Controlling Microbial Growth

Osmotic Pressure and Filtration

Osmotic Pressure

Surrounding cells with salt or sugar draws water from the cells to the outside environment.

Osmotic pressure describes the pressure needed to stop osmosis, or the diffusion of water across a membrane. Living cells require water in order to carry out life's processes, but water will diffuse across a membrane from areas of low solute concentration (inside the cells) to areas of high solute concentration (outside the cells), solute being a substance dissolved in water. Thus, salts and sugars can be used as solutes to draw water from microbial cells, causing them to die. This is the reason sugars are added to jams and fruit preserves, and it underlies the process of salt-curing meats and pickling meats, fish, and vegetables. Honey, which is composed of 80 percent sugar, remains remarkably preserved without refrigeration because of these considerations.

However, a few extremophiles, such as halophiles, bacteria that thrive in high-salt environments, make microbial control by osmotic pressure imperfect. Molds and yeasts are especially resistant to preservation via osmotic pressure. Furthermore, microbes can still live on the surfaces of jars and bottles used in pickling and making jam. Additionally, viruses are largely unaffected by osmotic pressure methods of preservation. The addition of salt or sugar is thus often combined with other methods, such as heating or desiccation, to ensure longer and more complete preservation of foods.


Filtration can be used to remove bacteria, and even viruses, from fluids and air.

Filtration describes passing liquid through a solid porous device that allows some but not all material to pass through. Filtration for disinfection or sterilization requires the pores of the filter to be very small. One of the benefits of filtration is that it does not require changes in temperature, which can affect the material being filtered. Historically, sterilization filters have been made of porcelain, asbestos, diatomaceous earth, and glass. Modern sterilization filters are usually made of plastic polymers such as nylon, polyethersulfone (PES), and polytetrafluoroethylene (PTFE).

Filtration offers advantages and disadvantages. Filters can be autoclaved so they are sterile, and they are relatively inexpensive. However, they may absorb some of the filtrate, or they may leach metallic ions into it.

It is essential when filtering a fluid to choose the correct pore size and material of the filter. Pore size depends on application. Very tiny pores can filter out viruses but slow the process of filtration as compared to larger pores that can trap bacteria and fungi but not viruses. When filtering liquids, care must be taken to choose a filter made of a material compatible with the material of the filtrate. Incompatible materials may alter the composition of the filtrate or cause filtration to fail entirely.

Because filters can be selective about what is captured by the filter and what is not, they can be used to select particular microbes. For example, during the manufacture of vaccines, a filter can be used to separate bacteria and other cells from the virus in question, rendering the filtrate free of contaminants.

Filters are commonly used to filter air in environments where sterile air is desirable or necessary, such as in operating rooms. High-efficiency particulate air (HEPA) filters are ideal for this application, having a pore size of 0.2 micrometers, small enough to trap a majority of viruses as well as bacteria and fungi. These filters must be changed regularly in order to maintain proper function.

HEPA Filtration

Air filters, such as those that are HEPA certified, remove particulates from the air that are larger than 0.2 micrometers.