From the point of view of an intracellular bacteria, the human body really is no more than just a habitat in which they must grow and thrive. While this particular habitat might have stable internal conditions, and less competition than the big open world, it has its disadvantages in continuous attacks from the immune system, and the lack of usable nutrients. In the soil, nutrients can be found and exploited, but within a living organism they are being used and locked away by the host cells.
In order to eat, therefore, the internal bacteria must find ways of stealing and sequestering nutrients from the infected cells. A recent paper from, PLoS Pathogens (reference 1) shows how Salmonella infecting rat cells manage to find enough nutrients to grow and develop.
In order to explore what pathways the bacteria could use to intake nutrients, the researchers used both computational and in vitro experiments to look at proteins and genes suitable for metabolism. What they found was that following an infection the Salmonella was able to mobilise a large section of its genome in order to carry out metabolic reactions.
Rather than just concentrating on using one particular metabolite from the host cell (i.e having lots of pathways to metabolise glucose) the Salmonella was able to exploit a diverse range of host molecules, without preferring one to the other. Adding either glucose or mannitol to established Salmonella colonies caused an increase in growth. Figure A below shows a schematic of the nutrients being added to the cells, while figure B shows the results. The more CFU (colony forming units) found, the more growth was seen:
Further experiments, combined with the computational data, indicated that the Salmonella bacteria use a wide range of chemically diverse nutrients inside the host cell in order to grow; including different lipids, carbohydrates, amino acids, nucleosides, and various pro-vitamins. This does mean that the Salmonella requires a large number of genes to deal with nutrition, but on the plus side, once it gets into the cell it's far more likely to grow and survive with any small amount of food it can scavenge.
Why does Salmonella require such a large range of different metabolic pathways while other intracellular bacteria (including E.coli) are happier to rely on far few? One potential explanation given in the paper is due to the conditions in which Salmonella lives inside the host cell. In order to protect itself from attacks from the cells defence system, the bacteria stays safely wrapped up in a vacuole inside the cell. While this does stop the cell destroying it, it may make it harder for the bacteria to cannibalise available nutrients, meaning it has to have the capacity to utilise whatever it can get hold of.
For more details on how Salmonella invades the host cell, along with a great animation, go here.
Reference 1: Steeb B, Claudi B, Burton NA, Tienz P, Schmidt A, et al. (2013) Parallel Exploitation of Diverse Host Nutrients Enhances Salmonella Virulence. PLoS Pathog 9(4): e1003301. doi:10.1371/journal.ppat.1003301
Reference 2: (2005) A Novel Data-Mining Approach Systematically Links Genes to Traits. PLoS Biol 3(5): e166. doi:10.1371/journal.pbio.0030166