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Transcript
Bacteriology
Dr. Bara‫ ׳‬Hamid Hadi
Haemophilus and Legionella
Haemophilus is a genus of Gram-negative, pleomorphic, coccobacilli bacteria belonging to
the Pasteurellaceae family. These organisms inhabit the mucous membranes of the upper
respiratory tract, mouth, vagina, and intestinal tract. The genus Haemophilus includes a number
of species that cause a wide variety of infections but share a common morphology and a
requirement for blood-derived factors during growth that has given the genus its name.The
impotant one and the major pathogen is Haemophilus influenza.
Other Haemophilus species cause disease less frequently. Haemophilus parainfluenzae
sometimes causes pneumonia or bacterial endocarditis. Haemophilus ducreyi causes chancroid.
Haemophilus aphrophilus is a member of the normal flora of the mouth and occasionally causes
bacterial endocarditis. Haemophilus aegyptius, which causes conjunctivitis and Brazilian
purpuric fever, and Haemophilus haemolyticus
Haemophilus influenzae is a small, nonmotile Gram-negative bacterium this bacteria is
classified as type-able (encapsulated) and non-type-able (non encapsulated) according to the
presence or absence of capsule .Encapsulated strains of Haemophilus influenzae are
coccobacilli, similar in morphology to Bordetella pertussis, the agent of whooping cough. ,and
are further classified into 6 serotypes(a-f) , with the Haemophilus influenzae serotype b being
the most pathogenic for humans, responsible for respiratory infections, ocular infection, sepsis
and meningitis. While non encapsulated strains are pleomorphic and often exhibit long threads
and filaments. The organism may appear Gram-positive unless the Gram stain procedure is very
carefully carried out. Furthermore, elongated forms may exhibit bipolar staining, leading to an
erroneous diagnosis of Streptococcus pneumoniae
Haemophilus "loves heme", more specifically it requires a precursor of heme in order to grow.
Nutritionally, Haemophilus influenzae prefers a complex medium and requires preformed growth
factors that are present in blood, specifically X factor (i.e., hemin) and V factor (NAD or
NADP). In the laboratory, it is usually grown on chocolate blood agar which is prepared by
adding blood to an agar base at 80oC. The heat releases X and V factors from the RBCs and turns
the medium a chocolate brown color. The bacterium grows best at 35-37oC and has an optimal
pH of 7.6. Haemophilus influenzae is generally grown in the laboratory under aerobic conditions
or under slight CO2 tension (5% CO2), although it is capable of glycolytic growth and of
respiratory growth using nitrate as a final electron accept.
Pathogenesis:
The pathogenesis of H. influenzae infections is not completely understood, but the important
Virulence factors of this bacteria are:
1. Capsule: all of the 6 encapsulated strains of H. influenzae have capsule as a virulence
factor. The capsule is composed of polysaccharide containing two hexose sugers as
subunits carbohydrates. But type b polysaccharide is the only capsular type which has
two pentose monosaccharide instead of the two hexose sugars in other serotypes called
polyribosyl-ribitol-phosphate (PRP)
2. outer membrane proteins: designated as P1 and P2.
3. lipooligosaccharide
4. neuraminidase .
5. IgA protease.
6. Fimbriae
So the presence of the capsule is known to be the major factor in virulence. Encapsulated
organisms can penetrate the epithelium of the nasopharynx and invade the blood capillaries
directly. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the
non immune host. Non encapsulated strains are less invasive, but they are apparently able to
induce an inflammatory response that causes disease.
Clinical features:
Naturally-acquired disease caused by H. influenzae seems to occur in humans only. In infants
and young children (under 5 years of age), H. influenzae type b causes bacteremia and acute
bacterial meningitis. Occasionally, it causes epiglottitis (obstructive laryngitis), cellulitis,
osteomyelitis, and joint infections. Nontypable H. influenzae causes ear infections (otitis
media) and sinusitis in children, and is associated with respiratory tract infections
(pneumonia) in
infants,children
and
adults.
Diagnosis:1- Isolation of the organism from clinical material (such as CSF, blood, throat swab,
sputum) on chocolate agar enriched with two growth factor X and V.
2- A capsule swelling test (Quelling test) with specific antiserum is analogous to quelling
test of pneumonia
3- Capsular antigen may be detected CSF or other body fluid using immunologic tests such
as latex agglutination.
