出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2013/12/21 10:56:00」(JST)
結核菌 | ||||||||||||||||||||||||
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結核菌のコロニー
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Mycobacterium tuberculosis Zopf 1883 |
結核菌(けっかくきん、Mycobacterium tuberculosis、ヒト型結核菌)は、ヒトの結核の原因となる真正細菌。1882年、細菌学者ロベルト・コッホにより発見された。ヒトの病原菌としては、コッホの原則に基づいて病原性が証明された最初のものである。グラム陽性桿菌である抗酸菌の一種であり、細胞構造や培養のための条件など多くの点で他の一般的な細菌と異なる。特に、ミコール酸と呼ばれる特有の脂質に富んだ細胞壁を持つため消毒薬や乾燥に対して高い抵抗性を有する。保菌者の咳やくしゃみなどの飛沫、あるいはそれが乾燥したものを含むほこりなどから空気感染して、肺胞マクロファージの細胞内に感染し、肺結核をはじめとする各種の結核の原因となる。
結核菌は、マイコバクテリウム科マイコバクテリウム属に属し、他のマイコバクテリウム属細菌とともに抗酸菌と呼ばれる細菌群の一種である。芽胞、鞭毛、莢膜を持たない、大きさ2-4 x 0.3-0.6 µmの好気性桿菌である。
細胞壁にミコール酸と呼ばれる脂質を多量に含有し、通常のグラム染色では染まりにくく結果が安定しないため、グラム不定と呼ばれることもある。ただし、細胞壁の構造と加温グラム染色法からグラム陽性菌として分類するのが一般的である。一方、媒染剤を加えて加温しながら染色を行うチール・ネールゼン染色などの強力な方法を用いると、染色が可能になるだけでなく、一旦染まった色素液が脱色されにくいという特徴を持ち、強い脱色剤である塩酸アルコールに対しても脱色抵抗性を示す。この染色法を抗酸性染色と呼び、本法で染色されるマイコバクテリウム属は抗酸菌(acid-fast bacteria, この場合のfastは「退色しない」「固定された」の意)とも呼ばれている。抗酸菌は、増殖の遅い(コロニーが肉眼で判別可能なまで増殖するのに1週間以上かかる)遅発育菌群 (slow growers) と、増殖の早い迅速発育菌群 (rapid growers)、培養不能菌(らい菌のみ)の3つに大別される。結核菌はこのうち遅発育菌群に属し、分離培養には3週間以上かかることがある、
抗酸菌のうち、結核菌(Mycobacterium tuberculosis、ヒト型結核菌)、ウシ型結核菌(M. bovis、ウシ型菌、ウシ菌)、マイコバクテリウム・アフリカンス (M. africans)、ネズミ型結核菌 (M. microti) の4菌種は、(1) 37℃で増殖可能だが28℃で増殖しない、(2)耐熱性のカタラーゼを持つ、などの点で他の抗酸菌とは鑑別される。この4種を結核菌群 (M. tuberculosis complex) と呼ぶ。結核菌群と癩菌(らいきん)以外の抗酸菌を非結核性抗酸菌(古くは非定型抗酸菌)と呼んで区別する。結核菌群の4種はいずれも遅発育菌群であり、同定が困難であった。現在は分子生物学的手法を用いて、遺伝子増幅により同定可能となっている。
生化学的性状および病原性の点で、4種それぞれに相違点を持つ。このうち、結核菌 (M. tuberculosis) が結核の原因菌としてヒトへの病原性を示すほか、M. bovisとM. africansがまれにヒトに感染する。M. microtiはヒトに対する病原性を持たない。またM. bovisを長期間継代培養して弱毒化したものがBCGであり、結核予防のためのワクチン(弱毒生菌ワクチン)として利用されている。
結核菌は人のくしゃみ、咳などで空気中に飛散し、空気感染を引き起こすことが多い。(正確には飛沫核感染である。)結核菌を吸い込んだとしても、免疫がしばらく菌を閉じ込めてしまい、すぐには発症しない。しかし、ほうっておくと、咳や微熱が出る。病状が進行していくと、最悪の場合死に至る。
イソニアジド(INH),リファンピシン(RFP),ストレプトマイシン(SM)(注射剤)またはエタンブトール(EB)(経口剤)のどちらか,ピラジナミド(PZA)の4剤の短期併用(2ヶ月)を行い、その後2剤(イソニアジド+リファンピシン)を4ヶ月または6ヶ月、もしくは3剤(短期にEB使用の際、イソニアジド+リファンピシン+エタンブトール)併用を行うことが近年推奨されている。しかしPZAは肝障害を合併しやすく、薬剤の選択については患者によって異なる。またストレプトマイシンは聴覚障害、エタンブトールは視神経障害が副作用として有名である。
この項目「結核菌」は、真正細菌(バクテリア)に関連した書きかけの項目です。加筆・訂正などをして下さる協力者を求めています。(Portal:生き物と自然/ウィキプロジェクト 生物) |
Mycobacterium tuberculosis | |
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M. tuberculosis bacterial colonies | |
Scientific classification | |
Kingdom: | Bacteria |
Phylum: | Actinobacteria |
Class: | Actinobacteria |
Order: | Actinomycetales |
Suborder: | Corynebacterineae |
Family: | Mycobacteriaceae |
Genus: | Mycobacterium |
Species: | M. tuberculosis |
Binomial name | |
Mycobacterium tuberculosis Zopf 1883 |
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Synonyms | |
Tubercle bacillus Koch 1882 |
Mycobacterium tuberculosis (MTB) is a pathogenic bacterial species in the family Mycobacteriaceae and the causative agent of most cases of tuberculosis (TB).[1] First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface (primarily mycolic acid), which makes the cells impervious to Gram staining. Acid-fast detection techniques are used instead. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, MTB infects the lungs. The most frequently used diagnostic methods for TB are the tuberculin skin test, acid-fast stain, and chest radiographs.[1]
The M. tuberculosis genome was sequenced in 1998.[2][3]
