Microbial Control - Physical Means Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives You should be able to: Contrast sterilization, disinfection, antisepsis, sanitization , and pasteurization . Compare the effects of "-static" versus "-cidal" control agents on microbial growth. Define microbial death rate, and describe its significance in microbial control. Describe how antimicrobial agents act against cell boundaries, proteins, and nucleic acids. Identify the three most-resistant groups of microbes, and explain why they are resistant to many antimicrobial agents. Describe types of physical methods of microbial control and discuss their advantages and disadvantages.

Microbial Control Antisepsis – reduction in number of microorganisms and viruses on living tissue; ex: alcohol, iodine Disinfection – destruction of most microorganisms and viruses on non-living tissue; ex: alcohol, phenol Pasteurization – use of heat to destroy pathogen; ex: pasteurized milk and fruit juice Sanitation – removal of pathogens from objects to meet public health standards; ex: washing table wares in restaurants Sterilization – destruction of all microorganisms and viruses in or on an object; by autoclave, incineration, or ethylene oxide gas; ex: preparing culture media

Microbial Death Rates Microbial death – permanent loss of reproductive ability MDR is constant over time for any microbes under a particular set of conditions Ex in graph: 90%/min.; requires 9 mins. to achieve sterilization

Physical Methods of Microbial Control Heat – moist; dry Low temperature – refrigeration; freezing Filtration Desiccation Osmotic pressure radiation

Heat Heat – oldest and common means Thermal death point -lowest temp. that kills all cells in10 mins. Thermal death time – minimum time required to kill all organisms at given temp. (ex: 22.5 mins) Decimal reduction time (D) - time required to kill 90%of organisms; useful in food processing to eliminate endospores of C. botulinum

Moist Heat Moist Heat – used to disinfect, sanitize, sterilize, & pasteurize: Boiling – vegetative bacteria & fungi, trophozoites of protozoas,and most viruses in 10 mins. Autoclave – uses steam heat under pressure (121˚C, 15 psi at 15 mins.) to sterilize chemicals and objects. Pasteurization – heating food/beverages to kill pathogens and control spoilage organisms.

Dry Heat Dry Heat – at higher temp. for longer times: Oven - 160˚C for 2 hrs., for powders and oils bunsen burner complete incineration

Refrigeration and Freezing Refrigeration = between 0˚C and 7˚C, halts growth of most pathogens Slow Freezing = below 0˚C , inhibits microbial metabolism; but many vegetitive bacterial cells, bacterial endospores, and viruses can survive for years

Desiccation & Lyophilization Desiccation – drying to preserve foods; inhibits growth because metabolism requires liquid water Lyophilization – freeze-drying; preserve microbes and other cells for many years; freeze culture in liquid nitrogen or frozen CO2, then remove water via a vacuum

Filtration & Osmotic Pressure Filtration – passage of air or liquid through material that traps and removes microbes; 0.3 µm for O.R.; 0.22 µm for bacteria; 0.01 µm for viruses Osmotic pressure – use of high concentration of salt/sugar; drawing out of water; ex. honey, jams, salted fish, and pickles

Radiation Ionizing radiation – wavelengths shorter than 1 nm; electron beams, gamma rays, X-rays; effects leading to denaturation (destroy DNA); medical/lab equipment sterilization Nonionizing radiation – wavelengths longer than 1 nm; UV light, visible light, infrared light, radio waves; UV causes pyrimidine dimers which kills cells; sterilization of surfaces and of transparent fluids and gases Microwaves – heat kills microbes; Trichinella spiralis in pork resist heating

Homework Define terms: sterilization, disinfection, antisepsis, sanitation, bacteriostatic, and bactericidal. Compare moist heat and dry heat method of microbial control and discuss advantages and disadvantages. Compare uses/advantages of microbial control of desiccation and lyophilization; and filtration and osmotic pressure. Compare ionizing and non-ionizing radiation. What is microbial death rate? Decimal reduction time?