Filter Sterilization

Air and Liquid media

Filter Sterilisation

Filtration is used for the removal of microbes from solutions that cannot easily be treated in other fashions. Typically heat-sensitive compounds such as antibiotics and vitamins are filtered before addition to sterile cool media. In the food industry, filtration finds utility in beer making to remove yeast before final bottling.

Filter Sterilisation

Filtration physically removes microbes because it employs membranes whose precisely defined pores are too small to allow their passage. It is obviously only useful for liquids and gases. Filtration does not effectively remove viruses from solution because they are typically too small. A filter system with sufficiently small pores to remove viruses would have an extremely slow flow rate due to the viscosity of water. Because viruses are not removed, filtration cannot technically be considered a form of sterilization, although it is common to refer to the process as “filter sterilization.”

Types of Filters

There are three major types of filters. Depth filters, are the oldest type and consist of overlapping layers of fibrous sheets of paper, asbestos or glass fibers. The random nature of the fibers laying on one top of another creates torturous paths through the filter that trap many particles.

Depth Filters

Due to their nature they do not trap all particles of a given size, but they find utility as pre-filters before final filtration. That is, their ability to remove the vast majority of things means that this removed material does not plug up the more efficient filters noted below. They are also useful for the filter sterilization of gases. Glass and asbestos depth filters can tolerate high heat and can be conveniently sterilized with steam.

Membrane Filters

The membrane filter is the most common type of filtration system used in modern microbiology laboratories. These are made from high tensile strength polymers of cellulose acetate, cellulose nitrate, polycarbonate, polyester, polypropylene or polysulfone.

Membrane Filters

At high magnification the polymers appear as a thick mesh of interconnected strands with rather precisely defined spaces between the polymer. By adjusting polymerization conditions, the size of the spaces or pores can be controlled to create membranes with various maximal sizes.

Nucleopore membranes

Nucleopore Membranes are created by exposing a very thin polycarbonate film (10 µm) to nuclear radiation, which creates areas of weakness in the polymer. The membrane is then treated with an etching solution that degrades the weak areas creating pores in the membrane.

Nucleopore Membranes

Changing the strength of the etching solution or the time of exposure of the membrane controls the size of the pores created and results in remarkably precise pore sizes. Nucleopore membranes are particularly useful for trapping bacteria on the surface of a membrane for subsequent microscopic examination.

Sterilization of Air

An aerobic process requires large amount of air ( of the order .1-1 volume of air per volume of liquid media). Since the cost incurred for sterilizing such huge amount of air would be enormous air filters are used to reduce costs. These filters are periodically changed when blocked dirty.

Air Sterilization

Filters come in many shapes and sizes from small (13 mm) filters that are used for the filtration of a few milliliters of liquid to industrial filters with surface areas of several square meters that can process hundreds of liters a minute. The figure below illustrates several different types of filters and filtration apparatus.

Some Filter Types

Diagram of Air Filter ( membrane Type)

Critical oxygen concentration

Critical oxygen concentration is the term used to indicate the value of specific oxygen absorption rate which permits the respiration without hindrance

Dissolved Oxygen Concentration

QO2

Ccritical

Effect of dissolved O2 concentration on the QO2 of a microorganism

Specific O2 uptake increases with increase in dissolved O2 levels to a certain point Ccrit.

QO2= Oxygen Consumption rated

Critical dissolved oxygen levels for a range of microorganisms

Organism Temperature Critical dissolvedoC Oxygen concentration

(mmoles dm -3)

Azotobacter sp. 30 0.018

E. coli 37 0.008

Saccharomyces sp. 30 0.004

Penicillium chrysogenum 24 0.022

Critical dissolved oxygen levelsTo maximise biomass production you must satisfy the

organisms specific oxygen demand by maintaining the dissolved O2 levels above Ccrit

Cells become metabolically disturbed if the level drops below Ccrit

High dissolved O2 levels also promote product formation

Amino acid biosynthesis by Brevibacterium flavum

Cephalosporium synthesis by Cephalosporium sp.

FACTORS AFFECTING OXYGEN DEMAND

• Rate of cell respiration• Type of respiration (aerobic vs anaerobic)• Type of substrate (glucose vs methane)• Type of environment (e.g pH, temp etc.)• Surface area/ volume ratio

large vs small cells (bacteria v mammalian cells)

hyphae, clumps, flocs, pellets etc.• Nature of surface area ( shape)

Size of sparger gas bubble

Gas composition, volume & velocity

Design of Impellersize, no. of bladesrotational speed Baffles

width, number

FACTORS INFLUENCING OXYGEN SUPPLY

Foam/antifoam

Temperature

Type of liquid

Height/width ratio

‘’Hold up’’

Process factors