Germicidal chemicals can be used to disinfect and, in some cases, sterilize, but they are less reliable than heat. They are especially useful for destroying microorganisms and viruses on heat-sensitive items and large surfaces.

■ Describe four factors that must be considered when selecting a germicidal chemical.

■ Explain why it is essential to dilute iodophores properly.

■ Why would a heavy metal be a more serious pollutant than most organic compounds?

Chapter 5 Control of Microbial Growth

5.5 Removal of Microorganisms by Filtration

Organisms in heat-sensitive fluids can be removed by filtration (figure 5.7). This procedure has many uses such as in space technology, the production of unpasteurized beer, and the sterilization of sugar solutions. Filtration of the air entering specialized hospital rooms removes organisms that can otherwise cause disease in patients who have been extensively burned or have undergone chemotherapy.

Filtration of Fluids

Filters capable of removing bacteria from fluids were developed during the last decade of the nineteenth century. They were made from materials such as porcelain, glass particles, diatoma-ceous earth, and asbestos. Today, two types of filters are in general use, depth filters similar to those used over the last century, and membrane filters.

Depth Filters

Depth filters have complex, tortuous passages that retain microorganisms while letting the suspending fluid pass through the small holes. The diameter of the passages is often considerably larger than that of the microorganisms they retain, and trapping of microbes is partly a result of electrical charges on the walls of the filter passages.

Although the filters developed in the nineteenth century remove most species of bacteria from their suspending media, they have several drawbacks. For example, they cannot reliably remove viruses. They also cannot be used to separate bacteria from desired products such as enzymes because they may retain proteins in addition to bacteria. Proteins can also interfere with filtering action by neutralizing electrostatic charges on the filter. Too much pressure applied to speed filtration can also overcome



Sterilized fluid



Figure 5.7 Filtration The liquid to be sterilized flows through the filter on top of the flask in response to a vacuum produced in the flask by means of a pump. Scanning electron micrograph (5,000x) shows a polycarbonate nucleopore filter retaining cells of Pseudomonas.

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