Concepts Microorganisms are too small to be seen without the use of a microscope.The techniques-such as sterilization and the use of culture medium are required to isolate and grow these microbes. Bacteria may be spherical (cocci),rod-shaped (bacilli),spiral,or filamentous. Most bacteria can be divided into gram-positive and gram- negative groups based on their cell wall structure and response to the Gram stain.Bacteria such as mycoplasmas lack a cell wall. Concepts • Microorganisms are too small to be seen without the use of a microscope. The techniques-such as sterilization and the use of culture medium are required to isolate and grow these microbes. • Bacteria may be spherical (cocci), rod-shaped (bacilli), spiral, or filamentous. • Most bacteria can be divided into gram-positive and gram￾negative groups based on their cell wall structure and response to the Gram stain. Bacteria such as mycoplasmas lack a cell wall