Jill: alpha, beta and gamma haemolysis

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http://www.nuigalway.ie/bac/resp_appendix.htm

Classification of Streptococci

When people began to study streptococci it was noticed that whne

grown of agar plates containing blood, three possible effects were

noted on the blood.

Greenish discolouration of the media around the bacterial colony

(alpha-haemolysis)

Complete clearing of the red cells around the colony so that light

could pass through the agar (beta-haemolysis)

No effect on the red cells (non-haemolytic or gamma-haemolysis).

Looking at the alpha-haemolytic group of organisms it was noted that

one type seemed to be frequently associated with lobar pneumonia,

this was called S. pneumoniae. The others did not seem to have such

an obvious common disease association and were at that time all

lumped together as Streptococcus viridans. Those alpha-haemolytic

streptococci other than S. pneumoniae are nor called " viridans group

streptococci " .

Lancefield studied ways to sub-divide the beta-haemolytic

streptococci and developed a system based on detection of cell wall

carbohydrate antigens. The groups were labelled with capital letters

A, B, C etc. The beta-haemolytic streptococci with a group A antigen

were noted to be associated with severe pharyngitis and with

subsequent Rheumatic Fever. Dr. Lancefields classification is the

basis of the expression " Group A, beta-haemolytic streptococci " .

Over the years the systems for classification of microorganisms have

become very much more sophisticated using many biochemical

parameters and now also molecular parameters. These kind of

parameters are now the definitive basis of grouping bacteraia into

species.

The beta-haemolytic streptococci with a Group A antigen, recognised

by Lancefield essentially all belong to the species we now call S.

pyogenes so that the term Group A beta-haemolytic streptococci and

S. pyogenes are synonymous. Likewise Group B, beta-haemolytic

streptococcus is synonymous with S. agalactiae.

For may other species of streptococci the application of the older

systems of classification results in great confusion because some

species, for example Streptococcus anginosus include some strains

that are alpha-haemolytic, (viridans group) some are beta-

haemolytic, some express Lancefields group C antigen and some

express Lancefields group F antigen. The older classification

systems (haemolysis and Lancefields antigen) however are cheap, fast

and provide a " good enough " level of classification for may clinical

purposes and so the terminology of the older classification system

remains current in clinical practice.

> > > > What is the zone? IE does it destroy red blood cells?