MICROBIOLOGY Chapter 7 The Control of Microbial Growth

MICROBIOLOGY Chapter 7 The Control of Microbial Growth Dr. Abdelraouf A. Elmanama Ph. D Microbiology Medical Technology Department, Faculty of Science, Islamic University-Gaza 2008

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. 2008

Terminology • Sterilization: Removal of all microbial life • Commercial Sterilization: Killing C. botulinum endospores • Disinfection: Removal of pathogens • Antisepsis: Removal of pathogens from living tissue • Degerming: Removal of microbes from a limited area • Sanitization: Lower microbial counts on eating utensils • Biocide/Germicide: Kills microbes • Bacteriostasis: Inhibiting, not killing, microbes 2008

• Bacterial populations die at a constant logarithmic rate. 2008 Figure 7. 1 a

Effectiveness of antimicrobial treatment depends on: • Number of microbes • Environment (organic matter, temperature, biofilms) • Time of exposure • Microbial characteristics 2008 Figure 7. 1 b

Actions of Microbial Control Agents • Alternation of membrane permeability • Damage to proteins • Damage to nucleic acids 2008

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 • Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature 2008

Heat • Moist heat denatures proteins • Autoclave: Steam under pressure 2008 Figure 7. 2

Physical Methods of Microbial Control • 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 2008

Physical Methods of Microbial Control • Dry Heat Sterilization kills by oxidation • Flaming • Incineration • Hot-air sterilization Equivalent treatments 2008 Hot-air Autoclave 170˚C, 2 hr 121˚C, 15 min

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 2008

Physical Methods of Microbial Control • Radiation damages DNA • Ionizing radiation (X rays, gamma rays, electron beams) • Nonionizing radiation (UV) • (Microwaves kill by heat; not especially antimicrobial) 2008

2008 Figure 7. 5

Chemical Methods of Microbial Control • Principles of effective disinfection • Concentration of disinfectant • Organic matter • p. H • Time 2008

Chemical Methods of Microbial Control • Evaluating a disinfectant • Use-dilution test 2008 1. Metal rings dipped in test bacteria are dried 2. Dried cultures placed in disinfectant for 10 min at 20°C 3. Rings transferred to culture media to determine whether bacteria survived treatment

Chemical Methods of Microbial Control • Evaluating a disinfectant • Disk-diffusion method 2008 Figure 7. 6

Types of Disinfectants • Phenolics. Lysol • Bisphenols. Hexachlorophene, Triclosan • Disrupt plasma membranes 2008 Figure 7. 7

Types of Disinfectants • Biguanides. Chlorhexidine • Disrupt plasma membranes 2008

Types of Disinfectants • Halogens. Iodine, Chlorine • Oxidizing agents • Bleach is hypochlorous acid (HOCl) 2008

Types of Disinfectants • Alcohols. Ethanol, isopropanol • Denature proteins, dissolve lipids 2008 Table 7. 6

Types of Disinfectants • Heavy Metals. Ag, Hg, Cu • Oligodynamic action • Denature proteins 2008

Types of Disinfectants • Surface-Active Agents or Surfactants Soap Degerming Acid-anionic detergents Sanitizing Quarternary ammonium compounds Bactericidal, Denature proteins, Cationic detergents disrupt plasma membrane 2008

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 2008

Types of Disinfectants • Aldehydes • Inactivate proteins by cross-linking with functional groups (–NH 2, –OH, –COOH, —SH) • Glutaraldehyde, formaldehyde 2008

Types of Disinfectants • Gaseous Sterilants • Denature proteins • Ethylene oxide 2008

Types of Disinfectants • Peroxygens • Oxidizing agents • O 3, H 2 O 2, peracetic acid 2008

Microbial Characteristics and Microbial Control 2008 Figure 7. 11

Microbial Characteristics and Microbial Control Chemical agent Effectiveness against Endospores Mycobacteria Phenolics Poor Good Quats None Chlorines Fair Alcohols Poor Good Glutaraldehyde Fair Good 2008