SIR — Your News Feature 'The inside story' (Nature 453, 578–580; 2008) highlights the potential importance of the gut microbiome in understanding health. As you discuss, human profiles differ in obese and lean individuals and are being scrutinized for their possible influence on weight.

A factor you don't mention is the routine administration over some 50 years of antibiotics and probiotics as growth promoters to farm animals. Use of antibiotics for this purpose was banned in 2005 in the European Union, but it continues in many places (including the United States).

Compounds with antibiotic activity and bacteria with probiotic activity have been widely tested as growth promoters (see, for example, M. Khan et al. Brit. Poultry Sci. 48, 732–735; 2007). The probiotics used in agricultural industries are mainly Lactobacillus and Bifidobacterium species.

Humans, particularly children, have been taking these same probiotics for many years, especially in fermented diary products. They also frequently take broad-spectrum antibiotics.

Given the current obesity pandemic among humans and the impact of antimicrobials on weight gain in animals such as poultry and pigs, there may be a case for evaluating the effects of routinely adding bacteria to our food and of long-term consumption of antibiotics.

See also article Human microbiome: hype or false modesty?