Which one of the following microorganisms is not involved with the production of weak organic acids associated with enamel demineralisation?

Staphylococcus aureus is the correct choice as it is not typically associated with the production of weak organic acids that contribute to enamel demineralization. This bacterium is primarily known for its role in various infections and does not play a significant role in dental caries or the acidic environment that leads to enamel breakdown.

In the context of dental health, Lactobacillus, Streptococcus mutans, and Streptococcus sanguis are all bacteria commonly found in the oral cavity that are associated with tooth decay. Streptococcus mutans, in particular, is well-known for its ability to produce lactic acid from the fermentation of sugars, which directly contributes to the demineralization of tooth enamel. Lactobacillus also produces lactic acid and can further contribute to caries progression once the enamel has already been compromised. Streptococcus sanguis is less virulent than Streptococcus mutans but still participates in the formation of dental biofilms and can produce acids as well.

Thus, Staphylococcus aureus does not belong to the group of microorganisms typically involved in enamel demineralization, making it the best answer to this question.