出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2016/09/06 23:43:22」(JST)
「インフルエンザウイルス」とは異なります。 |
インフルエンザ菌 | |||||||||||||||||||||
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血液寒天培地上のインフルエンザ菌
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学名 | |||||||||||||||||||||
Haemophilus influenzae (Lehmann & Neumann 1896) |
インフルエンザ菌(インフルエンザきん、Haemophilus influenzae)とは、主に呼吸器や中耳に感染する細菌の1種である。b型菌のことをHib(ヒブ)と呼ぶ。歴史的な理由によりインフルエンザという名称が付けられてはいるが、インフルエンザの病原体ではない。
1800年代のインフルエンザの大流行の際に、原因菌として分離された細菌である。そのためインフルエンザ菌という名称が付いているが、その後否定されたため名称だけが残ることとなった(インフルエンザの真の病原体は、RNAウイルスのインフルエンザウイルスである)。ただし、インフルエンザに引き続いて二次的感染を起こすことがある。1995年にH. influenzae Rd.株の全ゲノム配列が解析され[1]、その後データが改定されることにより、本菌のゲノムは1,830,138塩基の環状染色体からなり、染色体上には1,657のタンパク質配列がコードされている事が明らかとなった。なお、インフルエンザ菌は、初めて全ゲノム配列が明らかとなった生物である。
ヘモフィルス属のグラム陰性桿菌である。フィラメント状や球菌状の形態も呈する多形性という性質がある。発育因子としてX因子(ヘミン)とV因子(NAD)の両方を必要とする。ヘミン(hemin)を要求することは属名(Haemophilus)の由来ともなっている。通常はブレインハートインフュージョン等の培地にヘミンとNAD、または羊脱線維血液を加えて培養する。
生物型ではI~VII型までの8つに分類され、このうちII型とIII型は莢膜を持たない。莢膜の血清型はa~fの6型に分けられる。血清型bの莢膜の構成成分である莢膜多糖体抗原(phosphoribosylribotol phosphate)は病原因子として重要である。
非莢膜株は血清型分類できないという意味でnon-typable(NT)株とも呼ばれる。これに学名Haemophilus influenzaeの頭文字を略した"Hi"をつけて、b型菌を Hib、非莢膜株をNTHiなどと略すこともある。
非莢膜株と莢膜株とで大きく異なる病原性を持つ。
非莢膜株は健康なヒト、特に乳幼児の上気道(咽頭、鼻腔)にも常在している。感染症としては中耳炎、副鼻腔炎、気管支炎、肺炎などの気道感染症が多い。小児では気道感染症の3大起炎菌のひとつ(他は肺炎球菌、モラクセラ・カタラーリス)とされている。
莢膜株も上気道に保菌されていることがあるが、気道感染症を起こすことは少なく、直接血流中に侵入して感染症を起こすものと考えられている。莢膜株の感染症ではほとんどの場合b型が起炎菌で、敗血症、髄膜炎、結膜炎、急性喉頭蓋炎、関節炎などを起こす。b型以外の莢膜株が人に感染症を起こすことは稀であるが、Hibワクチンの普及によりb型以外による感染症が目立つようになってきている。
感染病巣からの培養による菌の分離、同定が基本である。血清型b型は迅速診断法として共同凝集反応、酵素抗体法、PCR法などが用いられる。ラテックス凝集法はb型菌の迅速診断法として広く行われており、髄液(髄膜炎の場合)、尿(敗血症の場合)などを対象とする。
一般にはペニシリン系抗生物質のアンピシリンなどが有効である。ただし、後述のとおり耐性菌の出現が問題となっている。
βラクタマーゼ産生菌(BLPAR)やβラクタマーゼ非産生アンピシリン耐性(BLNAR)インフルエンザ菌が報告された。BLNARの存在が報告されたのは1980年であり、それほどBLNARの発生はそれほど古い話ではない[2]。しかし、2004年の北里大学の報告によると、検出されたインフルエンザ菌のうち21.3%(2002年)がBLNARであり[3]、また2007年の長崎大学による報告では、19.5%(1995-1997年)がBLNARであり[4]、近年の高い出現率が問題になっている。耐性機構としては、ペニシリン結合タンパク質であるPBP-3(ftsI)が重要な役割を果たしており、ftsIの変異と薬剤耐性の関係は遺伝子工学的アプローチにより部分的であるが明らかになっている[5]。BLNARのftsIによる変異については、現在、Ubukataらによると3グループ(グループI、II、III[6])、さらに、Ubukataらの報告を発展する形で、Dabernetらにより6グループ(I、IIa、IIb 、IIc、IId、III[7])に分類されている。Ubukataらの報告によると、グループI、IIは比較的弱いセフェム系への耐性を、IIIは高度耐性を有するものとされている。これについては、グループI、IIとIIIの間でのミスセンス変異数の違いに起因するという考察がある[8]。 その場合はβラクタマーゼ阻害薬配合ペニシリン系抗生物質、第2、第3世代セフェム系、ニューキノロン系が一般的に用いられる。実際に、ニューキノロン系抗菌薬のレボフロキサシンにはBLNAR グループI、II、III全てMICが非常に低い値を示している[9]。
なお、インフルエンザ菌b型「Hib」の感染症、特に髄膜炎の場合には第3世代セファロスポリンであるセフトリアキソン、セフォタキシムが第一選択とされる。
b型菌の莢膜多糖体抗原を輸送蛋白に結合させたワクチンは、b型菌(Hib)による重症感染症(Hib感染症)の予防に極めて有効である。世界100カ国以上でこのHibワクチンは導入されており、導入された国では Hib による髄膜炎、喉頭蓋炎がほとんど消失している。
2007年1月26日、Hib莢膜多糖体蛋白結合ワクチン(販売名アクトヒブ®;[10])が厚生労働省により承認され、問題となっている乳幼児のインフルエンザ菌感染[11]への予防の切り札となることが期待されている。日本では2008年12月より任意接種可能となり、2013年4月より予防接種法による定期接種の対象となった。
接種年齢は、2ヶ月齢以上になれば受けられる。望ましい接種スケジュールは、初回免疫として生後2ヶ月から7ヶ月になるまでに接種を開始し、4~8週間間隔で3回、追加免疫として3回目接種から1年後に1回の合計4回接種する。合計4回接種を受けた人のほぼ100%に抗体(免疫)が出来るため最適な予防接種プランとされている。生後7ヶ月から1歳未満の場合は、4~8週間間隔で2回、追加免疫として2回目接種から1年後に1回の合計3回接種となる。1歳以上の場合は追加免疫はなく1回接種のみで抗体獲得となる。
