I've written before about the many ways that bacteria can move around. Considering that they're just one cell long, micro-organisms have a whole range of ways to travel through their little world. Movement is useful for finding food and for changing your environment when all nearby resources have been exhausted. For bacteria that can't move, however, or that don't want to move, there is a second option; they can park themselves on a nearby surface and settle down to wait.
There are several advantages to this. For a start, other things like food and nutrients tend to accumulate at surfaces as well, bringing the bacteria a regular supply of food. A surface is a more stable environment, the bacteria that adhere to your teeth do so because to get swept away into the stomach is to be pulled down into a very literal lake of acid. For bacteria that form biofilms, sticking to a surface is the first stage in this process.
So how do the bacteria know when they reach a surface? They can't see it, after all, and they don't really have a sense of touch. It's been proposed that they react to deformation stresses - i.e they notice when parts of their membrane are being squashed on a surface. Enough of these membrane deformations can act to switch the bacterial phenotype, turning it from a free moving bacteria into a bacteria stuck onto a surface.
Work done to study this involved looking at how well bacteria stick to different surfaces, and seeing how much of an adhesion force is produced. Surfaces such as polymer brushes or hydrogels don't exert enough pressure and the bacteria (in this case E. coli) doesn't even recognise that it's near a surface. The strongest adhesion forces are found with positively charged surfaces. As the outer membrane of bacteria is usually negatively charged, this extra charge attraction helps with adhesion.
Once the bacteria has attached to the surface, the adhesion forces increase dramatically to permanently fix the bacteria in place. The first adhesion event is likely to be not with the surface at all, but to a film of water covering the surface. Once attached to that, the bacteria can pull itself closer and start sticking to the actual surface of the object. The paper points out that these changes in force strength are unlikely to be changes in genetic expression, or metabolism, but instead are just the strengthening of the connection at the membrane surface. This will be followed by changes in bacterial metabolism and gene expression to best support this attachment.
The reference for this paper (below) is taken from PLoS Pathogens. As there's been a bit of a thing lately about journals and scientific publishing, I thought I'd join in by limiting my blogging exclusively to open access journals. From now on anything I blog about, you will be able to read. And if you've written (or read!) anything bacterial related in an open access journal feel free to get in touch if you want me to cover it.
Ref: Busscher HJ, & van der Mei HC (2012). How do bacteria know they are on a surface and regulate their response to an adhering state? PLoS pathogens, 8 (1) PMID: 22291589
Credit link for first image