Unit 13 Micro

Chapter 13: Control of Microbial Growth

Introduction

The control of microbial growth is a fundamental aspect of microbiology, essential for preventing infection, contamination, and spoilage in medical, laboratory, and everyday settings. This chapter explores the principles, terminology, and methods used to reduce or eliminate microorganisms from various environments.

General Considerations in Microbial Control

Decontamination and Its Importance

• Decontamination refers to the use of physical, chemical, or mechanical methods to destroy or reduce undesirable microbes in a given area.

• Primary targets are microbes capable of causing infection or spoilage.

Range of Microbial Targets (from most to least resistant):

• Vegetative bacterial cells and endospores

• Fungal hyphae and spores, yeast

• Protozoan trophozoites and cysts

• Helminths

• Viruses

• Prions

Relative Cleanliness and Microbial Load

"Clean" is a relative term. Different environments require different levels of microbial control:

• Household surfaces: Normally disinfected

• Medical instruments: Sterilized

• Food preservation: Microbial numbers are reduced, not eliminated

Example: A cell phone can have more bacteria than a public restroom. Car interiors also harbor significant microbial loads.

Car Surface	Average number CFUs per 6.5 x 6.5 cm area
Seat belt	256
Door handle	14
Window control	182
Gear shift	482
Radio control button	69
Steering wheel	239
Cup holder	399
Turn signal/engine knob	370
Center console	506

Terminology and Methods of Microbial Control

Key Definitions

• Sterilization: The process of destroying all viable microbes, including endospores and viruses.

• Commercial Sterilization: Killing Clostridium botulinum spores from canned goods.

• Disinfection: The destruction or removal of vegetative pathogens but not bacterial endospores, usually on inanimate objects.

• Antisepsis: Chemicals applied to body surfaces to destroy or inhibit vegetative pathogens.

• Sanitization: Any cleansing technique that reduces the number of microorganisms on inanimate surfaces to safe levels.

• Degermation: Reduction of microbial load from living tissue by mechanical means (e.g., handwashing).

• Asepsis: The absence of significant contamination; aseptic techniques prevent microbial contamination of wounds.

• -cidal agents: Agents that kill microbes (e.g., bactericide, fungicide, virucide, sporicide).

• -static agents: Agents that inhibit microbial growth (e.g., bacteriostatic, fungistatic).

Examples

• Using Lysol to disinfect a lab bench (disinfection)

• Using hand sanitizer on skin (antisepsis)

• Washing dishes with hot water (sanitization)

• Rinsing a wound with water (degermation)

Relative Resistance of Microbes

Levels of Resistance

• Highest resistance: Bacterial endospores (e.g., Bacillus, Clostridium)

• Moderate resistance: Protozoan cysts, naked viruses, bacteria with resistant cell walls (e.g., Mycobacterium, Pseudomonas, Acinetobacter), Staphylococcus

• Least resistance: Most bacterial vegetative cells, fungal spores and hyphae, enveloped viruses, protozoan trophozoites

Microbial Death and Death Curves

Definition and Detection

• Microbial death is defined as the permanent loss of reproductive capability, even under optimum growth conditions.

• Death occurs exponentially, similar to microbial growth curves.

Factors affecting death rate:

• Number of microbes

• Nature of microbes in the population

• Temperature and pH of environment

• Concentration or dosage of agent

• Mode of action of the agent

• Presence of solvents, organic matter, or inhibitors

Practical Concerns in Microbial Control

Choosing a Control Method

• Does the application require sterilization?

• Will the item be reused?

• Can the item withstand heat, pressure, radiation, or chemicals?

• Is the method suitable and cost-effective?

• Can the agent penetrate to the necessary extent?

Modes of Action of Antimicrobial Agents

Targets of Physical and Chemical Agents

• Cell wall: Becomes fragile and lyses (e.g., some drugs, detergents, alcohols)

• Cell membrane: Loses integrity (e.g., surfactants)

• Protein and nucleic acid synthesis: Inhibition of replication, transcription, translation (e.g., chloramphenicol, UV radiation, formaldehyde)

• Protein function: Disruption or denaturation of proteins (e.g., alcohols, phenols, heat, certain metals)

Physical Methods of Microbial Control

Heat

• Moist heat: Lower temperatures and shorter exposure time; causes coagulation and denaturation of proteins.

• Dry heat: Higher temperatures; causes dehydration, alters protein structure, and incineration.

