MICRO-f11_Ch07narr

Chapter 7 : 

Chapter 7 The Control of Microbial Growth

Ch. 7 Objectives: : 

Ch. 7 Objectives: Students should be able to….. Ch. 7: List 5 physical methods of controlling microbial growth, and give an example of each. *** Why is moist heat much more effective than dry air? Describe how to measure the effectiveness of a chemical disinfectant. List 6 different types of chemical disinfectants and how they damage microbial cells. *** Distinguish between biocidal and biostatic treatments. What are the two most Resistant, and the two most Sensitive microbial structures to antiseptic treatments? Why are these so?

The Control of Microbial Growth : 

The Control of Microbial Growth Sepsis refers to microbial contamination. Asepsis is the absence of significant contamination. Aseptic surgery techniques prevent microbial contamination of wounds.

Terminology : 

Sterilization: Removal of all microbial life. Commercial Sterilization: process kills C. botulinum endospores. (= the test organism!) Disinfection: Removal of pathogens. Fomite = nonliving surface or inanimate object that may carry infectious microbes. Antisepsis: Removal of pathogens from living tissue. (Treatments = less harsh, to preserve tissue.) Degerming: Removal of microbes from a limited area. Sanitization: Lowering microbial counts on eating utensils. Biocide/Germicide/Bacteriocide: Kills microbes. Germistasis/Bacteriostasis: Inhibiting, not killing, microbes. Terminology

Slide 5: 

Bacterial populations die at a constant, logarithmic rate. Figure 7.1a Microbial Death

Effectiveness of antimicrobial treatment depends on: : 

Number of microbes. Environment (organic matter, pH, temperature, biofilms) Time of exposure Microbial characteristics Effectiveness of antimicrobial treatment depends on: Figure 7.1b

** Actions of Microbial Control Agents ** : 

Alteration of Membrane permeability. Leakage Dissolution of PL bilayer Damage to Proteins. Destruction Denaturation Damage to Nucleic Acids. Chemical alterations Destruction or mutations ** Actions of Microbial Control Agents **

7.1) Physical Methods of Microbial Control : 

Heat: Thermal death point (TDP): Lowest temperature at which ALL cells in a culture are killed in 10 min. Thermal death time (TDT): Time to kill ALL cells in a culture (at a given temperature!). Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature. Drop population by an order of magnitude!! 7.1) Physical Methods of Microbial Control

A. Moist Heat : 

denatures proteins Autoclave: Steam under pressure Much higher temps reached at > 1 atm A. Moist Heat Figure 7.2

Moist Heat: (#2) : 

Pasteurization reduces spoilage organisms and pathogens. Equivalent treatments: 63°C for 30 min. High-temperature short-time: 72°C for 15 sec. Ultra-high-temperature: 140°C for <1 sec. Thermoduric organisms survive (& endospores). Moist Heat: (#2)

B. Dry Heat : 

Dry Heat sterilization kills by oxidation. Flaming Incineration Hot-air sterilization Not as effective as Autoclave (moist heat, high press.) B. Dry Heat

Other Physical Methods of Microbial Control : 

Filtration removes microbes. Low temperature inhibits microbial growth. Refrigeration Deep freezing Lyophilization High pressure denatures proteins. Desiccation prevents metabolism. Osmotic pressure causes plasmolysis. Other Physical Methods of Microbial Control

Physical Methods of Microbial Control : 

Radiation damages DNA Ionizing radiation (X rays, gamma rays, electron beams)  •OH Nonionizing radiation (UV) (Microwaves kill by heat; not especially antimicrobial) Physical Methods of Microbial Control Figure 7.5

7.2) Chemical Methods of Microbial Control : 

Principles of effective disinfection – Factors for consideration: Concentration of disinfectant Organic matter pH Time 7.2) Chemical Methods of Microbial Control

A. Chemical Methods of Microbial Control: Evaluation : 

Evaluating a Disinfectant: Use-dilution test: Metal rings dipped in test bacteria are dried. Dried cultures placed in disinfectant for 10 min at 20°C. Rings transferred to culture media to determine whether bacteria survived treatment. A. Chemical Methods of Microbial Control: Evaluation

Evaluating a Disinfectant: : 

Evaluating a Disinfectant: Figure 7.6 Disk-diffusion method: Zone of Inhibition/ZOI = diameter of clearing / sensitivity in mm.

B. Types of Disinfectants : 

B. Types of Disinfectants Figure 7.7 Phenol: Denature proteins. “chaotropic agents” Disrupt plasma membranes. Phenolics: Lysol Bisphenols: Hexachlorophene, Triclosan

Types of Disinfectants : 

Biguanides: Chlorhexidine. Disrupt plasma membranes. Halogens: Iodine, Chlorine. Oxidizing agents. Bleach is hypochlorous acid (HOCl). Iodophores = betadine (organic combo w/ I). Alcohols: Ethanol, Isopropanol. Denature proteins, dissolve lipids. Types of Disinfectants

Types of Disinfectants : 

Heavy Metals: Ag, Hg, Cu, Zn Oligodynamic action: Very small amounts exert antimicrobial activity! Denature proteins. Highly cationic. Large, attractive surfaces. Types of Disinfectants

Types of Disinfectants : 

Surface-Active Agents or “Surfactants”: Detergents Emulsifiers Types of Disinfectants COMPOUND ACTIVITY

Types of Disinfectants : 

Chemical Food Preservatives: Organic Acids Inhibit metabolism. Sorbic acid, benzoic acid, calcium propionate Control molds and bacteria in foods and cosmetics Nitrite prevents endospore germination Antibiotics: Nisin and natamycin prevent spoilage of cheese Aldehydes: Inactivate proteins by covalently cross-linking with reactive functional groups (–NH2, –OH, –COOH, —SH). Glutaraldehyde, formaldehyde Gaseous Sterilants: Denature proteins. Ethylene oxide Peroxygens: Oxidizing agents. O3, H2O2, peracetic acid Types of Disinfectants

Microbial Characteristics and Microbial Control : 

Microbial Characteristics and Microbial Control Figure 7.11 Acellular & Cellular Microbes: Prion 3D prot. Strx (acellular) Refolds easily Must be incinerated! Endospore resistance & Protistan cysts Gram– : LPS/OM porins Unenveloped viruses (acell.) All above have relatively enhanced resistance to chemical biocides.

Microbial Characteristics and Microbial Control : 

Microbial Characteristics and Microbial Control ** Microbial control methods, especially biocides, are not equally effective against all microbes. **