16 Chapter 1 Introduction: An Overview of Microbiology and Cell Biology (b) Figure 1. 14 Colonies formed on an agar-containing medium on a I'etri plate(a)Red colonies of the bacterium Serratia marcescens(top half of plate)streaked alongside colonies of a mutant derivative unable to make the red pigment(bottom half of plate).(b)Mixed colonies of ye low and purple pigmented bacteria