Control of Microbes by Physical and Chemical Methods One Shot Notes and MCQs

Control of Microbes by Physical and Chemical Methods

The control of microbes is essential in various fields like healthcare, pharmaceuticals, food processing, and water treatment to prevent contamination and infections. This involves the use of physical and chemical methods to eliminate or reduce the microbial population to a safe level.

1. Disinfection

Disinfection refers to the process of killing or inhibiting most pathogenic microorganisms on inanimate surfaces (non-living objects).
Disinfectants are chemical agents used on surfaces and objects to destroy microbes but are not safe for use on living tissues.
Antiseptics, on the other hand, are used on living tissues (e.g., skin) to prevent infection by inhibiting microbial growth.

Types of Disinfection:

High-level disinfection: Kills all microorganisms, including viruses and some spores (e.g., glutaraldehyde).
Intermediate-level disinfection: Eliminates most bacteria, fungi, and viruses but not all spores (e.g., alcohols).
Low-level disinfection: Targets vegetative bacteria and some viruses but does not eliminate spores (e.g., quaternary ammonium compounds).

2. Factors Influencing Disinfectants

Several factors affect the efficiency of disinfectants:

Concentration and Contact Time: A higher concentration and longer exposure time typically result in better disinfection.
Temperature: Warmer temperatures can enhance the activity of disinfectants.
pH: The activity of some disinfectants is pH-dependent (e.g., hypochlorite is more effective at lower pH).
Organic Matter: Blood, pus, or other debris can reduce the effectiveness of disinfectants by inactivating them.
Type of Microorganism: Spores and biofilms are more resistant to disinfectants compared to vegetative cells.
Surface Characteristics: Porous surfaces may harbor microbes that are harder to eradicate than smooth surfaces.
Presence of Water: Some chemical disinfectants require moisture to be effective.

3. Dynamics of Disinfection

The dynamics of disinfection explain how microbes are killed over time. The rate of disinfection follows first-order kinetics, meaning that the number of microbes killed per unit of time is proportional to the microbial load.

Killing Curve:

Microbial death is usually exponential, meaning a constant fraction of microbes is destroyed in each unit of time.
D-value (Decimal reduction time): The time required to kill 90% of the microbial population under specific conditions.

4. Chemical Disinfectants and Antiseptics

Chemical Disinfectants:

Alcohols:
- Ethanol and isopropanol are used as surface disinfectants.
- They denature proteins and disrupt lipid membranes.
- Effective concentration: 60-90%.
Chlorine and Hypochlorites:
- Used for water disinfection (e.g., bleach).
- Release chlorine, which disrupts microbial enzymes and proteins.
Phenolics:
- Used in hospitals to clean surfaces and instruments.
- They denature proteins and disrupt cell membranes.
Quaternary Ammonium Compounds (QACs):
- Used in household disinfectants and hand sanitizers.
- Disrupt cell membranes and are effective against bacteria and some viruses.
Aldehydes (e.g., Formaldehyde, Glutaraldehyde):
- Used to sterilize medical instruments.
- Act by cross-linking proteins and nucleic acids.
Hydrogen Peroxide:
- Used for wound cleaning and surface disinfection.
- Produces reactive oxygen species that damage cellular components.
Iodine and Iodophors:
- Used as skin antiseptics.
- Iodine disrupts protein synthesis and cell membranes.
Chlorhexidine:
- Used as a surgical scrub and antiseptic.
- Effective against bacteria but not spores.

5. Physical Methods for Control of Microbes

Heat:
- Moist heat (autoclaving): Uses steam under pressure to kill microbes and spores (121°C for 15 min).
- Dry heat (hot air oven): Kills microbes by oxidation (170°C for 2 hours).
Filtration:
- Used to sterilize heat-sensitive liquids and gases by physically removing microorganisms.
- Membrane filters with pore sizes of 0.22 µm are common.
Radiation:
- Ultraviolet (UV) radiation: Damages DNA by forming thymine dimers.
- Ionizing radiation (gamma rays): Used to sterilize medical equipment.
Low Temperature:
- Refrigeration (4°C) and freezing (-20°C) slow microbial growth but do not kill all microbes.
Desiccation:
- Removing water from microorganisms inhibits growth.

6. Evaluation of Disinfectants and Antiseptics

Several methods are used to evaluate the effectiveness of disinfectants and antiseptics:

Phenol Coefficient Test:
- Compares the effectiveness of a disinfectant to that of phenol under identical conditions.
- A coefficient greater than 1 indicates a more effective disinfectant than phenol.
Use-Dilution Test:
- Determines the minimum concentration of a disinfectant required to kill a standard microbial population under controlled conditions.
In-Use Test:
- Evaluates the effectiveness of a disinfectant under real-world conditions (e.g., in hospitals).
Filter Paper Method:
- Paper disks soaked in a disinfectant are placed on an agar plate inoculated with a test organism.
- The zone of inhibition around the disk indicates the effectiveness of the disinfectant.
Minimum Inhibitory Concentration (MIC):
- The lowest concentration of a disinfectant or antiseptic required to inhibit visible growth of a microorganism.

Summary

Disinfection involves killing or inhibiting microbial growth on non-living surfaces, whereas antiseptics are used on living tissues.
Chemical disinfectants include alcohols, chlorine, phenolics, QACs, and hydrogen peroxide.
Physical methods like heat, filtration, and radiation play a vital role in sterilization and microbial control.
The effectiveness of disinfectants depends on several factors like concentration, temperature, organic matter, and microbial type.
Evaluation methods like the phenol coefficient test, use-dilution test, and filter paper method ensure that disinfectants are effective in various settings.

