Bacilloscopy: what is it and how is it used in medicine

Bacilloscopy is a test used in medicine to detect bacillus-like bacteria 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 be able to treat an infectious disease, it is essential to find the causative agent quickly before it grows out of control. This can be straightforward in parasitic processes such as tapeworm, for example, where the parasite is over a meter tall.

Unfortunately, the bacteria are much smaller and questionable in many cases. For this reason, sophisticated methods have been devised for its detection, such as the bacilloscopy which belongs to us today. If you would like to learn more about the topic, we encourage you to keep reading.

    What is a bacilloscopy? Characteristics of this test

    Since we can’t start building the house on the roof, we first need to understand what a bacillus is, so that we can then delve into how to detect it.

    The word “bacillus” is used to describe a any rod or rod shaped bacteria. It is therefore a morphological classification which does not include species, genera and orders. However, there is one group that understands this term, bacteria of the genus Bacillus.

    It may sound confusing, but this terminology complex can be summed up in what all bacteria of the genus Bacillus are bacilli, but not all bacilli belong to this genus. Without going any further, the bacterium responsible for tuberculosis belongs to the genus Mycobacterium, regardless of its stick shape. This is not the only one, because among many others, the genera Salmonella, Moraxella or Yersinia are also considered as bacilli due to their elongated morphology.

    We have defined the first key term for understanding smear microscopy: the bacillus. This is the causal principle of the test, but the objective of course boils down to the detection of tuberculosis.. Therefore, this disease requires prior special mention.

    The bacillus of death and tuberculosis

    The World Health Organization (WHO) gives us some relevant data regarding tuberculosis. Some of them are:

    • This disease is one of the 10 most common causes of death in the world, the first of which is infectious in nature.
    • In 2018, no more and no less than 10 million people were entrusted, of which 1.5 million ended up dying.
    • Tuberculosis is the leading cause of death for people with HIV.

    As we can see we are ahead a pathogenic bacteria that is extremely harmful to human society. The growth of Mycobacterium tuberculosis in the patient’s airways results in a productive cough, chest pain, weakness, weight loss, and night sweats. Bacteria can spread to other organs through blood or lymph, further aggravating the clinical picture.

    One of the biggest problems with tuberculosis is that the most obvious symptoms begin to appear when the damage to the lung tissue is already severe and the infection is at an advanced stage.. Therefore, tests such as bacilloscopy become essential for working as quickly and efficiently as possible. This is what this detection method consists of.

      Differential diagnosis

      We already know the beginning (the bacillus Mycobacterium tuberculosis) and the end (TB tuberculosis). Now, of course, we still have to immerse ourselves in the world of diagnostics which correlates the microorganism with the patient’s disease. To do this, you need to follow a series of steps.

      1. Sample collection

      First of all, it should be noted that to perform a bacilloscopy a sputum sample from the patient is required. According to clinical studies, it should be stored in a standard container (wide mouth, airtight seal and break-resistant plastic).

      As the elimination of bacilli with sputum is not constant, it is advisable to take a total of three samples per patient. The first detects about 80% of positive cases, the second 15% and the third the remaining 5%. Of course, samples should be taken at different times of the day to maximize the possibility of detecting the pathogen.

      If the infection is suspected to have spread to other organs, samples of cerebrospinal fluid, urine, or pus from an abscess may be taken.

      Once obtained, transported and fixed on a slide, it is time to search for the sample in the microorganism causing the disease.

      2. Coloring

      In order to be able to observe the bacillus in the sample, the sample must be subjected to a staining process., More precisely in the Ziehl-Neelsen coloring.

      The basis of this technique is based on the fact that the cell wall of certain bacteria (such as Mycobacterium tuberculosis) has resistance to acids and alcohols, that is to say that these bacteria have a basic dye retention property despite exposure to bleaching agents such as the acid-alcohol complex.

      Therefore, a dye called fuchsin is applied to the expanded sputum sample, which will then go through a discoloration process. After that, a new dye will be used.

      Bacteria that show a red color after the discoloration process (due to fuchsin retained in its cell wall) are the ones sought after, while the others are seen with a blue stain (since methylene blue is used as a posterior contrast dye. ).

      So, under the microscope, you can see a series of small curved elements isolated or aggregated, shaped like a fuchsia red stick between one and 10 microns long. This allows a clear differential diagnosis: if there is a density of red microorganisms among the bruises in the patient sample, tuberculosis is insured.

        3. Subsequent considerations

        Not all that glitters are gold, because despite the speed and inexpensive cost of bacilloscopy, the World Health Organization is warning us that it it only detects half of TB cases and is unable to report whether there is drug resistance from the microorganism.

        Of course, observing whether or not the bacteria is present in the patient’s sample is the first step, but also understanding whether it is a strain resistant to drugs like rifampicin (multidrug-resistant tuberculosis) can lead to the disease. difference between life and death. of the patient.

        Therefore, this organization recommends performance of the “Xpert MTB / RIF” test, Which simultaneously detects disease and rifampicin resistance in less than two hours. While the sensitivity of this new test is very high for lung infections, its ability to diagnose declines as the infection spreads to other organs.


        As we have seen, smear microscopy is a method of detecting bacteria in the form of bacilli like Mycobacterium tuberculosis, the pathogen responsible for the world’s deadliest infectious disease.

        However, due to the emergence of drug-resistant bacterial strains, it becomes necessary to further refine the detection methods: It is no longer worth knowing that the bacteria is present in the patient’s sample, but also to which drugs it responds to and which drugs it is resistant to.

        For all these reasons, this form of detection is considered relatively archaic and increasingly obsolete, Although it is still a good choice in hospitals in low income countries without sophisticated means.

        bibliographical references

        • Barrera, L., Sequeira, M., Balandrano, S., and Velazco, M. (2008). Manual for diagnostics bacteriological tuberculosis. Rules and theoretical advice. Part II. Cultivate. INEI, ANLIS Dr. Carlos Malbrán. Argentina. Pan American Health Organization. Regional office of the World Health Organization.
        • Díaz, JML, Aréchiga, AF, Guerra, MGM and Martínez, PCC (2003). Bacilloscopy and culture in the diagnosis of extrapulmonary tuberculosis. Journal of Public Health and Nutrition, 4 (3).
        • Gómez, AV, González-Martín, J., and García-Basteiro, A EL (2017). Xpert® MTB / RIF: Useful in the diagnosis of tuberculosis and rifampicin resistance. Clinical Medicine, 149 (9), 399-405.
        • Xpert® MTB / RIF test for the diagnosis of extrapulmonary tuberculosis and rifampicin resistance, Collected August 24 at
        • Tuberculosis, World Health Organization (WHO). Retrieved August 24, from

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