Final answer:
Proteobacteria belong to the phylum Proteobacteria within the domain Bacteria and are characterized as Gram-negative. These are divided into five different classes and include both nonpathogenic and pathogenic types.
Step-by-step explanation:
Proteobacteria in the domain Bacteria are described as Gram-negative. This phylum, which was suggested by microbiologist Carl Woese in 1987, includes a large and diverse group of bacteria that share similar nucleotide sequences in their genomes. They are further divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria. A notable example of their influence can be seen in the fact that eukaryotic mitochondria are considered to be the descendants of alphaproteobacteria.
It's also important to note that the majority of Gram-negative bacteria can be found within the phylum Proteobacteria. Nonproteobacteria include other classes of Gram-negative bacteria such as the spirochetes, Chlamydia, and others. While Proteobacteria form a large part of the normal human microbiota, this phylum also includes many known pathogens.