Controlling Microbial Growth

How Antimicrobial Agents Affect Cells

• Cell wall damage → cell bursts

• Cell membrane disruption → contents leak out

• Protein denaturation → structural or enzymatic failure

• Nucleic acid damage → prevents replication/ protein synthesis

Factors When Choosing Control Methods

• Site to treat (living tissue? hospital instruments?)

• Microbial susceptibility

• Environmental conditions (temp, pH, organic matter)

• Cost, safety, efficiency, stability

Most Resistant Microbes

• Prions

• Endospores

• Mycobacteria

Prions

• misfolded proteins, resist heat/chemicals

Endospores

• tough coat and dormant state

Mycobacteria

• waxy mycolic acid cell wall

Environmental Effects

• Temperature

• pH

• Organic matter

Temperature

• warmer = faster action

pH

• extreme pH enhances some disinfectants

Organic Matter

• blocks chemicals → must clean surfaces first

Measuring Effectiveness of Disinfectants

• Phenol coefficient test

• Use-dilution test

• Disk diffusion test (Kirby-Bauer)

Physical Methods (HEAT)

• Moist heat (autoclave, boiling) – denatures proteins, sterilizes

• Dry heat (oven, incineration) – oxidizes, sterilizes

Physical Methods (COLD)

• Refrigeration – slows growth

• Freezing – stops growth, may kill by ice crystals

Physical Methods (DRYING)

• Desiccation – inhibits growth

• Lyophilization – freeze-drying for long-term storage

Physical Methods (FILTRATION)

• Removes microbes from liquids or air (HEPA)

Physical Methods (OSMOTIC PRESSURE)

• High salt/sugar → plasmolysis

Physical Methods (RADIATION)

• Ionizing (X-rays, gamma) – DNA breaks

• Non-ionizing (UV) – thymine dimers