M. tuberculosis requires oxygen to grow. It does not retain any bacteriological stain due to high lipid content in its wall, hence Ziehl-Neelsen staining, or acid-fast staining, is used. Despite this, it is gram-positive bacteria. While mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint (i.e., they do not retain the crystal violet stain), they are classified as acid-fast Gram-positive bacteria due to their lack of an outer cell membrane.[1]
M. tuberculosis divides every 15–20 hours, which is extremely slow compared to other bacteria, which tend to have division times measured in minutes (Escherichia coli can divide roughly every 20 minutes). It is a small bacillus that can withstand weak disinfectants and can survive in a dry state for weeks. Its unusual cell wall, rich in lipids (e.g., mycolic acid), is likely responsible for this resistance and is a key virulence factor.[4]
When in the lungs, M. tuberculosis is taken up by alveolar macrophages, but they are unable to digest the bacterium. Its cell wall prevents the fusion of the phagosome with a lysosome. Specifically, M. tuberculosis blocks the bridging molecule, early endosomal autoantigen 1 (EEA1); however, this blockade does not prevent fusion of vesicles filled with nutrients. Consequently, the bacteria multiply unchecked within the macrophage. The bacteria also carried the UreC gene, which prevents acidification of the phagosome.[5] The bacteria also evade macrophage-killing by neutralizing reactive nitrogen intermediates.[6]
The ability to construct M. tuberculosis mutants and test individual gene products for specific functions has significantly advanced our understanding of the pathogenesis and virulence factors of M. tuberculosis. Many secreted and exported proteins are known to be important in pathogenesis.[7]
M. tuberculosis comes from the genus Mycobacterium, which is composed of approximately 100 recognized and proposed species. The most familiar of the species are M. tuberculosis and M. leprae (leprosy).[8]
M. tuberculosis is genetically diverse, which results in significant phenotypic differences between clinical isolates. Different strains of M. tuberculosis are associated with different geographic regions. However, phenotypic studies suggest that strain variation never has implications for the development of new diagnostics and vaccines. Microevolutionary variation does affect the relative fitness and transmission dynamics of antibiotic-resistant strains.[9]
Typing of strains is useful in the investigation of tuberculosis outbreaks, because it gives the investigator evidence for-or-against transmission from person to person. Consider the situation where person A has tuberculosis and believes that he acquired it from person B. If the bacteria isolated from each person belong to different types, then transmission from B to A is definitively disproved; on the other hand, if the bacteria are the same strain, then this supports (but does not definitively prove) the theory that B infected A.
Until the early 2000s, M. tuberculosis strains were typed by pulsed field gel electrophoresis (PFGE).[10][11] This has now been superseded by variable numbers of tandem repeats (VNTR), which is technically easier to perform and allows better discrimination between strains. This method makes use of the presence of repeated DNA sequences within the M. tuberculosis genome.
