Lab Diagnosis of Lower Respiratory Tract Infections

By Acharya Tankeshwar •  Updated: 05/04/22 •  3 min read

This post contains key points regarding laboratory diagnosis of lower respiratory tract infections  (LRTI) so will be helpful for the revision.

Streptococcus pnuemoniae: Gram positive diplococci

First morning expectorated sputum is always best for bacterial culture. Expectorated sputum has been the primary means of determining the causes of lower respiratory tract infections. Induced sputum, endotracheal or tracheostomy suction specimen, transtracheal aspirates are also used. Sometimes samples can be obtained using bronchoscopy e.g. bronchial washings, aspirates, bronchoalveolar lavage (BAL).

Direct visual examinations of Lower respiratory specimens: KOH preparation or periodic acid-schiff-stained smears for fungal element detection, Gram staining (for bacteria and yeasts), Ziehl Neelsen staining for Mycobacterium tuberculosis (or Kinyoun carbolfuchsin stain, auramine or auramine-rhodamine) is commonly used direct examination methods for the detections of pathogen causing lower respiratory tract infections. Selection of a particular method depends on the clinical presentation as per the request of a physician.

If a few colonies of Gram-negative rods are isolated from the sputum or throat swab of a hospitalized patient, further identification and anti-microbial susceptibility testing are of no clinical relevance, since neither procedure will have any effect on the treatment of the patient.

Culture of Respiratory tract sample: Common etiologic agents of lower respiratory tract infections will be isolated on routinely used media: 5% sheep blood agarMacConkey agar (for isolation and differentiation of gram-negative bacilli), and chocolate agar (for Haemophilus and Neisseria spp). For suspected cases of legionnaires’ disease, buffered charcoal-yeast extract agar (BCYE) and for tuberculosis, Lowenstein Jensen medium is used.

Streptococcus pneumoniae with optochin sensitivity
Optochin sensitive colonies of S. pneumoniae

List of Bacteria causing Respiratory Tract Infection

  1. Definitive pathogens
    1. Bordetella pertussis
    2. Chlamydia trachomatis
    3. Chlamydophila pneumonia
    4. Mycoplasma pneumoniae
    5. Mycobacterium tuberculosis
    6. Corynebacterium diphtheriae (toxin-producing)
    7. Legionella spp

B: Possible pathogens

  1. Streptococcus pneumoniae
  2. Staphylococcus aureus
  3. Neisseria meningitidis
  4. Haemophilus influenzae
  5. Haemophilus parainfluenzae
  6. Pseudomonas spp
  7. Enterobacteriaceae
  8. Moraxella catarrhalis

List of fungal pathogens causing lower respiratory tract infections

  1. Pneumocystis jiroveci (Pneumocystis carinii)
  2. Nocardia spp
  3. Histoplasma capsulatum
  4. Coccidioides immitis
  5. Cryptococcus neoformans (may also be recovered from patients without disease)
  6. Blastomyces dermatitidis

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

One response to “Lab Diagnosis of Lower Respiratory Tract Infections”

  1. kusuma says:

    culture and maintainnce of ATCC strain as QC in diagnosis center?

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