MCQs on Control of Microbes by Physical and Chemical Methods

Which of the following is used to sterilize heat-sensitive materials?
a) Autoclaving
b) Dry heat sterilization
c) Filtration
d) UV radiation
Answer: c) Filtration
Which microorganism is most resistant to chemical disinfectants?
a) Gram-positive bacteria
b) Fungi
c) Viruses
d) Bacterial spores
Answer: d) Bacterial spores
The primary action of alcohol as a disinfectant is:
a) Protein coagulation
b) DNA damage
c) Membrane disruption
d) Spore destruction
Answer: c) Membrane disruption
Phenol coefficient measures the effectiveness of a disinfectant in comparison to:
a) Ethanol
b) Hydrogen peroxide
c) Phenol
d) Formaldehyde
Answer: c) Phenol
Which test evaluates the real-world effectiveness of disinfectants?
a) Use-dilution test
b) Phenol coefficient test
c) In-use test
d) MIC test
Answer: c) In-use test
Which of the following is an example of high-level disinfectant?
a) Isopropyl alcohol
b) Glutaraldehyde
c) Quaternary ammonium compounds
d) Chlorhexidine
Answer: b) Glutaraldehyde
Which type of sterilization is achieved by autoclaving?
a) Dry heat sterilization
b) Moist heat sterilization
c) Filtration
d) Radiation
Answer: b) Moist heat sterilization
What is the minimum effective concentration of alcohol used for disinfection?
a) 30%
b) 50%
c) 70%
d) 90%
Answer: c) 70%
Which physical method of sterilization is most effective for liquids?
a) UV radiation
b) Filtration
c) Hot air oven
d) Autoclave
Answer: d) Autoclave
Which chemical agent is used for water disinfection?
a) Alcohol
b) Phenol
c) Hypochlorite
d) Formaldehyde
Answer: c) Hypochlorite
Which of the following methods sterilizes without heat?
a) Autoclave
b) UV radiation
c) Membrane filtration
d) Hot air oven
Answer: c) Membrane filtration
Which property is most important for a disinfectant to be effective in the presence of organic matter?
a) High pH
b) Low temperature
c) Good penetration ability
d) Stability
Answer: c) Good penetration ability
The term “thymine dimer” is related to which method of microbial control?
a) UV radiation
b) Alcohol disinfection
c) Autoclaving
d) Quaternary ammonium compounds
Answer: a) UV radiation
Which of the following disinfectants works by releasing reactive oxygen species?
a) Alcohol
b) Chlorhexidine
c) Hydrogen peroxide
d) Iodine
Answer: c) Hydrogen peroxide
In the use-dilution test, what is the objective?
a) To determine the microbial load
b) To evaluate disinfectant performance under field conditions
c) To find the minimum effective concentration of a disinfectant
d) To test resistance to antibiotics
Answer: c) To find the minimum effective concentration of a disinfectant
Which disinfectant class disrupts cell membranes by lowering surface tension?
a) Alcohols
b) Quaternary ammonium compounds
c) Aldehydes
d) Halogens
Answer: b) Quaternary ammonium compounds
Which gas is used for the sterilization of heat-sensitive instruments?
a) Ethylene oxide
b) Nitrous oxide
c) Carbon dioxide
d) Ozone
Answer: a) Ethylene oxide
What is the role of surfactants in disinfectants?
a) Disrupt microbial enzymes
b) Reduce surface tension
c) Form a protective film
d) Coagulate proteins
Answer: b) Reduce surface tension
Which disinfectant can cross-link proteins and nucleic acids?
a) Alcohol
b) Glutaraldehyde
c) Chlorhexidine
d) Hydrogen peroxide
Answer: b) Glutaraldehyde
Which of the following is not a sterilization method?
a) Hot air oven
b) Autoclave
c) Use of antiseptic
d) Radiation
Answer: c) Use of antiseptic
A D-value refers to the:
a) Time required to kill 50% of microbes
b) Time needed to kill 90% of the microbial population
c) Concentration of a disinfectant required for sterilization
d) Minimum time needed to inhibit microbial growth
Answer: b) Time needed to kill 90% of the microbial population
Which method is ideal for sterilizing heat-sensitive culture media?
a) Autoclaving
b) Dry heat sterilization
c) Filtration
d) UV radiation
Answer: c) Filtration
Which disinfectant is known for leaving a residual antimicrobial effect on surfaces?
a) Alcohol
b) Quaternary ammonium compounds
c) Chlorine
d) Iodine
Answer: b) Quaternary ammonium compounds
What is the purpose of chlorhexidine in healthcare settings?
a) Surface disinfection
b) Skin antiseptic
c) Air sterilization
d) Water purification
Answer: b) Skin antiseptic
Which sterilization method uses gamma rays?
a) Autoclave
b) UV radiation
c) Ionizing radiation
d) Filtration
Answer: c) Ionizing radiation
Which of the following works best in low pH environments?
a) Quaternary ammonium compounds
b) Alcohol
c) Chlorine
d) Phenol
Answer: c) Chlorine
Which method of microbial control is based on first-order kinetics?
a) Disinfection
b) Sterilization
c) Pasteurization
d) Radiation
Answer: a) Disinfection
Which disinfectant is effective against spores?
a) Alcohol
b) Chlorhexidine
c) Glutaraldehyde
d) Iodophors
Answer: c) Glutaraldehyde
The zone of inhibition around a disinfectant-soaked disk measures:
a) Antimicrobial resistance
b) Microbial growth rate
c) Effectiveness of the disinfectant
d) Minimum inhibitory concentration
Answer: c) Effectiveness of the disinfectant
Which physical method is often used for sterilizing plastic syringes?
a) Filtration
b) UV radiation
c) Gamma radiation
d) Autoclaving
Answer: c) Gamma radiation