Hibワクチン(ヒブワクチン)接種後、次のワクチンを接種する場合には、6日間以上の間隔をあける必要がある。但し、このワクチンは他のワクチンと同時接種が可能である。諸外国では三種混合と同時接種スケジュールが組まれ、定期予防接種に認定されている[12]。
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Haemophilus influenzae | |
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H. influenzae on a blood agar plate. | |
Scientific classification | |
Domain: | Bacteria |
Kingdom: | Eubacteria |
Phylum: | Proteobacteria |
Class: | Gammaproteobacteria |
Order: | Pasteurellales |
Family: | Pasteurellaceae |
Genus: | Haemophilus |
Species: | H. influenzae |
Binomial name | |
Haemophilus influenzae (Lehmann & Neumann 1896) |
Haemophilus influenzae (formerly called Pfeiffer's bacillus or Bacillus influenzae) is a Gram-negative, coccobacillary, facultatively anaerobic pathogenic bacterium belonging to the Pasteurellaceae family. H. influenzae was first described in 1892 by Richard Pfeiffer during an influenza pandemic.[1]
The bacterium was mistakenly considered to be the cause of influenza until 1933 when the viral cause of influenza became apparent, and is still colloquially known as 'bacterial influenza'. H. influenzae is responsible for a wide range of localized and invasive infections. This species was the first free-living organism to have its entire genome sequenced.[2]
In 1930, two major categories of H. influenzae were defined: the unencapsulated strains and the encapsulated strains. Encapsulated strains were classified on the basis of their distinct capsular antigens. There are six generally recognized types of encapsulated H. influenzae: a, b, c, d, e, and f.[3] Genetic diversity among unencapsulated strains is greater than within the encapsulated group. Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes; however, they can be classified by multilocus sequence typing. The pathogenesis of H. influenzae infections is not completely understood, although the presence of the capsule in encapsulated type b (Hib), a serotype causing conditions such as epiglottitis, is known to be a major factor in virulence. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the nonimmune host. The unencapsulated strains are almost always less invasive; they can, however, produce an inflammatory response in humans, which can lead to many symptoms. Vaccination with Hib conjugate vaccine is effective in preventing Hib infection, but does not prevent infection with NTHi strains.[4]
Haemophilus influenzae infection | |
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Classification and external resources | |
Specialty | Infectious disease |
ICD-10 | A49.2 |
ICD-9-CM | 041.5 |
DiseasesDB | 5570 |
MedlinePlus | 000612 (Meningitis) |
eMedicine | topic list |
Patient UK | Haemophilus influenzae |
[edit on Wikidata]
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Most strains of H. influenzae are opportunistic pathogens; that is, they usually live in their host without causing disease, but cause problems only when other factors (such as a viral infection, reduced immune function or chronically inflamed tissues, e.g. from allergies) create an opportunity. They infect the host by sticking to the host cell using trimeric autotransporter adhesins.
