Mycoplasma/Legionella

Question 1

Atypical pneumonia: definition

Answer 1

• 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

Question 2

Atypical pneumonia: presentation

Answer 2

• 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

Question 3

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

Answer 3

• 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

Question 4

Atypical pneumonia: approach

Answer 4

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

Question 5

Atypical pneumonia: culpable organisms

Answer 5

• 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

Question 6

Most common causative agents of atypical pneumonia in the elderly

Answer 6

• S. pneumoniae
• Mycoplasma pneumoniae

Question 7

Biological characteristics of Ureaplasma

Answer 7

• 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

Question 8

Biological characteristics of Mycoplasma

Answer 8

• 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

Question 9

Mycoplasma: mechanisms of injury

Answer 9

• 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

Question 10

Diagnostic tests for Mycoplasma & Ureaplasma

Answer 10

• 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

Question 11

Biological characteristics of Legionella & related species

Answer 11

• 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

Question 12

Diagnostic tests for Legionella species

Answer 12

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

Question 13

Legionella: demonstration of bacteria in tissue or secretions

Answer 13

• 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

Question 14

Legionella: serology

Answer 14

• 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

Question 15

Legionella: molecular diagnostic tests

Answer 15

• 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