4- PCR to detect bacteria and to differentiate into capsulated and non capsulated one
Treatment:
The recommended treatment for H. influenzae meningitis is ampicillin for strains of the
bacterium that do not produce ß-lactamase, and a third-generation cephalosporin or
chloramphenicol for strains that do. Amoxicillin, together with a substance such as clavulanic
acid, that blocks the activity of ß-lactamase, has been unreliable in treatment of meningitis,
although it is effective in treatment of sinusitis, otitis media and respiratory infections.Vaccine
conjugated to diphtheria toxin or to carrier protein. The vaccine is more effective in children.
Haemophilus ducreyi
This is a significant cause of genital ulcers (chancroid). The infection is asymptomatic in women
but about a week following sexual transmission to a man, it causes appearance of a tender papule
with erythematous base on the genitalia or the peripheral area. The lesion progresses to become a
painful ulcer with inguinal lymphadenopathy. The H. ducreyi lesion (chancroid) is distinguished
from a syphilitic lesion (chancre) in that it is a comparatively soft lesion.
Legionella
Legionella cells are thin, somewhat pleomorphic Gram-negative bacilli that measure 2 to 20 μm.
Long, filamentous forms may develop, particularly after growth on the surface of agar.
Ultrastructurally, Legionella has the inner and outer membranes typical of Gram-negative
bacteria. It possesses pili (fimbriae), and most species are motile by means of a single polar
flagellum. It include the speciesL. pneumophila,which is the primary human pathogenic
bacterium in this group and is the causative agent of Legionnaires' disease, also known as
legionellosis. Although more than 70 Legionella serogroups have been identified among 50
species, L pneumophila causes most legionellosis. Transmission occurs by means of
aerosolization or aspiration of water contaminated with Legionella organisms. Wounds may
become infected after contact with contaminated water.
Pathogenesis
The pathogenesis of Legionella infections begins with a supply of water containing virulent
bacteria and with a means for dissemination to humans. Person-to-person transmission has never
been demonstrated, and Legionella is not a member of the bacterial flora of humans.
Infection begins in the lower respiratory tract. Alveolar macrophages, which are the primary
defense against bacterial infection of the lungs, engulf the bacteria; however, Legionella is a
facultative intracellular parasite and multiplies freely in macrophages. The bacteria bind to
alveolar macrophages via the complement receptors and are engulfed into a phagosomal vacuole.
However, by an unknown mechanism, the bacteria block the fusion of lysosomes with the
phagosome, preventing the normal acidification of the phagolysosome and keeping the toxic
myeloperoxidase system segregated from the susceptible bacteria. The bacilli multiply within the
phagosome. Thus, a cellular compartment that should be a death trap instead becomes a nursery.
Eventually, the cell is destroyed, releasing a new generation of microbes to infect other cells.
The symptoms of Legionella infection undoubtedly result from a combination of physical
interference with oxygenation of blood, ventilation-perfusion imbalance in the remaining lung
tissue, and release of toxic products from bacteria and inflammatory cells. Bacterial factors
include a protease that may be responsible for tissue damage. Cellular factors include
interleukin-1, which produces fever after it is released from monocytes, and tumor necrosis
factor, which may be responsible for some of the systemic symptoms.
Virulence appears to be multifactorial. An outer membrane protein that functions as a
metalloprotease and a cytoplasmic membrane heat-shock protein elicit protective immune
responses, but are not essential for expression of virulence.
Clinical Features:
The most common presentation of Legionella pneumophila is acute pneumonia (legionellosis).
The clinical manifestations of Legionella infections are primarily respiratory. Two very different
kinds of respiratory illness may result from infection; the reasons for this dichotomy are not
understood. The most common presentation is acute pneumonia, which varies in severity from
mild illness that does not require hospitalization (walking pneumonia) to fatal multilobar
pneumonia. Typically, patients have high, unremitting fever and cough but do not produce much
sputum. Extrapulmonary symptoms, such as headache, confusion, muscle aches, and
gastrointestinal disturbances, are common. Most patients respond promptly to appropriate
antimicrobial therapy, but convalescence is often prolonged (lasting many weeks or even
months).
The second form of respiratory illness is called Pontiac fever after the city in Michigan where the
first epidemic was recognized. This uncommon manifestation of infection resembles acute
influenza, including fever, headache, and severe muscle aches. It is self-limited, and
convalescence is uneventful.
Bacteremia occurs during Legionella pneumonia, and symptomatic infection outside the lungs
occasionally develops. Under special conditions, bacteria introduced through portals other than
the lungs, such as surgical wounds, may cause disease.
Diagnosis:
Legionellosis can be suspected clinically.
The preferred method is culturing on special charcoal-containing agar.
Direct detection of bacterial antigen in clinical specimens is potentially much faster than
culturing.
Control:
Decontamination of identified environmental sources is of primary importance for prevention.
The drug of choice is erythromycin.