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PHARMACEUTICAL MICROBIOLOGY (BP303T)
Unit-IiI
Part-4
Factors affecting action of Disinfectants and Factors
Affecting Choice Of Antimicrobial Agent.
Name: Mrs. Pooja Deepak Bhandare
Assistant Professor
G H RAISONI UNIVERSITY
SCHOOL OF PHARMACY

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 Factors affecting action of Disinfectants:
• The rate and extent of antibacterial action of the disinfectant depends on
many factors like,
1. Concentration of the disinfectant.
2. Chemical Structure of the disinfectant.
3. Formulation of the disinfectant.
4. Interfering substances in the environment.
5. pH of the surrounding.
6. Potentiation and antagonism of the disinfectants.
7. Surface Tension.
8. Temperature.
9. Time of Contact.
10. Type and no. of microbes present.

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1.Concentration of the disinfectant:
• The rate of killing the microbes varies directly with concentration of the disinfectant.
• The rate of killing is related exponentially with the concentration of the
disinfectant, not linearly.
• There is an optimum concentration of the disinfectant at which it shows
maximum efficacy, below and beyond this concentration the efficacy decreases.
• The Dilution Coefficient is an important characteristic of a disinfectant which
determines how much dilution is to be made for maximum efficiency.
• The dilution coefficient is calculated using following formula,

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Where,
• n= Dilution Coefficient of the Disinfectant.
• t1= Death time @ Concentration C1.
• t2= Death time @ Concentration C2.

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2. Chemical Structure of the disinfectant
• Chemical activity is largely dependent on the chemical structure of the
disinfectant.
• Introduction of an alkyl chain at para position of Phenol increases the
activity, however, when the chain increases more than 6 carbon atoms it
decreases the solubility and disinfectant action.
• Halogenation increases antimicrobial action of the phenol while nitration
reduces it.

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3.Formulation of the disinfectant:
• A good formulation increases the effectiveness of the disinfectant.
• Iodine is virtually insoluble in water and hence is made to dissolve by using
alcohol and potassium iodide solution.
• Addition of a surfactant in iodine solution decreases its odour, staining
problem and increases stability of the preparation.
• Chlorhexidine and Quaternary ammonium compounds show increased
efficiency in 70% alcohol solution than in the aqueous solution.

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4. Interfering substances in the
environment:
• The organic materials like pus, blood etc present at site of action of
disinfectant greatly reduces the activity of disinfectant.
• Presence of fats and oils at the site of action of phenol greatly
reduces its activity.

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5. pH of the surrounding:
• Most bacteria show optimum growth at 6-8 pH.
• Acidic disinfectant shows maximum activity at an acidic pH as they
remain ionized.
• Basic dyes like Acridine and Quaternary ammonium compounds show
maximum activity at basic pH as they remain ionized at that pH.
• Amphoteric surfactants e.g. Tego compounds show good activity at
variety of pH.

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6. Potentiation and antagonism of
the disinfectants:
• Some disinfectants potentiate the activity of other disinfectants while
some antagonize actions of each other.

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7.Surface Tension:
• Surface tension is the tendency of liquid surfaces to shrink into the minimum
surface area possible.
• The lowered surface tension in an aqueous solution of the disinfectant
increases its adsorption on the microbial cell and increases wetting properties
and solubility of the solution.
• A combination of Phenol with soap shows increased disinfectant action as
soap by its property lowers surface tension.

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8. Temperature:
• The action of disinfectant normally increases with temperature at a certain
point above which it decreases.
• The effect of temperature on disinfectant action is expressed by using
“Temperature coefficient” which is denoted by “𝞡” , where as per 10℃ is
denoted by Q10.
• The Q10 is calculated using following formula,

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9. Time of Contact:
• Sufficient time of contact must be allowed to the disinfectant to show
its action.
• The lesser time of contact results in decreased activity of the
disinfectant.

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10. Type and no. of microbes
present:
• Disinfectants are mainly active against vegetative forms of microbes and not
their spore form.
• Bacterial spores are very difficult to destroy however, aldehydes like
formaldehyde are known sporicidals.
• Acid fast bacilli due to presence of fats in their cell membrane are virtually
immune to aqueous solutions of disinfectants but can be killed using
phenols, aldehydes and halogen derivatives

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FACTORS AFFECTINGCHOICE OF ANTIMICROBIAL
AGENT
• Selection of the most appropriate antimicrobial compound for specific
practical application depends on :
1. Properties of chemical agents
2. Environment
3. Types of microorganisms
4. Intended application
5. Toxicity agents
6. Culture state

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1. Properties Of The Chemical Agent
• The process of killing or inhibiting the growth of microorganisms using an
antimicrobial agent is basically that of a chemical reaction and the rate and
extent of this reaction will be influenced by concentration of agent,
temperature, pH and formulation. Tissue toxicity influences whether a
chemical can be used as an antiseptic or preservative, and this limits the
range of agents for these applications or necessitates the use of lower
concentrations of the agent.

