Unit 4 - Bacterial Pneumonia: Atypicals Flashcards
describe the bacteriology of Legionella
poorly staining G- rods (need silver or IF)
- faculative intracellular parasites
- -free-living motile w/ flagella (infectious phase)
- -intracellular nonmotile (replicative phase)
when has the accidental opportunistic lifestyle of legionaires arisen?
recently (1970s)
- natural: form biofilms in stagnant freshwater, parasite protozoa (waterborne), full life cycle
- unnatural: biofilms in HVAC systems, enter human lungs (aspirated), parasitize alveolar MP as dead end host (hard to remove once established)
how do simultaneously-exposed cases of legionella occur?
form in an outbreak, but are not contagious
-most common source is contaminated water supply
how do legionella replicate within phagocytic cells
- begin living in biofilm on warm water or in soil
- taken up by phagocytes
- “contained” in altered phagosome (not merged to lysosome)
- becomes motile and escapes the phagosome
- lyses the cells and spreads to another
what are Legionella virulence factors
- mip - unknown, required to invade monocytes
- Dot/Icm locus - T4SS (involved in altering endosome)
- pilE/pilD - pilus formation (attachment)
- Mak/Mil/pmi - unknown
- Pep/pro - Zn metalloprotease (escape)
what are the 3 outcomes of Legionella infection in humans
- asymptomatic seroconversion
- Pontiac fever
- flulike in previously healthy patient
- incubates hours to 2 days
- symptoms immunogenic as immune system kills live and/or clears dead bacteria
- resolves w/o complication - Legionnaires disease
- pneumonia in previously ill or immunosuppressed
- suppression of kidney function; diarrhea
- incubates 2-10 days
- usually resolves w/ hospitalization and treatment
- can be fatal
what do the different outcomes of Legionella depend on?
- all 3 caused by same bacteria
- differences are in the hosts, possibly also dosage
what are risk factors for Legionnaire’s disease?
- increasing age
- immunosuppression
- smoking
- chronic heart/lung disease
- chronic swallowing disorder
- male
why is mortality of Legionnaire’s disease decreasing?
- prompt diagnosis
- early use of appropriate antibiotics
how do nosocomial LD outbreaks occur?
hospitals are hot spots for LD outbreaks
- large numbers of at-risk individuals
- old complex plumbing
- hot water tanks at reduced temperature to prevent scalding
- -fix with thermal mixing valves
what does the presentation of LD look like?
- altered mental status
- headache
- high fever/chills
- pneumonia/cough/chest pain (not clinically distinct from other pneumonias, so need lab to differentiate LD)
- pancreatitis
- acute renal failure
- diarrhea
what lab do you use to diagnose LD?
urine antigen test (commercial ELISA test)
- fast
- cell wall component excreted starting 3 days after symptom onset
- test complete in hours
- reiiably detects LP1 strain of L. pneumophila
- -doesn’t detect other strains/species
- causes 90% of LD in US
- testing significantly associated w/ reduced mortality
how does the urine Ag test work?
Ag capture ELISA
- petri dish coated with Ab for LD
- patient sample is added; if Ag is present, Ab binds it
- patient sample is removed
- tagged secondary Ab binds other side of Ag
- unbound Ab is removed and TAg is assayed
how do you culture Legionella?
respiratory secretions
- much slower; fastidious, requires special nutrients
- technically demanding
- detects many strains and species of Legionella
- 27% fatality rate among culture+, urine-test- patients
- needed to trace outbreaks
what is treatment for Legionella?
- Pontiac fever: often resolves w/o treatment
- LD: needs antibiotic that penetrates infected cells
- -Levofloaxin (M. pneumoniae and S. pneumoniae)
- -Azithromycin
- -Old-school erythromycin
