Pyrex: construction, property, and uses in microbiology

Pyrex is a brand name of a type of glass that is constructed of borosilicate. The Corning Glass Company of Corning, New York, developed Pyrex. Chemically, as borosilicate implies, this type of glass is composed of silica and at least five percent (of the total weight of the elements in the glass) of a chemical called boric oxide. The combination and concentrations of these constituents confers great resistance to temperature change and corrosion by harsh chemicals, such as strong acids and alkalis, to whatever vessel is made of the borosilicate glass. This durability has made Pyrex glassware extremely useful in the microbiology laboratory.

The development of Pyrex in 1924 by scientists at the Corning Company satisfied the demand for high quality scientific glassware that had began in the nineteenth century. Then, the glassware in existence was degraded by laboratory chemicals and became brittle when exposed to repeated cycles of heating and cooling. The formulation of Pyrex minimized the tendency of the material to expand and contract. This maintained the accuracy of measuring instruments such as graduated cylinders, and overcame the brittleness encountered upon repeated autoclave sterilization of the laboratory glassware.

Pyrex glassware immediately found acceptance in the microbiology research community. The popularity of the glassware continues today, despite the development of heat and chemical resistant plastic polymers. Glass is still the preferred container for growing bacteria . This is because the glass can be cleaned using harsh chemicals, which will completely remove any organic material that might otherwise adhere to the sides of the vessel. For applications where the chemical composition and concentrations of the medium components are crucial, such organic contaminants must be removed.

Pyrex glassware is also used to manufacture graduated cylinders that are extremely accurate. In some applications, the exact volume of a liquid is important to achieve. This type of glassware is known as volumetric glassware. Plastic still cannot match the accuracy or the unchanging efficiency of volume delivery that is achieved by Pyrex volumetric glassware.

Another application for borosilicate glass is in the measurement of optical density. For this application, typically specially designed vials are filled with the solution or suspension of interest and then placed in the path of a beam of light in a machine known as a spectrophotometer . The amount of light that passes through the sample can be recorded and, with the inclusion of appropriate controls, can be used, for example, to determine the number of bacteria in the sample. Plastic material does not lend itself to optical density measurements, as the plastic can be cloudy. Thus, the vial itself would absorb some of the incoming light. Pyrex, however, can be made so as to be optically transparent. Growth flasks have even been made in which a so-called "side arm," basically a test tube that is fused onto the flask, can be used to directly obtain optical density measurements without removing the culture from the flask.

In the same vein, the use of optically transparent slabs of Pyrex as microscope slides is a fundamental tool in the microbiology laboratory. The heat resistance of the slide allows a specimen to be heated directly on the slide. This is important for stains such as the acid-fast stain for mycobacteria, in which heating of the samples is essential for the accurate staining of the bacteria. Also, as for the optical density measurements, the light microscopic examination of the bacterial sample depends upon the transparency of the support surface. Plastic is not an appropriate support material for slides.

Another area in which Pyrex glassware is essential in a microbiology laboratory is in the pipelines required for the delivery of distilled water. Distillation of water is a process that requires the boiling of the water. The pipelines must be heat resistant. Also, because physical scrubbing of the pipelines is not feasible, the pipes must withstand the application of caustic chemicals to scour organic material off the interior surface of the pipes.

Other applications of borosilicate glassware in the microbiology laboratory include nondisposable Petri plates for the use of solid media, centrifuge tubes, titration cylinders, and the stopcocks that control the flow rate.

Heat and chemically resistant plastics are widely used in the typical microbiology laboratory, particularly for routine, high-volume operations where cleaning and preparation of glassware for re-use is time-consuming and prone to error.

However, the accuracy and advantages of Pyrex glassware ensure its continued use in the most modern of microbiology laboratories.

See also Laboratory methods in microbiology; Microscopy