Q&A: Microbiological Diagnosis Of Chest Infection

Lower respiratory tract infections are among the most common infectious diseases of humans worldwide. In the United States alone, pneumonia and influenza rank as the sixth leading cause of death. Changes in the characteristics of the population as it ages and the swelling numbers of patients with immunocompromising conditions have increased the number of individuals at risk. An expanded variety of emerging pathogens likewise provides challenges for the microbiology laboratory.

This article is for Medical Students & Professionals

This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Pneumonia article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.

In this article:
The laboratory in diagnosing chest infections
MCQ: clinical scenario
MCQ: answer
MCQ: explanation

The laboratory in diagnosing chest infections

The role of the microbiology laboratory in the diagnosis of community-acquired pneumonia (CAP) remains controversial. Limitations of diagnostic tests have led to the development of guidelines for empirical treatment approaches. Less controversial is the need to establish an aetiology in the hospitalized patient and the immunocompromised host with lower respiratory tract infection.

MCQ: clinical scenario

Sputum sample was collected from a 60 year old man suffering from a purulent cough and fever. The man was later found to have lobar pneumonia. The sputum was subjected to a gram staining technique. Gram positive, capsulated, flame shaped diplococci were seen when the stained smear was examined under the microscope. Sputum culture on blood agar revealed dome shape centrally umblicated colonies with evidence of alpha hemolysis.

The organism causing the infection is likely to be:

a) Staphylococcus aureus
b) Klebsiella pneumoniae
c) Streptococcus pneumoniae
d) Corynebacterium diptheriae
e) Mycoplasma pneumoniae

MCQ: answer

The correct answer is C

MCQ: explanation

Streptococcus pneumoniae is a gram-positive flame shaped organism and is seen characteristically as diplococci (i.e. is in pairs) and capsulated. The organism grows on blood agar to form dome shaped centrally umblicated colonies and produces alpha hemolysis of the RBCs.

Staphylococcus aureus is not the correct answer since it is typically seen in clusters under the microscope when gram stained and also its colonies are raised and not dome shaped. It produces beta hemolysis on blood agar rather than alpha hemolysis.

Klebsiella pneumoniae is also unlikely since it is a gram negative rather than gram positive organism. It will be seen under the microscope as rods rather then cocci.

Corynebacterium diphtheriae only causes an upper respiratory tract infection. It does not cause pneumonia so it cannot be brought up in the sputum.

Mycoplasma pneumoniae can cause a pneumonia but is incorrect since it does not take up gram stain and also it does not grow on blood agar.

Reference(s)
1). Karen C. Carroll: Laboratory Diagnosis of Lower Respiratory Tract Infections: Controversy and Conundrums J Clin Microbiol. 2002 Sep; 40(9): 3115–3120. doi: 10.1128/JCM.40.9.3115-3120.2002