Mycobacteria

Mycobacteria Definition

Mycobacteria are rod shaped aerobic ,acid fast bacilli (rods). There are more than 200 species of mycobacteria. They are neither gram positive or gram negative. Mycobacteria are stained poorly by the dyes used in gram stain. They are virtually the only bacteria that are acid fast (one exception is Nocardia asteroides, the major cause of nocardiosis, which is also acid fast). The term acid fast refers to an organism's ability to retain carbofuchin stain despite subsequent treatment with an ethanol-hydrochloric acid mixture. The high lipid content (approximately 60% of their cell wall makes mycobacteria acid-fast ) The major pathogens are mycobacteria tuberculosis the cause of tuberculosis and mycobacteria laprae the cause of leprosy . Atypical mycobacteria such as mycobacterium avium-intracellulare complex and mycobacterium kansasii, can cause tuberculosis like disease but are less frequent pathogens.Rapidly growing mycobacteria such as mycobacterium chelonei occasionally cause human disease in immunocompromised patients or those in whom prosthetic devices have been planted . The clinical features of three important mycobacteria are described below

Clinical Features of Important Mycobacteria

Organism	Main Site Of Infection	Skin Test In Common Use	Multiple Drug Therapy Use	Vaccine Available
Mycobacterium Tuberculosis	Lungs	yes	yes	yes
Mycobacterium avium-intracellulare	Lungs	no	yes	no
Mycobacterium leprae	Skin,Nerves	No	yes	no

Mycobacterium Tuberculosis

Disease

Mycobacterium Tuberculosis

Mycobacterium tuberculosis causes tuberculosis worldwide.It is the number one infectious killer disease
worldwide. Mycobacterium tuberculosis causes more deaths than any other single microbial agent.Approximately one third of the worlds population is infected with this organism.Each year it is estimated that 3 million people die of tuberculosis and 8 million new cases occur

Important Properties

Mycobacteria tuberculosis grows slowly (i.e it has a doubling time of 18 hours in contrast to most bacteria which can double in a 1 hour or less) .Because growth is so slow cultures of clinical specimens must be held for 6-8 weeks before being recorded as negative. Saprophytic forms tend to grow more rapidly, to proliferate well at 22–33°C, to produce more pigment, and to be less acid fast than pathogenic forms.

• Mycobacterium tuberculosis can be cultured on bacteriologic media whereas mycobacteria leprae cannot.
• Mycobacterium tuberculosis organisms are straight or slightly curved rods occurring singly, in pairs, or in small clumps.
• They measure about 3x0.3 m in size; sometimes long filamentous forms are also seen.Mycobacterium bovis is usually shorter and stouter than Mycobacterium tuberculosis

Species	Growth on Bacteriologic Media	Preferred Temperature in Vivo	Source or Mode of Transmission
Mycobacteria Tuberculosis	Slow(weeks)	37	Respiratory droplets
Mycobacteria bovis	Slow(weeks)	37	Milk From infected animals
Mycobacteria Leprae	None	32	Prolonged close contact
Atypical Mycobacteria M.Kansasii	Slow(weeks)	37	Soil and Water
Mycobacteria Marinum	Slow(weeks)	37	Water
Mycobacteria avium-intracellulare complex	Slow(weeks)	32	Soil and Water
mycobacteria fortuitum-chelonei complex	Rapid(days)	37	Soil and Water

Atypical Mycobacteria

Several species of mycobacteria are characterized as atypical,because they differ in certain respects from the prototype mycobacterium tuberculosis.For example atypical mycobacteria are widespread in the environment and are not pathogenic for guinea pigs,whereas mycobacterium tuberculosis is found only in humans and is highly pathogenic for guinea pigs.

Classification of Atypical Mycobacteria

Atypical mycobacteria are classified into four groups according to their rate of growth and whether they produce pigment under certain conditions. Group 1 organisms produce a yellow-orange pigment colony pigmented colony only when exposed to light (Photochromogens) Group 2 organisms produce the pigment chiefly in the dark (Scotochromogens). Group 3 mycobacteria produce little or no yellow-orange pigment,irrespective of the presence or absence of light (Nonchromogens). In contrast to the previous 3 groups  which grow slowly the Group 4 organisms grow rapidly producing colonies in fewer than 7 days.

Group	Growth Rate	Light	Dark	Typical Species
1	Slow	+	-	Mycobacteria kansasii,Mycobacteria Marinum
2	Slow	+	+	Mycobacteria scrofulaceum
3	Slow	-	-	Mycobacteria Avium-intracellulare complex
4	Rapid	-	-	Mycobacteria Fortuitum-chelonei complex

Group 1 (Photochromogens)

Photochromogen strains do not produce any pigments in the colonies that are incubated in dark, but form pigments when the young culture is exposed to light for 1 hour in the presence of air and is reincubated for 24–48 hours. Photochromogen bacteria produce yellow-orange pigments during such conditions. These mycobacteria are slow-growing ones, but grow faster (after 7 days) than the tubercle bacilli. The most common species are Mycobacterium kansasii.

