Pneumococcus : Introduction, morphology, pathogenecity, laboratory diagnosis and treatment

Pneumococcus in gram stain showing gram positive diplococci

Introduction of Pneumococcus

Pneumococcus or Streptococcus pneumoniae in Gram stain showing Gram positive diplococci or Gram positive cocci in pairs as shown above picture.

Scientific classification of Pneumococcus

Domain: Bacteria

Phylum: Firmicutes

Class: Bacilli

Order: Lactobacillales

Family: Streptococcaceae

Genus: Streptococcus

Species: S. pneumoniae

Binomial name

Streptococcus pneumoniae

It is a Gram-positive elongated cocci ususally found in pairs i.e diplococci or lanceolate , alpha-hemolytic on blood agar  facultative anaerobic , non-sporulated, non-motile but capsulated member of the genus Streptococcus.  It resides asymptomatically in healthy carriers typically colonizing the respiratory tract, sinuses, and nasal cavity.

Pathogenicity of Pneumococcus 

Pneumococcus is the most common cause of bloodstream infections, pneumonia, meningitis, and middle ear infections ( Otitis media) in young children. Severe infections can occur in the elderly and those already in
poor health or immunosuppressed. Risk of infection is increased following splenectomy. In tropical and developing countries, S. pneumoniae is a major pathogen, responsible for acute life-threatening pneumonia and bacteraemia in those co-infected with HIV The invasive pneumococcal diseases include bronchitis, rhinitis, acute sinusitis, otitis media, conjunctivitis, meningitis, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess.Childhood pneumonia and serious infections are
in patients with sickle cell disease.

Serotypes: Over 80 capsular serotypes where as  less than 15 serotypes are responsible for most infections.

Laboratory Diagnosis of Pneumococcus

Specimens: Depending on the site of infection, specimens may
include sputum, exudate, blood for culture, and cerebrospinal fluid.

Gram stain of specimen 
Gram positive elongated (lanceolate) diplococci and may show evidence of capsule but no evidence of spore.

Blood agar: Following overnight incubation. S. pneumoniae forms translucent or mucoid colonies, 1–2 mm in diameter. In young cultures the colonies are raised but later become flattened with raised edges, giving them a ringed appearance i.e ‘draughtsman’. Strains of some serotypes (e.g. serotype 3) produce large mucoid colonies. Pneumococci show alpha-haemolysis, i.e. colonies are surrounded by an area of partial haemolysis with a green-brown discoloration in the medium (reduced haemoglobin)

Optochin sensitivity
Pneumococci are sensitive to optochin (ethylhydrocupreine
hydrochloride). Placing a disc (5 µg) on a primary sputum culture and culturing the plate aerobically (not in CO2) can help to provide a rapid presumptive identification of pneumococcus.

Agglutination test: A latex reagent to detect S. pneumoniae capsular antigen.

Treatment 

Useful antibacterial drugs are ampicillin, amoxycillin,  erythromycin, co-trimoxazole, doxycycline, ofloxacin, vacomycin, chloramphenicol (nitrofurantoin in case of urine ), teicoplanin, linezolid. Penicillin-resistant
strains are becoming an increasing problem.

Key notes

  1. Streptococcus pneumoniae shows alpha haemolysis on blood agar under aerobic conditions where as beta-hemolysis  under anaerobic condition
  2. Absence of capsules in Gram stain  if  organisms from cultures.
  3. It also forms short chains in Gram stain particularly following culture but lacking such features in specimens.

References

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Hand book Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Text book of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  7.  Text book of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  8. Topley & Wilsons Principle of Bacteriology, Virology and immunology Vol I, II, III, IV & V. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
  9. Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  10. District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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