Section 2: Mycobacteria Flashcards
What are the characteristics of Mycobacterium
- Nonmotile
- Aerobic
- Non spore forming
- Cell morphology= bacilli
- Cell wall is 40% lipid
- Acid fast
What is Mycobacterium tuberculosis
Discovered as a causative agent ofTB by Robert Koch in 1882
• At the time TB killed I in 7 people, currently infects I in 3 worldwide. Total about 2 billion, kills about 3 million
What are the characteristics of Mycobacterium tuberculosis
-Thin pleomorphic rods
-No capsules
-Appear “beaded” when stained due to high lipid content of cell wall
• Very resistant to drying and chemicals
• Can remain alive and viable for weeks or months - Obligate aerobe
-Colony morphology is unique-rough irregular
-Growth is very slow, divide every 12 to 24 hours
What is the cell wall chemistry of Mycobacterium tuberculosis
composed of mycosides-glycolipids found ONLY in acid fast bacteria
What is Cord Factor
formed by the union of 2 mycolic acids with a disaccharide (trehalose)
• Cord factor is found only in virulent strains of M tuberculosis
• Its presence results in a parallel growth of the bacteria, so they appear as cords.
• Exactly how leads to virulence is unknown
What are Sulfatides
mycosides that resemble cord factor with sulfates attached to the disaccharide • Inhibit lysosome function in phagocytic cells
What is Wax D
complicated mycoside that acts as an adjuvant
What is the epidemiology of Mycobacterium tuberculosis
-Tuberculosis is primarily a disease of humans
-Transmitted by close person to person contact via aerosolized droplets
• Large droplets are trapped on mucosal surfaces and removed
• Small droplets containing 1-3 M tuberculosis organisms can reach alveoli and establish an infection
-Transmission via objects is also possible
-A person with untreated, active tuberculosis can infect an estimated 20 other people per year
What is the pathology of Mycobacterium tuberculosis (long)
-Primary infection
• Bacteria inhaled in droplet nuclei, land in the lower respiratory tract
• Infiltration of macrophages and neutrophils
• Phagocytosed bacteria are taken to regional lymph nodes
• Bacteria are not killed, multiply and survive in the phagocytes= intracellular
• Move through the lymphatic system to infect other tissue
• Some macrophages do successfully destroy bacteria and present antigens to helper T cells to activate cell mediated immunity
• T cells enter circulation and search forM. tuberculosis.
• When T cells encounter bacteria they release lymphokines to attract and activate macrophages. • Activated macrophages can now destroy M tuberculosis
• Activated macrophages attack, lung tissue becomes damaged and local necrosis of lung tissue occurs
• Foci of the infection is called the Ghon complex and the necrosed tissue is called caseous necrosis The entire structure is called a granuloma or a tubercle
• Within the tubercle bacteria are kept from spreading, but remain viable
• As cell mediated immunity develops
What is the secondary infection of Mycobacterium tuberculosis
- Most cases ofTB occur after the bacteria are dormant for some time
- The infection can occur in any of the organ systems “seeded” during the primary infection
- The temporary weakening of the immune system causes reactivation-many AIDS patients develop TB in this way
- I /3 of HIV infected person are also infected with M tuberculosis worldwide
What are sites of reactivation
- Pulmonary TB-most common site. Areas of infection grow, liquidity and cavitate. Clinically= weight loss, night sweats, cough with blood
- CNS -TB causes meningitis and forms granulomas in the brain
- Genital infection
- Bone infections-usually involves the spine, destroying intervertebral discs (Pott’s disease) • Joint infection
- Skin infection
- Miliary TB-tiny millet-seed sized tubercles (granulomas) are disseminated all over the body. The kidneys, liver, lungs and other organs are filled with tubercles
How is Mycobacterium tuberculosis clinically diagnosed
Tuberculin Mantoux Test
• Tests skin sensitivity to TB antigens
How is Mycobacterium tuberculosis diagnosed un the laboratory
w-Isolation of tubercle bacilli using selective media
- Use of biochemical tests
- Acid fast stain
What is Mycobacterium bovis
- Pathogen of cattle
- Can be transmitted to humans in raw milk
- Human infection with M bovis is often not pulmonary, causes infection of bone, cervical lymph nodes
What are atypical Mycobacteria
- Atypical = non-tuberculosis
- Found in many places in the environment: tap water, fresh and ocean water, milk, bird droppings, soil, and house dust.
- The manner in which these bacteria are transmitted is not completely understood.
- There is no evidence of person to person transmission
What are the 4 groups of Atypical Mycobacteria
Photochromogens
Scotochromogens
Non-photochromogens
Rapid growers
What are Photochromogens
produce yellow pigment when exposed to light
What are Scotochromogens
produce orange pigment in light OR dark
What are Non-photochromogens
- Closely related toM tuberculosis
- Usually infects birds (can infect other animals)
- Major cause of bacterial infection in HIV patients
What are Rapid Growers
See growth in 2-3 days
- M. phlei-widely distributed in soil and dust and on plants
- M smegmatis-originally isolated from human smegma
What is Mycobacterium leprae
- Acid fast rod shaped bacteria
* CANNOT be grown in laboratory culture, intracellular microbe
What is the epidemiology of Mycobacterium leprae
• Children more susceptible
-Cell mediated immune response is not as strong
• Males more susceptible
-Bacteria grows in cooler body temperature, testes often infected
• Transmission
-Poorly understood, not highly infectious. Attempts to infect human volunteers have failed.
-Direct contact
-Found in respiratory secretions, so droplet transmission is possible
-Very slow grower, incubation is from 3-10 years
What are the two types of Mycobacterium leprae
1: Lepromatous leprosy
2: Tuberculoid leprosy
What is Lepromatous leprosy
Progressive
• Most severe, patient can’t mount cell-mediated immune response
• Infects every organ, but the primarily the skin, nose, testes and nerves
• Large numbers of bacteria present
• Changes are seen mostly in skin
What is Tuberculoid leprosy
Less severe, patients can mount a cell-mediated response
• Skin damage is not as excessive and disease is milder and non-progressive
• Nerve involvement is common, but in patches.
• Most frequently enlarged nerves are closest to the skin, the auricular, the ulnar, the posterior tibial and the peroneal.
• Difficult to find bacteria in tuberculoid leprosy
How is Mycobacterium leprae diagnosed
Unable to grow M leprae in laboratory culture, must grow in
• Mouse food pads
• Armadillos