There are three generations of VNTR typing for M. tuberculosis. The first scheme, called ETR (exact tandem repeat), used only five loci,[12] but the resolution afforded by these five loci was not as good as PFGE. The second scheme, called MIRU (mycobacterial interspersed repetitive unit) had discrimination as good as PFGE.[13][14] The third generation (MIRU2) added a further nine loci to bring the total to 24. This provides a degree of resolution greater than PFGE and is currently the standard for typing M. tuberculosis.[15]
Mycobacterium outbreaks are often caused by hypervirulent strains of M. tuberculosis. In laboratory experiments, these clinical isolates elicit unusual immunopathology, and may be either hyperinflammatory or hypoinflammatory. Studies have shown the majority of hypervirulent mutants have deletions in their cell wall-modifying enzymes or regulators that respond to environmental stimuli. Studies of these mutants have indicated the mechanisms that enable M. tuberculosis to mask its full pathogenic potential, inducing a granuloma that provides a protective niche, and enable the bacilli to sustain a long-term, persistent infection.[16]
M. tuberculosis is characterized by caseating granulomas containing Langhans giant cells, which have a "horseshoe" pattern of nuclei. Organisms are identified by their red color on acid-fast staining.
The genome of the H37Rv strain was published in 1998.[17] Its size is 4 million base pairs, with 3959 genes; 40% of these genes have had their function characterised, with possible function postulated for another 44%. Within the genome are also six pseudogenes.
The genome contains 250 genes involved in fatty acid metabolism, with 39 of these involved in the polyketide metabolism generating the waxy coat. Such large numbers of conserved genes show the evolutionary importance of the waxy coat to pathogen survival.
About 10% of the coding capacity is taken up by the PE/PPE gene families that encode acidic, glycine-rich proteins. These proteins have a conserved N-terminal motif, deletion of which impairs growth in macrophages and granulomas.[18]
Nine noncoding sRNAs have been characterised in M. tuberculosis,[19] with a further 56 predicted in a bioinformatics screen.[20]
The Mycobacterium tuberculosis complex evolved in Africa and most probably in the Horn of Africa.[21] The M. tuberculosis group has a number of members that include Mycobacterium africanum, Mycobacterium bovis (Dassie's bacillus), Mycobacterium caprae, Mycobacterium microti, Mycobacterium mungi, Mycobacterium orygis and Mycobacterium pinnipedii. This group may also include the Mycobacterium canettii clade.
The M. canettii clade—which includes Mycobacterium prototuberculosis—are a group of smooth colony Mycobacterium species. Unlike the established members of the M. tuberculosis group they undergo recombination with other species. The majority of the known strains of this group have been isolated from the Horn of Africa.
The established members of the M. tuberculosis complex are all clonal in their spread. The main human infecting species have been classified into seven spoligotypes: type 1 contains the East African-Indian (EAI) and some Manu (Indian) strains; type 2 is the Beijing group; type 3 consists of the Central Asian (CAS) strains; type 4 of the Ghana and Haarlem (H/T), Latin America-Mediterranean (LAM) and X strains; types 5 and 6 correspond to Mycobacterium africanum and are observed predominantly and at very high frequency in West Africa. A seventh type has been isolated from the Horn of Africa.[21] The other species of this complex belong to a number of spoligotypes and do not normally infect humans.
Type 2 and 3 are more closely related to each other than to the other types. Types 5 and 6 are most closely aligned with the species that do not normally infect humans. Type 3 has been divided into two clades: CAS-Kili (found in Tanzania) and CAS-Delhi (found in India and Saudi Arabia).
The most recent common ancestor of the M. tuberculosis complex evolved ~40,000 years ago.[22] The most recent common ancestor of the EAI and LAM strains has been estimated to be 13,700 and 7,000 years ago respectively. The Beijing- CAS strains diverged ~17,100 years ago. All types of the M. tuberculosis began their current expansion ~5000 years ago—a period that coincides with the appearance of Mycobacterium bovis. The Beijing strain appears to have been the most successful with a ~500 increase in effective population size (Ne) since its expansion began. The least successful of the main linages appears to be have been those limited to Africa where they have undergone an Ne increase of only 5 fold. Since its initial evolution M. bovis has undergone an expansion of its Ne of ~65 fold.
M. tuberculosis, then known as the "tubercle bacillus", was first described on 24 March 1882 by Robert Koch, who subsequently received the Nobel Prize in physiology or medicine for this discovery in 1905; the bacterium is also known as "Koch's bacillus".[23]
Tuberculosis has existed throughout history, but the name has changed frequently over time. In 1720, though, the history of tuberculosis started to take shape into what is known of it today; as the physician Benjamin Marten described in his A Theory of Consumption, tuberculosis may be caused by small living creatures that are transmitted through the air to other patients.[24]
The BCG vaccine has been developed with success in preventing tuberculosis.
