The most common bacterial shapes are spherical (cocci), rod-shaped (bacilli) and spiral (spirilla). Microbiologists use light microscopy to observe the shape and cellular arrangement of bacteria, and use this information as a starting point for species identification.
A number of staining techniques aid scientists in identifying bacteria. Gram staining differentiates bacteria into Gram-positive and Gram-negative groups, according to the molecular structure of their cell walls. The Schaeffer-Fulton stain detects the endospores formed by certain bacterial species. The acid-fast stain tests for the presence of mycolic acid, which is a waxy substance in the cell wall that aids in bacterial infection and adhesion.
Microbiologists also use biochemical tests to differentiate bacteria species. For example, some bacteria use aerobic respiration to produce energy, some use fermentation and some -- like cyanobacteria -- are photosynthetic. Staphylococcus aureus ferments the sugar glucose, while Escherichia coli ferments lactose to produce lactic acid. Microbiologists use biochemical assays to identify the substrates as well as the products of these metabolic processes.
Microbiologists can distinguish bacteria species by their ability to grow on specific culture media, and by colony morphology (i.e., the size and appearance of bacterial colonies). Many Streptococci species will grow on blood agar. Vibrio cholera will grow in a medium with a high pH, while Staphylococcus aureus will tolerate an elevated sodium chloride concentration. Bacillus cereus produces fuzzy white colonies on tryptic soy agar.