6. Control of microorganisms Flashcards
Antimicrobial Control:
chemical method:
sterilization vs. Inhibition vs. decontamination vs. disinfection
- Sterilization: kill all visible organisms (including endospores )
- Inhibition: effectively inhibit microbial growth
- Decontamination: treatment of an object to make it safe to handle
- Disinfection: directly remove all pathogens, not necessarily all microorganisms
- external surface: sterilants, disinfectants, sanitizers, antiseptics
- internal use: antibiotics, antivirals, antifungals
Physical method:
- heat
- what is decimal reduction time (D)?
- High temperatures denature macromolecules
- decimal reduction time (D): amount of time required to reduce viability tenfold
- higher temperature, less time needed to kill microorganisms
- The time necessary to kill a defined fraction is independent of the initial cell concentration (90%)
- Different microorganisms have different decimal reduction times
D=1h, CFU at time=0 is 10^7
how much time is needed to have <1 CFU?
at time=0, 10^7
=1, 10^6
=2, 10^5
=3, 10^4
=4, 10^3
=5, 10^2
=6, 10^1
=7, 10^0=1
=8, <1
add 1 hour so the CFU is smaller than 1Heat sterilization:
- What is Thermal death time?
- endospores are resistant?
- Autoclave?
- Thermal death time: time needed to kill all cells at a given temperature.
» depends on population size - higher temperature and longer boiling time is needed to kill endospores
- Autoclave: a sealed device that uses steam under pressure:
– Allows temperature of water to get above 100°C.
– Not suitable for heat- sensitive object/liquid.
– It is not the pressure that kills microorganisms, but the high temperature.
Physical methods:
1. Pasteurization
Flash pasteurization vs. Bulk pasteurization
- using precisely controlled heat to reduce the microbial load in heat-sensitive liquids.(sterilization)
» does NOT kill all organisms. - Flash pasteurization: 72°C for 15s.
- Bulk pasteurization: 65°C, 30min.
- Radiation
- UV has sufficient energy to cause modifications and breaks in DNA
–> inhibit replication, transcription and cause death. - Microwaves, UV, X-rays, gamma rays, and electrons can reduce microbial growth (decontamination)
••Cannot penetrate solid, opaque, or light-absorbing surfaces
Radiation:
- Ionizing radiation
- Ionizing radiation: electromagnetic radiation that produces
ions and other reactive molecules
– Generates electrons and hydroxyl radicals causing damage to DNA and proteins.
– Higher amount of energy required to reduce viability tenfold, longer decimal reduction time
• Sources of radiation include cathode ray tubes (electrons), X-rays, and radioactive nuclides
• Radiation is used for sterilization in the medical field and food industry
- Filter sterilization
- depth filter vs. membrane filter?
- No use of heat–> suitable for sensitive liquids and gases
- -> small presentation of filter that only liquid and gas can pass through
• Depth filters
– Fibrous sheet or mat made from an array of fiber (paper or glass).
– Used to sterilize liquid, air.
e.g. HEPA filters
• Membrane filters
– Function more like a sieve
e.g. nucleation track (nucleopore) filter.
- Filtration speed can be increase by syringe, pump, or vacuum
Chemical methods:
2. Antimicrobial agents
Bacteriostatic vs. Bacteriocidal vs. Bacteriolytic
• Bacteriostatic: inhibit growth of microorganism
» total cell count & viable cell count are even off on the graph of log cell number vs. time /`````````````
• Bacteriocidal: kill microorganism.
»viable cell count decrease BUT total cell count even off
/\ vs. /```````````
• Bacteriolytic: kill microorganism by inducing lysis.
»total &viable cell count both decrease
/\
Measuring antimicrobial activity 1. MIC? MLC? MBC?
- Minimum inhibitory concentration (MIC): smallest amount of an agent needed to inhibit growth of a microorganism
- Minimum lethal concentration: lowest concentration of an agent to kill a test Organism
- Minimum bacteriocidal concentration: lowest concentration of an agent to kill a test Bacterium
- use viable counts for MLC & MBC
- Decimal reduction time (at concentration x),
- Decimal reduction concentration (after x minutes).
- Disc diffusion assay
- Zone of inhibition?
- Zone of inhibition: Area of no growth around the disc. - Antimicrobial agent added to filter paper disc. • MIC is reached at some distance from the disc.
sterilants
disinfectants
antiseptics
antimicrobial drugs
• Sterilants: destroy all forms of microorganisms, including endospores. Called COLD STERILIZATION.
• Disinfectants/Sani:zers: applied to nonliving objects or surface (can be toxic for animals/humans). Do not kill endospores.
• An:sep:cs: applied to the surface of living :ssues or skin (must not be toxic for animals/humans). Do not kill endospores.
• Antimicrobial drugs: Antibiotics, antifungals, antivirals: applied outside or inside the body of animals/humans (must not be toxic for animals/humans). Do not kill endospores.
** ONLY sterilants kill endospores
Antimicrobial drugs
pros & cons?
• Can be used internally in humans or animals with minimal side effects (low toxicity).
• Can be either bacteriostatic or bactericidal. Usually have a specific target.
• A good antimicrobial drug has:
– NO severe side effects, must be far more toxic for bacteria than mammalian cells.
– Low risk/benefit ratio.
– Broad spectrum of activity to facilitate rapid medical intervention.
