Francisella tularensis - MegaMicro

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Francisella tularensis

Microbiology > Bacteriology > Gram-negative bacteria
Francisella tularensis  

1.       Structure
a.       Weakly staining Gram negative coccobacilli (.2-.7┬Ám)
b.      Non-motile
c.       Contains thick capsule
                           i.      50-70% lipid concentration
                           ii.      Large amounts of long-chain saturated and monoenoic C20 to C26 fatty acids as well as alpha and beta hydroxyl fatty acids.

2.       Pathobiology
a.       Oligate aerobe
b.      Requires cysteine and iron for growth
c.       Infection overview:
                             i.      F. tularensis enters the cell.
                              ii.      Proliferation inside acidified compartments containing iron.
                             iii.      High levels of viable bacteria induce cytopathagenesis and apoptosis.
                              iv.      Inflammatory response due to pathogen entry attracts large numbers of macrophages. These macrophages are not activated and are easier to infect.
                              v.      Due to bacterial capsule, immunity to the effect of neutrophils and complement.
                                              vi.      Renewed infection in arriving macrophages.
                                             vii.      The accumulation of macrophages without removal of bacteria initiate granuloma formation and the continued activation of the immune system.
                                           viii.      Host death due to complications due to pnuemonia and/or due to septic shock  due to the large quantity of cytokines released.

3.       Epidemiology
a.       Infection through tick and deer fly bites
                              i.      dog tick (Dermacentor variabilis)
                              ii.      wood tick (Dermacentor andersoni)
                              iii.      lone star tick (Amblyomma americanum)
                               iv.      Deer flies (Chrysops spp.)
b.      Skin contact with infected animals (usually rabbits)
c.       Ingestion of contaminated water
d.      Inhalation of contaminated aerosols or agricultural dusts

4.       Disease manifestations of Tularemia: Almost all forms are accompanied by fever

a.       Ulceroglandular
                i.      This is the most common form of tularemia and usually occurs following a tick or deer fly bite or after handing of an infected animal. A skin ulcer appears at the site where the bacteria entered the body. The ulcer is accompanied by swelling of regional lymph glands, usually in the armpit or groin.

b.      Glandular
                  i.      Similar to ulceroglandular tularemia but without an ulcer. Also generally acquired through the bite of an infected tick or deer fly or from handling sick or dead animals.

c.       Oculoglandular
                   i.      This form occurs when the bacteria enter through the eye. This can occur when a person is butchering an infected animal and touches his or her eyes. Symptoms include irritation and inflammation of the eye and swelling of lymph glands in front of the ear.

d.      Oropharyngeal
                    i.      This form results from eating or drinking contaminated food or water. Patients with orophyangeal tularemia may have sore throat, mouth ulcers, tonsillitis, and swelling of lymph glands in the neck.

e.      Pneumonic
                    i.      This is the most serious form of tularemia. Symptoms include cough, chest pain, and difficulty breathing. This form results from breathing dusts or aerosols containing the organism. It can also occur when other forms of tularemia (e.g. ulceroglandular) are left untreated and the bacteria spread through the bloodstream to the lungs.

f.      Typhoidal
                     i.      This form is characterized by any combination of the general symptoms (without the localizing symptoms of other syndromes)

5.       Diagnosis
a.       Growth of F. tularensis in culture is the definitive means of confirming the diagnosis of tularemia. Appropriate specimens include swabs or scrapings of skin lesions, lymph node aspirates or biopsies, pharyngeal swabs, sputum specimens, or gastric aspirates, depending on the form of illness.
b.      A presumptive diagnosis of tularemia may be made through testing of specimens using direct fluorescent antibody, immunohistochemical staining, or PCR.
c.       The diagnosis of tularemia can also be established serologically by demonstrating a 4-fold change in specific antibody titers between acute and convalescent sera. Convalescent sera are best drawn at least 4 weeks after illness onset; hence this method may be useful for confirming the diagnosis but not for clinical management.

6.       Therapy
a.      Streptomycin is the drug of choice based on experience, efficacy and FDA approval.
b.      Gentamicin is considered an acceptable alternative, but some series have reported a lower primary success rate. Treatment with aminoglycosides should be continued for 10 days.
c.       Tetracyclines may be a suitable alternative to aminoglycosides for patients who are less severely ill. Tetracyclines are static agents and should be given for at least 14 days to avoid relapse.
d.      Ciprofloxacin and other fluoroquinolones are not FDA-approved for treatment of tularemia but have shown good efficacy in vitro, in animals, and in humans.
Related concepts

  1. Francisella,
  2. Tularemia,
  3. F. tularensis,
  4. tick-borne disease,
  5. gram negative bacteria
Web links:
a.       Great for basic info on disease
a.       Detailed presentation about F. tularensis
Tularemia cases 2014
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