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Microbiology 12 |
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Microbiology 12 |
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NOTE: INFORMATION ON THIS PAGE IS INTENDED FOR EDUCATIONAL USE ONLY. FOR MEDICAL ADVICE YOU SHOULD CONSULT WITH A PHYSICIAN
Introduction
Vibrio spp make up a significant portion of heterotrophic bacteria of marine and estuarine environments. Some species live as harmless endosymbionts of marine organisms (e.g., Vibrio fischeri in squid). Various pathogenic species cause devastating diseases in finfish, shellfish and mammals. Vibrio parahaemolyticus and Vibrio vulnificus cause acute diarrheal disease associated with consumption of fecally-contaminated, undercooked shellfish, oysters, clams and crabs.
The focus of this lecture is the bacterium, Vibrio cholerae, a diarrheal pathogen that has afflicted humans for over two thousand years and has caused the flushing of many a toilet!
V. cholerae, the causative agent of cholera, has a rich history. John Snow, the "Father of Epidemiology" became famous after tracing a large outbreak of cholera to the London Broad Street water pump in 1845. The organism was isolated by researchers guided by the German microbiologist, Robert Koch. An assistant of Louis Pasteur was the first to develop an "attenuated" form of vaccine against chicken cholera in 1879.
V. cholerae is able to survive in marine environments between epidemics of disease and has give rise to seven pandemics of cholera since 1817. A recent dramatic example involved the Peruvian Cholera epidemic of 1991-1992. By April 1992 the epidemic had spread to 16 Latin American countries, infected over half a million people and casued nearly 5,000 deaths. The origins of the epidemic? Contaminated ballast waters released from a Southeast Asian fishing ship into the Peruvian seaport of Chimbote. Since then ballast water from large commercial ships has been implicated in spreading a number of disease pathogens around the world.
Cultural and molecular characteristics
The term vibrio refers to a back and forth vibration of these gram negative, comma-shaped bacteria that are motile by a polar flagellum (Fig1). The organisms prefer alkaline and high salt environments. There are a wide variety of strains, both pathogenic and nonpathogenic, that vary in their virulence gene content. The major biotypes responsible for disease are known as:
The classical dogma of the bacterial chromosome, based on studies with E. coli, held that all bacterial contain a single circular chromosome. This view has now changed because of the discovery of bacteria like V. cholerae that have more than one chromosome. The complete genome sequence of the organism is known and has revealed the existence of two circular chromosomes (Fig 2). Chromosome 1 is larger (2.96 million base pairs) and carries many genes for essential cell functions and housekeeping. It also contains important virulence genes, most of which have been acquired by lateral gene transfer from other species. A fascinating discovery revealed that chromosome one actually carries two bacteriophages. One virus is called the V. cholerae pathogenicity island phage (VPI), which infects and inserts its DNA into the bacterial chromosome. This event allows the synthesis of a pilus which the bacteria use to attach to the host intestine. The same pilus serves as a coreceptor for a second virus called the cholera-toxin phage (CTX). The CTX phage then inserts itself into chromosome one and the bacterium is now capable of secreting a powerful enterotoxin. Amazing!
Chromosome 2 is smaller (about 1.07 million base pairs). Some researchers think it could have originated as a large plasmid, while others propose it could be a large piece of excised chromosome. The integron region is often found on plasmids and serves as a "gene capture system." This region may contain antibiotic resistance genes.
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Fig 1. Vibrio cholerae . Mag x 10,000 |
Fig 2. The two Vibrio cholerae chromosomes showing gene loci of virulence markers from other sources in red. Source of images: Nature. Vol 406. August 3rd, 2000. pp 469-470. |
Disease transmission and characteristics
Transmission occurs through direct or indirect fecal contamination of water or food or by heavily soiled hands and eating utensils.
Vibrio cholera causes a severe secretory diarrhea by means of a secreted enterotoxin. The inubation period is about 12-72 hours. V. cholerae does not invade the intestine. Rather the organism attaches to epithelial cells of the small intestine by pili and the toxin makes the cell membrane of the target cell highly permeable to water. A significant loss of water and chloride ions results and sodium reabsorption is inhibited. The intestinal lining becomes damaged and the patient experiences severe painful watery diarrhea and vomiting. Mucus plugs can give the appearance of " rice water" to the stool. Up to 20 liters of fluid can be lost a day so anyone is at risk of death from dehydration and shock . Children can experience seizures caused by hypoglycemia and may experience cardiac arrhythmia from loss of potassium. Death may occur in up to 50% of untreated cases.
Treatment
Rehydration and electrolyte replacement is critical in treating cholera. Oral rehydration salt solutions are available for this purpose. Tetracycline or doxycycline can be given to diminish symptoms but does not eliminate the organism or toxin. Ciprofloxacin can be given for tetracycline resistant strains.
Prevention
