4.2 Proteobacteria  (Page 4/11)

The Pasteurellaceae also includes several clinically relevant genera and species. This family includes several bacteria that are human and/or animal pathogens. For example, Pasteurella haemolytica causes severe pneumonia in sheep and goats. P. multocida is a species that can be transmitted from animals to humans through bites, causing infections of the skin and deeper tissues. The genus Haemophilus contains two human pathogens, H. influenzae and H. ducreyi . Despite its name, H. influenzae does not cause influenza (which is a viral disease). H. influenzae can cause both upper and lower respiratory tract infections, including sinusitis, bronchitis, ear infections, and pneumonia. Before the development of effective vaccination, strains of H. influenzae were a leading cause of more invasive diseases, like meningitis in children. H. ducreyi causes the STI known as chancroid .

The order Vibrionales includes the human pathogen Vibrio cholerae . This comma-shaped aquatic bacterium thrives in highly alkaline environments like shallow lagoons and sea ports. A toxin produced by V. cholerae causes hypersecretion of electrolytes and water in the large intestine, leading to profuse watery diarrhea and dehydration. V. parahaemolyticus is also a cause of gastrointestinal disease in humans, whereas V. vulnificus causes serious and potentially life-threatening cellulitis (infection of the skin and deeper tissues) and blood-borne infections. Another representative of Vibrionales, Aliivibrio fischeri , engages in a symbiotic relationship with squid. The squid provides nutrients for the bacteria to grow and the bacteria produce bioluminescence that protects the squid from predators ( [link] ).

The genus Legionella also belongs to the Gammaproteobacteria. L. pneumophila, the pathogen responsible for Legionnaires disease , is an aquatic bacterium that tends to inhabit pools of warm water, such as those found in the tanks of air conditioning units in large buildings ( [link] ). Because the bacteria can spread in aerosols, outbreaks of Legionnaires disease often affect residents of a building in which the water has become contaminated with Legionella. In fact, these bacteria derive their name from the first known outbreak of Legionnaires disease, which occurred in a hotel hosting an American Legion veterans’ association convention in Philadelphia in 1976.

Enterobacteriaceae is a large family of enteric (intestinal) bacteria belonging to the Gammaproteobacteria . They are facultative anaerobes and are able to ferment carbohydrates. Within this family, microbiologists recognize two distinct categories. The first category is called the coliforms, after its prototypical bacterium species, Escherichia coli . Coliforms are able to ferment lactose completely (i.e., with the production of acid and gas). The second category, noncoliforms , either cannot ferment lactose or can only ferment it incompletely (producing either acid or gas, but not both). The noncoliforms include some notable human pathogens, such as Salmonella spp ., Shigella spp ., and Yersinia pestis .