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Wikipedia says:

When alpha hemolysis (α-hemolysis) is present, the agar under the colony is dark and greenish. Streptococcus pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci) display alpha hemolysis. This is sometimes called green hemolysis because of the color change in the agar. Other synonymous terms are incomplete hemolysis and partial hemolysis. Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green methemoglobin.

From this it seems that in alpha hemolysis , it's not the lysis that's the defining event rather the chemical change of the hemoglobin molecule.

Wikipedia states:

Beta hemolysis (β-hemolysis), sometimes called complete hemolysis, is a complete lysis of red cells in the media around and under the colonies: the area appears lightened (yellow) and transparent.

I am unsure of the reason for color change (blood agar plate) in beta hemolysis, my guess it's due to diffusion as I have found no mention of chemical change in hemoglobin molecule anywhere (yet). So far my position is chemical change in hemoglobin without lysis is alpha hemolysis and lysis followed by diffusion of the hemoglobin molecule is beta hemolysis. Problem is my support is shaky at best.

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Alphahemolysis simply oxidizes hemoglobin to methemoglobin and causes a color change on blood-agar. The RBC membrane is left in tact as far as I know. If you look at a picture of the 3 types of hemolysis, you will see that there is no zone of clearing in the alphahemolysis plate. Compare that to the obvious zone of clearance observed on the betahemolysis plate, in which complete lysis occurs. Here is a nice picture

Hemolysis

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Anaerobic condition and CO production

Hemolytic and nonhemolytic bacteria were incubated aerobically and anaerobically with the following substrates: erythrocytes, hemoglobin, myoglobin, cytochrome c, hematin, iron hematoporphyrin, copper hematoporphyrin, protoporphyrin, and bilirubin. After 18 hr at 37 C the evolved CO was measured by gas chromatography. None of the bacteria formed CO anaerobically.

CO and alpha hemolysis

Because our two strains of S. mitis differed with regard to H202 production and hemolysis, we also compared an H20 2-producing strain of Streptococcus faecalis with a peroxide-negative mutant, kindly supplied by Beulah Gray Holmes. In both cases the H20 2-producing organisms generated alpha hemolysis and formed CO from heme compounds, whereas the peroxide-negative bacteria did not demonstrate alpha hemolysis or CO formation. If, in alpha hemolysis, hemoglobin is converted into a green pigment without lysis of the red cell membrane, then the failure of this pigment to diffuse away from the colony would be explained.

Color change in beta-hemolysis can be explained with diffusion of content from lysed erythrocyte.

All six strains of bacteria that formed hemolytic zones on blood agar plates also produced CO from erythrocytes, and conversely the four strains that were nonhemolytic did not make CO. Despite this concordance, it appears that the formation of beta-hemolytic zones around colonies grown on blood agar plates does not depend on hemoglobin catabolism. Thus, prominent beta-hemolytic zones were formed under anaerobic conditions that prevented CO formation. Also, hemolysis but not CO formation was produced by sterile filtrates of the extracellular products from beta-hemolytic bacteria.

Clear zones resembling beta hemolysis can be produced by simply freeze-thawing a spot on a blood agar plate and then waiting a day for hemoglobin to diffuse away from the lysed cells. Although hemolytic B. cereus grown on aerobic hemoglobin agar plates gradually produced diffuse color changes, the discrete zones observed on plates with sheep erythrocytes were absent.

http://jb.asm.org/content/112/3/1310.full.pdf+html

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