Microbiology-Atypical CAP Flashcards
A patient presents to your clinic and the medical student diagnoses him with atypical community acquired pneumonia. What symptoms would you expect to see when you review his history? What would you expect to see in his labs?
Constitutional symptoms will predominate over respiratory ones because of multi system pathology. (Fever, malaise, headache, myalgia, non-productive cough, sore throat). His chest x-ray would show greater pulmonary involvement than you would expect from his pulmonary symptoms. Sputum sampling often does not indicate any predominate organisms.
What mycoplasms exist in our normal flora? How are they different from other bacteria?
Oropharynx: M. salivarium, M. orale. Urinary tract: M. Hominis, M. fermentans.
Why wouldn’t you treat a mycoplasm infection with penicillin?
Mycoplasms are different from regular bacteria. They are much smaller and lack a cell wall…which means they do not make peptidoglycans…the target of penicillin.
How does the small size of mycoplasms limit their biosynthetic capacity?
Except for M. pneumoniae, they must have O2. They grow slowly and require sterols to survive (in their plasma membranes).
A 17 year old boy is telling you that he felt kind of ill for a few months. He had a non-productive cough, muscle aches, a sore throat, headache and general malaise. It resolved itself after about 3 months. What is your diagnosis in this patient?
Note the insidious onset and combination of constitutional and respiratory symptoms. This is indicative of community acquired atypical (walking) pneumonia caused by Mycoplasma pneumoniae.
What are the consequences of leaving atypical pneumonia caused by mycoplasma pneumoniae untreated?
Permanent lung damage, asthma and sometimes death.
Why does mycoplasm pneumoniae normally present with a non-productive cough?
The bacteria does not invade the epithelium of the lower respiratory tract, but is tightly associated with it because P1 adhesin on the bacteria tightly adheres to the neuraminic acid-containing glycoprotein receptor on the epithelium. This makes it very difficult to cough out and the bacteria causes ciliostasis.
Below is an image of normal respiratory epithelium before infection by mycoplasm pneumoniae and a post-infection image. What is causing the differences seen in these two images?
M. pneumoniae produces a toxin called CARDS. This toxin causes vacuolation of epithelial cells and ciliostasis. The toxin also activates macrophages which in turn release many cytokines that cause further damage to the respiratory epithelium.
What populations are at highest risk for infection by M. pneumoniae? What is its incubation period?
Young adults, college students military recruits. It is spread with close contact and respiratory droplets. Incubation period is 2-3 weeks.
How is M. pneumoniae usually diagnosed?
The patient’s history. It is such a slow-growing bacteria, blood culture takes too long. Serology and analysis of IgG antibodies to CARDS and P1 can show infection retrospectively. Cold hemagglutinins (IgM binding RBCs 1-2 wks after initial infection) are not very specific.
How do you treat someone with suspected M. pneumoniae infection?
Tetracycline, erythromycin, or azythromycin. Beta-lactams are not effective due to lack of peptidoglycans.
What are the properties of chlamydiaceae?
Obligate intracellular bacteria. Carries plasmids. Membrane similar to gram - w/outer and inner membrane, but has no detectable peptidoglycan.
What part of chlamydia is the infectious form?
It has two developmental forms: EB (elementary body) and RB (reticulate body). The elementary body is the extracellular form and has disulfide cross-liked outer membrane proteins making it more durable and infectious. RB is contained within the cell and replicates.
How does chlamydia cause disease?
It takes over the host cell, killing it. It also elicits inflammation and scarring of tissue from the host immune response.
A patient comes to see you with continued non-productive cough, malaise, sore throat and muscle aches. Chest x-ray shows diffuse fluid accumulation and you diagnose the patient with walking pneumonia. Recently there has been an outbreak of c. pneumoniae in the community so you treat him accordingly. How do you confirm that this is what he is infected with?
Serology IgM titer > 1:64 or a 4x increase in IgG titer from 4 weeks prior measurement.
A 22 year old veterinarian comes to see you after an acute bout of pneumonia. She said she had fever, headache, myalgia, mild cough and confusion. She was pregnant and the fetus spontaneously aborted. What was the likely offending agent in this woman’s pneumonia? How would you prove this? What medications should she have received during active infection?
Chlamydia psittaci that caused psittacosis. It most often is transmitted via respiratory droplets or dried feces of birds. You could test for this with serology analysis of IgM and IgG titers or microimmunofluorescence. Common medications that work are doxycycline and macrolides.
