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Pathogens Primarily Within the Enteric Tract
This Sqadia video is the elucidation of Gram-Negative Rods Related to the Enteric Tract – II. Shigella species cause enterocolitis. Enterocolitis caused by Shigella is often called bacillary dysentery. Shigella are non–lactose-fermenting, gram-negative rods that can be distinguished from salmonellae by three criteria. Shigella are the most effective pathogens among the enteric bacteria. Shigellosis is only a human disease. The organism is transmitted by the fecal–oral route. After an incubation period of 1 to 4 days, symptoms begin with fever and abdominal cramps, followed by diarrhea, which may be watery at first but later contains blood and mucus. Shigella form non–lactose-fermenting colonies on MacConkey’s or EMB agar. On TSI agar, they cause an alkaline slant and an acid butt, with no gas and no H2S. The main treatment for shigellosis is fluid and electrolyte replacement. In mild cases, no antibiotics are indicated. In severe cases, a fluoroquinolone (e.g., ciprofloxacin) is the drug of choice. Prevention of shigellosis is dependent on interruption of fecal–oral transmission.
V. cholerae, the major pathogen in this genus, is the cause of cholera. Vibrio vulnificus causes cellulitis and sepsis. V. cholerae is divided into two groups according to the nature of its O cell wall antigen. V. parahaemolyticus and V. vulnificus are marine organisms. They live primarily in the ocean, especially in warm salt water. V. cholerae is transmitted by fecal contamination of water and food, primarily from human sources. The main animal reservoirs are marine shellfish, such as shrimp and oysters. Ingestion of these without adequate cooking can transmit the disease. A major epidemic of cholera, which spanned the 1960s and 1970s, began in Southeast Asia and spread over three continents to areas of Africa, Europe, and the rest of Asia. The genes for cholera toxin and other virulence factors are carried on a single stranded DNA bacteriophage called CTX. The pili that attach the organism to the gut mucosa are the receptors for the phage. Watery diarrhea in large volumes is the hallmark of cholera. There are no red blood cells or white blood cells in the stool. During an epidemic, a clinical judgment is made and there is little need for the laboratory. For diagnosis of sporadic cases, a culture of the diarrhea stool containing V. cholerae will show colourless colonies on MacConkey’s agar. Treatment consists of prompt, adequate replacement of water and electrolytes, either orally or intravenously. Prevention is achieved mainly by public health measures that ensure a clean water and food supply. V. cholerae is also diagnosed by presumptive diagnosis and retrospective diagnosis.
Types of Vibrio
V. parahaemolyticus is a marine organism transmitted by ingestion of raw or undercooked seafood especially shellfish such as oysters. It is a major cause of diarrhea in Japan, where raw fish is eaten in large quantities, but is an infrequent pathogen in the United States. An enterotoxin similar to choleragen is secreted and limited invasion sometimes occurs. V. parahaemolyticus is distinguished from V. cholerae mainly on the basis of growth in NaCl. No specific treatment is indicated, because the disease is relatively mild and self-limited. Disease can be prevented by proper refrigeration and cooking of seafood. V. vulnificus is also a marine organism. It causes severe skin and soft tissue infections such as cellulitis. The recommended treatment is doxycycline.
C. jejuni is a frequent cause of enterocolitis, especially in children. C. jejuni infection is a common antecedent to Guillain-Barré syndrome. Campylobacters are curved, gram-negative rods that appear either comma- or S shaped. C. jejuni grows well at 42°C, whereas Campylobacter intestinalis does not. It is an observation that is useful in microbiologic diagnosis. Transmission is usually fecal–oral. Domestic animals such as cattle, chickens, and dogs serve as a source of the organisms for humans. C. jejuni is a major cause of diarrhea in the United States. Enterocolitis begins as watery, foul-smelling diarrhea. The symptoms of bacteremia are associated with no specific physical findings. A stool specimen is cultured on a blood agar plate containing antibiotics that inhibit most other fecal flora. Unlike Shigella and Salmonella, lactose fermentation is not used as a distinguishing feature. Erythromycin or ciprofloxacin is used successfully in C. jejuni enterocolitis. There is no vaccine or other specific preventive measure. Proper sewage disposal and personal hygiene are important.
Helicobacter pylori causes gastritis and peptic ulcers. Infection with H. pylori is a risk factor for gastric carcinoma and is linked to mucosal-associated lymphoid tissue (MALT) lymphomas. In particular, helicobacters are strongly urease-positive, whereas campylobacters are urease negative. The natural habitat of H. pylori is the human stomach, and it is probably acquired by ingestion. H. pylori attaches to the mucus-secreting cells of the gastric mucosa. Loss of the protective mucus coating predisposes to gastritis and peptic ulcer. Most patients with these diseases show H. pylori in biopsy specimens of the gastric epithelium. Antibiotic treatment directed against the organism often causes the tumor to regress. Gastritis and peptic ulcer are characterized by recurrent pain in the upper abdomen, frequently accompanied by bleeding into the gastrointestinal tract. The organism can be seen on Gram-stained smears of biopsy specimens of the gastric mucosa. If the organism is present, urease will cleave the ingested urea, radio-labelled CO2 is evolved, and the radioactivity is detected in the breath. A combination of two antibiotics is used because resistance, especially to metronidazole, has emerged. There is no vaccine or other specific preventive measure.