Bacterial Pathogens of Humans
Station Location: Hog Bay, GPS Coordinates: 38°57’07.02”N, 76°14’09.32”W
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- Domain: Bacteria
- Kingdom: Bacteria
- Phylum: Proteobacteria
- Class: Gammaproteobacteria
- Order: Vibronales
- Family: Vibrionaceae
- Genus/Species: Vibrio vulnificus
Some estimates suggest that there are as many as ten times the number of bacterial cells in the human body as there are human cells.¹ Without these organisms, human life would not be possible, since the human microbiota contribute in numerous, essential ways to our life processes. It’s important to keep this in mind, when considering the relatively small number of bacteria that are pathogenic to humans.
Bacteria, particularly human pathogens, are frequently divided into two main groups, based on their staining characteristics. These groups are called “Gram-positive” and “Gram-negative”, terms named for the Danish microbiologist who developed the staining technique. The color of the organism, following staining (violet for Gram-positive organisms, reddish pink for Gram-positive organisms), reflects a distinct difference in the type of cell wall that encloses the bacterium in question, and a separate evolutionary development of the two types of microbes. Other general ways of describing bacteria include their type of motility (or lack of motility), their atmospheric requirements (e.g. whether oxygen is required or toxic), their morphology (e.g. shape and size), their rate of growth (i.e. some bacteria reproduce in minutes, while others take weeks or longer), and their metabolic chemistry (what type of compounds are metabolized, and what sort of waste products are produced). The genus Vibrio includes several important human pathogens, V. cholerae (the causative agent of cholera),
V. vulnificus, V, parahaemolyticus, and V. alginolyticus. Vibrio species are Gram-negative, motile, rod-shaped bacteria. The Vibrio species that cause infections in the U.S. (all of those listed, except V. cholerae), are halophilic organisms (salt-loving) and are typically found in warm, coastal waters.
Vibriosis (infection caused by non-cholera Vibrio species) occurs in approximately 80,000 Americans each year, with about 100 deaths reported. Infections involving V. vulnificus are particularly concerning, since the morbidity and mortality associated with these infections is quite high (approximately 25% of V. vulnificus infections result in death). The principle types of infections caused by V. vulnificus are acute gastroenteritis (normally associated with consumption of contaminated shellfish), necrotizing (“flesh-eating”) wound infections (usually caused by infection of a previously-existing wound to contaminated water), and invasive septicemia (also associated with consumption of contaminated shellfish). The CDC reports that approximately 80% of the reported cases of vibriosis occur between May and October, when coastal water temperatures are warmer. Evidence suggests that as global temperatures have risen and the warm season is extended, the bacterial load of Vibrio species in shellfish has increased. A recent journal article reports that “a warming trend in sea surface temperature is strongly associated with spread of vibrios, an important group of marine prokaryotes, and emergence of human diseases caused by these pathogens”.²
Notes & References:
¹American Academy of Microbiology FAQ: Human Microbiome January 2014
²Vezzulli L, Grande C, et al. “Climate influence on Vibrio and associated human diseases during the past half-century in the coastal North Atlantic”. PNAS, Vol 113 No 34, Aug 23, 2016.
Photo attribution: https://commons.wikimedia.org/wiki/Category:Vibrio_cholerae#/media/File:Vibrio_cholerae.jpg (distributable under Creative Commons license)