Microbial Growth Control

Microbial Growth Control

• Microbial growth can be controlled in various ways using physical and chemical methods
• Sterilization and sanitization are key to good public health

Sterilization

• Sterilization involves destroying or removing all living organisms, spores, and viruses
• Microbicidal/germicidal agents kill microbes, including bactericidal (killing bacteria) and fungicidal (killing fungi) agents
• In the food industry, commercial sterilization aims at destroying potential human pathogens, although nonpathogenic species may remain

Sanitization

• Sanitization reduces or inhibits microbial growth
• Microbiostatic agents reduce pathogenic microbes to safe levels but need regular reapplication
• Bacteriostatic agents reduce bacteria on a surface, while fungistatic agents reduce fungus on a surface

Control Methods

• Control methods affect the bacterial cell envelope by dissolving phospholipids, inactivating membrane proteins, and causing loss of permeability control in the cell membrane
• Cell walls are weakened or damaged, leading to loss of structural integrity and potentially cell lysis
• Environmental conditions can alter the 3-D shape of DNA and RNA, interfering with growth/division, preventing gene expression, and leading to cell death
• Heat, pH, and salt alter 3-D structure of proteins; denaturation unfolds proteins from their active shape, causing enzymes to lose function and disrupting structural proteins

Physical Control Methods

• Thermal death time refers to the time needed to kill all microbes at a specific temperature
• Thermal death point is the lowest temperature that kills all microbes in 10 minutes
• Decimal reduction time (D value) is the time needed to kill 90% of viable organisms at a specific temperature
• Microbes die at exponential rates

Dry Heat

• Dry heat denatures proteins, but oil and grease can insulate against heat penetration
• Incineration uses direct flame, turning microbes to ashes
• Hot air needs long exposure at high heat, such as 160°C for 2 hours in an oven

Moist Heat

• Moist heat penetrates materials more rapidly than dry heat
• It works at lower temperatures and shorter times than ovens
• Boiling water can kill vegetative cells in minutes and protozoan cysts, some viruses, and fungal spores in a 30-minute boil
• It is not considered a sterilant

Pressurized Steam

• Pressurized steam is the most dependable method for sterilization, destroying bacterial endospores
• Autoclaves need 15 psi at 121°C for 15 minutes to kill bacteria. 30 minutes is required for liquids and dense materials
• Plastics, oils, and some chemicals cannot be autoclaved

Pasteurization

• Pasteurization, developed in the 1860s, destroys wine- and beer-contaminant microbes
• Effective pathogen killing in milk and fruit juice
• Holding/batch method uses 63°C for 30 minutes
• Flash pasteurization uses 72°C for 15 seconds
• Ultra-high temperature sterilization uses 140°C for 3 seconds

Refrigeration and Freezing

• Refrigeration at or near 5°C retards the growth of spoilage organisms due to lower metabolic rate, but it does not affect psychrophiles
• Freezing foods below 0°C inhibits metabolism/growth and destroys many cells with ice crystal penetration
• Some bacteria remain dormant indefinitely

Filtration

• Filtration involves the mechanical separation of microbes from liquids or gasses
• Nitrocellulose or polycarbonate membranes with pores from 25µm to 25nm trap organisms larger than pores
• It sterilizes heat-sensitive liquids that cannot be autoclaved and also monitors water quality by culturing trapped organisms
• Air filters include surgical masks, biological safety cabinets, and HEPA filters

Irradiation

• UVA, UVB, and UVC categories have wavelengths from 100-400 nm, with UVA being the most damaging
• Absorbed DNA generates high energy; adjacent thymine molecules form cross bridges, distorting DNA shape
• Replication and transcription are interfered, thereby disrupting metabolism and reproduction

Ionizing Radiation

• X-rays and gamma rays cause ions to form by forcing molecular electrons out of shells
• Ions combine with water to form new products in the cytoplasm, interfering with metabolism and physiology and causing mutations
• It can sterilize heat-sensitive materials and extend the shelf life of some foods

Preservation Methods

• Freeze drying/lyophilization: Food is flash frozen with liquid nitrogen (-80°C); no ice crystals form, water is drawn off by vacuum, and water will reconstitute later
• Drying and salting desiccate foods by removing water to support cellular activity
• Salt increases osmotic pressure, causing water to diffuse out of cells

