The three most common morphological types are bacilli (rodshaped), cocci (spherical), and spirillum (helical rods). Some bacteria are photosynthetic and obtain their energy directly from the sun. Others oxidize inorganic compounds to supply their energy needs. Still other bacteria generate energy by breaking down organic compounds such as amino acids and sugars. Some bacteria require oxygen (aerobes), while others are unable to tolerate it (anaerobes). Some bacteria can grow either with or without oxygen (facultative anaerobes).
Bacteria are frequently divided into two broad classes based on their appearance when subjected to Gram stain. Gram-negative bacteria appear pink after staining (e.g.,Salmonella typhi, which causes typhoid fever, and Yersinia pestis, which causes plague). Gram-positive bacteria stain purple (e.g., Staphylococcus aureus, which causes skin, respiratory, and wound infections, and Clostridium tetani, which produces a toxin that can be lethal for humans).