Tuberculosis

 

 Dylan Hershkowitz and Peter Satin

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Introduction

 

The most common form of the disesase Tuberculosis is caused by the bacteria Mycobacterium tuberculosis which belongs to the slow growing subset of the Mycobacterium bacteria. Tuberculosis is primarily an infection of the lungs, but can infect the central nervous system and other body systems as well.  M. Tuberculosis has recently emerged as a mutliple drug resistant bacterium, a trait possibly conferred by its unique cell wall, characteristic among the genus Mycobacterium.  Treatment options for tuberculosis exist though, with the development of multiple drug resistant tuberculosis, curing tuberculosis has become more difficult.  This article will provide an overview tuberculosis, examining the cell morphology, the pathology of the disease, and the treatment options for the disease.

 

Tuberculosis   Introduction Morphology Cell Wall Components Pathology High Infection Rate Detection Treatment Current Treatments Multiple Drug Resistance BCG Vaccine Natural Immunity Developing Treatments Conclusion References

 

Morphology

 

M. tuberculosis is a rod shaped bacteria whose colonies often form serpentine chains (Todar, 2005).  These tight, filamentous chains have been suggested as the casue of the extreme latency of the bacterium to appear on agar plates (Madigan and Martink 2006).  The cell wall of M. tuberculosis has a high concentration of lipids, creating a waxy layer that is resistant to disinfectants (Madigan and Martinko, 2006, p 734).  While technically considered Gram-posititive, Tuberculosis cells don't stain well, usually resulting in a very faint stain or none at all (Todar, 2005).  M. tuberculosis is an obligate aerobic bacteria and therefore finds the lungs to be the most favorable environment for infection (Madigan and Martinko, 2006, p702).

 

Cell Wall Components

 

The cellular wall of M. tuberculosis is unique among prokaryotes.  Though, like all bacteria, the cell wall conains peptidoglycan, the majority (60%) is composed of three complex lipids (Todar, 2005). 

 

Mycolic acids are found in many of the macromolecules in the cell wall.  These acids, which comprise a majority of the cell's dry weight, are strongly hydrophobic, helping to create a highly impermeable shell around the bacterium (Todar 2005).  The hydrophobic nature of mycolic acids may arise from their particular folding conformations, which can resemble cholesterols, another extremely hydrophobic moleculte (Benadie 2008).  Many drugs designed to kill M. tuberculosis inhibit the formation of these mycolic acids at any number of the steps along their synthesis pathway. 

 

Cord Factor (trehalose-6,6'-dimycolate) is responsible for M. tuberculosis' filamentous structure (Todar 2005).  It is highly immunogenic and specific antibodies are an initial clue of TB infection.  Cord factor is composed of trehalose and one of two mycolic acids, depending on the species of bacterium (Benadie 2008).  Cord factor is a primary virulence factor of M. tuberculosis, and is necessary for proper growth of the bacterium in vivo (Behling 1993).  When cord factor is removed from the surface of the bacteria, no infection results.  While the specific toxicity is unknown, it has been demonstrated that to act as a virulence factor, cord factor must be grouped into a monolayer, as it is found in the cell wall (Behling 1993).  Cord factor is used as an adjuvant in many TB vaccines (Behling 1993).

 

Wax D is responsible for the bacterium's waxy coat (Todar 2005).  Wax D may also be responsible for host detection of the bacteria.  Buffalo rats injected with isolated Wax D from M. tuberculosis demonstrated an inflammation response (Kawabata 1998).  Wax D also caused arthritis in another species of rats.

 

Pathology

 

Tuberculosis typically affects the human respiratory system, although it can also infect other parts of the body such as the brain, the kidneys, the spine, or the skin ("Tuberculosis Fact Sheet", cdc.gov).  The common symptoms of Tuberculosis include feelings of weakness, weight loss, fever, night sweats, and, in the case of a respiratory infection, coughing, chest pain, and coughing up blood ("Tuberculosis Fact Sheet", cdc.gov).  Tubeculosis is typically transmitted through the air when an infected or carrying individual coughs, releasing small particles of the bacteria (Hart, 2004).   Infection can also occur through ingestion.  Due to their small size, these particles are able to remain in the air for long periods of time, which increases the chance of infection.  It is estimated that one in six people exposed to a case of Tuberculosis will become infected. 

