Bacteria exhibit a variety of shapes, including: cocci, bacilli, and spirochetes. Cell shape influences the spatial and temporal dynamics of processes that are essential for growth and replication in bacteria and is connected to pathogenicity and evasion of the mammalian immune system. Several bacterial cell-shape determinants have been proposed, including: 1) the cytoskeleton as an internal scaffolding, and 2) the peptidoglycan layer of the cell wall that resists osmotic pressure and maintains cell shape. However, some bacteria lack these subcellular components and yet retain distinct cellular shapes. This observation raises the question of whether bacteria use other shape-determining strategies and provides an opportunity to explore the biochemical evolution of cell shape across bacteria. The phospholipid membrane is another cellular structure that may regulate the shape of bacteria, and yet the impact of this cellular structure on bacterial morphology has been largely overlooked. Bacterial cell membranes consist of the three major families of phospholipids: phosphatidylethanolamine is zwitterionic, and phosphatidylglycerol and cardiolipin are anionic. The composition of cell membranes plays a fundamental role in bacterial cell biology. This dissertation describes how cardiolipin regulates cell morphology and influences bacterial adaptation to environmental stress. We observed that a cardiolipin deficiency in Rhodobacter sphaeroides changes the shape of cells and impairs biofilm formation. We demonstrated that cardiolipin participates in bacterial cell shape determination by regulating peptidoglycan precursor biosynthesis. In this dissertation, we also developed a new method for optimizing the production of recombinant proteins in Escherichia coli by engineering its cell shape. The insights gained from these studies will enable a wide range of applications spanning the identification of novel antibiotic targets and the optimization of biomaterials production by bacterial cells.