Selection Pressure

Influencing Bacterial Development - Oral Biofilms

The idea of selection pressure relates to the fact that there are certain “environmental factors”, that will affect what type of bacterial organisms or fungi, survive or not, within the mouth.

We see this phenomenon within our garden on a daily basis. Some plants need direct sunlight, an abundance of water and warm temperatures to survive, while others will flourish in a dry climate. Yet others will grow happily in a dark, cold, moist environment.

In the mouth we have the same concept of selection pressure, but the factors are different. In the oral cavity we have pressures such as saliva quantity, quality, and pH, diet, professional and home cleaning, fluoride and antimicrobial products, etc. It is these selection pressures that will determine the type of bacteria that grows in our mouths and more importantly within the biofilm.

Dental Caries a New Understanding

Current research has shown that of all the selection pressures we have in the mouth, pH is the most significant.

Earlier research demonstrated that sugar availability was the key selection pressure for the overpopulation of cariogenic bacteria. Newer more sophisticated research has expanded on this concept and has demonstrated that there is a pressure even greater than the presence of sugar, it is the resulting drop in pH that occurs when carbohydrates are ingested. Prolonged exposure and or frequent low pH experiences will drive the bacterial population within the biofilm towards cariogenic strains.

Collectively these studies have shown conclusively that it was the low pH generated from sugar metabolism rather than sugar availability that led to the breakdown of microbial homeostasis in dental plaque. This finding has important implications for caries control and prevention; the data suggests that the selection of cariogenic bacteria could be prevented if the pH changes following sugar metabolism could be reduced.

Dental plaque as a biofilm and a microbial community – implications for health and disease, Philip D Marsh, BMC Oral Health 2006, (15 June 2006)

The following quote from the above referenced article outlines how we can in turn utilize pH selection pressure to drive the population of bacteria with the biofilm back toward health.

“A key element of the ecological plaque hypothesis is that disease can be prevented not only by targeting the putative pathogens directly, e.g. by antimicrobial or anti-adhesive strategies, but also by interfering with the selection pressures responsible for their enrichment.”