18.3.2 Soil
Food also supplies us with organisms from the soil, especially in farmers’ markets
where food has been subjected to little, if any, washing. But it turns out that soil
consumption (geophagy) is an evolved behaviour, and extremely ancient in an
evolutionary sense. It is likely that all vertebrates do it, especially in early life, and
the green iguana has been much studied in this context (Troyer 1984). More relevant
to humans is the fact that many primate species eat soil, including the closest
relatives of humans, gorillas, orangutans, and chimpanzees (Krishnamani and
Mahaney 2000; Sing and Sing 2010). Similarly, there is some archaeological
evidence that early hominids included soil in their diets (Clark 2001). So what
about modern humans? In the 1990s, it was noted that in western Kenya ~70% of
207 schoolchildren aged 5–18 years consumed soil on a daily basis (median ingestion of 28 g/day, range 8–108 g) (Geissler et al. 1997). This behaviour was more
prevalent in girls and continued into adolescence. It has been noted in numerous
other cultures, on all continents (Sing and Sing 2010; Geissler et al. 1997). Geophagy is also common in pregnancy, not only in undeveloped rural cultures, but also as
a manifestation of the psychological disorder “pica” in Westernised ones. Babies of
all cultures consume astonishing quantities of soil if adults do not intervene (Ngure
et al. 2013). Interestingly soil was also an integral part of food preparation in many
cultures, possibly because added soil adsorbed toxic glycoalkaloids present in foods
based on acorns or early varieties of potatoes (Sing and Sing 2010). However, this
does not explain the consumption of soil as an independent dietary component.
Some authors suggest it was a desperate act driven by starvation but this is clearly
not usually the case. Others suggest that it is an evolved behaviour that increases
intake of minerals. But since evolution turns the inevitable into a necessity it is
reasonable to suggest that whatever the initial motivation, intake of soil was an
important source of inevitable microbial intake.
18.3.3 Skin, Wounds, and Abrasions
In addition to intake of microorganisms via the airways and via the oral route, we can
assume that early humans encountered environmental microorganisms in cuts and
abrasions much more often than does modern urban man, and would not have had
the facilities for effective wound cleaning. Therefore, it is interesting that when
tested, probiotic Lactobacilli were shown to be active via a parenteral route (subcutaneous) in models of arthritis and colitis (Sheil et al. 2004). Similarly, subcutaneous
immunization with a saprophytic environmental soil mycobacterium blocked stressinduced colitis and behavioural changes, and partially inhibited stress-induced
changes to the microbiota. These effects were dependent upon induction of Treg
(Reber et al. 2016). Parenteral administration is not pursued by pharmaceutical
336
G. A. W. Rook
Food also supplies us with organisms from the soil, especially in farmers’ markets
where food has been subjected to little, if any, washing. But it turns out that soil
consumption (geophagy) is an evolved behaviour, and extremely ancient in an
evolutionary sense. It is likely that all vertebrates do it, especially in early life, and
the green iguana has been much studied in this context (Troyer 1984). More relevant
to humans is the fact that many primate species eat soil, including the closest
relatives of humans, gorillas, orangutans, and chimpanzees (Krishnamani and
Mahaney 2000; Sing and Sing 2010). Similarly, there is some archaeological
evidence that early hominids included soil in their diets (Clark 2001). So what
about modern humans? In the 1990s, it was noted that in western Kenya ~70% of
207 schoolchildren aged 5–18 years consumed soil on a daily basis (median ingestion of 28 g/day, range 8–108 g) (Geissler et al. 1997). This behaviour was more
prevalent in girls and continued into adolescence. It has been noted in numerous
other cultures, on all continents (Sing and Sing 2010; Geissler et al. 1997). Geophagy is also common in pregnancy, not only in undeveloped rural cultures, but also as
a manifestation of the psychological disorder “pica” in Westernised ones. Babies of
all cultures consume astonishing quantities of soil if adults do not intervene (Ngure
et al. 2013). Interestingly soil was also an integral part of food preparation in many
cultures, possibly because added soil adsorbed toxic glycoalkaloids present in foods
based on acorns or early varieties of potatoes (Sing and Sing 2010). However, this
does not explain the consumption of soil as an independent dietary component.
Some authors suggest it was a desperate act driven by starvation but this is clearly
not usually the case. Others suggest that it is an evolved behaviour that increases
intake of minerals. But since evolution turns the inevitable into a necessity it is
reasonable to suggest that whatever the initial motivation, intake of soil was an
important source of inevitable microbial intake.
18.3.3 Skin, Wounds, and Abrasions
In addition to intake of microorganisms via the airways and via the oral route, we can
assume that early humans encountered environmental microorganisms in cuts and
abrasions much more often than does modern urban man, and would not have had
the facilities for effective wound cleaning. Therefore, it is interesting that when
tested, probiotic Lactobacilli were shown to be active via a parenteral route (subcutaneous) in models of arthritis and colitis (Sheil et al. 2004). Similarly, subcutaneous
immunization with a saprophytic environmental soil mycobacterium blocked stressinduced colitis and behavioural changes, and partially inhibited stress-induced
changes to the microbiota. These effects were dependent upon induction of Treg
(Reber et al. 2016). Parenteral administration is not pursued by pharmaceutical
336
G. A. W. Rook