Control of Microbial Growth: Principles, Methods, and Agents

Chapter 7: The Control of Microbial Growth

Introduction

The control of microbial growth is a fundamental aspect of microbiology, essential for preventing infection, contamination, and spoilage. This chapter covers the terminology, principles, physical and chemical methods, and mechanisms by which microbial growth is controlled in laboratory, medical, and industrial settings.

Terminology of Microbial Control

Key Definitions

• Sepsis: Refers to bacterial contamination.

• Asepsis: Absence of significant contamination.

  • Aseptic techniques are used in surgery to prevent microbial contamination of wounds.

• Sterilization: Removal and destruction of all microbial life.

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

• Disinfection: Destroying harmful microorganisms on inanimate objects.

• Antisepsis: Destroying harmful microorganisms from living tissue.

• Degerming: Mechanical removal of microbes from a limited area.

• Sanitization: Lowering microbial counts on eating utensils to safe levels.

• Biocide (germicide): Treatments that kill microbes.

• Bacteriostasis: Inhibiting, not killing, microbes.

The Rate of Microbial Death

Factors Affecting Effectiveness of Treatment

• Number of microbes: Larger populations require longer treatment times.

• Environment: Presence of organic matter, temperature, and biofilms can affect efficacy.

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Some microbes are more resistant than others.

Understanding the Microbial Death Curve

Microbial death is often plotted logarithmically. If the rate of killing is constant, it takes longer to kill all members of a larger population than a smaller one, regardless of the method used (heat or chemicals).

• Death curve equation: $N_t = N_0 imes 10^{-kt}$ Where $N_t$ is the number of surviving cells at time $t$, $N_0$ is the initial number of cells, and $k$ is the death rate constant.

Actions of Microbial Control Agents

Mechanisms of Action

• Alteration of membrane permeability: Causes cell contents to leak out, interfering with cell growth.

• Damage to proteins (enzymes): Heat and chemicals can denature proteins, causing loss of function.

• Damage to nucleic acids: Heat, chemicals, and radiation can damage DNA and RNA, preventing replication and normal metabolic functions.

Physical Methods of Microbial Control

• Heat

• Filtration

• Low Temperature

• High Pressure

• Desiccation

• Osmotic Pressure

• Radiation

Moist Heat Sterilization

• Denatures proteins

• Uses steam and pressure (e.g., autoclave)

• Kills endospores

• Effectiveness depends on volume and sterilization time

• Test strips are used to indicate sterility

Pasteurization

• Reduces spoilage organisms and pathogens

• High-temperature short-time (HTST): 72°C for 15 seconds

• Thermoduric organisms may survive

Filtration

• Passage of substance through a screen-like material

• Used for heat-sensitive materials

• High-efficiency particulate air (HEPA) filters remove microbes >0.3 μm in diameter

• Membrane filters remove microbes >0.22 μm; pores as small as 0.01 μm are available for viruses and large proteins

Low Temperature & Desiccation

• Low temperature: Bacteriostatic effect; slows metabolic rate

  • Refrigeration

  • Deep-freezing

  • Lyophilization (freeze drying)

• Desiccation: Absence of water prevents metabolism; bacteria cannot grow or reproduce but remain alive; endospores can survive for years

Principles of Effective Disinfection

• Concentration of disinfectant

• Presence of organic matter

• pH

• Time of exposure

Chemical Methods of Microbial Control

The Disk-Diffusion Method

• Evaluates efficacy of chemical agents

• Filter paper disks are soaked in a chemical and placed on a culture

• Zone of inhibition around disks indicates effectiveness

Alcohols

• Denature proteins and dissolve lipids

• No effect on endospores and nonenveloped viruses

• Ethanol and isopropanol require water for effectiveness

Biocidal Action of Ethanol (Table 7.6)

Concentration of Ethanol (%)	10	20	30	40	50
Time of Exposure (sec)	G	NG	NG	NG	NG
Note	G = growth, NG = no growth

Heavy Metals

• Oligodynamic action: denature proteins

• Examples: Silver (Ag), Mercury (Hg), Copper (Cu), Zinc (Zn)

Surface-Active Agents

Agent	Action
Soap	Degerming; emulsification
Acid-anionic sanitizers	Anions react with plasma membrane
Quaternary ammonium compounds (quats)	Cations are bactericidal, denature proteins, disrupt plasma membrane; broad spectrum, but ineffective against endospores and mycobacteria

Chemical Structures

• Ammonium ion: $NH_4^+$

• Benzalkonium chloride: $C_6H_5CH_2N(CH_3)_2C_{12}H_{25}Cl$

Effectiveness of Chemical Antimicrobials (Table 7-7)

Chemical Agent	Effect against Endospores	Effect against Mycobacteria
Glutaraldehyde	Fair	Good
Chlorines	Fair	Fair
Alcohols	Poor	Good
Iodine	Poor	Good
Phenolics	Poor	Good
Chlorhexidine	None	Fair
Bisphenols	None	None
Quats	None	None
Silver	None	None

Summary

Effective microbial control requires understanding the terminology, mechanisms, and methods used to inhibit or destroy microorganisms. Both physical and chemical approaches are employed, with their effectiveness influenced by environmental and microbial factors. Selection of appropriate methods is critical in clinical, laboratory, and industrial contexts.