|The clinical microbiology laboratory utilizes a number of
different media for the growth and identification of bacteria. Some of these
media contain blood or blood products to enhance the growth of potential
pathogens. In addition, the presence of blood in these media allows for
discernment of organisms based on their ability to alter or destroy the red
blood cells and create a color change in the medium. This process is known
as hemolysis and it can be described as alpha- or beta-hemolysis depending
on the effect on the red blood cells. In alpha-hemolysis, colonies are
surrounded by a zone of intact erythrocytes that have a greenish color.
Bacterial peroxide production, which results in the reduction of hemoglobin
to methemoglobin, is generally responsible for this appearance. In
beta-hemolysis, colonies are surrounded by a clear zone in which few or no
intact erythrocytes can be found. Beta-hemolysis is caused by the action of
hemolysins, erythrocyte-lysing enzymes, produced by the bacteria. In the
photograph below, note how the cross-hatched pattern of the fluorescent
panel in the ceiling is visible -- but only in the areas of hemolysis. In
the photos below, the plate on the left shows beta-hemolysis and the plate
on the right shows alpha-hemolysis.
Perhaps the most useful distinction among species of hemolysis-causing bacteria are in the genus, Streptococcus. Group A streptococci (S. pyogenes) are beta-hemolytic. They produce two hemolysins: streptolysin O and streptolysin S. Streptolysin O is hemolytically active in the reduced stated (when there are available –SH groups), but is rapidly inactivated in the presence of oxygen. It is thus most often responsible for hemolysis observed when growth is found in deep cuts made into the agar (where oxygen is scarce). Streptolysin O is antigenic. Antistreptolysin O is an antibody that appears in humans after infection with Group A streptococci. It blocks hemolysis produced by streptolysin O, and can be quantitatively measured in a person suspected of having recent infection with the organism (especially helpful in the evaluation of possible rheumatic fever). Streptolysin S is responsible for the hemolysis observed around colonies growing on the surface of blood agar plates. While it is not antigenic, it may be inhibited by a non-specific inhibitor this is frequently present in human and other animal sera. The presence of antistreptolysin O and non-specific inhibitors of streptolysin S may explain in part why hemolysis may be present on sheep blood, but not on human blood agar.
Because of these differences sometimes noted on different animal blood
agar preparations, the hemolytic reactions of beta-hemolytic streptococci on
sheep blood agar are in some parts of the world used as the standard for
defining hemolytic reactions of streptococci. Streptococci, however, are not
the only hemolytic bacteria. Some of the other bacteria that can produce
hemolytic reactions include: Staphylococcus aureus, Enterococcus spp.,
Bacillus cereus, Clostridium perfringens, Corynebacterium diphtheriae,
Listeria monocytogenes, and certain strains of E. coli and Pseudomonas
aeruginosa. Differences in patterns of hemolysis can also be observed on
different animal blood agars when some of these other bacteria are cultured.
For example, in a genomic study of various strains of Enterococcus faecium,
beta-hemolytic activity was observed on both sheep and human blood in only
one strain, while four strains showed beta-hemolysis on human blood, but not
on sheep blood, and the remainder of the strains studied showed no hemolysis
on either medium.
Brooks, G., Butel, J., & Ornston, L. (1995). Jawetz, Melnick, &
Chandar, A. et al. Pig and goat blood as substitutes for sheep blood in
Ryan, K. (Ed.). (1994). Sherris Medical microbiology: An introduction to
Vancanneyt, M. et al. Intraspecies genomic groups in Enterococcus faecium