 

Tuberculosis has evolved several mechanisms that aid in the pathogenicity of the bacteria.  Once established in the host environment, the M. tuberculosis bacteria will begin the secretion of two proteins that inhibit the response of the immune system (Wu 2008).  These two proteins, Protein Kinase G and MptpB, make it difficult for the host macrophages to engulf the bacteria (Szekely 2008).  These proteins are essential to the pathogenicity of the bacteria because without them, the host macrophages can simply engulf the bacteria and inhibit the spread of infection.

 

High Infection Rate

 

Nearly one third of the world’s population is believed to carry tuberculosis (“Extensively drug-resistant TB,” http://www.who.int).  The reason for such a high infection rate is that tuberculosis is transmitted through the air and is highly contagious.  The World Health Organization estimates that a person carrying tuberculosis can infect around 15 new people in one year (“Ten Facts about Tuberculosis,” http://www.who.int).  Although one third of the world’s population carries the bacteria, only one in ten of those infected will develop tuberculosis (“Tuberculosis,” http://www.who.int).  The primary reason for this lower rate of disease is that the tuberculosis bacteria are very slow growing and a healthy immune system is able to keep the bacteria in check.  When the immune system is compromised, the risk of developing symptoms increases.  This is particularly problematic in HIV patients or other individuals with immunodefficiency diseases.

 

Detection

 

Purified Protein Derivative (PPD) Test involves injecting tuberculin -- the name given to extracts of either M. tuberuculosis, M. bovis, or M. avium -- under the skin.  The PPD test results in a raised bump due to the fluid injected under the skin.  The state and size of the bump after 48 hours is used to determine the presence of tuberculin antibodies in your system.  Individuals who have tuberculosis, either latent or active, will develop a skin reaction to the injection of the antibodies within the 48 hour period (Haholu 2008).  The test lacks specificity, as it does not determine whether you have been exposed to M. tuberculosis in particular, or just a member of the mycobacterium family, nor does it differentiate between active TB infection and recent vaccination (Division of Tuberculosis Elimination 2007).  The test also lacks specificity in that it is not able to determine the progression of the illness (Haholu 2008).  Individuals with latent tuberculosis will demonstrate the same response as individuals with active tuberculosis.  There has been some suggestion that the size of the bump could be an indicator of the severity of the infection, but further research is needed before any conclusions can be reached.

 

Treatment

 

One reason Tuberculosis is so difficult to treat is the relative impermeability of the cell wall of M. tuberculosis (Hett 2008).  Lipophillic molecules pass through the rigid cell wall more easily than other molecules, thus lipophilic drugs such as fluoroquinolones and rifamycins have had better success at combatting TB infections (Hett 2008).

 

Current Treatments

 

Currently, most drugs effective against TB target synthesis of molecules in the cell wall.  A brief list of current drugs and their method of action follows:

 

Ethambutol inhibits the polymerization of polysaccharide arabinogalactan.  Arabinogalactan anchors the mycolic acid layer to the peptidoglycan layer.

 

Isoniazid is a prodrug, meaning that it is administered in a nonactive form that must be transformed into an active configuration in the body.  Isoniazid is activated by mycobacterially encoded KatG catalase, and inhibits mycolic acid synthesis.

 

Ethionamide is another prodrug that inhibits the synthesis of certain fatty acids required for mycolic acid synthesis.

 

Standard treatment of TB involves a combination of these drugs over a 6-9 month period.  This method has remained largely unchanged since the 1960s (Hett 2008).  This very long treatment time often results in patients not completing their treatment because the antibiotics work fairly quickly to alleviate their negative symptoms.  Failure to complete tuberculosis treatment can lead to the formation of antibiotic resistant strains of the bacteria, which pose significant treatment complications.

