<P> The proteobacteria are a major phylum of gram - negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella etc . Other notable groups of gram - negative bacteria include the cyanobacteria, spirochaetes, green sulfur, and green non-sulfur bacteria . </P> <P> Medically relevant gram - negative cocci include the four types that cause a sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis, Haemophilus influenzae). </P> <P> Medically relevant gram - negative bacilli include a multitude of species . Some of them cause primarily respiratory problems (Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa), primarily urinary problems (Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens), and primarily gastrointestinal problems (Helicobacter pylori, Salmonella enteritidis, Salmonella typhi). </P> <P> Gram - negative bacteria associated with hospital - acquired infections include Acinetobacter baumannii, which cause bacteremia, secondary meningitis, and ventilator - associated pneumonia in hospital intensive - care units . </P>

Example of gram positive and gram negative bacteria