Mycobacterium Kansasii

Mycobacterium kansasii reduces nitrates to nitrites  and shows a positive Tween 80 hydrolysis test within

Mycobacterium Kansasii

3–6 hours, while other mycobacteria species produce positive test within 3–10 days. Mycobacterium kansasii is a photochromogen  that requires complex media for growth at 37°C and  causes lung disease clinically resembling tuberculosis.This species is the second most common Non Tuberculous Mycobacteria, next to Mycobacterium avium complex (MAC), as causative agent of lung diseases.Because it is antigenic ally similar to mycobacterium tuberculosis , patients are frequently tuberculin skin test positive. These strains have been frequently isolated from tap water, and infected tap water is believed to be the major source of infection. Mycobacterium kansasii grows rapidly on LJ medium, producing visible colonies within 2 weeks. It is sensitive to rifampicin and other anti-tubercular drug.

Mycobacterium Marinum

Mycobacterium Marinum

Mycobacterium marinum closely resembles M. kansasii but is differentiated from it by its

     poor growth at 37°C,
     failure to reduce nitrate to nitrite, and
     failure to produce the enzyme catalase.

Mycobacterium marinum causes "swimming pool granuloma" also known as "fish tank granuloma" .These granulomatous ,ulcerating lesions occur in the skin at the site of abrasions incurred at swimming pools and aquariums .They grow best at low temperature (31°C). The natural habitat of the organism is both fresh and salt water .Treatment with a tetracycline such as minocycline is effective.

Mycobacterium Simiae

Mycobacterium Simiae

Mycobacterium simiae is associated with pulmonary disease in humans was originally isolated from monkeys.

This species grows well at 37°C and is niacin positive, like Mycobacterium tuberculosis.

Group 2 (Scotochromogens)

Scotochromogen  mycobacteria are characterized by their ability to produce yellow, orange, or red pigmented colonies on the LJ medium even when incubated in dark . Scotochromogen species are widely distributed in the environment.Mycobacterium scrofulaceum,Mycobacterium gordonae, and Mycobacterium szulgai are the important species.

Mycobacterium Scrofulaceum

Mycobacterium Scrofulaceum

Mycobacterium Scrofulaceum causes scrofula a granulomatous cervical adenitis usually in children
(Mycobacterium tuberculosis also causes scrofula). The organism enters through the oropharynx and infects the draining lymph nodes. Its natural habitat is environmental water sources but it has also been isolated as a saprophyte from the human respiratory tract .scrofula can often be cured by surgical excision of the affected lymph nodes.

Mycobacterium Gordonae

Mycobacterium gordonae rarely causes pulmonary disease. It is often found as a contaminant in tap water, hence called tap water scotochromogen. Mycobacterium gordonae is also found as a contaminant in clinical specimens. Mycobacterium. gordonae does not hydrolyze urea, nicotinamide, and pyrazinamide; in this respect, it differs from Mycobacterium scrofulaceum.

Mycobacterium Szulgai

Mycobacterium szulgai occasionally may cause pulmonary disease and bursitis.It is a scotochromogen when grown at 37°C and photochromogen when grown at 27°C.

Group 3 (Nonphotochromogens)

Nonphotochromogens are the mycobacteria that do not produce pigment in dark or on exposure to light. These include Mycobacterium avium complex (MAC), Mycobacterium xenopi , and  Mycobacterium ulcerans . Group III nonphotochromogens also includes Mycobacterium terrae , Mycobacterium triviale, and Mycobacterium nonchromogenicum , which rarely cause infections in humans.

Mycobacterium Avium-Intracellulare Complex

Mycobacterium avium intracellulare complex is composed of two species.

Mycobacterium Avium Complex

1. Mycobacterium Avium
2. Mycobacterium intracellulare

Both of the above mentioned organisms are very difficult to distinguish from each other by standard laboratory tests.These organisms grow optimally at 41℃. Mycobacterium avium complex organisms are strongly acid-fast mycobacteria with a cell wall rich in lipids.They are weakly positive aerobic bacteria. This complex possesses 28 agglutination types. Types 1, 2, and 3 are considered as Mycobacterium avium and other types as Mycobacterium intracellulare.Mycobacterium avium complex cause pulmonary disease clinically indistinguishable from tuberculosis, primarily in immunocompromised patients such as those with AIDS who have CD4 cell counts of less than 200/μL.Ability to grow inside the cells is the main virulence determinant of the bacteria. In infected humans, development of the disease depends primarily on response of host to infection. Mycobacterium avium intracellare complex is the most common bacterial cause of disease in AIDS patients. The organisms are widespread in the environment including water and soil(brackish, ocean, and drinking water), Particularly in the south eastern United States.They are highly resistant to anti tuberculosis drugs and as many as six drugs in combination are frequently required for adequate treatment. Current drugs of choice are clarithromycin plus one or more of the following ethambutol, rifabutin or ciprofloxacin.clarithromycin is currently recommended for preventing disease in AIDS patients.

