Gram Negative RodsRelated to the respiratory tract

IntroductionThree medically important gm neg rods are

associated with the diseases of the respiratory tract.

They are Haemophilus influenzae, Bordetella pertussis and Legionella pneumophia.

H.influenzae and B.pertussis found only in humans.

L.pneumophila found in water sources.

HaemophilusImportant cause of Upper respiratory tract

infections like otitis media, sinusitis, conjunctivitis and epiglottis.

Sepsis in children.Causes pneumonia in adults with obstructive

lung disease.H.ducreyi is an agent causing Chancroid (STD).H.influenzae used to be a leading cause of

meningitis in children, but the incidence is greatly reduced due to an effective conjugate vaccine.

H.influenzae is a small. Gm neg rod.It has a polysaccharide capsule.Along with pneumococcus and meningococus it

is the third important encapsulated pyogens.Six serotypes based on their capsular antigen.Type b causes the most severe, invasive

diseases such as meningitis and sepsis.Unencapsulated strains usually cause a non-

invasive disease of the upper respiratory tract such as sinusitis and otitis media.

Growth on chocolate agar with heme (factor X) and NAD (factor V) for adequate energy production.

Pathogenesis:

Pathogenesis involves the antiphagocytic capsule and endotoxin. No exotoxin produced.

It produces IgA protease that degrades secretory IgA, facilitating its attachment to the respiratory tract mucosa. After establishing itself there, it can now enter the blood stream and spread to the meninges.

Encapsulated type b strains cause 95% of the meningitis caused by Haemophilus.

Its route of entry is through inhalation or air borne droplets into the respiratory tract, causing otitis media, sinusitis or pneumonia.

Infects only humans. No animal reservoir.

Clinical findings:For meningitis: Symptoms are typically

similar to pneumococcal and meningococcal species, like fever, headache, stiff neck along with drowsiness.

Children between 6 months to 6 years are most susceptible, as they lose maternal IgG and generate sufficient antibodies against the capsular antigens themselves.

They are second only to pneumococcus as the cause of Sinusitis and Otitis media.

Other serious infections include arthritis, cellulitis and sepsis (sepsis especially in splenectomy cases).

Lab diagnosis: Chocolate agar with X and V- factors sufficient identification. Other identification to identify capsular antigen such as Quellung reaction,

fluorescent antibody staining, slide agglutination.

Treatment: Drug of choice for H.influenzae infections is Ceftriaxone as 20-30% strains

produce β lactamase. Antibiotic treatment should be prompt as infection can be followed by a

neurologic involvement like subdural empyema. Fatality rate of 90% in meningitis, if untreated. Upper respiratory tract diseases like otitis media and sinusitis treated with

Amoxicillin-clavulanate or teimethoprim-sulfamethoxazole.

A more effective conjugated vaccine containing the type b capsular polysaccharide conjugated to diphtheroid toxoid or other carrier protein is available. Depending on the carrier protein can be given to the age group of 2 and 15 months and is safe.

Rifampin for close contacts of the patients as better secreted in the saliva than ampicillin.

Bordetella B.pertussis causes Whooping cough. They are small, encapsulated gm neg rods.

Pathogenesis:

I) The orgs. attach to the cilia of the epithelial cells of the upper respiratory tract but do not invade the underlying tissue. This attachment is mediated by a protein on the pili called ‘filamentous hemagglutinin’. Antibodies against these hemagglutinins offer protection.

Decreased cilia activity followed by death of the ciliated cells are imp aspects of pathogenesis.

Highly contagious and spreads through air borne droplets during severe coughing episodes.

A pathogen only for humans.

II) Pertussis toxin binds with its B-sub unit and ADP-ribosylates the Gi protein (the inhibitory unit of G protein complex), inhibits the inhibitory subunit and with stimulation of adenylate cyclase and a consequent rise in cAMP enhance the activity of the dependent protein kinase.

The ADP-ribosylation also inhibits the signal transduction by chemokine receptors. This results in the failure of lymphocytes to enter the lymphoid tissue such as spleen and lymph nodes. This causes the striking phenomena of Leucocytosis in the blood of patients.

III) The orgs also synthesize and export Adenylate cyclase. This enzyme inhibits the bactericidal activity of phagocytic cells like neutrophils. Bacterial mutants lacking cyclase activity are avirulent.

IV) It is a tracheal cytotoxin (a fragment of the bacterial peptidoglycan) that damages the ciliated cells and act in concert with the endotoxin to induce nitric oxide, which kills the ciliated epithelial cells.

Clinical findings:

Whooping cough is an acute tracheobronchitis that begins with a mild upper respiratory tract symptoms.

This is followed by severe paroxysmal cough lasting for 1-4 weeks.

The paroxysmal pattern is characterized by a series of hacking cough accompanied by large amount of mucous. This hacking cough ends with an inspiratory whoop as air passes through the narrowed glottis.

