hour. It is frequently used by campers lacking access to uncontaminated
drinking water.
Heavy Metals
For many years
the ions of heavy metals such as mercury, silver, arsenic, zinc, and copper
were used as germicides. A 1%
solution of silver nitrate is often added to the eyes of infants to prevent
ophthalmic gonorrhea. Silver sulfadiazine is used on burns. Copper sulfate is
an effective algicide in lakes and swimming pools. Heavy metals combine with
proteins, often with their sulfhydryl groups, and inactivate them. They may
also precipitate cell proteins.
The biocidal effect of
metals, especially heavy metals, that occurs even in low concentrations is
known as oligodynamic effect (Greek oligos, "few", and dynamis,
"force").
Quaternary Ammonium Compounds
Detergents
are
organic molecules that serve as wetting agents and emulsifiers because they
have both polar hydrophilic and nonpolar hydrophobic ends. Due to their
amphipathic nature, detergents solubilize otherwise insoluble residues and are
very effective cleansing agents. They are different than soaps, which are derived
from fats.
Although anionic detergents
have some antimicrobial properties, only cationic
detergents are effective disinfectants. The most popular of these
disinfectants are quaternary ammonium compounds characterized by a positively
charged quaternary nitrogen and a long hydrophobic aliphatic chain. They
disrupt microbial membranes and may also denature proteins.
Cationic detergents like
benzalkonium chloride and cetylpyridinium chloride kill most bacteria but not M.
tuberculosis or endospores.
They do have the advantages
of being stable, nontoxic, and bland but they are inactivated by hard water and
soap. Cationic detergents are often used as disinfectants for food utensils and
small instruments and as skin antiseptics. Several brands are on the market.
Zephiran contains benzalkonium chloride and Ceepryn, cetylpyridinium chloride.
Aldehydes
Both of the commonly used
aldehydes, formaldehyde and glutaraldehyde, are highly reactive molecules that
combine with nucleic acids and proteins and inactivate them, probably by
crosslinking and alkylating molecules. They are sporicidal and can be used as
chemical sterilants. Formaldehyde is usually dissolved in water or alcohol
before use. A 2% buffered solution of glutaraldehyde is an effective
disinfectant. It is less irritating than formaldehyde and is used to disinfect
hospital and laboratory equipment. Glutaraldehyde usually disinfects objects
within about 10 minutes but may require as long as 12 hours to destroy all
spores.
Sterilizing Gases
Many heat-sensitive items
such as disposable plastic petri dishes and syringes, heart-lung machine
components, sutures, and catheters are now sterilized with ethylene oxide gas.
Ethylene oxide (EtO) is both microbicidal and sporicidal and kills by combining
with cell proteins. It is a particularly effective sterilizing agent because it
rapidly penetrates packing materials, even plastic wraps.
Sterilization is carried out
in a special ethylene oxide sterilizer, very much resembling an autoclave in
appearance, that controls the EtO concentration, temperature, and humidity.
Because pure EtO is explosive, it is usually supplied in a 10 to 20%
concentration mixed with either CO2 or dichlorodifluoromethane. The ethylene
oxide concentration, humidity, and temperature influence the rate of
sterilization. A clean object can be sterilized if treated for 5 to 8 hours at
38°C or 3 to 4 hours at 54°C when the relative humidity is maintained at 40 to
50% and the EtO concentration at 700 mg/liter. Extensive aeration of the
sterilized materials is necessary to remove residual EtO because it is so
toxic.
Betapropiolactone (BPL) is
occasionally employed as a sterilizing gas. In the liquid form it has been used
to sterilize vaccines and sera. BPL decomposes to an inactive form after
several hours and is therefore not as difficult to eliminate as EtO. It also
destroys microorganisms more readily than ethylene oxide but does not penetrate
materials well and may be carcinogenic. For these reasons, BPL has not been
used as extensively as EtO.
Recently vapor-phase hydrogen
peroxide has been used to decontaminate biological safety cabinets.
Dyes
Two
groups of dyes, aniline dyes and acridine dyes are used as skin and wound
antiseptics which are bacteriostatic in action. Aniline dyes in use are
malachite green, brilliant green and crystal violet; these are more active
against gram positive organisms and are inhibited by organic material. Their lethal action is due to the reaction
with the acid group in the cell.
Acridine
dyes such as proflavine, acriflavine, euflavine and aminacrine are active
against gram positive bacteria and are not affected by organic materials. Their
action is by impairing DNA replication.