– Appropriate bioavailability and pharmacokinetics (must reach the site of infection).
– Low cost
Synthetic antimicrobial drugs
- Selective toxicity
- the ability to inhibit or kill a pathogen without affecting the host
- Paul Ehrlich studied selective toxicity in the early 1900s
- Salvarsan–one of the first antimicrobial drugs, used to treat syphilis (Treponema pallidum)
Synthetic antimicrobial drugs
- Growth factor analogs?
• Growth factor analogs are structurally similar to growth factors but do not function in the cell. Analogs similar to vitamins, amino acids, and other compounds
e.g. Isoniazid: a growth analog effective only against Mycobacterium. Interferes with synthesis of mycolic acid.
Synthetic antimicrobial drugs
- Nucleic acid base analogs
- Nucleic acid base analogs have been formed by the addition of bromine or fluorine. –>Stop DNA replication, translation.
- Quinolones: antibacterial compounds that interfere with DNA gyrase (control DNA supercoiling).
Antibiotics?
- Antibiotics: antimicrobial agents naturally produced by a variety of bacteria and fungi to inhibit or kill other microorganisms.
- <1% are clinically useful
- can be modified to enhance the efficacy
- Gram-positive and Gram-negative bacteria vary in their sensitivity to antibiotics. due to their different cell wall
β-Lactam Antibiotics
1. Penicillins
- Primarily effective against Gram-positive bacteria
- Some synthetic forms are effective against some Gram-negative bacteria–> has another cell wall that prevents the penetration of penicillins
» Inhibit cell wall synthesis - Discovered by Alexander Fleming, isolated from Penicillium chrysogenum
(mold).
❗️mold does not affect by penicillin because mold do not have peptidoglycan in their cell walls–> NO target
β-Lactam Antibiotics:
2. Cephalosporins
- Produced by the fungus Cephalosporium
» Inhibit cell wall synthesis
–>same mode of action as the penicillins - Transpeptidase (TPase) is a penicillin-binding protein.
• β-lactams are bactericidal or bacteriolytic
• Can be bacteriostatic (in isotonic solutions). - Commonly used to treat gonorrhea (Neisseria gonorrhea)
Antibiotics from Prokaryotes
- Aminoglycosides
- Chloramphenicol:
- Macrolides: erythromycin
- Tetracyclines
- Daptomycin
- Platensimycin
- Aminoglycosides: kanamycin, neomycin, amikacin, streptomycin.
– Target 30S subunit of the ribosome
»cause misreading of mRNA.
– Bactericidal. - Chloramphenicol:
– bind to 23S rRNA
»block peptide elongation.
– Bacteriostatic. - Macrolides: erythromycin
– Broad-spectrum antibiotics that target the 50S subunit of the ribosome
»block protein synthesis
– Bacteriostatic.
4. Tetracyclines – Broad-spectrum, – Inhibit 30S ribosomal subunit >>block protein synthesis. – Bacteriostatic.
5. Daptomycin – Also produced by Streptomyces – Used to treat Gram-positive bacterial infections >> Forms pores in cytoplasmic membrane
- Platensimycin
– New structural class of
antibiotics
– Broad-spectrum, effective against MRSA and vancomycin-resistant enterococci
Antimicrobial drug resistance?
– The acquired ability of a microorganism to resist the effects of a chemotherapeutic agent to which it is normally sensitive
Antibiotic producers are tolerant due to……?
• Lack target sites (i.e. no
peptidoglycan)
• Modify target sites
• Lack of uptake mechanism
Antibiotic resistance mechanism:
• Destruction or modification of the antibiotic (i.e. β- lactamase)
• Modification of the target site
• Modification of uptake
mechanism
• Efflux pumps: reduce intracellular concentration
Acquisition of resistance
- R plasmids?
-
- the acquisition of a new gene provides the cells with a new function (i.e. an:microbial resistance)
- drug resistance genes located on R plasmids
- The use of an:bio:cs in medicine, veterinary medicine, and agriculture selects for the spread of R plasmids.
- R plasmids can be transferred between bacteria of the same species or related species. (pass the resistance)
Antimicrobial Drug Resistance
• all pathogenic microbes have acquired resistance to some chemotherapeutic agents
•A few pathogens have developed resistance to all known antimicrobial agents
e.g. Methicillin-resistant S. aureus (MRSA)
• Resistance can be minimized by using antibiotics correctly and only when needed (reduce selection).
Antiviral Drugs
• high risk of side effect–> target hist structures causes toxicity,
–>NO need for antiviral drugs for flus, due to high risk and strong side effect
•vaccination is the best way against viruses.
1. nucleoside analogs
(e.g., AZT, Acyclovir):
- block reverse transcriptase and production of viral DNA (RNA viruses).
2. Protease
- inhibitors inhibit the processing of large viral proteins into individual components.
3. Fusion inhibitors
- prevent viruses from successfully fusing with the host cell.
Antifungal drugs
• Fungi need special chemotherapy
–>because they are eukaryotes: much of the cellular machinery is the same as that of animals and humans.
• A few drugs target unique metabolic processes not found in mammals:
> > Ergosterol synthesis (Nystatin, Fluconazole).
– Cell wall synthesis (inhibitor of chitin synthesis).
- stability of plasma membrane: fugus—>ergosterol, animal—>cholesterol
