Bacterial Disease in Humans

What are pathogens?

They are disease-causing agents.

Bacterial Disease in Humans

• What are the two general ways that bacteria cause disease? – Some damage the tissues of the Some damage the tissues of the

infected organism directly by breaking infected organism directly by breaking them down for food. them down for food.

– Others release toxins that harm the Others release toxins that harm the body. body.

Bacterial Disease in Humans

• What kind of tissue do the bacteria that cause tuberculosis break down? –They break down lung tissue.

World TB Day - March 24th

Mycobacterium tuberculosis

• Acid fast aerobic rod Gram +ve• Multi- lobate colony morphology• Doubling time 24-30 h

History

• TB has been known as Pthisis, King’s Evil, Pott’s disease, consumption, and the White Plague.

• Egyptian mummies from 3500 BCE have the presence of Mycobacterium tuberculosis

• 460 BC: Hippocrates identifies “consumption” as “most widespread disease of the

time” and “is always fatal”• 1679: Physician Sylvius describes TB lung pathology• 1702: First reference to infectious nature of disease and first description of disinfection to stop transmission• 1720: Physician Marten conjectures about “wonderfully minute living creatures” as cause of consumption• 1854: Brehmer opened first sanitorium,

Isolated the infected in sanitariums, which served as waiting rooms for death

• 1865: Villemin demonstrated human to cow to rabbit transmission• 1882: Koch isolated agent, Mycobacterium tuberculosis, in pure

culture• 1895: Calmette and Guerin developed BCG vaccine• 1943: Streptomycin discovered

Disease progression- Stage 1

• Stage 1– Droplet nuclei are inhaled,

and are generated by talking, coughing and sneezing.

– Once nuclei are inhaled, the bacteria are non-specifically taken up by alveolar macrophages.

– The macrophages will not be activated, therefore unable to destroy the intracellular organism.

– The large droplet nuclei reaches upper respiratory tract, and the small droplet nuclei reaches air sacs of the lung (alveoli) where infection begins.

– Disease onset when droplet nuclei reaches the alveoli.

Disease Progression- Stage 2

• Begins after 7-21 days after initial infection.

• TB multiplies within the inactivated macrophages until macrophages burst.

• Other macrophages diffuse from peripheral blood, phagocytose TB and are inactivated, rendering them unable to destroy TB.

Disease Progression- Stage 3

• Lymphocytes, specifically T-cells recognize TB antigen. This results in T-cell activation and the release of Cytokines, including interferon (IFN).

• The release of IFN causes the activation of macrophages, which can release lytic enzymes and reactive intermediates that facilitates immune pathology.

• Tubercle forms, which contains a semi-solid or “cheesy” consistency. TB cannot multiply within tubercles due to low PH and anoxic environment, but TB can persist within these tubercles for extended periods.

Disease Progression- Stage 4

• Although many activated macrophages surround the tubercles, many other macrophages are inactivated or poorly activated.

• TB uses these macrophages to replicate causing the tubercle to grow.

• The growing tubercle may invade a bronchus, causing an infection which may spread to other parts of the lungs. Tubercle may also invade artery or other blood supply.

• Spreading of TB may cause milliary tuberculosis, which can cause secondary lesions.

• Secondary lesions occur in bones, joints, lymph nodes, genitourinary system and peritoneum.

Disease Progression- Stage 5

• The caseous centers of the tubercles liquefy.

• This liquid is very crucial for the growth of TB, and therefore it multiplies rapidly (extracellularly).

• This later becomes a large antigen load, causing the walls of nearby bronchi to become necrotic and rupture.

• This results in cavity formation and allows TB to spread rapidly into other airways and to other parts of the lung.

Virulent Mechanisms of TB

TB mechanism for cell entry– The tubercle bacillus can bind directly to

mannose receptors on macrophages via the cell wall-associated mannosylated glycolipid (LAM)

TB can grow intracellularly– Effective means of evading the immune

system– Once TB is phagocytosed, it can inhibit

phagosome-lysosome fusion– TB can remain in the phagosome or escape

from the phagosome ( Either case is a protected environment for growth in macrophages)

Antibiotic Mechanisms

• Inhibition of mRNA translation and translational accuracy (Streptomycin and derivatives)

• RNA polymerase inhibition (rifampicin) – inhibition of transcript elongation

• Gyrase inhibition in DNA synthesis (fluoroquinolone)

Antibiotic Mechanism II

• Inhibition of mycolic acid synthesis for cellular wall (isoniazid)

• Inhibition of arabinogalactan synthesis for cellular wall synthesis (ethambutol)

• Sterilization – by lowering pH (pyrazinamide)

Resistance Mechanisms of TB

• TB inactivates drug by acetylation – effective on aminoglycoside antibiotics (streptomycin)

• Also, through attenuation of catalase activity, in this way TB has developed resistance against certain drugs (asonizid)

• TB microbe has accumulated mutations that resist antibiotic binding (rifampicin and derivatives)

“The co-epidemic”HIV & TB

• HIV is the most powerful factor known to increase the risk of TB

• HIV promotes both the progression of latent TB infection to active disease and relapse of the disease in previously treated patients.

