Task6. Prove that various culture techniques have been developed to aid in species identification.

Task7. Read the text and describe the method of staining bacteria devised by Hans Christian Joachim Gram.

Gram’s stain is a widely used method of staining bacteria as an aid to their identification, devised by Hans Christian Joachim Gram, a Danish physician. In Gram's method, bacteria are first stained with gentian violet (a dye consisting of a methyl derivative of pararosaniline) and then treated with Gram's solution, consisting of 1 part iodine, 2 parts potassium iodide, and 300 parts water. After being washed with ethyl alcohol, the bacteria will either retain the strong blue color of gentian violet or be completely decolorized. Sometimes a counter stain such as fuchsine or eosin is applied to give the decolorized bacteria a reddish color to make them more visible.

Bacteria that retain the blue stain are known as gram-positive; those that do not are known as gram-negative. Organisms that sometimes retain the blue color and sometimes do not are known as gram-variable. Typical gram-positive bacteria are those staphylococci that produce boils; typical gram-negative bacteria are the bacilli that cause whooping cough; typical gram-variable bacteria are the bacilli that cause tuberculosis.

Task8. Read the article about the microscope, which is one of the most important tools used in studying bacteria. This article will look at some of the sophisticated new microscopes and the fascinating images they create.

Seeing the Invisible: The New Microscope

By Samuel H. Cohen

In the Beginning

The first microscopes were of two kinds: simple and compound. A simple microscope has just one lens (or lens combination)—a magnifying glass can be thought of as such a microscope. Compound microscopes were devised in the same place and at the same time as the early telescopes—among the lens grinders and experimenters of the Netherlands in the first years of the 17th century. A compound microscope consists of an eyepiece (technically known as an ocular) and a second lens (called an objective), near which the sample to be studied is positioned.

Electron Microscopes

Optical microscopes generally cannot resolve objects smaller than one-half the wavelength of the light they employ. Using an electron microscope, scientists, for the first time, could peer into the structure of crystals and fibers and explore the world of viruses, DNA, and objects of like scale.

There are two basic types of electron microscopes: transmission and scanning. In transmission electron microscopes an electron beam passes through a thin sample; the resulting image is either observed on a fluorescent screen or captured by a video camera and viewed on a TV screen. In a scanning electron microscope (SEM) a finely focused electron beam is literally scanned back and forth over the sample, knocking so-called secondary electrons off the sample’s surface. The secondary electrons are picked up by a detector, converted to an image of the sample's surface, and displayed on a TV monitor.

Images From Ultrasound

In the late 1960s, shortly after the introduction of ultrasonic imaging for medical purposes, a microscope was developed that also used ultrasound, or high-frequency sound waves. With this new "scanning laser acoustic" instrument researchers were able to obtain images from within samples as thick as 1 inch (about 2.5 centimeters).

The sample sits on a "transducer" (the device that produces the sound), immersed in a liquid such as water, which is better than air at transmitting ultrasound. A clear plastic cover, its bottom covered with a thin gold film, is placed on the sample. The high-frequency sound, attenuated to varying degrees by the different internal features within the sample, strikes the gold film, creating waves or wrinkles that form a kind of shadowlike acoustic image. This is scanned with a laser beam, and a visual image is created based on the brightness of the light reflected from each spot on the film. The ultrasound may be reflected from the sample's surface or transmitted through the sample. Both approaches have yielded important information in a nondestructive manner about internal structural characteristics of samples.