Temperature	Time to Sterilize (Moist Heat)	Time to Sterilize (Dry Heat)
121°C	15 min	600 min
125°C	10 min	—
134°C	3 min	—
130°C	—	150 min
160°C	—	120 min
170°C	—	60 min

Methods of Moist Heat Control

• Autoclaving: Steam under pressure (15 psi, 121°C, 10-40 min); sterilizes by denaturing proteins and destroying membranes and DNA.

• Intermittent sterilization (tyndallization): Free-flowing steam for 30-60 min, incubated, and repeated for 3 days; used for heat-sensitive substances.

• Boiling: 100°C for 30 min; disinfects but does not sterilize (endospores survive).

• Pasteurization: Reduces microbial load in food and beverages without destroying flavor or value; does not sterilize.

• Ultra-high-temperature (UHT) pasteurization: 134°C for 2-5 sec; produces sterile milk.

Methods of Dry Heat Control

• Incineration: Direct flame or electric heating coil; used for loops in labs and waste disposal.

• Hot air (dry) ovens: 150-180°C for 2-4 hours; used for glassware and metal instruments.

Cold and Desiccation

• Cold: Microbistatic (slows growth); refrigeration (0-15°C) and freezing (

  • Desiccation: Gradual removal of water inhibits metabolism; not reliable for sterilization.

  • Lyophilization: Freeze-drying for preservation.

Radiation

• Ionizing radiation: Deep penetration; causes DNA damage (gamma rays, X-rays).

• Non-ionizing radiation: Little penetration; causes pyrimidine dimers in DNA (UV light).

Filtration

• Physical removal of microbes by passing a gas or liquid through a filter.

• Used for heat-sensitive liquids and air purification.

Chemical Methods of Microbial Control

Types of Chemical Agents

• Disinfectants, antiseptics, sterilants, degermers, and preservatives

Desirable qualities of germicides:

• Rapid action in low concentration

• Solubility in water or alcohol, stability

• Broad spectrum, low toxicity

• Penetrating, noncorrosive, nonstaining

• Affordable and readily available

Levels of Chemical Decontamination

• High-level germicides: Kill endospores; sterilants for items contacting sterile tissues

• Intermediate-level: Kill fungal spores, tubercle bacilli, viruses; for items contacting mucous membranes

• Low-level: Eliminate vegetative bacteria, fungi, some viruses; for surfaces contacting skin

Factors Affecting Germicidal Activity

• Nature of material being treated

• Degree of contamination

• Time of exposure

• Concentration and chemical action of agent (expressed as dilution factor or percentage)

Mechanisms of Action of Chemical Agents

• Halogens: Denature proteins by disrupting disulfide bonds

• Phenolics: Disrupt cell walls and membranes, precipitate proteins

• Chlorhexidine: Surfactant and protein denaturant

• Alcohols: Dissolve membrane lipids, coagulate proteins

• Hydrogen peroxide: Produces reactive oxygen species, damages proteins and DNA

• Aldehydes: Alkylate proteins and nucleic acids

• Gases: Alkylating agents, inactivate proteins and DNA

• Detergents: Disrupt membranes, lower surface tension

• Heavy metals: Inactivate enzymes by binding functional groups

• Dyes and alkalis: Denature proteins, inhibit growth

Soap and Mechanical Removal

• Soaps do not kill microbes but remove them mechanically (degerming).

• Antibacterial soaps contain additional agents (e.g., phenols) for enhanced activity.

Examples of Commercial Antimicrobial Products

Product	Active Ingredient	Type
Clorox Sanitizing Wipes	Dimethyl benzyl ammonium chloride	Detergent (quaternary ammonium)
Lysol Mildew Remover	Sodium hypochlorite	Halogen
Bacteria Hand Soap	Benzalkonium chloride	Detergent (quaternary ammonium)
Disinfecting Spray	Alkyl dimethyl benzyl ammonium ethanol	Detergent (quaternary ammonium)
Contact Lens Solution	Polyaminopropyl biguanide	Chlorhexidine
Antibacterial Moist Towelettes	Benzethonium chloride	Detergent (quaternary ammonium)
Hand Wash	Ethanol	Alcohol
Hand Sanitizer	Ethanol	Alcohol
Pine Oil and Surfactant	Aldehydes	Aldehyde
Eye Drops	Sodium chlorite	Halogen

Summary

Effective microbial control requires understanding the types of microbes present, their resistance levels, and the appropriate physical or chemical methods for their removal or destruction. Selection of methods depends on the context, desired level of control, and safety considerations.