The easiest way to select a certain range of brightness is to use the "Blend IF" sliders. Here's a short intro, but many more are available:
http://blogs.adobe.com/jkost/2013/02/blend-if-sliders-in-photoshop.html .
One puts your image on a layer above a solid color layer. Usually, this background layer is either white or black, but even having no layer, just transparency is also an option. Then one adjusts the blendIF sliders to select the upper and lower brightness bounds that you want to see, e.g.,
Here is the result of the above:
Unfortunately, as you can see, such a simple approach rarely, if ever, yields good, clean results. One reason is that even the best photomicrographs have variations in illumination level over the field of view. In addition, in no photomicrograph will particles ever have truly sharp edges. Consider a very dark, uninteresting particle on a white background. In the transition region between the background and this particle, there will always be a thin ring of gray pixels that encircle the particle, and this ring will always show up when using the BlendIF approach.
The way it's usually done is using a full blown image analysis program such as ImageJ from NIH or the image processing toolbox for Matlab:
http://imagej.nih.gov/ij/
http://www.mathworks.com/help/images/index.html
Many, many algorithms, scripts and plugins have been developed for these programs to count single bacteria, inorganic particles, fibers, stars, colonies of bacteria, etc.. Many of these incorporate a priori knowledge of the shape / morphology of the particle to aid in the selection process. If you have other versions of your images that contain color information, that can also usually be incorporated into the selection criteria to improve the results.
For more info, Google search terms like {imagej count bacteria}, or for a more general search, omit "imagej" from the search criteria and search for other types of particles.
HTH,
Tom M