Naturally acquired disease caused by H. influenzae seems to occur in humans only. In infants and young children, H. influenzae type b (Hib) causes bacteremia, pneumonia, epiglottitis and acute bacterial meningitis. On occasion, it causes cellulitis, osteomyelitis, and infectious arthritis.
Due to routine use of the Hib conjugate vaccine in the U.S. since 1990, the incidence of invasive Hib disease has decreased to 1.3/100,000 in children. However, Hib remains a major cause of lower respiratory tract infections in infants and children in developing countries where the vaccine is not widely used. Unencapsulated H. influenzae strains are unaffected by the Hib vaccine and cause ear infections (otitis media), eye infections (conjunctivitis), and sinusitis in children, and are associated with pneumonia.
Clinical features may include initial symptoms of an upper respiratory tract infection mimicking a viral infection, usually associated with fevers, often low-grade. This may progress to the lower respiratory tract in a few days, with features often resembling those of a wheezy bronchitis. Sputum may be difficult to expectorate and is often grey or creamy in color. The cough may persist for weeks without appropriate treatment. Many cases are diagnosed after presenting chest infections do not respond to penicillins or first-generation cephalosporins.
Clinical diagnosis of H. influenzae is typically performed by bacterial culture or latex particle agglutinations. Diagnosis is considered confirmed when the organism is isolated from a sterile body site. In this respect, H. influenzae cultured from the nasopharyngeal cavity or sputum would not indicate H. influenzae disease, because these sites are colonized in disease-free individuals.[6] However, H. influenzae isolated from cerebrospinal fluid or blood would indicate H. influenzae infection.
Bacterial culture of H. influenzae is performed on agar plates, the preferable one being chocolate agar, with added X (hemin) and V (nicotinamide adenine dinucleotide) factors at 37 °C in a CO2-enriched incubator.[7] Blood agar growth is only achieved as a satellite phenomenon around other bacteria. Colonies of H. influenzae appear as convex, smooth, pale, grey or transparent colonies.
Gram-stained and microscopic observation of a specimen of H. influenzae will show Gram-negative, rod shapes with no specific arrangement. The cultured organism can be further characterized using catalase and oxidase tests, both of which should be positive. Further serological testing is necessary to distinguish the capsular polysaccharide and differentiate between H. influenzae b and nonencapsulated species.
Although highly specific, bacterial culture of H. influenzae lacks in sensitivity. Use of antibiotics prior to sample collection greatly reduces the isolation rate by killing the bacteria before identification is possible.[8] Beyond this, H. influenzae is a finicky bacterium to culture, and any modification of culture procedures can greatly reduce isolation rates. Poor quality of laboratories in developing countries has resulted in poor isolation rates of H. influenzae.
H. influenzae will grow in the hemolytic zone of Staphylococcus aureus on blood agar plates; the hemolysis of cells by S. aureus releases factor V which is needed for its growth. H. influenzae will not grow outside the hemolytic zone of S. aureus due to the lack of nutrients such as factor V in these areas. Fildes agar is best for isolation. In Levinthal medium, capsulated strains show distinctive iridescence.
The latex particle agglutination test (LAT) is a more sensitive method to detect H. influenzae than is culture.[9] Because the method relies on antigen rather than viable bacteria, the results are not disrupted by prior antibiotic use. It also has the added benefit of being much quicker than culture methods. However, antibiotic sensitivity testing is not possible with LAT alone, so a parallel culture is necessary.
Polymerase chain reaction (PCR) assays have been proven to be more sensitive than either LAT or culture tests, and highly specific.[9] However, PCR assays have not yet become routine in clinical settings. Countercurrent immunoelectrophoresis has been shown to be an effective research diagnostic method, but has been largely supplanted by PCR.
