Is gas produced by bacteria always mainly methane? Or, are there bacteria out there that produce some biogas composed mainly of hydrogen, natural gas, propane, butane?
Microbes can produce several gasses other than methane.
- All microbes produce $CO_2$ through the oxidation of reduced carbon
Additionally some metabolic pathways produce other gases.
Photosynthetic microbes produce $O_2$
Sulfur reducing bacteria can produce $H_2S$ (as @ohcanada points out)
Denitrifying bacteria produce $NO$, $N_2O$, and $N_2$
Fermenters produce $H_2$ (as @mart indicates)
There may be others that I am not thinking of but these are some of the major players...
Well, some bacteria can produce Hydrogen Sulfide gas. For example, Proteus and Salmonella. The presence of $H_2S$ producing bacteria is actually clinically significant and we have a way to test for this, which is via the use of TSI (Triple Sugar Iron) media.
Assume you set up the test correctly, $H_2S$ producing bacteria will generate dark deposit within the media.
In the clinical setting, there are other choices beside TSI, such as SIM media and API20E.