 

 

Multiple Drug Resistance

 

The World Health Organisation reported that multiple drug resistant (MDR) TB is at a record high, with a virtually untreatable strain now present in at least 45 countries (Foulkes, BBC.com).  MDR-TB is resistant to isoniazid and rifampicin, the two most common treatment drugs, and must now be treated with lengthier, more toxic treatments (Zager 2008).  The highest prevalence of MDR-TB is seen in Eastern Europe, China, and Sub-Saharan Africa.  Treating MDR-TB is especially difficult because the areas affected most by tuberculosis are areas with higher poverty levels that have limited medical resources. 

 

    A severe version of MDR-TB now exists that, in addition to isoniazid and rifampicin resistance, is resistant to at least one injectable antibiotic and to fluoroguinolone (Zager 2008).  Due to its extremely high resistance, this strain is considered virtually untreatable.  Organizations like the WHO are watching trends of XDR-TB and attempting to raise money for treatment research.

 

 

BCG Vaccine

The BCG vaccine, developed by Calmette and Guèrin in 1921, uses an attenuated strain of M. bovis to induce an immune response (CDC 1996).  Because the actual effectiveness of the vaccine is uncertain, it in not widely used in the United States, but is still in use in the developing world as means to slow the spread of TB (CDC 1996).  With the emergence of multiple-drug resistant strains (see below), however, the CDC is reevaluating its stance on the use of BCG as a preventive measure for TB infection.  Vaccination has been shown to limit the cases of TB in children under the age of 5, as well as in those working in areas where the risk of infection is high (CDC 1996).  As it stands, the CDC recomends that only persons who meet specific criteria be given the vaccine, due to the questionable success, as well as the interference it causes in PPD tests (CDC 1996).

 

Natural Immunity

Research suggests that there is a genetic component to tuberculosis succeptibility.  So far, two categories of resistance have been described: high responders and low responders, with high responders making up approximately 90% of the total population (Gaikwad 2008).  High responding individuals have been associated with lower levels of the M. tuberculosis detected when the bacteria is exposed to host macrophages.  The exact mechanism by which a higher degree of natural immunity exists is yet to be discovered.  There are, however, two potential components that may increase natural resistance.  High responding individuals typically possess higher levels of both TNF-a and IL-12 than low responding individuals (Gaikwad 2008).  The higher levels of these proteins also correspond to a higher level of antibodies detected in the blood.  

 

 

Developing Treatments

 

The increasing threat of multi-drug resistant tuberculosis has begun a new search for a treatment.  The goal of developing a new treatment is not only to provide a cure for multi-drug resistant tuberculosis, but also to reduce the dependence on antibiotics for treatment.  One of the new treatment ideas to cure M. tuberculosis involves particular protein kinases embedded in the bacteria’s cell wall. M. tuberculosis has eleven eukaryotic-like protein kinases, three of which are considered essential for the survival of the bacteria (Szekely, 2008).  The goal of one potential treatment of tuberculosis is to inhibit two of these essential kinases, PknB and PknG.    PknB is one of these essential kinases.  It is believed to be essential in cell division and cell growth, as the bacterial cells cannot grow without it (Szekely, 2008).  The PknG kinase is responsible for allowing M. tuberculosis to survive when engulfed by macrophages because it inhibits the binding of the bacteria to the lysosome, preventing the bacteria from being digested (Szekely, 2008).  Inhibition of the PknG kinase, however, has been shown to allow the lysosome to bind to and digest the bacteria (Szekely, 2008).  This dual treatment is believed to be able to target all M. tuberculosis in the host because the PknB inhibitor would prevent the growth of the bacteria while the PknG inhibitor would allow the host macrophages to digest the bacteria.