Chemical Control

• The use of chemical agents can be microbicidal or microbiostatic
• Iodine was recognized in 1830 as antiseptic and In the 1860s, Joseph Lister used carbolic acid for treating wounds
• Disinfectants are used on inanimate surfaces; antiseptics are used on living tissues
• Degerming is a mechanical process which removes organisms

Chemical Agent Factors

• Consideration should be paid to any microbial susceptibility
• Vegetative cells are killed by low-level agents
• Endospores require chemical sterilants
• Optimal temperature and pH conditions give better outcomes
• Concentration is more effective at higher levels, but also may be toxic

Microbe Numbers

• Greater bioburden requires longer disinfection time
• Biofilms are extremely resistant
• Endospore formers can escape destruction which requires increased measures
• Chemical agents should be nonstaining, noncorrosive, and odorless, and easy to buy

Chemical Agent Evaluation

• The Phenol Coefficient Test has been used as a historical test of effectiveness
• Compares chemical agent to phenol
• A PC number higher than 1 indicates it is more effective than phenol
• Tested bacterial cultures are Staphylococcus aureus and Salmonella enterica ser. Typhi

Use-Dilution & In-Use Tests

• Use-dilution tests use stainless-steel cylinders coated in specific test microbes. Cylinders exposed to chemical strengths for 10 minutes, cultured and assessed by the most effective dilution of agent which is the highest with no growth.
• In-use tests include swab samples which are taken before and after spraying chemical; the swabs are incubated with growth where if no growth occurs in the after swab, the chemical is effective

Disk Diffusion

• Absorbent paper disks containing chemical agents are adhered to bacterial lawn agar plate
• Plates are incubated, effective chemical agents show a zone of inhibition, illustrated by a clear ring in agar

Chemical Agents

• Intermediate-level chemical agents are highly reactive- denaturing cytoplasmic proteins.
• Effective against bacteria and fungi, sporicidal at high concentrations, used in water treatment
• Used to sanitize dairy or restaurants
• Iodine is more reactive germicidal- commonly used as an antiseptic

Phenol and Phenolic Compounds

• Operate between low and intermediate levels- disrupting membrane proteins
• This process works against many bacteria, fungi and some viruses
• Phenol has draw backs; expensive, pungent, causing issues
• Phenol derivatives are greater than germicidal activity; Chlorhexidine is important while Triclosan was banned by the FDA in 2016

Heavy Metals

• Low-level chemical agents, inactivate enzymes or denature structural proteins
• Mercury toxicity is lowered with carrier compounds, used at a preservative
• Copper can be used in water or plumbing
• Silver keeps transmission of gonorrhea in check

Alcohols

• Works at intermediate levels- denaturing proteins
• Dissolving lipids is effective at dehydration
• Bactericidal, fungicidal, viricidal
• Water helps prevent alcohol from evaporating better and quicker
• 75% effective in ethenol hand sanitizer

Soaps and Detergents

• Soaps are fatty acids with potassium or sodium hydroxide
• There Ph typically has around 8%, microbial distruction occurs due to alkalinity of the cleanser.
• Degerming is the process of removing microorganisms or emulsifying particles
• Cleaners will loosen surface tension and clean surface

Detergents

• Cationic, drawn to phosphate membrane groups, kill gram + bacteria, can stop effects fungi and and enveloped viruses
• Most are made by ammonium chloride and substitute hydrogen atoms in organic groups

Peoxide

• Works in high level chemical agents-
• Hydrogen Peroxide is foam catalyst in water and oxygen to loosen dead tissue but deadly and anaerbic
• Topical treatment effective for extended exposure
• Acne treatment or whitener

Aldehydes

• High level used to cross link with NH2 and OH groups
• Denatures proteins
• Formailin: 37% of formaldahyde solution
• Glutaraldehydes; give of fumes rinsing treatmeant

Sterilizing Gasses

• EtO
• Sterilizing gas
• Chlorhexidine dioxide-
• Nontoxic
• Decontaminates surfaces