Microbial Growth Control
Talk about various methods to control the growth of bacteria, e.g., on food to prevent spoilage.
Begin with physical methods to control growth and then look at chemical methods.
Heat Sterilization - Two important parameters: How long? and What temperature? Both of these can vary depending on the sensitivity of the microorganisms to temperature. Temperature effects are obvious when you look at what is called decimal reduction time - which is the time required to reduce the population by 10 fold at a given temperature.
Thermal Death time - is the time required to kill all bacteria in a sample at a specific temperature. Need to standardize the population size of your sample before determining the thermal death time.
Heat killing is most effective if the heat is a moist heat. Dry heat killing requires higher temperatures for longer periods of time.
Heating must kill spores which are heat resistant structures. The environment the spores are in affects their sensitivity to heat killing. For example a solution rich in sugars and proteins increases spores resistance to heat killing. Also the amount of water in the spores affects their sensitivity - the more water the more sensitive.
The autoclave - 15 lbs/sq. inch which yields a temperature of 121 oC for 10 -15 minutes.
Not all materials can withstand autoclaving, e.g., milk, so they are pasteurized. Pasteurization is a mild heating to kill sensitive spoilage bacteria. Milk is heated to 71oC for 15 seconds in a continuous flow system.
Electromagnetic irradiation - microwave, UV, X-rays, gamma rays, and electrons are all forms of electromagnetic radiation. They all work in different ways, for example microwaves heat up the sample, UV breaks the DNA up causing death.
Ionizing radiation - electromagnetic radiation that produces electrons, hydroxyl radicals, and hydride radicals. These products react with macromolecules such as DNA and proteins which are detrimental to the cell. Common sources of ionizing radiation are 60Co and 137Cs. Radiation is becoming accepted for foods.
Filter sterilization - useful for sterilizing heat labile liquids. The filters have pores of specific sizes down to 0.2 micron in diameter.
Three types of filters
Depth filters - routine filters made of paper, asbestos or glass fibers. Just a mat of the fibers.
Membrane filters - cellulose acetate or cellulose nitrate materials. Contains lots of tiny holes that act live sieves to retain bacteria.
Nucleopore filters - polycarbonate filters. Very uniform pore sizes. Great for microscopy work since the filters are flat in nature.
Chemical growth control - chemical agents that either kill or inhibit growth. Bacteriocidal agents kill bacteria and bacteriostatic agents inhibit growth.
Bacteriostatic agents usually target a cellular process such as protein synthesis where they inhibit ribosomes
Bacteriocidal agents kill cells but do not cause lysis of cells. These agents bind tightly to their target.
Bacteriolytic agents kill cells by lysis.
Minimal inhibitory concentration - concentration of chemical agent that inhibits growth. Can determine it using a series of dilutions of the chemical introduced into tubes with growth medium and inoculated. This can be standardized so that results can be compared. Agar diffusion method is used where a disc is loaded with the chemical and placed on a plate with the bacteria growing. Results in a zone of inhibition around the disc with the chemical.
Antiseptics and disinfectants
Antiseptics are chemicals sufficiently non-toxic to use on living tissue. Includes silver nitrate, iodine, alcohol (70%), and hydrogen peroxide.
Disinfectants are chemicals that kill microorganisms and are used on inanimate objects. Includes copper sulfate, mercuric chloride, chlorine gas, ethylene oxide and ozone.
Growth factor analogs - chemotherapeutic agents that can be taken internally and mimic a growth factor needed for normal growth.
Sulfa drug - a growth factor analog that mimics p-amino benzoic acid which is part of the vitamin folic acid. Sulfanilamide inhibits bacteria since they synthesize their own folic acid whereas higher animals obtain it from the food.
Other growth factor analogs block DNA metabolism such as nucleotide synthesis.
Antibiotics - chemicals that are produced by bacteria that kill or inhibit other bacteria.
Gram positive bacteria are more sensitive to antibiotics than Gram negative bacteria generally. Broad spectrum antibiotics act on both types of bacteria.
Targets of antibiotics
cell wall synthesis
cytoplasmic membrane
protein and nucleic acid biosynthesis
[beta]-lactam antibiotics includes penicillin, cephalosporins and cephamycins. First one discovered was penicillin G effective against Gram positive cells. Gram negative bacteria are impermeable to this drug. Other penicillins are effective against gram negative bacteria for example carbenicillin and ampicillin.
Mode of action - inhibit cell wall synthesis specifically the transpeptidation reaction between adjacent glycan chains. Works only of growing cells which are actively synthesizing peptidoglycan layers.
Aminoglycosides - are amino sugars bonded by glycosidic linkages. Include streptomycin, kanamycin, neomycin, and gentamicin. Used against gram negative bacteria. Not commonly used today.
Mode of action - inhibit protein synthesis at the 30S subunit of the ribosome.
Macrolide antibiotics - lactone rings connected to sugars. Includes erythromycin.
Mode of action - inhibits protein synthesis at the 50S subunit of the ribosome.
Tetracylines - first of the broad spectrum antibiotics.
Mode of action - inhibits protein synthesis. Used on poultry feed.
