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Infectious Disease: Unit 2 > Mycoplasma/Legionella > Flashcards

Flashcards in Mycoplasma/Legionella Deck (15):

Atypical pneumonia: definition

  • Acute infectious pulmonary disease caused by:
    • Mycoplasma pneumoniae
    • Species of Rickettsia
    • Chlamydia
    • Various viruses - adenoviruses, parainfluenza viruses
  • Marked by extensive but tenuous pulmonary infiltration  + fever, malaise, myalgia, sore throat, cough
    • Cough at first nonproductive but becomes productive and paroxysmal
  • Endemic or epidemic (closed population, military, schools), world-wide distribution, all seasons, higher relative incidence in late summer and fall


Atypical pneumonia: presentation

  • Insidious onset:
    • Fever, malaise, myalgia, headache, sore throat/cough (usually non-productive)
  • Constitutional symptoms often predominate over respiratory symptoms
  • Extrapulmonary manifestations 2/2 infection dissemination:
    • Stevens-Johnson syndrome
    • Erythema syndrome


Atypical pneumonia: lab results (sputum, path, CXR)

  • Sputum:
    • Variable amount
    • Gram stain and routine culture often show only oral flora
  • Pathology:
    • Bronchopneumonia
    • Involved area = respiratory mucosa lining airways
    • Inflammatory response predominantly lymphocytic
  • CXR:
    • Pulmonary involvement often greater than expected from mild physical findings
    • Patchy or peribronchial infiltrates (unilateral or bilateral)
    • Often with predominantly lower-lobe involvement
    • Lobar consolidation, pleural effusions common


Atypical pneumonia: approach

  • Laboratory results
    • Confirmation of clinical susupicion
  • Tx with broad-spectrum antibiotics
    • Appropriate range
  • Timing is a limitation with regards to elucidation of causative agent


Atypical pneumonia: culpable organisms

  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Legionella pneumophila
  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Coxiella burnetii --> Q fever
  • Chlamydia psittaci --> Psittacosis
  • Viral agents: adenovirus, parainfluenza virus, Epstein-Barr virus, respiratory syncytial virus


Most common causative agents of atypical pneumonia in the elderly

  • S. pneumoniae
  • Mycoplasma pneumoniae

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Biological characteristics of Ureaplasma

  • Distinguished by unique ability to metabolize urea
  • Small, lacks cell coat - allows to pass through filters
  • High affinity for epithelial surfaces
  • High resistance against dryness and temperature changes
  • High detection rate during pregnancy


Biological characteristics of Mycoplasma

  • Small genome size, smallest free-living organism
    • Require sterols & osmotically active medium
  • "Eaton agent" = MP --> most common cause of primary atypical pneumonia syndrome
  • No cell wall (no peptidoglycan) --> marked pleiomorphism (variability in shape) allowing organism to pass through filter
    • Not amenable to gram stain
    • Resistant to B-lactam antibiotics
  • Fried-egg appearance - grow down into agar
    • Exception: M. pneumoniae with "mulberry" appearance (smaller granular colonies)
  • Specialized tip structure - binary division + gliding motility
    • P1 protein (attachment factor) concentrated on cell membrane, functions in adhesion to target cell respiratory epithelium, forming tight association


Mycoplasma: mechanisms of injury

  • Immobilizes cilia of respiratory epithelial cells (ciliostasis)
    • Impairs clearance of microbes and secretions
  • Reactive oxygen species generated
    • Facilitate tissue damage
    • Cytokines contribute to disease process + clearance
  • CARDS TX = significant virulence factor
    • AA sequence homology with S1 active of pertussis toxin (ADP-ribosylated toxin)
    • Precipitates cytopathic effects & vacuolization


Diagnostic tests for Mycoplasma & Ureaplasma

  • ELISA (IgM, IgG) + PCR
    • ​Gold standard
  • Complement fixation - sensitive, but cumbersome
  • Serologic tests must be able to demonstrate rise in titer to be employed
  • Test for cold hemagglutinins (autoAbs that react with T antigen of RBCs): poor sensitivity/specificity, primarily of historical interest
  • Isolation of M. pneumoniae from sputum = slow (2-6 weeks), insensitive, not done routinely


Biological characteristics of Legionella & related species

  • 85% of legionellosis cases caused by L. pneumophilia, mostly serotype 1
  • Gram (-), motile, pleomorphic, coccobacilli
    • Stains poorly with gram stain --> might be interpreted as lack of bacterial prevalence in sputum
  • Direct fluorescent Ab methods can improve visualization
    • Comparable to Mycoplasma
  • Distinct fatty acid composition
  • Oxidase (+), catalase (+)
  • Resistant to B-lactam antibiotics - produces B-lactamase
  • Produces hemolysin + cytotoxin
  • Evades host defenses by living within macrophages - facultative intracellular pathogen
  • Inhibits endosome-lysosome fusion --> endosome becomes associated with ER to form ribosome-studded phagosomes
  • Macrophage employs coiling phagocytosis - zippering mechanism 


Diagnostic tests for Legionella species 

  • Demonstration of bacteria in tissue or secretions
  • Serology 
  • Molecular diagnostic tests

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Legionella: demonstration of bacteria in tissue or secretions

  • Sensitivity: culture > DFA > silver stain
  • Requires cysteine and iron for growth on buffered charcoal-yeast extract (BCYE) medium at 35C with charcoal absorbing fatty acids
    • 2-6 days for growth
  • Intracellular Legionella can be visualized via silver staining methods (very nonspecific)
  • Direct immunofluorescence can detect Legionella in sputum, pleural fluid, or lung tissue 


Legionella: serology

  • Purpose: epidemiological > patient management
  • Detect 4-fold rise in indirect fluorescent Ab titer
  • Serum Ab rises 5-7 days after onset of symptoms and may be delayed in some patients


Legionella: molecular diagnostic tests

  • Rapid tests based on immunoassay for L. pneumophila serogroup 1 Ag in urine = commercially available, recommended for patients sick enough to be hospitalized
  • PCR tests for L. pneumophila have also been developed, not yet widely available in standardized format
  • Urinary antigen must be serotype 1
  • IgM can remain positive - cannot suggest recurrent versus recent infection