Are the large numbers of microorganisms found in the mouth cause for concern? Can someone explain this please. Does not it lead to tooth decay

Not so much. There's good bacteria, and there's bad bacteria, and there's neutral bacteria which is also good bacteria because they don't leave room for bad bacteria. As for tooth decay, it is due to two problems:

1) bacteria that generate a sticky layer of plaque for other bacteria to stick to

2) anaerobic fermentation of mostly sugars (and starches in your mouth also become sugars), which produces a localized acidic spot which dissolves some of the calcium from the tooth.

 

The tooth decay is countered because you have some calcium in the saliva to reverse this, and also with fluorine which is part of a mineral that does not dissolve as easily. I have heard suggestion of a bacteria-based solution to tooth decay, by replacing the plaque forming bacteria with something more benign.

 

But other than tooth decay and pathogenic bacteria that would actually cause disease, there's no cause for concern. But having harmless bacteria there leaves less room for harmful bacteria.

Mr Skeptic, have you ever considered a career in dentistry?

One of the most interesting things I learnt in biochemistry was with regard to anaerobic respiration in the context of tooth decay. If you have a build up of plaque on your tooth, made up of layers of bacterial cells, the bacteria closest to the tooth don't have access to oxygen. As a result of this they have to survive through anaerobic respiration, which produces lots of lactic acid and the acid gets excreted directly onto your tooth, dissolving the enamel and causing tooth decay/cavities.