Despite the severity, orgs restricted to the respiratory tract and blood cultures are neg.

Pronounced leucocytosis with upto 70% lymphocytes are sen.

The whooping not seen in adults, and cough lasting several weeks(100 day cough) should be evaluated for B.pertussis infection.

Lab diagnosis:

Orgs. Isolated from the naso-pharyngeal swab taken during the paroxysmal stage.

Bordet-Gengou agar with a high % of blood (20-50%) to inactivate inhibitors in the agar.

Identification of the isolated org. by agglutination with specific antiserum of by fluorescent antibody staining (can be used for diagnosis as orgs grow very slowly)

PCR based tests are highly sensitive and specific and must be used whenever possible.

Detecting antibodies in the serum of a case of prolonged cough may be diagnostic.

Treatment:

Macrolides such as Erythromycin or Azithromycin used.

These antibiotics reduce the no. of the orgs. And decrease the risk of secondary infection, but have little influence on the course of the disease as the toxins have already damaged the respiratory mucosa.

Supportive care such as oxygen therapy and suction of mucous may be necessary.

Prevention:

2 vaccines are available:A acellular vaccine containing 5 purified proteins from the

org( antigens). The maintoxoid ( antigenic part of the toxin) is the inactivated pertussis toxin. They are inactivated genetically (first vaccine to have genetically inactivated toxoid) by introducing 2 amino acid changes in their protein. This eliminates their ADP-ribosylating activity but retains their antigenicity. The other antigens are the filamentous hemagglutinin, pertactin and fimbriae types 2 and 3. The acellular vaccine has fewer side effects.

Pertussis vaccine given in combination with diphtheria and tetanus toxoids(DTaP) in 3 doses from 2 months of age (2,4,6 mths of age) with boosters at 1 and 6 years of age.

A killed vaccine containing killed orgs themselves.

LegionellaLegionella pneumophila and other legionellae causes

pneumonia both community acquired and hospital acquired in immunocompromised patients. Famous Legionaire’s disease among people attending the American legion convention in 1976.

Legionella are gm neg rods that stain faintly with gm stain. They do however have a gm neg type cell wall.

Legionellae lung biopsy sections stained by Dieterle silver impregnation stain to visualize orgs. As do not stain with the standard hematoxylin and eosin (H and E) procedure.

Require a high concentration of iron and cysteine, hence the charcoal yeast agar, which has been supplemented by these.

L.pneumophila cause 90% of the pneumonia caused by this genus.

Many spp upto 30 are known,L.micdadei and L.bozemanii are two other species that account for most of the remaining 10%.

Pathogenesis:

The portal of entry is the respiratory tract through air conditioners and water cooling towers (environmental water sources).

Pathological changes are primarily in the lungs. In severe case, bacteremia may result accompanied by damage to the vascular endothelium in multiple organs, especially brain and kidneys.

Major virulent factor is a LPS (endotoxin).Cell-mediated immunity is the most defense mechanism

and individuals with compromised cell mediated immunity like cancer, AIDS or transplant patients(on corticosteroids), old smoker and drinking individual are all predisposed to this infection.

Person to person spread does not occur.

Clinical findings:

Clinical manifestations vary from mild influenza like to severe pneumonia accompanied by mental confusion, non bloody diarrhea, proteinuria, and microscopic hematuria.

Although cough is prominent, mucous is scant and non purulent.

Hyponatremia (serum sodium < or= 130mEq/L) is an imp. Lab finding that occurs more in Legionellosis than other pneumonia.

Most cases resolve spontaneously in 7-10 days, but can be fatal in older and immunocompromised individuals.

Legionellosis is an Atypical pneumonia hence has to be distinguished from other atypical pneumonia like Mycoplasma pneumonia, viral pneumonia, psittacosis and Q-fever.

Pontiac fever is a mild flu like infection of legionella. It does not result in pneumonia.

Lab findings:

Sputum gm stain reveal many neutrophils but no orgs.Grows on charcoal yeast agar supplemented with iron

and cysteine.Detecting antigens in urine is a rapid way of diagnosis.It is possible to demonstrate legionella antigens in the

infected lung tissue by fluorescent antibody staining.Cold agglutinin titer (patients develop auto immune

antibodies that agglutinate human cells at 4 degree centigrade and not at 37 degrees.)does not rise as in pneumonia caused by Mycoplasma.

Treatment:

Azithromycin or Erythromycin with or without Rifampin is treatment of choice.

Fluoroquinolones such as Levofloxacin and trovafloxacin also drugs of choice.

These drugs effective against Mycoplasma and Stre.pneumoniae as well.

Penicillins and cephalosporins less effective as orgs produce βlactamase.

Prevention:

No vaccine.Eliminating aerosols from water sources and

in hospital water sources by using high temp and hyperchlorination.