Staphylococci are alpha-haemolytic

General microbiology
Nature of microbes
Bacterial structure
Bacterial growth
Bacterial cultivation


False. Staphylococci are not alpha-haemolytic but most strains of Staph. aureus produce a membrane damaging haemolysin which causes haemolysis similar to that produced by Strep pyogenes. These haemolysins are not produced by strains of Staph. epidermidis which is a normal skin commensal. Staph. aureus produces four types of haemolysin which are also exotoxins: alpha, beta, gamma and delta. Do not confuse the production of alpha haemolysin with alpha-haemolysis.


When some bacteria are grown on a culture medium containing blood they produce a change in the appearance of the blood in the area around the colony. This is known as haemolysis and two particular types have been recorded. Alpha-haemolysis is recognised as a slight clearing of the blood around the colony coupled with a slight green colouration.

Beta-haemolysis is a distinct, sharp-edged clearing of the blood around a colony.

For a fuller explanation of haemolysis see below
see also the Kiss Plates tutorial

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Fuller explanation

Here are 3 pictures of different bacteria growing on growth medium containing horse blood. You should be able to make out individual small colonies.

This is Streptococcus pyogenes. The bacteria have secreted something into the growth media which has lysed the red blood cells and broken down the haemoglobin. This has caused a clearing of the Blood Agar and is known as beta-haemolysis . Beta-haemolysis is characterised by a very definite clearing, to almost complete transparency, of the Blood Agar and is caused by the bacteria secreting haemolysins which lyse red blood cell walls. Haemolysins secreted by streptococci are known as "streptolysins".

This picture shows the other type of haemolysis known as alpha-haemolysis and is characterised by the Blood Agar turning a rather subtle shade of green. This is very difficult to show in a photograph because of inherent contrast problems between red and green, especially when the green is not so bright.

In alpha-haemolysis there is no really definite clearing of the blood. Alpha-haemolysis is caused by certain bacteria such as many of the oral streptococci, eg Streptococcus sanguis, secreting a metabolic by-product called hydrogen peroxide which bleaches the haemoglobin. In fact this is how "viridans" streptococci got their name (viridis is Latin for green).You will appreciate that this is a very different mechanism from that of beta-haemolysis which is caused by haemolysins and which is related to the virulence of the microbe.

Just to be really confusing, there has been a third type of haemolysis described called gamma-haemolysis. In this picture you should be able to make out tiny colonies at the lower edge. Again this is difficult to show clearly for the reasons already mentioned. Gamma-haemolysis is characterised by no change in the blood of Blood Agar. In other words, gamma-haemolysis is no haemolysis. If this leaves you wondering why someone bothered to give it a name, rest assured you are not alone.







Concepts to Grasp

Beta-haemolysis is caused by haemolysins (eg streptolysin) and is related to pathogenicity. Alpha-haemolysis is caused by a metabolic by-product (hydrogen peroxide) which bleaches haemoglobin green and gamma-haemolysis is no-haemolysis.

Terms to Learn

selective media
indicator media
lactose fermenter
non lactose fermenter

Concepts to Grasp

Selective media are a useful and commonly used means of isolating bacteria of interest from a biological sample. However, by creating conditions which are at best inhibitory and sometimes actually toxic to some microbes you there is always the risk of inhibiting the growth, albeit to a lesser extent, of the microbe you are interested in. For this reason you have to be careful about using selective media when using it to measure the actual numbers of a particular microbe in a sample.

Concepts to Grasp

Indicator media are called "Indicator" because they differentiate or indicate different species of bacteria, usually because of some difference in their phenotype. They are not called "Indicator" because they contain a pH indicator. Bit confusing that, so it's as well it is cleared up. Remember also that there are many many different examples. MacConkey Agar has been used because it is a good and useful example of a growth medium which is both a selective and ani indicator medium.


Streptococcus pyogenes




Streptococcus sanguis








Streptococcus salivarius

Selective media inhibit bacterial growth

True. Selective media are used to help in the isolation of particular microbes from a mixture containing many different kinds.

They do this by inhibiting the growth of the microbes which are not of interest.

Example 1

SALT AGAR contains 6% by weight of sodium chloride. Such a high concentration of salt inhibits most bacteria. Staphylococci which are normal commensals of the skin are resistant to it and are, therefore, selected for when a mixture of microbes is inoculated onto Salt Agar.

Why should staphylococci be resistant to such high concentrations of salt? The reason is that sweat is salty and when it dries on skin it leaves large deposits of salt behind. If staphylococci were not resistant to it they would not be able to colonise the skin.

