Kiss-plates are a good way of demonstrating that environmental factors exert very strong selective pressures on bacterial populations. Coincidentally they also demonstrate the specific nature of the oral bacterial population.

The bacteria which live on the skin and in the mouth are, mostly, fastidious. So the growth medium needs to be highly nutritious. For this reason we use one called Brain-Heart Infusion Agar with added yeast extract to provide extra vitamins and minerals. Although not absolutely necessary, some sterile horse blood is also added. This gives a little extra boost to the bacteria but the main reason it is added is because some of the bacteria which will grow are either alpha- or beta-haemolytic. These properties have different effects on blood which enables us to demonstrate their different distributions.

This is a typical Kiss-Plate after incubation
The Blood Agar does not appear its normal deep red colour because strong light is shining through it so that a good picture can be taken.

Below the picture is a map of the plate showing the main areas of bacterial growth.

Area "A" is where the nose touched the agar

Area "B" is where the lips touched the agar

Area "C" is where the inner lips and tongue touched the agar.

The bacteria growing here are mostly staphylococci. Most of these will be Staphylococcus epidermidis. These produce quite large white colonies and do not react in any visible way with the blood. Sometimes you may see very similar, bright yellow, golden-coloured, colonies which will probably be Staphylococcus aureus.

On the left side of region "A" above some colonies have produced a clear zone in the agar. This is known as beta-haemolysis. The bacteria have produced something called a "haemolysin" which has leached out of the colony and broken down the blood. It is not possible to identify these any further without extensive tests but it is unlikely that they are staphylococci.

The bacteria growing in region "C" are obviously very different.

Although it can not be seen in the photograph, the area contains so many bacterial colonies that they have merged together to give what is known as "confluent growth" This means that the bacterial density on the inner part of the lips and the tongue is very much higher than on the outer lips and the skin of the nose.

Where it is possible to see individual colonies they are much smaller than the ones in areas "A" and "B" and are light grey in colour. This is typical of streptococci.

This "greening" effect on blood was noticed by early bacteriologists who called the bacteria Streptococcus viridans because the Latin word for green is "viridis".

Later it was discovered that more than one species of alpha-haemolytic streptococus lived in the mouth so they were given the collective name "Viridans Streptococci". This is still in common use although strictly incorrect because not all the streptococcal species living in the mouth are alpha-haemolytic. The best term to use is "oral streptococci"

he most striking difference is in the reaction these streptococci have had on the blood which has been turned green. This reaction is known as alpha-haemolysis. Actually it isn't haemolysis at all because the haemoglobin has not been lysed. Instead, it has been bleached by the action of hydrogen peroxide produced as a metabolic waste product by some of the species of streptococci living in the mouth

The population of bacteria in the mouth is different to that of the skin even though the two sites are adjacent and cross-contamination must be very frequent. Also, the mouth must be continually exposed to contamination by other bacteria such as those living in the gut and soil yet these do not thrive sufficiently to affect the overall balance of the population.

This control is exerted primarily by saliva but other factors such as unique sites for attachment, the presence of gingival fluid and special gaseous condition will also be important.

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