Fastidious Gram Negative Coccobacilli
Haemophilus
The word Haem means blood, and philus means loving. So, this m.o. Requires blood for its
growth.
Classification:
There are 2 criteria for the classification of these m.o.:
1. Their ability to haemolyse blood.
2. Their requirments for X and/or V factors.
Morphology:
1. The typical m.o. are short bacilli but some times they are long bacilli and even
filamentous forms. This characteristic is called pleomorphism.
2. From young cultures the m.o. are coccobacilli and capsulated, while from old cultures
they are long bacilli with filamentous forms and are non-capsultated.
3. The capsule can be typed by anti-sera in a reaction called “Quellung Reaction” which is
capsule swelling test.
4. The capsular antigen can be identified by:
counter current electrophoresis (CCE) and immunofluorescent (IF) test.
CULTURES:
1. Media:
The media used for the cultivation of these
m.o. have to contain blood but not from
human or sheep because it contains
anti-Haemophilus antibodies. The most
important media used are:
A. Brain- heart infusion
B. Chocolate agar.
C. Levinthal’s agar which is nutrient agar
supplemented with blood.
Cultural requirments:
H. Influenzae requires:
A. Both X and V factors;
The X factor is haem or haematin which is a
heat stable factor important for the
respiratory enzymes of the m.o.
The V factor is nicotinamide adenine
dinucleotide (NAD) or NADP . The NAD is
coenzyme I and NADP is coenzyme II. These
two types of enzymes are heat labile factors
and are important for the oxidation-reduction
system of the m.o.
B. Temperature; these m.o. tolerate
temperatures of 25 - 40 C, but the optimum is
37 C.
C. CO2; these m.o. Require 10% CO2 for their
growth which is provided by candle jar.
D. Isovitalex (1%) added to the culture media
can enhance the growth of the m.o.
Cultural characteristics:
On broth medium; the medium does not
become turbid .
On solid media; in young cultures they have
strong iridescence or some times are
described as dew-like colonies, while in old
cultures the colonies are rough and not
glistening. Also, the growth have a bleach
–like or mousy odour. Such odours can be
smelled from the mouth of patients when
infected with H. influenzae.
Isolation of H. influenzae:
1) Bacitracin can be added to the media to
make more selective.
Satellite phenomenon (Satellitism); around
the growth of Staphylococcus aureus , the
colonies of H. influenzae will be larger. This
is because staph. provides an extra amount
of V factor which enhances the size of the
colonies .
Antigenic structure and factors of pathogenicity:
1. Capsule: composed of polyribose ribitol phosphate (PRP) the m.o. can be typed into 6
serotypes (a – f). The most important and virulent type is H. influenzae type b. Type ( a ) can
cause chronic sinusitis, and (e )and (f ) may cause post-operative infections.
It has anti-phogocytic function in the absence of anti-capsular antibodies.
2. Somatic antigens (P and M); P constitutes much of the bacterial body, while M is a labile
surface antigen.
3. Endotoxin
Pathogenesis of H. Influenzae type b:
The non-capsulated m.o. is not pathogenic ( may be part of the normal flora of the mouth
and upper respiratory tract).
The capsulated H. influenzae type - b is virulent m.o. and can cause :
1. suppurative R.T. Infections .
2. Otitis media; it is a common cause.
3. Meningitis; most common cause in children of age 5 months to 5 years.
4. It can cause bronchitis ,pneumonitis or empyema .
5. Septic arthritis and cellulitis.
This infection could start as Upper R.T. Infection and then spread to the middle ear,
meninges or joints.
Diagnostic laboratory tests:
1. Specimens: Nasopharyngeal swabs, pus, blood, and spinal fluid.
2. Direct identification; by
a) immunofluorescent (FA),
b) Quellung reaction for capsule swelling test,
c) or commercial kits for identification of H. influenzae antigens in the CSF.
3. Culture
Immunity:
1. Infants under age 3 months may have
antibodies transmitted from the mother.
During this time the infection is rare
2.Unimmunized children by the age of 3 - 5
years have anti- PRP antibodies.
3. Immunization of children with H.
influenzae type b conjugate vaccine induces
anti- PRP antibodies.
Treatment:
Mortality rate from H. influezae meningitis
may reach up to 90%.
Many strains of this m.o. are sensitive to
ampicillin and 25% of them are beta
lactamases producer. most strains are
susceptible to chloramphenicol. So,
combined use of ampicillin and
chloramphenicol in the treatment of
meningitis is important to prevent
neurological complications and. Moreover,
cefotaxime may also give excellent results .
Subdural fluid accumulation after meningitis
requires surgical drainage.
Epidemiology, Prevention & Control:
1. Haemophilus b conjugate vaccine to children can prevent infections. Also, this vaccine
reduces the carrier rates of H. influenzae.
2. Prophylactic rifampin to exposed nonimmune children under 4 years of age is
recommended.
3. The capsulated m.o. is transmitted from person to person by respiratory route.