Haemophilus influenzae
Scientific Classification
• Kingdom: Bacteria
• Phylum: Proteobacteria
• Class: Gamma Proteobacteria
• Order: Pasteurellales
• Family: Pasteurellaceae
• Genus: Haemophilus
• Species: H. influenzae
• Binomial Name: Haemophilus influenzae
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
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
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
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)
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
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
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.
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.
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
Visible growth of H.influenzae on a chocolate agar
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
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.
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”)
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.
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
Satellitism
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.
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.
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
LABORATORY DIAGNOSIS:
BIOLOGICAL SAMPLES
SEROLOGICAL METHODS
• 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
Treatment
Infections with H.influenza type b are treated with;
• 1st choice (ceftriaxone)
• Chloramphenicol
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.
REFERENCES
• “Haemophilus influenza Disease (Including Hib)”. Centre For Disease
Control And Prevention. 18th March, 2021.
• https://vk.ovg.ox.ac.uk/vk/hib-disease
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