Bacterial Classification

The taxonomy of bacteria is not very definitively worked out yet, especially the higher levels of classification. Some authorities believe that the degree of variance between different bacterial groups is sufficient to give them each 'Kingdom Status' of their own. Thus in the 9th edition of "Brock: Biology of Microorganism" you will find reference to 13 Kingdoms of Bacteria. From the point of view of these pages it is not really important whether you think of the different categories as Phyla or Kingdoms as long as you are aware that the bacteria are an incredible diverse group of organisms. Here I have followed the classification scheme laid out in the 2nd edition of "Bergey's Manual of Systematic Biology".

Gram Staining

You will find bacteria referred to as 'Gram +' or 'Gram Positive' and 'Gram -' or 'Gram Negative' this is a reference to how the bacteria responds to the Gram staining method. Staining methods are designed to make a staining agent bind to the cell wall of the bacteria. The Gram staining method is named after Christian Gram who invented the method in 1884.

In testing for gram stain response, microbiologists first spread some bacteria on a slide, then fire it by passing the slide through a flame briefly. The next step is to flood the slide with crystal violet solution for 1 minute. Then they add iodine solution for 3 minutes - at this stage all cells are purple. Adding alcohol for 20 seconds results in Gram negative cells becoming clear again, ie they lose their purple staining. Lastly, the cells are restained with safranin. This results in gram positive cells remaining purple and gram negative ones being red or pink. Gram staining is nearly always the first step in identifying a new sample or species of bacteria. Nowadays, gram staining can be done in one step using a fluorescent dye and a fluorescence microscope.