Microscopic Examination in Medical Biology

◉ Introduction

◉ Focusing with an Optical Microscope

Focusing in a microscope involves adjusting the distance between the microscope objective and the sample to get the sharpest image of what you are observing. Here are the general steps for tuning:

◉ Microscopic Examination Wet Mount

It is a rapid, easy, and cost-effective technique that allows visualization of the sample under a microscope without any preparation (neither fixation nor staining).

The wet mount allows visualization of the various components of the sample, but its greatest value lies in its ability to keep bacteria and parasites alive, allowing optimal observation of their numbers, movements and of their arrangement.

It can be performed directly from the pathological product (urine, cerebrospinal fluid, etc.) or from a bacterial suspension. Various supports can be used, such as slides and coverslips, Malassez cells, and Nageotte cells.

Observation is generally done with a ×40 objective with an almost closed aperture to increase contrast. It is advisable to wait a few moments until the fluid movements stop.

1- Wet Mount Between Slide and Coverslip

Wet mount is a common, easy, quick and widely used practice in bacteriology laboratories. Here are the main steps:

Wet Mount procedure
Wet Mount procedure
L'Wet Mount procedure

Depending on the type of sample and the purpose of the sample, wet mount examination can detect the presence or absence of several biological components such as:


2- Hemacytometer Cells

The counting of different elements of biological liquids is possible outside of any automatic machine thanks to the use of calibrated glass cells called hemocytometers.

These are thick glass blades hollowed out with channels, presenting a particular grid on the slightly lowered central platform. Covered with a coverslip placed on the raised side platforms, they allow, with a very precise and known volume, to count the cells of biological liquids.

There are several types of hemocytometers, all with different counting grids, among the most used are the Malassez cell, the Thoma cell, the Nageotte cell and the Neubauer cell.

To obtain a count close to reality, it is important to:

2. 1 Manipulation


2. 2 Counting

◉ Microscopic Examination After Staining

The coloring of biological products is a very common technique in medical biology. It allows you to provide more details or even make visible elements that are invisible when fresh.

There are several types of coloring, with different production procedures. The choice depends on the type of sampling and the element sought. Roughly, they can be classified into simple (non-differential) colorations and differential colorations

The common first step is usually taking a smear and fixing it. Then, after coloring, the reading is done at a magnification of X100 with an open diaphragm.

1- Simple staining

Simple stains, or direct staining, are quick-to-perform procedures that involve adding a single dye to the smear, leaving it to act, then washing, drying and observing under a microscope. They make it possible to determine the presence and morphology of biological elements such as bacteria, cells, etc.

Simple coloring uses basic dyes such as methylene blue, safranin, crystal violet, malachite green, etc.

2- Differential staining

Differential staining involves using multiple dyes to differentiate between various types of cells, structures, and microorganisms.

There are a number of differential staining methods, such as Gram staining, considered the gold standard in bacteriology, which allows bacteria to be differentiated into Gram positive and Gram negative. Ziehl-Neelsen staining makes it possible to detect and differentiate tuberculosis bacilli from other bacteria.

◉ Conclusion

In conclusion, microscopic examination proves to be an essential technique in the medical analysis laboratory. It allows detailed observation of biological samples, providing valuable information for diagnosis and patient management.

It is essential to emphasize the importance of rigor and precision in carrying out this review, as well as the need to take into account the various sources of potential errors.


  1. Holmskov, A. (2023, December 14). How to count cells with a hemocytometer - ChemoMetec. ChemoMetec. https://chemometec.com/how-to-count-cells-with-a-hemocytometer/
  2. Difference between Simple and Differential Staining. BYJUS. https://byjus.com/biology/difference-between-simple-and-differential-staining/
  3. Le microscope. (n.d.). http://urine.optmq.connexence.com/docfr/doc_009.html
  4. CSCQ Mars 2020 Evelyne Mertz, Dagmar Kesseler - FICHE TECHNIQUE - Microscope
  5. HERVÉ SAUER JOËLLE SURREL - Principes et utilisations de base du microscope