BIO205 - Chapter 5: Control of Microbial Growth

Joseph Lister

applied carbolic acid (phenol) directly onto damaged tissues, where it prevented infections

• oral antiseptic Listerine named for him

physical approaches to microbial control

heat treatment, irradiation, filtration, mechanical removal

chemical approaches to microbial control

variety of antimicrobial chemicals

sterilization

the process of killing or removing all of the microorganisms in or on a material

• item is free of microbes, endospores, and viruses

Pseudomonas species

resistant to and can actually grow in some disinfectant

• also Mycobacterium sps are endospore formers

• Clostridium and Bacillus sp. are non-specific enveloped viruses

Biocide/Germicide

kills microbes

Bacteriostasis

inhibiting, not killing microbes

nosocomial iinfections

acquired from hospital settings

Pasteurization

brief heating to reduce number of spoilage organisms, and destroy pathogens

• commercial canning process destroys the endospores of Clostridium botulinum

effects of moist heat

fast, cheap, reliable

• irreversibly denatures proteins in microbes - bactericidal

• breaks H bonds, changes shapes of enzymes

effect of boiling at 100 degrees C for 10 min

kills vegetative bacteria, fungi, many viruses

• does not sterilize, endospores can survive

pressurized steam (autoclave)

sterilization typically at 121 degrees C for 15 min at 15 psi

• longer for larger volumes

• prions thought destroyed at 132 degrees C for 1 hour

biological indicators of autoclave

heat-resistant endospores of Geobacillus stearothermophilus

effect of dry heat

not as effective as moist heat

• sterilization requires longer times and higher temperatures

membrane filters

some materials cannot withstand heat treatment

• filtration retains bacteria

  • filtration of fluids used extensively

HEPA (High-Efficiency Particulate Air Filter)

used for air, removes microbes larger than 0.3mm

• level IV contaminant at CDC (Ebola)

radiation

electromagnetic radiation: radio waves, microwaves, visible and ultraviolet rays, X rays, gamma rays

ionizing radiation

• shorter wavelength, high energy

• gamma rays, X-rays destroy DNA

• damages cytoplasmic membranes of G (-ve): Pseudomonas

• high energy gamma rays are used to sterilize or pasteurize

ultraviolet radiation

destroys microbes directly

• damages DNA (Thymine Dimer)

effects of alcohols (germicidal chemicals)

kills fungi and vegetative bacteria

• not reliable against endospores, some naked viruses

• pure alcohol is less effective than aqueous solution - 100% or 70%

aldehydes (germicidal chemicals)

• common sterilant = glutaraldehyde

  • immersion for 10-12 hrs kills all microbial life

  • is very good for use on heat-sensitive medical items

Biguanides

• chlorhexidine is most effective

• extensive use as antiseptics

• destroys vegetative bacteria, fungi, some enveloped viruses

• common in skin cream, mouthwash

Ethylene oxide

• useful gaseous sterilant

• destroys microbes including endospores and viruses

• useful in sterilizing heat or moisture-sensitive items

• limitations: toxic and potentially carcinogenic

Halogens

• chlorine: destroys all microorganisms and viruses

  • used as disinfectant

  • caustic to skin and mucous membranes

  • very low levels disinfectant drinking water

    • Cryptosporidium oocysts, Giardia cysts survive

• iodine: kills vegetative cells, unreliable on endospores

Metal compounds

• silver still used as antiseptic: creams and bandages

  • silver nitrate eye drops given at birth to prevent Neisseria gonorrhoeae infections

• compounds of mercury, tin, and copper

• extemsive use led to environmental pollution

Ozone (O3)

• disinfectant for drinking and wastewater

• peroxygens

  • hydrogen peroxide = effectiveness depends on surface, inactivated on living tissues, more effective on inanimate objects

  • peracetic acid: more potent when used in combination with H2O2

Phenolic compounds

• destroy cytoplasmic membranes, denatures proteins

• remains effective in presence of detergents and organic contaminants

• Triclosan: has been used widely in various soaps and lotions

Quaternary Ammonium compounds

cationic detergents

• disinfection of food preparation surfaces

• attracted to negative charges of microbial cell surface

two chemicals used in processed meats

nitrate and nitrite

function of nitrate and nitrite

• stops growth of Clostridium botulinum

• higher concentrations give meats pink color

• shown to be carcinogenic

significance of low temperature storage

freezing preserves by stopping all microbial growth

significance of reducing water ability

• salt addition: sugar increases environmental solutes

• some bacteria grow in high salt environments

• drying often supplemented by salting

lyophilization

freeze drying food

• coffee, milk, meats, fruits, vegetables

does pasteurization destroy endospores?

no - kills vegetative cells, but endospores remain intact

incineration

• method of dry heat sterilization

  • hot air ovens oxidize cell components to ashes, denature proteins

  • flaming laboratory inoculation loop incinerates organism

formaldehyde

• used to kill bacteria and inactive viruses for vaccines

• used to preserve specimens