**5.1 Nutrient agar**

*E. coli*, on NAM, forms large, thick, greyish white, moist, smooth, opaque, or translucent discs like colonies as shown in **Figure 3**. The smooth forms (S) of colonies seen in fresh isolation are easily emulsifiable in saline. The rough forms (R) of colonies seen in older cultures, with dull surfaces often auto-agglutinable in saline. S-R variation occurs as a result of repeated subcultures and is associated with the loss of surface antigens and usually of virulence [24].
