Hydrocarbons, in both ring and chain form, taken from a mix of sources
The general rule is that the shorter and fewer rings present, the more toxic the compound to bacteria (ethanol, for example, is deadly) however there are very few hydrocarbons that some bacteria somewhere won't consume. Like all living organisms, bacteria need a carbon source for energy, and when there are none of the coventional ones around (such as glucose and lactose) many of them will start consuming the ones in oil - crude oil contains a lot of carbon, which is why we burn it for energy in the first place.
Oil-eating bacteria have been found in a huge number of locations as well - seawater, freshwater, groundwater, water containing sludge, and land environments such as silt, soil and sand. They are found in arid deserts o the Middle East (which to be fair is a good place to hang out if you consume oil) and even in ice cores from Antarctica. And rather than being found widespread within seperate species, the capacity for oil consupmtion is often found in many different species within a particular location - in the bacterial world you are more likely to share habits with your nearest neighbours than with your closest relatives.
Using these bacteria for our own purposes, however, is proving slightly more problematic. You can purchse freeze-dried hydrocarbon-degrading bacteria and sprinkle them on your oil slick like minature Captain Planets, but their abilities are currently not nearly as effective as chemical detergents, or physical removal of the oil. This is mostly because these freeze-dried bacteria are specialised lab strains that (much like several of the academics that constructed them...) are often highly unsuited to the real world outside of the safe confines of the lab. Sometimes as well the pollutant is unaccessible to the bacteria, which may be floating on the top of the water while the oil is in droplets underneath the surface.
Looking into the potential of oil-eating bacteria is encouraging though, especially after incidents like the BP oil spill earlier this year. For the oil eating bactera that were hanging around, that must have been a feast day! And even though that event has now been largely forgotten by the media, there will still be coils and dropplets of oil floating around from it, and those will be cleaned up almost exclusively by bacteria.
Lena Ciric (2010). A natural solution: how bacterial communities can help clean up oil spills Microbiology Today, 229-231
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