One of the common sayings in microbiology that drives me up a wall is the notion that 99% of all bacteria can't be grown in the lab. This false statement stems from the observation that if you take any sample (soil, water, clinical samples) and look under a microscope we see many more bacterial cells that contain DNA than we can grow. The problem is that, if you look at the paper that claimed this, they attempted to grow bacteria on a single, rich medium.
One weekend, when I was a post-doc, I did a very simple comparison. I took standard rich lab medium ('nutrient agar' which is basically one of those high protein nutrient bars minus the artificial flavorings. That's why I don't eat them. Seriously, if you dilute one bar in a liter of water, you basically have nutrient agar). Anyway, the other medium was a home-brew very-low nutrient agar that had one-thousandth of the ingredients in the standar nutrient agar. I also used extremely high-grade agar (electrophoresis grade) to solidify the medium (standard agars actually contain a fair amount of nutrients). I did this because we know that many bacteria can't grow in environments with a lot of carbon or protein sources--they basically end up choking on their own excreta. I also added cycloheximide to kill off fungi, as well as catalase to protect the bacteria from their own metabolisms.
After 72 hours, my special plates had 20-40 times as many bacteria as the standard plates. So I've never bought into the 99% percent.