Bacteria are everywhere. An estimated five nonillion (5×1030) bacteria exist on Earth, a collective biomass that is greater than all plants and animals combined. The intestinal tract alone is home to 100,000,000,000,000(1014) bacteria, which is more than ten times the number of the body’s own cells.1 The diverse ecosystem in the average adult colon consists of colonies of roughly 1,000 – 1,200 different species of these single-cell organisms that do not contain a nucleus, and are much smaller than human cells.2

Interestingly, there is fluctuation in species diversity from person to person, and even within the same individual over his or her lifespan. Despite considerable scientific research, questions remain about what influences bacteria growth, and the potential impact they have on human health. Such factors shown to affect the number and variety of bacteria in the digestive tract include:

  • birth delivery method
  • diet and lifestyle
  • the presence of disease
  • the use of certain drugs (especially antibiotics)
  • geographic location of residence
  • nationality, and
  • normal physiological changes associated with age.

Science has even shown that individuals who live together are more likely to have similar gut microorganism ecosystems (microbiota).

Over time, our symbiotic relationship with bacteria has become so intricate that scientists still do not understand its full complexity. At the basic level, bacteria are able to live and grow inside of us by eating unabsorbed food residues (prebiotics) in our large intestine, which hosts the largest concentration of bacteria in the body. Stool is at least 60% bacteria.

As their hosts, we benefit from the bacteria because they help with digestion of carbohydrates (a chemical process known as fermentation) and the synthesis of essential vitamins and amino acids such as vitamin K and biotin. Large colonies of good bacteria (probiotics) prevent the overgrowth of bad (pathogenic) bacteria that can cause disease.


Probiotics are living organisms that provide a health benefit to the human host when ingested in adequate amounts.

Prebiotics are non-digestible food components, which can feed colonies of good bacteria, so they can thrive and beneficially affect the human host.


Balancing the Good and the Bad

To keep a balance between the good and the bad bacteria, the body’s immune system maintains a constant low-grade inflammation in the gut. This allows a strong immune response toward harmful disease-causing bacteria while remaining tolerant to beneficial bacteria. Certain diseases and disorders characterized by intestinal inflammation, such as allergies, diverticulitis, inflammatory bowel disease, and metabolic syndrome, can cause a breakdown in this equilibrium. In older people there is also an overall decline in the functionality of the immune system, resulting in a chronic, low-grade inflammatory status called ‘inflamm-aging’ throughout the body.1 It remains unclear whether an increase in the growth of pathogenic bacteria in older individuals promotes inflammaging, or if inflammaging itself encourages the growth of these bad bacteria. Nutritional deficiencies in the aged population, as well as tissue weakness, may also elicit a stronger immune response to symbiotic bacteria in the gut.


Population Shift

As of July 2009, there were 4.7 million seniors aged 65 and older living in Canada, accounting for 13.9% of the population.3 The most recent projections suggest this number will rise to 25% of the population by the end of the 2030s.3 As life expectancy continues to climb, more and more Canadians are living into their ninth and tenth decade. There were 6,000 Canadians aged 100 and older in 2009, and the number of centenarians could be greater than 17,000 by the beginning of the 2030s.3 Deterioration of health often comes with aging. Seniors frequently experience problems with their digestive tracts, whether it be motility issues (difficulty chewing or swallowing, and constipation), nutrient deficiencies caused by decreased absorption of certain nutrients (commonly calcium, iron, and vitamin B12), or the presence of disease (65% of those who are older than 85 years of age have diverticular disease). As our bodies age, the bacterial ecosystem in our intestines also undergoes changes, although the details of these transformations are still not fully understood.


Microbiota Through the Ages

A unique study recently published in the Public Library of Science investigatedthe age-related differences in both the gut microorganisms and the inflammatory status of 84 adults in a restricted geographical area in Italy.4 In contrast to previous studies, these researchers did not simply lump the subjects into vague young and old adult categories; they evaluated the entire adult lifespan by dividing the subjects into three specific groups: 20 young adults (25-40 years old), 43 elderly people (59-78 years old), and 21 centenarians (99-104 years old). Half of the elderly group were offspring of the centenarians and the researchers analysed them as a fourth group.

Interestingly, the results showed that changes in the gut ecosystem do not follow a linear relationship with age, but instead the difference between the gut microbiota of young adults and the elderly is very small compared to that observed between centenarians and adults 65 years of age and younger. The findings of this study suggest relative stability of the gut ecosystem until approximately age 65, after which the aging process begins to exert an effect.

Two major groups of bacteria, Bacteroidetes and Firmicutes, were the most prevalent residents of the gut across all the age groups, although there were significant changes in the relative proportion of the subgroups of Firmicutes in the centenarians. Firmicutes produce butyrate, a short chain fatty acid that serves as a major energy source for the body’s intestinal cells, and offers a protective role against inflammatory bowel disease. The researchers found lower quantities of butyrate producers in centenarians than in other age groups. In contrast, centenarians’ guts had a higher concentration of Proteobacteria, another bacterial group that, under some circumstances (e.g., inflammation), can induce disease. With regard to inflammation, the proportion of centenarians showing a high inflammation score was significantly greater than in the other age groups.


How to be a Good Host Organism

Increased understanding of the impact of the gut microbiota on human health has resulted in attempts to manipulate its composition, mainly by the use of probiotics and prebiotics. It is important to remember that each mechanism of action for probiotics is strain-dependent, meaning not all varieties will produce the same results.5 Another important factor to consider is that while there are now guidelines from Health Canada concerning the quality of probiotics sold in pharmaceutical dosage forms (e.g., tablets, capsules), there are currently no regulations concerning probiotic bacteria in food.

For probiotics to exert a benefit on the host’s health, they have to remain alive through food manufacturing and ingestion. Although foods containing probiotics most likely offer other nutritional benefits, it is unlikely they will supply sufficient probiotics to cause an effect. Typically, probiotic supplements work better than do probiotics found in food products, simply due to the quantity and stability of the organisms.

The human digestive tract does change with age, but there are strategies to promote balance of the intestinal bacteria. Consuming a nutritious diet, such as that outlined in Canada’s Food Guide, helps to prevent micronutrient deficiencies, which are common in the elderly. A registered dietitian can help create a meal plan to ensure your diet is adequate and minimize any drug-nutrient interactions. Although current scientific literature is limited, the use of probiotics may also be beneficial in preventing the disruption of the gut environment, especially after treatment with antibiotics.

First published in the Inside Tract® newsletter issue 179 – 2011
1. Rambaud JC et al. Gut Microflora Digestive Physiology and Pathology. Paris: John Libbey Eurotext; 2006.
2. Biagi E et al. Ageing of the human metaorganism: the microbial counterpart. Age. Published online: 24 February 2011.Available from Accessed 2011-08-31
3. Statistics Canada. Seniors page. Available at: Accessed September 12, 2011.
4. Biagi E et al. Through Ageing, and Beyond: Gut Microbiota and Inflammatory Status in Seniors and Centenarians. PLoS One. 2010;5(5):e10667. Available from Accessed 2011-08-31.
5. Saulnier D et al. Mechanisms of probiosis and prebiosis: considerations for enhanced functional foods. Current Opinion in Biotechnology. Published online: 24 February 2009. Available from Accessed 2011-09-14.