Filtration

Importance of Filter in Microbiology history:

In the early days of microbiology, hollow candle shaped filters of unglazed porcelain were used to filter liquids. The long and indirect passageways through the walls of the filter adsorbed the bacteria. Filters came into prominent use in microbiology as interest in viruses grew during the 1890s. Previous to that time, filters had been utilized to trap airborne organisms and sterilize bacteriological media, but now they became essential for separating viruses from other microorganisms. Among the early pioneers of filter technology was Charles Chamberland, as associate of Pasteur. His porcelain filter was important to early virus research. Another pioneer was Julius Petri (inventor of Petri dish), who developed a sand filter to separate bacteria from the air.

Introduction and Action:

Filtration is the passage of a liquid or gas through a screen like material with pores small enough to retain microorganisms (often the same apparatus used for counting. A vacuum that is created in the receiving flask helps gravity pull the liquid through the filter. As fluid passes through the filter, organisms are trapped in the pores of the filtering material, as (figure) shows.

The solution that drips into the receiving container is decontaminated or, in some cases, sterilized.They are usually made of nitrocellulose and have the great advantage that they can be manufactured with specific pore sizes from 25 µm to less than 0.025μm. Particles filtered by various pore sizes are summarized in table.

Table 4	Pore sizes of membrane filters and particles that pass through them
Pore Size in (µm)	Particles that pass through them
10	Erythrocytes, yeast cells, bacteria, viruses, molecules
5	Yeast cells, bacteria, viruses, molecules
3	Some yeast cells, bacteria, viruses, molecules
1.2	Most bacteria, viruses, molecules
0.45	A few bacteria, viruses, molecules
0.22	Viruses, molecules
0.10	Medium-sized to Small Viruses, molecules
0.05	Small viruses, molecules
0.025	Only the very smallest viruses, molecules
Ultra-filter	Small molecules

Uses of Filtration:

Membrane-filters are used to sterilize heat sensitive materials include media, special nutrients that might be added to media, enzymes, vaccines, and pharmaceutical products such as drugs, sera, and vitamins. They are also used to sterilize the things such things as beverages, intravenous solutions and bacteriological media. Some operating theaters and rooms occupied by burn patients receive filtered air to lower the numbers of air borne microbes.

Some filters can be attached to syringes so that materials can be forced through them relatively quickly. Filtration can also be used instead of pasteurization in the manufacture of beer. When using filters to sterilize materials, it is important to select a filter pore size that will prevent any infectious agent from passing into the product.

Advantages:

Membrane filters have certain advantages and disadvantages. Except for those with the smallest pore sizes, membrane filters are relatively inexpensive, do not clog easily, and can filter large volumes of fluid reasonably rapidly. They can be autoclaved or purchased already sterilized.

Disadvantages:

A disadvantage of membrane filters is that many of them allow viruses and some mycoplasmas to pass through. Other disadvantages are that they may absorb relatively large amounts of the filtrate and may introduce metallic ions into the filtrate.

Different types of filters will be discussed in some future post.