Bacilloscopy: what is it and how is it used in medicine?
These are the characteristics of bacilloscopy, a test used to detect certain bacteria.
A smear microscopy is a test performed in medicine to detect bacillus-type bacteria in a given sample. in a given sample. This methodology is very useful in the identification of the microorganism Mycobacterium tuberculosis, the causative agent of tuberculosis (TB).
In the world of microbiology, detection is the key to success. In order to treat an infectious disease, finding the causative agent early before it multiplies out of control is essential. This can be simple in parasitic processes such as taeniasis, for example, where the parasite is more than a meter long.
Unfortunately, bacteria are much smaller and more elusive in many cases. For this reason, sophisticated methods have been devised for their detection, such as the smear microscopy that concerns us today. If you want to learn more about this topic, we encourage you to read on.
What is a bacilloscopy? Characteristics of this test
Since we can't start building the house from the roof up, we must first understand what a bacillus is, so we can later dive into how to detect it.
The word "bacillus" is used to describe any rod-shaped or rod-like bacterium. any rod or rod-shaped bacterium. Thus, it is a morphological classification that does not understand species, genera or orders.. However, there is one grouping that does fall under this term, the bacteria within the genus Bacillus.
It may seem complicated, but this terminological complex can be summarized in the following way all bacteria of the genus Bacillus are bacilli, but not all bacilli belong to this genus.. Without going any further, the bacterium that causes tuberculosis belongs to the genus Mycobacterium, even though it is rod-shaped. This is not the only one, because among many others, the genera Salmonella, Moraxella or Yersinia are also considered bacilli due to their elongated morphology.
We have defined the first key term for understanding bacilloscopy: the bacillus. This is the causal principle of the test, but the purpose of course boils down to the detection of tuberculosis.. Thus, this disease requires special mention beforehand.
Death bacillus and tuberculosis
The World Health Organization (WHO) provides us with certain relevant data regarding tuberculosis. Some of them are the following:
- This disease is one of the 10 most frequent causes of death in the world, the first of an infectious nature.
- A whopping 10 million people were infected in 2018, of which 1.5 million ended up dying.
- Tuberculosis is the leading cause of death among HIV-positive people.
As we can see, we are dealing with an extremely harmful pathogenic bacterium for human society.. The growth of Mycobacterium tuberculosis in the patient's respiratory tract causes productive cough, chest pains, weakness, weight loss and night sweats. By Blood or lymphatic routes, the bacteria can spread to other organs, aggravating the clinical picture even more.
One of the biggest problems with tuberculosis is that the most obvious symptoms begin to appear when the lesions in the lung tissue are already severe and the infection is at an advanced stage.. For this reason, tests such as smear microscopy are essential to act as quickly and effectively as possible. Below, we explain what this detection method consists of.
The differential diagnosis
We already know the principle (the bacillus Mycobacterium tuberculosis) and the end (TB tuberculosis). Now, naturally, it remains for us to dive into the world of diagnostics that correlate the microorganism with the patient's disease. This requires a series of steps to be followed.
1. Sample collection
First of all, it is necessary to emphasize that in order to be able to perform the smear microscopy, a sputum sample is required. a sputum sample from the patient is required.. According to clinical studies, this should be stored in a standard container (wide-mouth, hermetically sealed and made of break-resistant plastic).
As the clearance of bacilli with sputum is not constant, it is advisable to collect a total of three samples per patient. The first detects approximately 80% of positive cases, the second detects 15% and the third detects the remaining 5%. Of course, the samples should be collected at different times of the day to maximize the possibility of pathogen detection, samples should be taken at different times of the day to maximize the possibility of pathogen detection..
In the case of suspected dissemination of the infection to other organs, samples can be taken from cerebrospinal fluid, urine or pus from an abscess.
Once obtained, transported and fixed on a slide, it is time to search the sample for the microorganism causing the disease.
In order to observe the bacillus in the sample, it is necessary to subject it to a staining processspecifically to the Ziehl-Neelsen staining.
The basis of this technique is based on the fact that the cell wall of certain bacteria (such as Mycobacterium tuberculosis) presents acid-fast capacities, that is to say, that these bacteria have a property of retaining basic dyes despite exposure to decolorants such as acid-alcohol complexes..
Therefore, a dye called fuchsin is applied to the extended sputum sample and then subjected to a decolorization process. After that, a new dye is used.
Bacteria that show a red color after the decolorization process (due to the fuchsin retained in their cell wall) are the ones sought, while the rest are observed with a blue coloration (since methylene blue is used as a subsequent contrast dye).
Thus, under the microscope a series of small isolated or aggregated, curved, rod-shaped, fuchsia-red elements between one and 10 micrometers long can be observed. This allows a clear differential diagnosis: if there is a density of red microorganisms among the blue ones in the patient's sample, tuberculosis is assured..
3. Subsequent considerations
All that glitters is not gold, because despite the speed and low cost of the smear test, the World Health Organization warns us that it only detects half of the cases of tuberculosis. only detects half of the cases of tuberculosis and is unable to report whether there is drug resistance on the part of the microorganism..
Of course, observing whether or not the bacterium is present in the patient's sample is the first step, but also understanding whether it is a strain resistant to drugs such as rifampicin (multidrug-resistant tuberculosis) can mean the difference between life and death for the patient.
For this reason, the organization recommends performing the "Xpert MTB/RIF" test.which detects the disease and rifampicin resistance simultaneously in less than two hours. While the sensitivity of this new test is very high for lung infections, its diagnostic ability decreases when the infection spreads to other organs.
As we have seen, smear microscopy is a method of detecting bacteria in bacillus form such as Mycobacterium tuberculosistuberculosis, the pathogen causing the infectious disease that causes the most deaths worldwide.
However, due to the emergence of drug-resistant bacterial strains, it has become necessary to further refine detection methods.It is no longer enough to know that the bacterium is present in the patient's sample, but also to which drugs it responds and to which it is resistant.
For all these reasons, this form of detection is considered relatively archaic and increasingly out of use.although it is always a good option in hospitals in low-income countries without sophisticated means.
- Barrera, L., Sequeira, M., Balandrano, S., & Velazco, M. (2008). Manual for the bacteriological diagnosis of tuberculosis. Standards and theoretical guide. Part II. Culture. INEI, ANLIS Dr. Carlos Malbrán. Argentina. Pan American Health Organization. Regional Office of the World Health Organization.
- Díaz, J. M. L., Aréchiga, A. F., Guerra, M. G. M., & Martínez, P. C. C. (2003). Smear microscopy and culture in the diagnosis of extrapulmonary tuberculosis. Journal of Public Health and Nutrition, 4(3).
- Gómez, A. V., González-Martín, J., & García-Basteiro, A. L. (2017). Xpert® MTB/RIF: utility in the diagnosis of tuberculosis and rifampicin resistance. Medicina Clinica, 149(9), 399-405.
- Xpert® MTB/RIF test for the diagnosis of extrapulmonary tuberculosis and rifampicin resistance, Cochrane.org. Retrieved August 24 from https://www.cochrane.org/es/CD012768/INFECTN_prueba-xpertr-mtbrif-para-el-diagnostico-de-la-tuberculosis-extrapulmonar-y-la-resistencia-la.
- Tuberculosis, World Health Organization (WHO). Retrieved August 24 from https://www.who.int/es/news-room/fact-sheets/detail/tuberculosis.
(Updated at Mar 28 / 2023)