slides #8 - Control of Microbial Growth: Principles and Practices

slides 8 - Control of Microbial Growth

Introduction

The control of microbial growth is essential in healthcare, laboratory, and domestic settings to reduce transmission and limit exposure to potentially harmful microorganisms. This process involves a combination of physical, chemical, and mechanical methods to either kill (microbicidal) or inhibit (microbistatic) microbial growth, as well as to physically remove microbes from surfaces.

Key Definitions and Concepts

Basic Terminology

• Sterilization: The complete removal or destruction of all forms of microbial life, including spores.

• Disinfection: Reduction of the number of vegetative organisms to a level considered safe.

• Antisepsis: Disinfection of skin or other living tissues.

• Preservation: Prevention of microbial growth that might damage a product.

• Cleaning: General removal of debris (e.g., dirt, food, blood), which reduces organic matter that supports microbial proliferation.

• Microbicidal: Agents or processes that kill microbes.

• Microbistatic: Agents or processes that inhibit microbial growth without killing.

Principles of Microbial Killing

Factors Affecting Death Rate

• Temperature: Higher temperatures generally increase the rate of microbial death.

• Type of Microbe: Some microbes, such as those in biofilms or endospores, are more resistant.

• Presence of Interfering Substances: Organic matter (e.g., vomit, feces) can protect microbes from control measures.

• Initial Microbial Load: The time to total elimination depends on the starting number of organisms.

Levels of Disinfection in Healthcare

Device Classification and Required Processing

• Critical Items: Require sterilization. These include items that enter sterile tissue or the vascular system (e.g., surgical instruments, catheters, implants, needles).

• Semi-critical Items: Require high-level disinfection or sterilization. These contact mucous membranes or broken skin (e.g., endoscopes, respiratory therapy equipment).

• Non-critical Items: Require intermediate or low-level disinfection. These contact only intact skin (e.g., blood pressure cuffs, stethoscopes, high-touch surfaces).

Linen Handling

• All linens are treated as contaminated and bagged at the point of use.

• Decontamination involves hot water (160°F for 25 min) or cold water (71–77°F) with chlorine bleach or equivalent detergent.

Physical Methods of Microbial Control

Temperature-Based Methods

• Cool Temperatures: Refrigeration slows or halts growth; freezing may kill some organisms (used for preservation).

• Moist Heat: Denatures proteins and destroys membranes. Methods include:

  • Boiling: Disinfection, kills vegetative cells but not endospores.

  • Pasteurization: Disinfection, removes pathogens/spoilage organisms (e.g., 63°C for 30 min, 72°C for 15 sec, or 140°C for 2 sec for UHT).

  • Autoclaving: Sterilization using steam at 121°C for 15 min at 15 psi. Steam must penetrate all surfaces; temperature, not pressure, is the lethal factor.

• Dry Heat: Includes flaming, incineration, and hot air baking (e.g., 171°C for 1 hr or 160°C for 2 hrs).

Autoclave Operation and Monitoring

• Autoclave effectiveness depends on container size, liquid volume, and exposure time.

• Monitoring involves chemical indicators (color change) and biological indicators (spore strips tested for viability).

Container Size	Liquid Volume	Sterilization Time (min)
Test tube (18 x 150 mm)	10 ml	15
Erlenmeyer flask (125 ml)	95 ml	15
Erlenmeyer flask (2000 ml)	1500 ml	30
Fermentation bottle (9000 ml)	6750 ml	70

Pressure (psi in Excess of Atmospheric Pressure)	Temperature (°C)
0	100
5	110
10	116
15	121
20	126
30	135

Radiation

• UV Radiation: Causes abnormal DNA bonds (pyrimidine dimers); poor penetration, mainly for surface disinfection.

• Ionizing Radiation: Causes DNA breaks; effective for heat-sensitive items but hazardous to humans.

• Microwaves: Kill by heat generation, not direct DNA damage.

Filtration

• Used for liquids or gases, especially heat-sensitive materials (e.g., media, vaccines).

• Removal is based on filter pore size (e.g., 0.22 μm or 0.45 μm for bacteria).

• Mechanical methods such as masks also filter microbes from air.

Chemical Methods of Microbial Control

Properties of an Ideal Chemical Disinfectant/Antiseptic

• Broad-spectrum activity

• Fast acting

• Stable and non-toxic to humans

• Effective at room temperature

• Odorless or pleasant smell

• Inexpensive

Modes of Action

• Alteration of cell walls

• Alteration of membrane permeability

• Damage to proteins

• Damage to DNA

Factors Affecting Efficacy

• Treatment site (e.g., skin vs. instruments vs. surfaces)

• Type of microbe (e.g., Gram-positive vs. Gram-negative, presence of endospores)

• Environmental conditions (e.g., presence of organic matter)

Major Classes of Chemical Agents

• Halogens (Iodine, Chlorine): Strong oxidizing agents; chlorine (bleach) is widely used but not for skin.

• Alcohols (Ethanol, Isopropanol): Disinfect skin and surfaces; evaporate quickly.

• Heavy Metals (Silver, Mercury): Denature proteins; silver used in medical products, mercury use limited due to toxicity.

• Quaternary Ammonium Compounds (Quats): Disrupt plasma membranes; more effective against Gram-positive bacteria; used for antisepsis and surface disinfection.

• Aldehydes (Formaldehyde, Glutaraldehyde): Denature proteins; highly effective but irritating; glutaraldehyde can sterilize with prolonged exposure.

• Chemical Sterilants (Ethylene Oxide): Denature proteins; effective at ambient temperatures; toxic, so items require aeration after treatment.

Evaluating Disinfectants

• Manufacturers must prove efficacy.

• Disk-diffusion tests measure the zone of inhibition to assess effectiveness.

Hand Hygiene and Practical Applications

Handwashing

• Soap and water with scrubbing physically remove microbes and reduce their numbers.

• Effective handwashing can prevent millions of deaths from infectious diseases worldwide.

Summary Table: Methods for Controlling Microbial Growth

Method	Purpose	Examples
Physical (Heat)	Sterilization/Disinfection	Autoclave, Pasteurization, Incineration
Physical (Radiation)	Sterilization/Disinfection	UV, Gamma rays
Mechanical	Removal	Filtration, Masks
Chemical	Sterilization/Disinfection/Antisepsis	Alcohols, Halogens, Quats, Aldehydes

Example Applications

• Contaminated wound dressings: Incineration or autoclaving before disposal.

• Used TSA slants: Autoclaving before disposal.

• Glass pipettes: Autoclaving or dry heat sterilization before reuse.

• Plastic petri dishes: Gamma irradiation or ethylene oxide for sterilization.

• Skin inoculation site: Antisepsis with alcohol or iodine.

• Culture spill in lab: Cover with disinfectant (e.g., bleach), allow contact time, then clean up.

• Heat-sensitive endoscope: High-level chemical disinfection (e.g., glutaraldehyde) or gas sterilization (ethylene oxide).

• Media with heat-sensitive components: Filtration sterilization.

• Soiled linens from infectious patient: Hot water wash with bleach or equivalent.

• Computer keyboard in patient room: Routine cleaning and disinfection with appropriate agents.