That is difficult to answer properly become sand grains come in all sorts of different sizes and bacteria are normally found in cracks and dips on the surface of the sand grain (so less bacteria on smoother grains). There are probably very few bacteria inside sand grains, because sand grains are just tiny little bits of rock, but the surface of the sand grains is really covered in bacteria! In every square millimetre on the surface of sand grains, there might be as many as 259,000 bacteria (although only about 40 of these might be viable, or ‘alive’)
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