Microbial Growth Control - DOC - DOC by T8QqX451

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									                           Microbial Growth Control
                                                 Chapter 20
                                           Terminology
• “decrease the microbial load by limiting growth” (inhibition) or reducing the
  load (killing)
   – Decontamination
      • Inanimate objects; inhibit growth, but not really reduce the initial load; not sterile, but safe to handle
      • Wiping a table after a meal
   – Disinfection
      • Inanimate object; use of a physical agent to inhibit growth and majorly reduce the initial load
      • Using bleach to clean a table
   – Antisepsis
      • Same as disinfection, but on external living tissue
      • Iodine on a wound
   – Antibiosis
      • In vivo killing or inhibition; internal tissues
      • Antibiotic given for an infection

                                    More Terminology
• Sterilization
   – A completely different process than the rest
   – Removing all viable organisms and spores by killing or removing
   – Surgical instruments
• Bacteriocidal versus Bacteriostatic versus bacteriolytic
   – Viri-
   – Fungi-



                         What to Attack (in general)?
• Plasma membrane
   – Consequences: PMF, permeability barrier
• Proteins (enzymes)
   – Metabolism and transport disrupted
• Nucleic acids
   – Cannot produce enzymes or make new membrane
• Cell Wall
   – Lysis
                             Physical Methods: Heat
• Moist heat versus dry heat
   – Denaturation versus oxidation effects
• Factors that effect sterilization by heat:
   –   Time
   –   Organism properties
   –   Temperature
   –   Concentration of microbe
   –   Presence of organic material
   –   Environmental
        • pH
        • Osmotic pressure
   – Type of heat
                                Physical Methods: Heat
• Heat is bacteriocidal
• 2 ways of describing
   – Decimal reduction time: time required for 10-fold reduction in population at a given
     temperature
        • Canning industry looks at 90% reduction
        • It increases for a population as temp. decreases
   – Thermal death time: time at which all cells are killed at a given temperature
        • Important for sterilization procedures

                                     Moist Heat Method
• Autoclave
   – Cidal
   – Heat above boiling (high pressure)
   – Vegetative cells and spores: 121º C (15 lbs/in2) for 10-15 min.
                               Moist Heat Method
                           Physical Methods: Radiation
• Ionizing
        o   Ex) gamma rays, X rays, high energy electrons
        o   Short wavelength (<1nm) high energy
        o   Produce ion (hydroxide, hydride, or electrons)
        o   Measured in absorbed radiation dose
              o Rad or gray
              o 100 rads = 1 gray
        o Damage to DNA or enzymes (cidal)
        o Food preservation, medical supply and some pharmaceutical sterilization
        o Some bacteria more resistant (Deinococcus radiodurans)

                           Physical Methods: Radiation
• Nonionizing radiation
   o Longer wavelength (>100 nm)less energy
   o EX) UV light (best at 260nm)
        o UVC
   o Thymine dimers inhibit replication (static)
        o Initiate the SOS response!!!
   o Lower energyLow penetration
   o Germicidal lamps and vaccine disinfection
                      Physical Methods: Filtration
o Heat sensitive liquid materials (media, some antibiotics, etc.)
o Separate microbes from fluid
o Passage of liquid through a filter pore size of 0.2m or less and collection in a
  sterile container
o Viruses and spores can slip by so sterilization often difficult

                                 Filtration
                          Other Physical Methods
• Low temperatures
   o Water in the cells crystallize below 0 C
   o Refrigeration slows the growth of most cells (not Listeria)
• Osmotic pressure
   o   Old method of preservation
   o   Slows the growth of many non-halophilic cells
   o   Salt or sugar
   o   Molds better suited to these conditions


                               Chemical Methods
• Antiseptic versus disinfectant
• Factors affecting efficiency of chemicals
   – Concentration
   – Initial load of microbes
   – Environmental: Temperature, pH, salt
   – Presence of organic matter
   – Time or exposure
   – Organism
      • Spores versus vegetative cells
      • Mycobacterium and lipid rich cell wall
      • Gram negative organisms and outer membrane
                               Chemical Methods
• Minimum inhibitory concentration (MIC)
   –   Not constant (dependent on many factors)
   –   Must standardize for comparison
   –   Does not distinguish cidal versus static
   –   Tube dilution technique
   –   Proportional to MIC
• Agar diffusion method
   – Zone of inhibition
   – Diameter proportional to amount of agent, solubility, diffusion coefficient, and
     effectiveness
      • Cannot distinguish ability to diffuse from actual effectiveness
   – Does not distinguish cidal versus static