Mycobacterium Xenopi

Mycobacterium xenopi was originally isolated from a cutaneous lesion of a South African toad (Xenopus laevis). The  organism is a thermophilic mycobacterium, which may occasionally cause chronic pulmonary lesions in humans. Most of these infections have been documented from London. Mycobacterium xenopi have been isolated from mostly hot water taps and also from main water supplies in hospitals.

Mycobacterium. ulcerans

Mycobacterium. ulcerans is a skin pathogen, which was originally isolated from human ulcerative skin lesions in Australia (1958).Subsequently, the species causing similar cutaneous lesions have been documented from Uganda (Buruli ulcer), Nigeria, Congo, Malaysia, New Guinea, and Mexico. Mycobacterium. ulcerans grows slowly on LJ medium in 4–8 weeks when incubated at 31–34°C.However, this species fails to grow when incubated at 37°C in primary culture.

Group 4 (Rapidly Growing Mycobacteria)

Group IV, or rapid growers, is a heterogenous group of mycobacteria that produce visible growth on LJ medium rapidly within 1 week of incubation at 37°C or 25°C. This group may also include photochromogens, scotochromogens, or nonphotochromogens species

Mycobacterium Fortuitum-Chelonei Complex 

Mycobacterium fortuitum-chelonei complex is composed of two similar species Mycobacterium chelonei Mycobacterium fortuitum. They are saphrophytes, found chiefly in soil and water and rarely cause human disease infections occur chiefly in two populations

Mycobacterium Chelonei

and

• Immuno compromised patients
• Individuals with prosthetic hip joints and indwelling catheters

skin and soft tissue infections occur at the site of puncture wounds. They grow rapidly (3–6 days) in culture and form no pigment. They are often resistant to anti-tuberculosis therapy and therapy with multiple drugs in combination plus surgical excision may be required for effective treatment current drugs of choice are amikacin and doxycycline.

Mycobacterium Abscessus

Mycobacterium abscessus is another rapidly growing mycobacteria acquired from the environment. Mycobacteria abscessus causes chronic lung infection, as well as infections of the skin, bones and joints.The individuals most commonly infected are elderly, white, female nonsmokers. Patients with cystic fibrosis are also at risk and may succumb to a fulminant, rapidly progressive form of the disease. Mycobacterium abscessus is highly antibiotic resistant. Clarithromycin, amikacin, and cefoxitin are usually used for treatment of Mycobacterium abscessus.

Mycobacterium Smegmatis

Mycobacterium smegmatis is a rapidly growing mycobacteria it is not associated with human disease. It is part of normal flora of smegma, the material that collects under the foreskin of the penis.

Mycobacterium phelei

Mycobacterium. phlei is mostly nonpathogenic. It produces buff-colored colonies, which on further incubation become yellow to orange.Mycobacterium phlei differs from Mycobacterium smegmatis by its ability to grow at 52°C and survive heating at 60°C for 4 hours.

Saprophytic Mycobacteria Not Associated With Human Illness

Mycobacterium phlei is frequently found on plants, in soil, or in water. Mycobacterium gordonae is similar. Mycobacterium smegmatis occurs regularly in human sebaceous secretions,and it might be confused with pathogenic acid-fast organisms. Mycobacterium paratuberculosis produces a chronic enteritis in cattle. There is renewed interest in this organism as a potential cause of inflammatory bowel disease

Mycobacterium Leprae

Mycobacterium Leprae is an organism that causes leprosy. Mycobacterium leprae was described by Arinuer Hansen in 1873 in Italy.Leprosy was not thought to be an infectious disease despite the discovery of mycobacterium leprae  Mycobacterium Leprae has not been grown in the laboratory either on artificial media or in cell culture.Mycobacterium leprae can be grown in the mouse footpad or in the armadillo.Mycobacterium leprae was the first bacterial pathogen to be associated with a specific  human disease.There are more than 10 million cases of leprosy mainly in Asia.

Humans are the natural hosts although the armadillo may also be a reservoir for human infection. The optimal temperature for growth (30 ℃) is lower than body temperature, it therefore grows preferentially in the skin and superficial nerves. Mycobacterium leprae grows very slowly with a doubling time of 14 days. This makes it the slowest growing human bacterial  pathogen. One consequence of this is that antibiotic therapy must be continued for a long time, usually several years.