Numerous bacteria can cause pneumoniae in the elderly or immunocompromised. The most common ones were covered in the section "Bacterial Diseases of the Lower Respiratory System." Devise a set of laboratory tests that could be used to distinguish between

Question

these bacteria in a clinical laboratory. Have as your goal the minimum number of tests necessary. (Hints: Try to make a dichotomous key to start. You may also need to recall material from previous chapters.)

What will be an ideal response?

Answer 1

A number of answers are possible, but all should start with a Gram stain and microscopic examination of a sputum or lavage sample.

Gram-positive bacterial possibilities include Staphylococcus aureus and Streptococcus species. A catalase test can distinguish Staphylococcus (positive) from Streptococcus (negative). A hemolysin test can be used to identify the most likely Streptococcus species: S. pyogenes is beta-hemolytic and S. pneumoniae is alpha-hemolytic.

Gram-negative bacteria that may cause pneumonia include Klebsiella pneumoniae, Haemophilus influenza, Mycoplasma pneumoniae, Chlamydophila species and Yersinia pestis. Close observation of the cell morphology may be sufficient. Klebsiella and Yersinia are bacilli and rather large compared to the others. Klebsiella is a non-motile coliform and Yersinia is a motile non-coliform, so observation of a bacterial wet mount may be used to distinguish them (or a lactose fermentation test). Haemophilus and Mycoplasma are pleomorphic, with Haemophilus tending to a coccobacillus shape while Mycoplasma does not. Chlamydophila are tiny intracellular parasites and thus will be observed inside host cells.
Bloom's Taxonomy: Application
Section: Bacterial Diseases of the Lower Respiratory System
Learning Outcome: 22.11, 22.12