On helical bacteria
These bacteria are a diverse collection of Gram-negative staining organisms that demonstrate a characteristic curved morphology. Three groups have been presented in class (Vibrio, Campylobacter, and Helicobacter) that all are transmitted via the fecal-oral route, and account for significant levels of disease in the United States and worldwide. In fact, epidemiologists have estimated that gastroenteritis infections due to Campylobacter outnumber infections due to all other food borne pathogens combined, with a whopping 2 million cases in the US alone. Since Campylobacter infections are not reportable to the CDC, the actual incidence is not known.
Members of the genus Vibrio are significant pathogens, and are reportable infections here in the United States. Many species of Vibrio are responsible for reported cases of seafood-bourne food poisoning, as they are common organisms in marine environments. The main virulence trait of the vibrios are the potent exotoxins that they produce. The course of disease then is very similiar to the diseases of many other exotoxin-producing pathogens: the patient is infected with a microorganism or only ingests the toxin, the toxin begins to produce a biological effect resulting in the signs and symptoms of the disease which may continue even after the actual organism has been eliminated from the body.
The exotoxin made by Vibrio cholerae is a fascinating compound. When cholera toxin binds to the membranes of intestinal epithelial cells (the cells lining the large intestine,) a portion of the toxin enters these cells and activates a protein (adenylate cyclase) involved in transmitting cellular signals. These cellular signals cause the epithelial cells to rapidly pump out sodium and chloride ions (salt) from the cytoplasm of the cells to the lumen of the intestine in an active transport process. Water then follows the salt from the body into the intestine by passive transport, resulting in an uncontrolled loss of water from the body. Patients with severe cholera toxemia can lose up to one liter of fluid per hour via extremely watery diahrrea, and lose up to half of their body weight during the infection. Oral fluid intake is not sufficient to counteract the fluid loss during diahrrea. If the patient become severely dehydrated, the only recourse is administer fluids intravenously.
Now here’s something interesting I learned from reading the textbook. Vibrio cholerae is not a hugely virulent pathogen. The organism is readily killed by the acidic conditions in the stomach, and it requires an inoculum of at least 108 organisms for the disease to develop. Additionally, recent research has found that virulence genes in Vibrio cholerae are activated by the organism being in the human body; that is, naturally occuring environmental isolates are less pathogenic than the organisms shed in human feces. This observation has been used to explain the extremely rapid spread of cholera outbreaks; epidemics actually do appear to become more severe as they progress!