Mycobacterium tuberculosis

 

1. Historical Perspective

·   Over a century ago, Robert Koch identified Mycobacterium tuberculosis as the cause of tuberculosis (TB).

·       TB was once widespread and caused a significant number of deaths in Europe and among young adults.

·    Today, TB remains a global health problem, with an estimated one-third of the world's population infected (around 2 billion people).

·     The HIV/AIDS pandemic has led to a dramatic increase in TB cases, as AIDS and TB are closely linked. TB is a leading cause of death among AIDS patients worldwide.

2. TB in the United States

·       In the United States, TB is most common among specific populations, including the homeless, elderly, malnourished, alcoholic individuals, minorities, immigrants, prison populations, and Native Americans.

·       Approximately one-fourth to one-third of active TB cases in the United States result from recent transmission, while most cases are due to the reactivation of old, dormant infections.

·       Infection occurs when bacteria are phagocytosed by lung macrophages, where they can survive normal antimicrobial processes.

3. Symptoms and Progression

·       TB has a slow incubation period of about 4 to 12 weeks.

·      Symptoms of TB include fever, fatigue, weight loss, and a characteristic cough that may produce bloody sputum.

·  M. tuberculosis does not produce typical virulence factors but has unique cell envelope components that contribute to its virulence.

4. Immune Response and Disease Stages

·       Caseous Lesion: A caseous lesion is a pathological condition characterized by the transformation of a tubercle (a small, hard nodule composed of bacteria, macrophages, T cells, and proteins) into a soft, cheese-like consistency. This change in the tubercle's texture is a distinctive feature of tuberculosis (TB).

 

·       Ghon Complexes: Ghon complexes are structures formed when caseous lesions calcify. These complexes are often prominently visible in chest X-rays and serve as radiological evidence of a previous or ongoing TB infection.

 

·       Tuberculous Cavities: In some instances, tubercle lesions may liquefy, leading to the formation of air-filled cavities within the affected tissue. These cavities can become sites from which the tuberculosis bacteria can spread to new areas in the body.

 

·    Miliary Tuberculosis: Miliary tuberculosis refers to the condition where the tuberculosis bacteria disseminate to various parts of the body from the air-filled tuberculous cavities. This dissemination is often characterized by the formation of numerous small tubercles, similar in size to millet seeds, in the infected tissues.

 

·    Reactivation Tuberculosis: Reactivation tuberculosis occurs when the tuberculosis bacteria, which may have remained dormant in the body after the initial infection, become active again. This can result in the progression of the disease even after a long period of latency.

5. Transmission and Treatment

·       Most TB cases in the United States are acquired through droplet nuclei in the respiratory route.

·   TB requires treatment with antimicrobial therapy, typically involving multiple drugs taken simultaneously.

·       Multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) are resistant to several drugs and pose serious public health challenges.

6. Drug Resistance

·       Drug-resistant TB arises due to spontaneous, predictable chromosomal mutations that confer resistance to drugs.

·       These mutations are unlinked, meaning resistance to one drug is not associated with resistance to another.

·       Directly observed treatment short course (DOTS) is a practice where each dose is taken in the presence of a healthcare worker to prevent resistance.

7. Tuberculin Skin Test

·       Persons infected with Mtb develop cell-mediated immunity.

·  Tuberculin skin tests involve injecting a purified protein derivative (PPD) of Mtb intracutaneously.

·     A positive reaction indicates exposure to Mtb, but further tests are needed to confirm a diagnosis.

8. Diagnostic Methods

·   Additional diagnostic methods include X-rays, bacterial isolation, microscopy of acid-fast bacteria, and DNA probes.

· This information provides an overview of tuberculosis, its historical context, symptoms, progression, transmission, treatment, drug resistance, and diagnostic methods.