Wikimedia Commons has media related to Mycobacterium tuberculosis. |
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国試過去問 | 「110A038」「096A056」「100B067」「109A010」 |
リンク元 | 「細菌」「ワクチン」「結核」「ラニヨン分類」「咳嗽」 |
拡張検索 | 「hot water extract of Mycobacterium tuberculosis」 |
関連記事 | 「Mycobacterium」 |
C
※国試ナビ4※ [110A037]←[国試_110]→[110A039]
B
※国試ナビ4※ [096A055]←[国試_096]→[096A057]
D
※国試ナビ4※ [100B066]←[国試_100]→[100B068]
B
※国試ナビ4※ [109A009]←[国試_109]→[109A011]
病原体 | 感染症 | ワクチン | 学校伝染病 | ワクチンの形状 | 潜伏期間 | 季節性 | 年齢 | 出席停止解除条件 | |
ジフテリア菌 | Corynebacterium diphtheriae | ジフテリア | ジフテリア,破傷風,百目咳混合ワクチン | トキソイド | |||||
百日咳菌 | Bordetella pertussis | 百日咳 | ○ | 不活化 | 6~14 | 咳の消失 | |||
結核菌 | Mycobacterium tuberculosis | 結核 | BCG | ○ | 不活化 | 伝染のおそれが無くなるまで | |||
ポリオウイルス | poliovirus | ポリオ | ポリオワクチン(経口) | 生 | |||||
麻疹ウイルス | measles virus | 麻疹 | 麻疹・風疹混合ワクチン | ○ | 生 | 10~12 | 0~2 | 解熱後3日 | |
風疹ウイルス | rubella virus | 風疹 | ○ | 生 | 18 | 春~初夏 | 4~9 | 発疹消失 | |
日本脳炎ウイルス | Japanese encephalitis virus | 日本脳炎 | 日本脳炎ワクチン | 不活化 | |||||
インフルエンザウイルス | influenza virus | インフルエンザ | インフルエンザワクチン | ○ | 不活化 | 1~5 | 冬期 | 解熱後2日 | |
インフルエンザ菌 | Haemophilus influenzae | 化膿性髄膜炎など | Hibワクチン | ||||||
肺炎球菌 | Streptococcus pneumoniae | ||||||||
水痘・帯状疱疹ウイルス | varicella zoster virus | 水痘 | ○ | 生 | 11~21 | 冬(12, 1) | 5~9 | 発疹の痂皮化 | |
ムンプスウイルス | mumps virus | 流行性耳下腺炎 | ○ | 生 | 18~21 | 耳下腺腫脹消失 | |||
B型肝炎ウイルス | hepatitis B virus | B型肝炎 | 成分 | 60~160 | |||||
A型肝炎ウイルス | hepatitis A virus | A型肝炎 | 不活化 | 15~40 | |||||
狂犬病ウイルス | rabies virus | 狂犬病 | 不活化 | ||||||
アデノウイルス | adenovirus | 咽頭結膜熱 | ○ | ||||||
黄熱病ウイルス | yellow fever virus | 黄熱病 | 生 |
ラニアン分類 : 1 件 ラニャン分類 : nothing ラニヤン分類 : ラニョン分類 : ラニヨン分類 : 4 件 ラニアン分類 Runyon : nothing ラニャン分類 Runyon : nothing ラニヤン分類 Runyon : ラニョン分類 Runyon : ラニヨン分類 Runyon : 3 件
分類 | 菌種名 | |||
培養可能菌 | 遅発育菌 | 結核菌群 | Mycobacterium tuberculosis | |
Mycobacterium bovis | ||||
Mycobacterium microti | ||||
Mycobacterium africanum | ||||
非結核性抗酸菌群 | I群菌 光発色菌 |
Mycobacterium kansasii | ||
Mycobacterium marinum | ||||
Mycobacterium simiae | ||||
Mycobacterium asiaticum | ||||
II群菌 暗発色菌 |
Mycobacterium flavescens | |||
Mycobacterium gordonae | ||||
Mycobacterium scrofulaceum | ||||
Mycobacterium szulgai | ||||
III群菌 非光発色菌 |
Mycobacterium avium complex | |||
Mycobacterium celatum | ||||
Mycobacterium haemophilum | ||||
Mycobacterium gastri | ||||
Mycobacterium genavense | ||||
Mycobacterium malmoense | ||||
Mycobacterium nonchromogenicum | ||||
Mycobacterium shimoidei | ||||
Mycobacterium terrae | ||||
Mycobacterium trivale | ||||
Mycobacterium ulcerans | ||||
Mycobacterium xenopi | ||||
迅速発育菌 | IV群菌 | Mycobacterium abscessus | ||
Mycobacterium fortuitum group | ||||
Mycobacterium chelonae group | ||||
Mycobacterium phlei | ||||
Mycobacterium smegmatis | ||||
Mycobacterium vaccae | ||||
培養不能菌 | Mycobacterium leprae |
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