Microbiology-Legionella & Pseudomonas Flashcards
By what mechanism does pseudomonas evade the immune system?
Production of antiphagocytic capsules that mimic self
By what mechanism does legionella evade the immune system?
Lives intracellularly and prevents phagosome-lysosome fusion.
In 1976 a group of veterans had a reunion. About 5% of them came down with fever, chills, pneumonia, nausea and vomiting. Of those 5%, 30% died. What were they infected with and why was this group of veteran susceptible?
Legionella pneumophila. it has the characteristic GI symptoms along with flu-like symptoms. Legionella infects those who have decreased cell-mediated immunity.
What are the characteristics of Legionella?
Gram-negative slender bacilli that do not gram stain well.
What is required to grow legionella in the lab?
Buffered charcoal yeast extract agar (it needs amino acids and iron in order to grow)
Why is legionella such a good nosocomial pathogen?
It replicates anywhere between 5 and 63 C, it is found in water sources and is very hard to eliminate due to biofilms.
Shown below is an image of a legionella bacterium being taken in by an ameba. How does this image represent the manner in which it infects our cells?
Coiling phagocytosis prevents fusion and acidification by the lysosome, sparing the bacterium. The phagosome actually becomes studded with ribosomes, the bacteria replicate, the phagosome ruptures and the bacteria lyse the cell and move on.
A 24 year old female presents with fever, chills, myalgia, headache and flu-like symptoms. If this is a bacterial infection spread via water and she has no evidence of pneumonia, what does she have?
Pontiac fever. It is less-severe, self-limiting type of legionella infection without pneumonia.
What tests are used to determine if a patient has been infected by legionella?
Urine sample (LPS antigen detection w/ELISA), Bronchoalveolar lavage (macrophage extraction w/saline wash and analysis w/fluorescence, silver stain, DNA probe or culture) and convalescent sera (4-fold antibody rise after the fact)
What drugs are used to treat legionella infections?
Macrolides (erythromycin, azithromycin) and Fluoroquinilones (ciprofloxacin, levofloxacin)
Why does penicillin usually have little effect on clearing a legionella infection?
Most species make beta-lactamase.
Your hospital administrator runs a PCR sample on the water system and finds legionella. How would he get rid of this bacteria?
Hyperchlorination or superheating of the water.
What are the characteristics of pseudomonas aeruginosa?
Gram negative, flagellated rod that is a strict aerobe.
You are going to the hospital to visit a friend who was severely burned in a house fire. You bring him a basket of goodies and a balloon telling him to get well soon. What foods can absolutely not go into the basket of goodies?
Fresh fruits and vegetables. Pseudomonas is ubiquitous in the environment and has a high rate of infection on people who have lost their external immune defenses (such as burn victims)
Why is pseudomonas such a difficult bug to treat?
It is resistant to most antibiotics.
What contributes to the virulence of pseudomonas?
LPS, alginate capsue, pili, flagella and exotoxins (A,S & T) that cause tissue damage.
Patients with what conditions are particularly susceptible to pseudomonas?
Neutropenia, diabetes, prematurity and cystic fibrosis (pneumonia).
What pathogen is the leading cause of swimmers ear (otitis externa)?
Pseudomonas
You and your girlfriend go hot tubbing on a Friday night. She comes out looking like this. You talk to the management office and they tell you the hot tub had not been chlorinated yet and was under maintenance. What is causing this condition?
This is hot tub folliculitis caused by pseudomonas.
A patient with a history of cystic fibrosis comes to see you in clinic with dyspnea, fever, chills and pleuritic chest pain. Sputum culture reveals a very sticky gram negative rod. Why would you treat this patient with surfactant and aerosolized antibiotics?
Patient’s with cystic fibrosis have defective Cl- channels that create a sticky environment for pseudomonas to adhere to and form biofilms. These biofilms are resistant to antibiotics and immune system attack. Surfactants break up the biofilms and aerosolized antibiotics provide for a more direct attack on the bacteria.
How do you identify pseudomonas in the laboratory?
Growth on MacConkey agar (it’s a non-fermenter and is lactose negative), oxidase-positive (strict aerobe and makes cytochrome oxidase), produces pyocyanin (green pigment), doesn’t oxidize other carbs and spells like grape.
What tests are important for determining the type of gram negative rods you see in a lab?
*
Why would you plate bacteria on MacConkey agar in the lab?
Contains bile salts (good for culturing enteric gram negatives), and organisms that ferment lactose will turn pink.
A 50 year old plummer who smokes and drinks comes down with a pneumonia that is also associated with GI complaints. What is the likely pathogen and what kind of pneumonia would this patient present with?
Legionella. It presents in typical and atypical pneumonias.
