It depends if the bacteria is obligative or faculative.
An example of a faculative bacterium is E.coli.
It primarily produces ATP through aerobic respiration, but under anaerobic conditions it can use fermentation to produce ATP.
The big difference between aerobic respiration and anaerobic fermentation, is how the ATP is generated and the amount produced.
In aerobic respiration, ATP is produced via oxidative phosphorlyation. This is done by using some sort of ionic gradient, the most common example is a proton gradient. However, there are other types of organisms that may use a sodium or nitrate gradient.
In anaerobic fermentation, the ATP is produced by substrate level phosphorylation. This is done by hydrolysis of a high energy biomolecule like 1,3-bpg, acetyl-coa, and succinyl coa.
In addition, the amount of ATP produced using aerobic respiration is usually a lot greater than fermentation.
ETC's in bacteria are located on the cell membrane of the bacteria. The gradient is produced between the cell wall and cell membrane of the bacterium and the ions involved are normally shuttled and stored in the intermembrane space between the two.
The book says multiple ETC's because the associated group of proteins that make up the ETC are spread throughout the cell membrane. There isn't just one ETC. =)
