Blood agar with alpha hemolytic colony: Introduction, Possible pathogen and its details

Blood agar


5% sheep blood agar(BAP) is with growth of Streptococcus pneumoniae showing alpha (α) haemolytic colony as shown above picture. incomplete (α) hemolysis: Streptococcus pneumoniae, Streptococcus salivarius, viridans are referred to collectively as viridans streptococci, a name derived from viridis (Latin for “green”), referring to the green pigment formed by the partial, α-hemolysis of blood agar. Encapsulated, virulent strains of S. pneumoniae often forming highly mucoid, glistening colonies (production of capsular polysaccharide) surrounded by a zone of α -hemolysis. S. pneumoniae  can also be isolated from above organisms being this sensitive towards the optochin and bile solubility test positive.

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.

Bile solubility test for S. pneumoniae

The bile (sodium deoxycholate) solubility test distinguishes S. pneumoniae from all other alpha-hemolytic (viridans) streptococci. It  is bile soluble
whereas all other alpha-hemolytic streptococci are bile resistant.

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


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.


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