There is a wide variety of Gram-negative pathogens. Although those described here are all found within the Proteobacteria phylum, they are spread across all the classes within this most diverse of bacterial phyla. Some can be free living under many environmental conditions as well as cause disease (Pseudomonas aeruginosa) whereas others are obligate intracellular pathogens (Rickettsia rickettsii). Almost all the common Gram-negative bacteria are rod-shaped (bacilli). The only commonly encountered genus of pathogenic Gram-negative cocci is Neisseria.