Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Electron microscopic examination of microorganisms

181

•

This technique is known as immunoelectron microscopy. Care

is required during the fixation and other preparation steps to

ensure that the antigenic sites are not changed so that antibody

is still capable of binding to the antigen.

Frozen samples can also be cracked open by allowing

the sample to strike the sharp edge of a frozen block. The

crack, along the path of least chemical resistance, can reveal

internal details of the specimen. This technique is called

freeze-fracture. Frozen water can be removed from the frac-

ture (freeze-etching) to allow the structural details of the spec-

imen to appear more prominent.

Samples such as virusesare often examined in the

transmission electron microscopeusing a technique called

negative staining. Here, sample is collected on the surface of

a thin plastic support film. Then, a solution of stain is flowed

over the surface. When the excess stain is carefully removed,

stain will pool in the surface irregularities. Once in the micro-

scope, electrons will not pass through the puddles of stain,

producing a darker appearing region in the processed image of

the specimen. Negative staining is also useful to reveal surface

details of bacteriaand appendages such as pili, flagella and

spinae. A specialized form of the staining technique can also

be used to visualize genetic material.

Electron microscopes exist that allow specimens to be

examined in their natural, water-containing, state.

Examination of living specimens has also been achieved. The

so-called high-vacuum environmental microscope is finding

an increasing application in the examination of microbiologi-

cal samples such as biofilms.

See alsoBacterial ultrastructure; Microscope and microscopy

Scanning electron micrograph of the dinoflagellate Gambierdiscus toxicus.

womi_E 5/6/03 2:12 PM Page 181