                           Chemotherapeutic Agents
• Used to decrease load in tissues
   – antibiosis
• Selective toxicity: injure bacteria and no harm to host
• Synthetic versus antibiotic
   – Synthetic: growth factor analogs
   – Antibiotic: produced by microorganisms
                              Growth Factor Analog:
                                  Sulfa Drugs
• Microbes normally produce folic acid from a para-aminobenzoic acid (PABA)
  substrate and an enzyme
• When sulfanilamide is present the enzyme binds to it instead
• Why is it selectively toxic?
                                       Other Analogs
• Nucleic Acid analogs
   – Similar to nucleic acids
   – DNA/RNA mutations
   – EX)5-bromouracil is a thymine analog
• Amino acid analogs
   – Similar to amino acids
   – Destroys protein structure
   – EX) p-Fluorophenylalanine is a phenylalanine analog
                                           Antibiotics
• Produced by microbes; kill or inhibit other microbes
• Mostly produced by fungi or streptomycetes (a group of bacteria)
• Spectrum of microbial activity
   – Broad spectrum: affect broad range of bacteria
      • EX) Tetracyline
      • Also destroys normal flora (why is this a problem?)
   – Narrow spectrum: effective against single group
   – Superinfection: overgrowth of an organism that is either not sensitive to an antibiotic or is
     resistant
      • EX) Candida albicans


                              Antibiotics: Cell Wall
• Penicillin G
   – Narrow (Gr +)
   – Inhibit cell wall synthesis
      • No transpeptidation
      • Cannot replace peptidoglycan lysed by autolysins during cell wall synthesis
                                Antibiotics: Proteins
• Inhibition of protein synthesis
   – Selective toxicitydifference in ribosomes
      • Bacteria= 70S ribosome (50S and 30S portions)
      • Ribosomes are key players in protein synthesis
      • Chloramphenicol (bacteriostatic and broad spectrum)
          – 50S portion
          – Inhibits formation of peptide bonds
      • Tetracycline (bacteriostatic and broad spectrum)
          – 30S portion
          – Interference with attachment of tRNA, which carries amino acid
      • Streptomycin (bacteriocidal and broad spectrum)
          – 30S portion
          – Changes shape of 30S portion and genetic code read wrong




                                             Antibiotics
• Injury to plasma membrane
   – Changes cell permeability
   – Loss of metabolites or cell bursting
   – EX) polymyxin B attaches to phospholipids (bacteriocidal and Gram neg.)
• Inhibition of nucleic acid synthesis
   – More difficult to find b/c of lack of selective toxicity
   – EX) Rifampin inhibits mRNA synthesis (bacteriocidal and Mycobacterium tuberculosis)



                                 Antibiotic Resistance
• Organisms that produce them are resistant to them
   – Pass these resistance genes on to others
• Resistance has always existed
   – Recent problems due to selection
   – Replacement of sensitive bacteria in a population by resistant bacteria*
• Resistance is genetically coded
   – Plasmids*
   – genomic




                  Mechanisms of Drug Resistance
• Lack a target structure
   – Cell wall for Mycoplasma–penicillin resistance
• Destruction or inactivation
   – Some Staphylococcus have enzymes that inactivate Penicillin
• Impermeable
   – Gram – organisms and penicillin resistance
• Modify the target site
   – Modify the ribosome to become erythromycin resistant
• Rapid efflux
• Genetic change in metabolic pathway
   – Resistance to sulfonamides: take up folic acid rather than make it
                 Reasons for Resistance Problems
• Lack of prescriptions
• Improper use (headaches and viruses)
   – CDC estimates percentage of wrong prescriptions
      • 30% ear infections, 50% sore throats, 100% colds
• Weak forms select for resistant bacteria
• Not taken long enough
• Promote animal growth

                              Drug Combinations
• In case the organism is resistant to one drug
• Synergism: drug assistance
   – EX) penicillin and Streptomycin for bacterial endocarditis
      • Penicillin disrupts cell wall and Streptomycin gets in faster
• Antagonism: One drug makes other ineffective
   – EX) penicillin and Tetracycline
      • Tet interferes with Penicillin
                             Viral Control (HIV)
• Difficult to be selectively toxic
• Nucleoside analog—AZT
   – Azidothymidine is analog of thymidine
   – Inhibits elongation of viral nucleic acid chain
   – Host toxicity
• Protease inhibitors
   – Viral protease enzyme used to cut up larger polypeptides to assemble new viruses
   – Binds to active site of protease enzyme
   – Inhibits viral maturation and processing of viral polypeptides
• Interferons
   – Made by uninfected host cells to prevent viral infection
   – Host specific (not viral specific)
   – Clinical aspects: needs to be in high concentration in local area
      • Interferon stimulation drugs

                           Chemotherapeutic Future
• Modify existing drugs
   – Extend spectrum
   – Prevent resistance
• New drugs
   – New targets of activity
   – Novel organisms
• Triplex agents
   – DNA that will selectively bind to site on DNA or mRNA where pathogenic protein is
     coded and prevent the production of the pathogenic protein
   – Prevent production rather than destroy product
   – Problem with penetration

								
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