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Haemophilus influenzae
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Scientific Classification
Kingdom: Bacteria Phylum: Proteobacteria Class: Gamma Proteobacteria Order: Pasteurellales Family: Pasteurellaceae Genus: Haemophilus Species: H. influenzae Binomial Name: Haemophilus influenzae
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History H. lnfluenzae is also called Pfeiffer's bacillus as it was discovered by Pfeiffer ( 1892) Was mistaken as the causative agent of Human Influenza Isolated in 1933 by Smith, Laidlaw and Andrewes
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Overview Haemophilus means "blood loving."
infiuenzae (the flu-an upper respiratory illness) Haemophilae are small Coccobascillus Pleomorphic Gram negative bacilli Are non-motile Non-sporing Catalse and oxidase positive
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Overview Present on the mucous membranes of humans
Normal flora of the human respiratory tract and the oral cavity Transmitted via droplets discharged from upper respiratory tract during the infectious period or contact with contaminated secretions
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Overview Infectious Period
As long as the organism is present even if there is no nasal discharge. Organism is noninfectious within 24 to 48 hours after the start of antibiotics. There are two major categories of H. influenzae; Encapsulated strain(typeable) Unencapsulated stains(nontypeable)
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Overview The Encapsulated strains are classified into six serotypes based on their capsular antigens (designated a to f ) Unencapsulated strains are called nontypeable because they lack the polysaccharide capsule and consequently capsular antigens Type b (Hib) causes about 95% of the invasive diseases. Important human pathogen
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Epidemiology Encapsulated strains colonize approximately 40-80% of children and adults Capsulated(Hib)Colonizes approximately 3-5% of children 2-5yrs By the age of 5–6 years more than 50 % of children will be colonized with this bacterium and most healthy adults (at least 75 %) will be Neonates – protected by maternal antibodies. Older children and adults develop bacteriocidal antibodies during carriage Closed communities e.g. nurseries – increased risk of infection Incidence has reduced considerably since introduction of conjugate vaccine – by about 95%. Cases exclusively seen in unimmunized children
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Cultural Characteristics
H. influenzae requires two accessory growth factors present in blood; Factor X and Factor V Factor X consists of group of heat-stable compounds such as hemin or other porphyrins required for the synthesis of enzymes such as cytochrome, catalase and peroxidase. It is involved in the aerobic respiration. It is not required when H. influenzae grows anaerobically.
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Cultural Characteristics
Factor V: It is a heat-labile, nicotinamide adenine dinucleotide (NAD), which is also produced by some animals, plant cells and other bacteria, such as Staphylococcus aureus. It gets inactivated by NADase present in sheep blood.
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Cultural Characteristics
. In the laboratory it is classically grown on chocolate agar Another way to grow it, is to grow with staphylococcus colonies, on blood agar which provides factor V via red blood cell hemolysis On both chocolate agar and blood agar, H.influenzae grows into convex, smooth, gray or transparent colonies
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Visible growth of H.influenzae on a chocolate agar
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Cultural Characteristics
The growth of H.influenzae vary in different media depending on the availability of X and V factors. Does not grow on ordinary media: Nutrient agar or peptone water lack X and V factors, hence does not support Haemophilus growth
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Cultural Characteristics
Growth is best on chocolate agar because it has both factor X and V While preparing chocolate agar, blood is poured into molten agar at 75°C which inactivates NADase and lyses RBCs releasing excess of factor V. Hence it supports the growth of H.influenzae.
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Cultural Characteristics
Growth is scanty on blood agar because only Factor X is available in this medium and Factor V is largely intracellular, present only inside the RBCs. Factor V is available in very minute quantities freely in the medium especially in sheep blood . More so, blood contains NADase that destroys factor V. Growth can be enhanced if medium is supplemented with an extra source of NAD: H.influenzae does not grow on sheep blood agar except around colonies of staphylococci (“satellite phenomenon”)
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Cultural Characteristics
Streaking S. aureus across surface of BA produces growth stimulation in its vicinity or Use of paper discs impregnated with X and V factors alone, and in combination The NADase in the blood may be in activated by heating to 70 – 80°C The enhanced growth around V factor – called SATELLITISM.
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Satellitism When Staph aureus is streaked across plate of blood agar with species containing H influenza The large colonies of H influenza develop along the streak of Staphylococcus aureus and the small colonies further away
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Satellitism
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Cultural Characteristics
In cultures, the morphology depends both on the length of incubation and on the medium. At 6–8 hours in rich medium, the small coccobacillary forms predominate. Later, there are longer rods and very pleomorphic forms. Facultative anaerobes but do not grow at a temperature less than 22°C Optimal growth is seen at 35-37°C. Better growth is obtained under aerobic conditions as compared to anaerobic environment. Presence of CO2 improves the growth.
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Pathogenesis Virulence Factors
Sword-like structure that helps it attack and destroy the host cell. Encapsulated strains of Haemophilus influenzae are covered by a polysaccharide layer called the capsule They also have pili and adhesion proteins (HMW1 and HMW2) for attachment to host cells. Both encapsulated and unencapsulated have an outer membrane which consists of lipooligosaccharide which inhibits self-clearing mechanism of the bronchi. Encapsulated and unencapsulated strains make IgA protease which destroys IgA. IgA protease neutralizes the first line of mucosa defense Unencapsulated strains have two other abilities to help invade the immune system 1 Phase variation where the strains consist of oligosaccharide 2 Ability to produce biofilms (Made of exopolysaccharides) within which H.I live and produce.
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Invasive diseases Epiglottitis-fever, sore throat, difficulty speaking and dyspnea Cellulitis-fever, warm and tender area of erythema or cheek or periorbital area. Bacteremia-fever, chills, hypotension and tachycardia Meningitis- fever, lethargy, irritability, vomiting, sore neck and altered mental status Osteomyelitis-fever, bone pain and weakness Septic arthritis- fever, pain, swelling and tenderness of the affected joint
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LABORATORY DIAGNOSIS:
SEROLOGICAL METHODS BIOLOGICAL SAMPLES Blood CSF Synovial fluid Pleural fluid Fluids retained by sinus aspiration Tympanocentesis Tracheal or lung aspiration Bronchoscopy Bronchoalveolar lavage Latex agglutination Enzyme immunoassay Coagglutination For epiglottitis, a laryngoscopy can be done which shows a cherry red and swollen epiglottis and an X-ray which shows a thumbprint sign on the epiglottis
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Treatment Infections with H.influenza type b are treated with;
1st choice (ceftriaxone) Chloramphenicol
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PREVENTION & CONTROL Vaccination( for H.inflenzae type b)
Recommended between 2 and 18 months of age Contains type b capsular polysaccharide conjugated to a Diphtheria toxoid component Used to greatly decrease H.influenzae meningitis The use of antibiotics. Contacts of children with H.inflenzae type b may be treated with chemoprophylaxis by using RIFAMPIN Avoid close contact. Practice good hygiene. Breathing support. Medication to treat low blood pressure. Wound care for parts of the body with damaged skin.
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REFERENCES “Haemophilus influenza Disease (Including Hib)”. Centre For Disease Control And Prevention. 18th March, 2021.
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