Bacterial GI infections Flashcards
Why do most people die from GI infections?
Death from dehydration
Describe the epidemiology of GI infections (2)
Major cause of morbidity and mortality in children in the developing world. In some areas of the world, 50% of childhood deaths are attributable to acute diarrheal disease
What is the most important thing to remember about incubation periods of bacterial GI infections?
If you get sick really fast, the sickness is probably toxigenic. If there’s a really long incubation period, most likely E. coli 0157
What are the laboratory diagnostic requirements for suspected GI infections? (3)
Selective media must be used because poop contains a lot bacteria. Stool that is greater than 72 hours old or formed (hardened) is rejected. TCBS agar can be requested for vibrios
Describe the morphology, growth requirements and 2 most common species of Campylobacter associated with disease
Campylobacter is a gram negative, highly motile, spiral rod. It is thermophilic and microaerophilic. It can be grown on blood or charcoal agar containing antibiotics. C jejuni and C coli are the species that most often cause diease.
What are the clinical features of campylobacter? (6)
Campylobacter usually takes 2-4 days to incubate, and requires 1000-10000 cells to cause disease. Fever and myalgia, diarrhoea that is occasionally bloody and abdominal pain are the most common symptoms. It usually resolves itself within a week but may be associated with guillain barre syndrome
What is the Kauffmann-White antigenic scheme? What 3 antigens is the scheme performed against?
Agglutination reactions with specific antisera performed against Salmonella antigens.
O antigen: repeating polysaccharide units of LPS
H antigen: flagellar proteins
Vi antigens: capsular polysaccharide
What are the symptoms, duration and treatment of salmonella gastroenteritis?
Nausea, vomitting, diarrhoea, abdominal cramps, myalgia and headache, fever. Usually clears itself in 2-7 days. The only time you would treat it is in patients who have low stomach acid levels such as people under 6 months and over 50 years
Describe the incubation period, symptoms, diagnosis and sequelae of salmonella enteric fever
Incubation period of 10-14 days. Bacteremic illness with headache, myalgia, fever, abdominal pain, splenomegaly, rose spots on abdomen. Positive tests from blood, bone marrow, urine and stool.
Sequelae: intestine hemorrage and perforation
How is salmonella isolate by culture?
Placed in pre-enrichment broths containing selenite or tetrathionate. Plate on selective and differential media: XLD, HE, SS, bismuth-sulfite agar.
What is XLD agar and what does it contain?
A selective and differential agar for the isolation of gram negative enteric pathogens. It contains xylose, lysine, sodium desoxycholate, sodium thiosulfate and ferric ammonium citrate
What are the principles of use for XLD media? What 3 reactions does it utilize to differentiate salmonella and shigella from non-pathogenic bacteria?
It is used to differentiate salmonella and shigella from non-pathogenic bacteria.
1) Xylose fermentation: all enterics (including salmonella) except shigella rapidly ferment xylose rapidly, causing the agar to turn yellow
2) Lysine decarboxylation: salmonella rapidly exhausts xylose and decarboxylates lysine, reverting back to alkaline conditions (red). Lactose and sucrose are added in excess to prevent lactose fermentors from also reverting to alkaline.
3) Hydrogen sulfide production: under alkaline conditions, H2S production will create black ppt, turning colonies black but under acidic conditions the reaction is inhibited.
Describe the incubation and symptoms of E. coli 0157 (4)
3-8 day incubation, diarrhoea that is mild to very bloody (haemorrhagic colitis), sever abdominal cramping and in 5% of patients hemolytic urenic syndrome
What HUS?
Hemolytic urenic syndrome. Causes anemia, thrombocytopenia and kidney damage.
What is the distinguishing virulence factor of E coli 0157?
Shiga-like toxin. It is a subunit toxin where A inhibits protein synthesis at the ribosomal level and B binds a glycolipid receptor in mammalian endothelial cells.
