Bacteria from a Smelly Sponge

Image and Text © 2005 Jeffrey C. May

This is a high-power SEM (4000x) from the surface of a kitchen sponge that reeked.  Numerous rod-shaped bacteria are visible.  Most of the bacteria are embedded in a plane that is draped over the surface of the sponge.  Other bacteria that were living in the sponge were deriving their nutrients from milk that had been soaked up by the sponge, but these bacteria are all within the surface of a human skin scale.

In any indoor air sample, the most common  aerosol particle is the human skin scale.  These are usually well defined, and about 25 microns in size.   When viewed through a light microscope after staining, they appear to be pink.  Very often in indoor air samples, bacteria can be seen in indistinct pink-stained fragments much smaller than 25 microns.  I believe that these are skin-scale fragments that have been partially digested by bacteria in damp reservoirs where dust has accumulated.  These reservoirs can include humidifiers, air-conditioners, and carpets or rugs that have been wet.