Alpha-hemolytic Streptococcus are spherical bacteria that produce hemolysins capable of partially lysing (bursting) red blood cells. When grown on sheep blood agar, colonies of alpha-hemolytic Streptococcus are encircled by green areas of partial clearing where alpha hemolysis has occurred. 

Alpha-hemolytic Streptococcus may be pathogens, such as Streptococcus pneumoniae, or part of the normal flora of the upper respiratory tract ("viridans Strep", e.g. Streptococcus mitis). Streptococcus pneumoniae is the major cause of community-acquired bacterial pneumonia and the most frequent cause of otitis media (middle ear infections) and bacteremia in infants and children. Thus it is important to distinguish Streptococcus pneumoniae from other alpha-hemolytic streptococci that may be normal flora.  Two tests are commonly employed for distinguishing Streptococcus pneumoniae from the viridans strep it resembles: the optochin test and the bile solubility test.  

The Optochin Test 

Optochin (ethylhydrocupreine hydrochloride), a quinine derivative, has a detergent-like action and causes selective lysis of pneumococci. A sterile disk impregnated with optochin is placed on the first sector of an isolation plate before incubation. A zone of inhibition (area with no growth) of 14 mm. or more in diameter will be seen around the disk after incubation if the organism is Streptococcus pneumoniae. Other alpha-hemolytic streptococci are resistant to (not killed by) optochin. Their colonies will thus grow right up to the disk of optochin or have zones of inhibition less than 14 mm. in diameter.  

The Bile Solubility Test 

Bile will selectively lyse colonies of Streptococcus pneumoniae while other strep are immune to bile's activity. When a bile salt such as desoxycholate is added directly to Streptococcus pneumoniae growing on an agar plate or in a broth culture the bacteria will lyse and the area become clear. Other alpha-hemolytic streptococci are resistant to (not lysed by) bile and will stay visible or turbid (cloudy). 

1. optochin sensitivity	2. bile solubility