 

Also emerging as a possible TB treatment is the compound nitroimidazopryan (NAP), which works by disrupting synthesis of key proteins and lipids found in the cell wall (Stover 2000).  It has shown promise in treating both normal and MDR TB (Stover 2000).  While it may share similarities with treatment drugs before it, namely dependence on bacterial activation to be effective, NAPs may also function via new mechanisms.  While older drugs disrupted also mycolate synthesis - the primary factor in TB virulence - NAP halts the process at a more terminal step.  Specifically, it prevents the oxidation of hydroxymycolates into ketomycolates, the mycolate that is ultimately implanted into the cell wall (Stover 2000).  Experimental evidence suggests that this alone does not hamper TB spread, but paired with the disruption in protein synthesis, may halt a TB infection.

 

Conclusion

 Tuberculosis is an ancient disease that continues to pose serious health threats worldwide.  Mycobacterium tuberculosis evolved many pathogenic attributes to survive in the human body and has incredible virulence.  Tuberculosis has always been a difficult disease to manage due to the long recovery time and easy spreading.  But now the efficacy of the traditional method of treating tuberculosis with antibiotics is threatened by the development of MDR-TB and now XDR-TB.  Research into new methods of treatment for these particularly difficult strains of tuberculosis is needed in order to prevent them from becoming a pandemic.

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References

 

Behling, Cynthia A, et al (1993).  Development of a trehalose 6,6'-dimycolate model which explainscord formation by Mycobacterium tuberculosis.  Infection and Immunity.  61, 2296-2203.

 

Benadie, Yolandy, et al (2008).   Cholesteroid nature of free mycolic acids on M. tuberculosis.  Chemistry and physics of lipids.  152, 95-103.

 

Brock: Biology of Mircoorganisms, Madigan, M.T., and Martinko, J.M. Prentice Hall, 11th edition, 2006.

 

Centers for Disease Controll (1996).  The role of BCG vaccine in the prevention and control of tuberculosis in the United States.  Morbidity and mortality weekly.  45.

 

Foulkes, Imogen (2008 February 28). Drug resistant TB 'at new high'. Retrieved March 25, 2008, from BBC News Web site:         http://news.bbc.co.uk/2/hi/health/7265464.stm

 

Gaikwad, A.N., and Sinha, S. (2008)  Determinants of natural immunity against tuberculosis in an endemic setting: factors operating at the level of macrophage-Mycobacterium tuberculosis interaction.  Clinical & Experimental Immunology 151 (3): 414-422.

 

Hett, Erik C. and Rubin, Eric J (2008).  Bacterial cell growth and division: A mycobacterial perspective.  Microbiology and molecular biology review.  72, 126-156.

 

 Kawabata, Y., et. al. (1998) Wax D of Mycobacterium tuberculosis induced osteomyelitis accompanied by reactive bone formation in Buffalo rats.  FEMS Immunology and Medical Microbiology. 22: 293-302.

 

Microterrors: the Complete Guide to Bacterial, Viral, and Fungal Infections that Threaten our Health, Hart, T.  Axis Publishing, 2004.

 

Stover, C. Kendall, et.al (2000).  A small-molecule nitroimidazopryan drug candidate for the treatment of tuberculosis. Nature. 405, 962-966.

 

Szekely, R. et. al. A novel drug discovery concept for tuberculosis: Inhibition of bacterial and host cell signaling. Immunology Letters. 2008.  Article in Press.

 

Todar, Kenneth (2005). Tuberculosis. Retrieved February 16, 2008, from http://www.textbookofbacteriology.net/tuberculosis.html

 

"Tuberculosis: General Information" The Center for Disease Control, http://www.cdc.gov/tb/pubs/tbfactsheets/tb.htm, 2007

 

"Tuberculosis" The World Health Organization, http://www.who.int/tb/en/, 2008.

 

Wu, H., Wang, A. H., and Jennings, M.P. (2008)  Discovery of virulence factors of pathogenic bacteria.  Current Opinion in Chemical Biology. 12 (1): 93-101.

 

Zager, E.M., and McNerney, R. (2008)  Multipledrug-resistant tuberculosis.  BMC Infectious Diseases. 8:10