Both H. influenzae and S. pneumoniae can be found in the upper respiratory system of humans. In an in vitro study of competition, S. pneumoniae always overpowered H. influenzae by attacking it with hydrogen peroxide and stripping off the surface molecules H. influenzae needs for survival.[10]
When both bacteria are placed together into a nasal cavity, within 2 weeks, only H. influenzae survives. When either is placed separately into a nasal cavity, each one survives. Upon examining the upper respiratory tissue from mice exposed to both bacteria species, an extraordinarily large number of neutrophils (immune cells) was found. In mice exposed to only one bacterium, the cells were not present.
Lab tests showed neutrophils exposed to dead H. influenzae were more aggressive in attacking S. pneumoniae than unexposed neutrophils. Exposure to dead H. influenzae had no effect on live H. influenzae.
Two scenarios may be responsible for this response:
It is unclear why H. influenzae is not affected by the immune response.[11]
Pneumonia occurs when the lungs become infected, causing inflammation (swelling). Symptoms of pneumonia usually include:
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Haemophilus influenzae produces beta-lactamases, and it is also able to modify its penicillin-binding proteins, so it has gained resistance to the penicillin family of antibiotics. In severe cases, cefotaxime and ceftriaxone delivered directly into the bloodstream are the elected antibiotics, and, for the less severe cases, an association of ampicillin and sulbactam, cephalosporins of the second and third generation, or fluoroquinolones are preferred. (Fluoroquinolone-resistant Haemophilus influenzae have been observed.)[13]
Macrolide antibiotics (e.g., clarithromycin) may be used in patients with a history of allergy to beta-lactam antibiotics.[citation needed] Macrolide resistance has also been observed.[14]
The serious complications of HiB are brain damage, hearing loss, and even death.[15]
Effective vaccines for Haemophilus influenzae Type B have been available since the early 1990s, and is recommended for children under age 5 and asplenic patients. The World Health Organization recommends a pentavalent vaccine, combining vaccines against diphtheria, tetanus, pertussis, hepatitis B and Hib. There is not yet sufficient evidence on how effective this pentavalent vaccine is in relation to the individual vaccines.[16]
Hib vaccines cost about seven times the total cost of vaccines against measles, polio, tuberculosis, diphtheria, tetanus, and pertussis. Consequently, whereas 92% of the populations of developed countries was vaccinated against Hib as of 2003, vaccination coverage was 42% for developing countries, and only 8% for least-developed countries.[17]
H. influenzae was the first free-living organism to have its entire genome sequenced. Completed by Craig Venter and his team at The Institute for Genomic Research - one of the institutes now part of the J. Craig Venter Institute. Haemophilus was chosen because one of the project leaders, Nobel laureate Hamilton Smith, had been working on it for decades and was able to provide high-quality DNA libraries. The genome consists of 1,830,140 base pairs of DNA in a single circular chromosome that contains 1740 protein-coding genes, 2 transfer RNA genes, and 18 other RNA genes. The sequencing method used was whole-genome shotgun, which was completed and published in Science in 1995.[18]
Unencapsulated H. influenzae is often observed in the airways of patients with chronic obstructive pulmonary disease (COPD). Neutrophils are also observed in large numbers in sputum from patients with COPD. The neutrophils phagocytize H. influenzae, thereby activating an oxidative respiratory burst.[19] However instead of killing the bacteria the neutrophils are themselves killed (though such an oxidative burst likely causes DNA damage in the H. influenzae cells). The lack of killing of the H. influenzae appears to explain the persistence of infection in COPD.[19]
H. influenzae mutants defective in the rec1 gene (a homolog of recA) are very sensitive to killing by the oxidizing agent hydrogen peroxide.[20] This finding suggests that rec1 expression is important for H. influenzae survival under conditions of oxidative stress. Since it is a homolog of recA, rec1 likely plays a key role in recombinational repair of DNA damage. Thus H. influenzae may protect its genome against the reactive oxygen species produced by the host’s phagocytic cells through recombinational repair of oxidative DNA damages.[21] Recombinational repair of a damaged site of a chromosome requires, in addition to rec1, a second homologous undamaged DNA molecule. Individual H. influenzae cells are capable of taking up homologous DNA from other cells by the process of transformation. Transformation in H. influenzae involves at least 15 gene products,[18] and is likely an adaptation for repairing DNA damages in the resident chromosome (as suggested in Transformation (genetics)#Transformation, as an adaptation for DNA repair).
Wikimedia Commons has media related to Haemophilus influenzae. |
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リンク元 | 「ヒブワクチン」「Haemophilus influenzae type b」 |
拡張検索 | 「Hib髄膜炎」「Familial Hibernian fever」 |
関連記事 | 「H」 |
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