Description
All organisms have a name consisting of two parts: the genus followed by the species (i.e., Homo sapiens). Bacteria have been grouped and named primarily on their morphological and biochemicaVmetabolic differences. However, bacteria are now also being classified according to their immunologic and genetic characteristics. This chapter focuses on the Gram stain, bacterial morphology, and metabolic characteristics, all of which enable the clinician to rapidly determine the organism causing a patient’s infection.
GRAM STAIN
Because bacteria are colorless and usually invisible to light microscopy, colorful stains have been developed to visualize them. The most useful is the Gram stain, which separates organisms into 2 groups: gram-positive bugs and gram-negative bugs. This stain also allows the clinician to determine whether the organism is round or rod-shaped.
For any stain you must first smear the substance to be stained (sputum, pus, etc.) onto a slide and then heat it to fix the bacteria on the slide.
There are 4 steps to the Gram stain:
1) Pour on crystal violet stain (a blue dye) and wait 60 seconds.
2) Wash off with water and flood with iodine solution. Wait 60 seconds.
3) Wash off with water and then “decolorize” with 95% alcohol.
4) Finally, counter-stain with safranin (a red dye). Wait 30 seconds and wash off with water.
When the slide is studied microscopically, cells that absorb the crystal violet and hold onto it will appear blue. These are called gram-positive organisms. However, if the crystal violet is washed off by the alcohol, these cells will absorb the safranin and appear red. These are called gram-negative organisms.