Unit 5: Controlling microbial growth
Culturing methods
Planktonic growth
Biofilm
Bacteria and fungi live in organized communities
Hard to get rid of
Quorum sensing
Environmental factors
Temperature
pH
Oxygen
Water
Control of microbial growth - overview
Principles
Sterilization
Disinfection
Pasteurization
removal of all microorganisms
elimination of most or all pathogens
heating of food etc to limit pathogens
Decontamination
Sanitization
reduce pathogens to levels considered sfe to handle
reduced microbial population that meets accepted health standards
Preservation
delaying spoilage of foods and other products
Methods
Daily life
Water and soap
Cooking
Refrigeration
Healthcare facilities
Sterilized instruments
Nosocomial infections
Microbiology laboratories
Aseptic techniques
Biosafety levels - different laboratories
Food production facilities
Heat, pasteurization
Irradiation
Chemical additives
Water treatment facilities
Chlorine
Selection of an antimicrobial procedure
Microbes
Bacterial endospores
Protozoan cysts and oocysts
Mycobacterium species
resistant to heat
killed by heat
Pseudomonas
resistant to some disinfections
Naked viruses
Lack lipid envelope, susceptible to chlorine
Environmental conditions
pH, temperature
Risk for infection
Different levels of cleanness
Physical methods of controlling microbial growth
Pasteurization
Sterilization with pressured steam
Canning
Moist heat is more effective than dry heat
Autoclave
Industrial-sized autoclave called retort
Destroys botulism pathogens
dry heat required longer times and higher temperatures
Filtration
Membrane filters
Depth filters
0.2 micrometer pore size or smaller
Liquid is sucked or pushed through material
Thick filtering material
Electrical charges on filter walls trap cells
Radiation
Gamma irradiation - ionizing radiation
Sterilizes heat-sensitive materials and can be used after packing
Removes electrons from atoms
Effectiveness depends on dose applied
Used for foods
Ultraviolet radiation
Damages DNA
Does not deeply penetrate
Effective when used at close range against expected microbes
Chemical methods of controlling microbial growth
Chemical formulants
Sterilant
High-level disinfectant
Intermediate-level disinfectant
Low-level disinfectant
Destroys all microbes
Destroys all viruses, vegetative cells
Destroys vegetative bacteria, mycobacteria, fungi, most viruses
Destroys fungi, vegetative bacteria, enveloped virus
Factors to consider when choosing a germicidal chemical
Toxicity
Activity
Compatibility
Residues
Cost and availability
Storage and stability
Environmental risk
Germicide classes
Alcohols
Denaturates essential proteins and damage lipid membranes
More effective when mixed in water
Aldehyde
Sterilants, toxic
Damage DNA
Biguanides
Chlorhexidine - low toxicity
Ethylene oxide gas
Penetrates fabrics etc
Reacts with proteins
Halogens
Chlorine, iodine
Ozone
Peroxygens
Phenolic compounds
Unstable, used in drinking water
Leave no residue, ex hydrogen peroxide
Strong irritant, denature proteins
Quaternary ammonium compounds
Cationic detergents, disinfect food prep surfaces