Example 2

MACCONKEY AGAR contains bile salts which are toxic to many species of microbes except those which normally inhabit the gut (Enterobacteria). Microbes living in the gut would have to be resistant to bile salts otherwise thay wouldn't be able to survive there!

MacConkey Agar is very extensively used by medical microbiologists to isolate pathogens such as salmonellae and the various pathogenic strains of Escherichia coli.

Example 3

TYC AGAR contains 5% by weight of sucrose and the amino acid cysteine which select for Streptococcus mutans. Sometimes the antibiotic, bacitracin, is also added.

Most oral microbes except Streptococcus mutans are sensitive to bacitracin and so are inhibited. Resistance to particular antibiotics is widely exploited in selective media.

Lactose fermenting bacteria turn MacConkey Agar pink

True. MacConkey Agar contains bile salts to inhibit non-enteric bacteria, a mixture of peptides, as a source of nitrogen, various minerals and vitamins needed to support bacterial growth and a pH indicator called Neutral Red which is pink in acid condiditons and yellow in alkali. It also contains the sugar lactose as the only available fermentable substrate.

The ability to ferment lactose is used to distinguish between the normally non-pathogenic gut commensal Escherichia coli and the pathogenic Salmonella spp.

When bacteria which can ferment lactose are grown on MacConkey Agar the colonies turn the media in their immediate vicinity pink because of the acid by-products of lactose fermentation. On the other hand, non-lactose fermenters, which are known in the trade as NLFs, have to look elsewhere for their energy and the only available source is the amino acids in the peptides. When amino acids are metabolised the by-product is ammonia which is basic and turns the pH indicator yellow . Clever stuff!

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The photograph on the right shows a MacConkey Agar plate which has been inoculated with Salmonella typhimurium and Escherichia coli. The colonies in the top half of the plate are E.coli and those in the bottom half are S.typhimurium (NLFs).

Because MacConkey Agar allows different species to be distinguished on the plate it is known as an "Indicator Medium" and practically, this is very useful.

To discover if someone is infected with a species of Salmonella all a microbiologist has to do is to inoculate some MacConkey Agar with a sample of the patients faeces, incubate it and look for any non-lactose-fermenting colonies which usually stick out like a sore thumb because they are yellow.

It's not quite as simple as that, of course, because the NLFs still have to be confirmed as Salmonella, usually using serological tests, but most of the heavy work has been done very simply and quickly.


If this all sounds like a lot of trouble just to isolate a few Salmonella bacteria, imagine how difficult it would be without indicator media like MacConkey Agar. Escherichia and Salmonella colonies look pretty much the same which means that many hundreds of colonies would have to be tested serologically before a clinician could be sure the patient did not have an infection. With MacConkey a quick eyeball of the plate usually reveals any NLFs which can be picked off and tested before morning coffee!


MacConkey Agar is an Selective Medium

True. MacConkey Agar contains bile salts which inhibit the growth of non-enteric bacteria. This makes it useful to isolate bacteria which normally inhabit the gut. MacConkey Agar is both a selective and an indicator medium. See above.

You can make MacConkey Agar even more selective by including a dye called "Brilliant Green" which inhibits E.coli, Proteus spp and many other enteric bacteria. This makes it very useful for isolating Salmonella spp when they are present in only very low numbers.


Photolithotrophs are fastidious microorganisms

False. Fastidious organisms are sometimes called "nutritionally exacting" and depend on the presence of certain growth factors. Exactly which growth factors depends to a large extent on the species but many are normally supplied by blood eg haemin. What the bacteria are after when they show a requirement for haemin, by the way, is usually the iron that's locked up in it. Blood also supplies lots of other growth factors (vitamins).

Very much less exacting in their growth requirements are prototrophs which require very little in the way of growth supplements and are quite happy feeding solely on inorganic matter. This does not mean that they do not cause disease.

Pseudomonas aeruginosa has very simple growth requirements and lives reasonably happily up the spouts of 3 in 1 syringes on dental clinics. Once inside the body, however, it can cause some pretty nasty abscesses.

The ultimate prototrophic bacteria are the photolithotrophs which can fix atmospheric carbon dioxide and get their energy from the sun. Examples are the cyanobacteria (also known as Blue-green algae....yes, they are, in fact, bacteria not plants). Between these very basic prototrophs and the most fastidious organotroph there is a complex array of microorganisms exhibiting a seemingly infinite variety of nutritional demands.