• TB is one of the leading causes of death in HIV-infected people.

TB/HIV Facts

• Individual infected with HIV has a 10 x increased risk in developing TB

• By 2000 nearly 11.5 million HIV-infected people worldwide were co-infected with M. tuberculosis

- 70% of these 11.5 million co-infection cases were in sub-Saharan Africa

Reasons for Fear

• Drug resistant strains of Mycobacterium tuberculosis have developed

• Underdeveloped countries are the most affected by TB

• 95% of reported cases come from underdeveloped countries

• High HIV rates in those areas contribute to the contraction of TB

What is MDR-TB ?

• It is a mutated form of the TB microbe that is extremely resistant to at least the two most powerful anti-TB drugs - isoniazid and rifampicin.

• People infected with TB that is resistant to first-line TB drugs will confer this resistant form of TB to people they infect.

• MDR-TB is treatable but requires treatment for up to 2 years.

• MDR-TB is rapidly becoming a problem in Russia, Central Asia, China, and India.

MDR-TB in the news:Man with tuberculosis jailed as

threat to health - USA Today 4-11-2007

• Russian-born man with extensively drug-resistant strain of TB, has been locked in a Phoenix hospital jail ward since July for not wearing face mask

Dpt. Infection and Tropical Medicine, Sheffield Teaching Hospitals

Case 1 – miliary tuberculosis

What will happen if treatment delayed? – gibbus formation (acute angulation of spine with or

without neurological damage)

The physical appearance – Potts disease of spine - gibbus

Diagnosis

Tuberculin skin test : which yields a delayed hypersensitivity type response to an extract made from  M. tuberculosis

Interferon-γ release assays : from blood sample QuantiFERON-TB Gold (licensed in US, Europe and Japan); andT-SPOT.TB, a form of ELISPOT (licensed in Europe).

 

Chest photofluorography has been used in the past for mass screening for tuberculosis (x-ray fluoroscopy of the thorax)

Molecular dignostic polymerase chain reaction assays for the detection of bacterial DNA

Amplified mycobacterium tuberculosis direct test :is highly sensitive and specific when used to test smears positive for acid-fast bacilli (AFB), which was approved by the FDA in 1996

Mycobacterium tuberculosis (stained red) in sputum

Microscopic examination

Prevention

• An experimental vaccine, with positive results in mouse models, may be effective in not only preventing infection, but also in eradicating the infection once established. A tuberculosis vaccine aimed at sterile Mtb eradication should be able to target latent Mtb as well as Mtb that causes early-stage tuberculosis. The vaccine is a combination of antigens Ag85B and ESAT-6 as well as the protein Rv2660c. Ag85B and ESAT-6 together form the vaccine Hybrid-1, while Rv2660c is a protein that is expressed even in late-stage infections, when protein transcription is generally reduced. The novel combination of Ag85B, ESAT-6, and Rv2660c allows for both short- and long-term protection as a result of the continued expression of target proteins. The new vaccine, currently referred to as H56, works by promoting a polyfunctional CD4+ T cell response against tuberculosis protein components. Phase I clinical trials are scheduled to begin in Cape Town, South Africa

•Bacillus Calmette-Guérin (BCG) vaccine 

Treatment

•Antibiotics to kill the bacteria- Isoniazid and Rifampicin or -Combination of several antibiotics

TB requires much longer periods of treatment (around 6 to 24 months) to entirely eliminate mycobacteria from the body

•The DOTS (Directly Observed Treatment Short-course) strategy of tuberculosis treatment recommended by WHO was based on clinical trials done in the 1970s by Tuberculosis Research Centre, Chennai, India•Is for MDR-TB•Technical strategy develop by Dr. Karel Styblo in 1980

This contributed to a steady global uptake of DOTS TB control servies over the subsequent decade. Whereas less than 2% of infectious TB patients were being detected and cured from TB with DOTS treatment services in 1990, approximately 60% are now benefiting from this care. Since 1995, 41 million people have been successfully treated and up to 6 million lives saved through DOTS and the Stop TB Strategy. 5.8 million TB cases were notified through DOTS programmes in 2009