Antimicrobial treatments
Control of microbial growth
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Microbes cause problems in almost any imaginable human activity: food growth and technology, medicine, paint industry, nuclear power plants to mention a just a few. Control of microbial growth is often used to prevent growth of microorganisms. This may be achieved by killing microorganisms or by inhibiting their growth. If we kiil all microorganisms we sterilize the object.
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Problems in biofilm control
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Microbial growth in planktonic state can be reasonable controlled with above mentioned methods. Much more problematic is control of microbial growth in biofilms. The effectivenes of biocide tratment is dramatically reduced and high resistance of biofilms for chemical challenges is a serious industrial and medical problem.
There are multiple mechanisms of bacterial resistance which vary with the bacteria present in the biofilm and the drug or biocide being applied. These mechanisms include physical or chemical reaction–diffusion barriers to antimicrobial penetration into the biofilm, slow growth of the biofilm cells due to nutrient limitation, activation of the general stress response, and the emergence of a biofilm-specific phenotype. The individual bacteria in a biofilm may undergo physiological changes that improve resistance to biocides, such as induction of the general stress response (e.g., rpoS-dependent process in Gram-negative bacteria), increased expression of multiple drug resistance pumps, activation of quorum-sensing systems, or changing profiles of outer membrane proteins. Furthermore, biofilms are rarely, as the name implies, continuous films of microbial material that cover large surface area but are uneven distributions of small and large patches of biofilm structures.
It is generally assumed that biocide effectiveness in biofilms is approximately three orders of magnitude lower compared to bacterial suspensions. However, this might be misleading as biofilms are inherently heterogeneous structures and different parts of the biofilm may have significantly different susceptibilities for biocides.
There are several sterilization methods:
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Incineration: burns organisms and physically destroys them. Used for needles , inoculating wires, glassware, etc. and objects not destroyed in the incineration process.
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Boiling: 100°C for 30 minutes. Kills everything except some endospores (Actually, for the purposes of purifying drinking water 100°C for five minutes is probably adequate though there have been some reports that Giardia cysts can survive this process). To kill endospores, and therefore sterilize the solution, very long or intermittent boiling is required.
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Autoclaving (steam under pressure or pressure cooker): 121°C for 15 minutes (15#/in2 pressure). Good for sterilizing almost anything, but heat-labile substances will be denatured or destroyed.
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Dry heat (hot air oven): 160°C/2hours or 170°C/1hour. Used for glassware, metal, and objects that won't melt.
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Ultraviolet light is usually used (commonly used to sterilize the surfaces of objects), although x-rays and microwaves are possibly useful.
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Filtration: involvres the physical removal (exclusion) of all cells in a liquid or gas, especially important to sterilize solutions which would be denatured by heat (e.g. antibiotics, injectable drugs, amino acids, vitamins, etc.)
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Chemical and gas: (formaldehyde, glutaraldehyde, ethylene oxide) toxic chemicals kill all forms of life in a specialized gas chamber.
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The effectivenes of biocide is dependent on biofilm surface cover density. Low in high density biofilms, and high on single attached cells.
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We have recently shown that that oxidative biocides work efficiently on a single attached cell or small aggregate of attached cells, but significantly less on biofilms that cover large surface areas. Biofilm biocide resistance increases with biofilm maturation which correlates with extracellular matrix production. Thus, more dense attached structures present lower log reductions (i.e., lower percentages of dead cells) compared to less dense ones. The observations further suggest that changing viscoelastic properties of biofilms may allow for a better diffusion and therefore increased effectiveness of biocide treatment, a hypothesis worth further testing.
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