Dependingon the componentsof the cell wall,bacteriawillretaintheprimarystainduringdecolorizing orlosetheprimarystain andtakeupthecounterstain. Organismsthatresistdecolorizingandretainthecrystal violet-iodinecomplexappearpurpleordarkblueunderthemicroscopeandarecalled Grampositive(G+). Conversely,bacteriathatdecolorize,orgiveupthecrystalviolet- iodinecomplexwillacceptthesafranincounterstainandappearred.TheyareGram negative (G-)bacteria.
The Gram stain allows for recognition of the shape and pattern or arrangement of bacteria. The shapes are cocci (Sing: coccus; round), bacilli (sing: bacillus; rod-shaped), and spirilla (spiral).Basedonhowthebacteriadividesduringreplication, differentpatternsorarrangementsmaybeproduced.Thecellularmorphology,orshapes,oftheindividualcellsandtheir arrangementinpairs,chains,orclustersareusefulintheidentificationofthebacteria.
d. tetrads– fourinasquare
Bacilli vary in size from very long oblongs to short rods. They may even have squared-off ends, or one end larger than another, forming a club. The patterns or arrangements they form are:
Spiral bacteria usually occur singly. Some are very tightly coiled; whereas others are long and slightly curved or only curved at one end.