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• Organic matter can have a drastic effect on antimicrobial capacity either by adsorption or
chemical inactivation, thus reducing the concentration of active agent in solution or by
acting as a barrier to the penetration of the disinfectant. Blood, body fluids, pus, milk,
food residues or colloidal proteins, even when present in small amounts, all reduce the
effectiveness of antimicrobial agents to varying degrees, and some are seriously affected.
In their normal habitats, microorganisms have a tendency to adhere to surfaces and are
thus less accessible to the chemical agent. Some organisms are specific to certain
environments and their destruction will be of paramount importance in the selection of a
suitable agent, e.g. Legionella in cooling towers and non-potable water supply
systems, Listeria in the dairy and food industry and HBV in blood-contaminated articles.
2. Environmental Factor

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• Dried organic deposits may inhibit penetration of the chemical
agent. Where possible, objects to be disinfected should be
thoroughly cleaned. The presence of ions in water can also affect
activity of antimicrobial agents, thus water for testing biocidal
activity can be made artificially ‘hard’ by addition of ions.

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3. Types of microorganism
• The types of microorganism present and the levels of microbial contamination
(the bioburden) both have a significant effect on the outcome of treatment. If the bioburden
is high, long exposure times and/or higher concentrations of antimicrobial may be required.
Microorganisms vary in their sensitivity to the action of chemical agents. Some organisms
merit attention either because of their resistance to disinfection or because of their
significance in cross-infection or nosocomial (hospital-acquired) infections. Of particular
concern is the significant increase in resistance to disinfectants resulting from microbial
growth in biofilm form rather than free suspension. Microbial biofilms form readily on
available surfaces, posing a serious problem for hospital infection control committees in
advising suitable disinfectants for use in such situations.

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• The efficacy of an antimicrobial agent must be investigated by appropriate
capacity, challenge and in-use tests to ensure that a standard is obtained which
is appropriate to the intended use. In practice, it is not usually possible to know
which organisms are present on the articles being treated. Thus, it is necessary
to categorize agents according to their antimicrobial activity and for the user to
be aware of the level of antimicrobial action required in a particular situation.

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4. Intended application
• The intended application of the antimicrobial agent, whether for
preservation, antisepsis or disinfection, will influence its selection and also
affect its performance. For example, in medicinal preparations the
ingredients in the formulation may antagonize preservative activity. The risk
to the patient will depend on whether the antimicrobial is in close contact
with a break in the skin or mucous membranes or is introduced into a sterile
area of the body.

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• In disinfection of instruments, the chemicals used must not adversely affect the
instruments, e.g. cause corrosion of metals, affect clarity or integrity of lenses, or
change the texture of synthetic polymers. Many materials such as fabrics, rubber
and plastics are capable of adsorbing certain disinfectants, e.g. quaternary
ammonium compounds (QACs) are adsorbed by fabrics, while phenolics are
adsorbed by rubber, the consequence of this being a reduction in the
concentration of active compound. A disinfectant can only exert its effect if it is
in contact with the item being treated. Therefore, access to all parts of an
instrument or piece of equipment is essential. For small items, total immersion in
the disinfectant must also be ensured.

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5. Toxicity Of The Agent
• In choosing an antimicrobial agent for a particular application some consideration must be given to
its toxicity. Increasing concern for health and safety is reflected in the Control of Substances
Hazardous to Health (COSHH) Regulations that specify the precautions required in handling toxic
or potentially toxic agents. In respect of disinfectants these regulations affect, particularly, the use of
phenolics, formaldehyde and glutaraldehyde. Toxic volatile substances, in general, should be kept in
covered containers to reduce the level of exposure to irritant vapour and they should be used with an
extractor facility. Limits governing the exposure of individuals to such substances are now listed,
e.g. 0.7 mg/m3 (0.2 ppm) glutaraldehyde for both short-and long-term exposure. Many disinfectants
including the aldehydes, glutaraldehyde less so than formaldehyde, may affect the eyes, skin
(causing contact dermatitis) and induce respiratory distress. Face protection and impermeable nitrile
rubber gloves should be worn when using these agents. Table 19.4 lists the toxicity of many of the
disinfectants in use and other concerns of toxicity are described below for individual agents.

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• The COSHH Regulations specify certain disinfectants that contain active substances
not supported under the BPD that had to be phased out by 2006. Specified
disinfection procedures applied to laboratories in relation to spills and routine use
state that certain phenolic agents (including 2,4,6-trichlorophenol and xylenol) can
no longer be employed in disinfectant products.
• Because of the historically high number of occupational asthma cases caused by
glutaraldehyde (an alkylating agent) products in chemical disinfection of
endoscopes, an HSE report (2007) sought alternatives to this agent. The report
recommended the preferential use of an oxidizing agent such as a chlorine-based or
peroxygen-based product rather than a product containing an alkylating agent.
However, it was recognized that consideration must be given to incompatibility of
disinfectants with endoscope construction materials in some cases

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6. Culture State:
• When the bacterial cell are actively dividing in the log phase of
growth, they are more sensitive to antimicrobial agents.
• The sensitivity may be due to a minor interference with the replication
of nucleic acid and with protein synthesis having a profound effect on
the continuation of high metabolic activity.
• All microorganisms are more susceptible to chemicals at the point of
division.

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THANK YOU