Microbiology of the Gastro-Intestinal System Flashcards

Question

Specimen Collection and Handling

Answer 1

Stool specimens: transported to the laboratory shortly after collection – Preservatives (polyvinyl alcohol or formalin) must be avoided if bacterial cultures are ordered Microscopy, Culture and Sensitivity performed on same specimen therefore, normal specimen container used – If rectal swabs are to be processed, Cary- Blair or a similar transport medium should be used

Answer 2

``` Rectal mucosa : scrapings, biopsies – parasites/ova  excreted  more direct specimens, e.g. schistosome ova; Ent. histolytica (trophozoites) ```

Answer 3

Helicobacter pylori take specimens from edge of ulcer, inflammationers to

Answer 4

E.g. giardiasis, strongyloides – Not readily detected in stools – Duodenal aspirates/biopsies required

Answer 5

TB esp. in children and debilitated adults Acute food poisoning- toxins

Answer 6

NB in pts with fever e.g. Salmonella infections – | invasive infection

Answer 7

Stained or unstained slides, wet mount preparations – Microscopic examination of the stool may reveal white blood cells in cases of inflammatory diarrhea (e.g. Salmonella, Shigella, Y ersinia, Campy lobacter, EIEC, and various Vibrio spp.) – Red blood cells may be present because of intestinal wall bleeding – The bacteria may be visible on direct microscopic examination of the stool –

Answer 8

Selective and differential culture media are commonly used to identify bacterial pathogens in stool – Selective media contain antimicrobials or chemicals, which limit the growth of normal bacterial flora and enhance thegrowth of pathogenic bacteria – The differential aspect of the medium often allows differentiation of bacterial species based on colony morphology • Differences in colony appearance are usually a result of different biochemical characteristics of the organisms

Answer 9

Antigen detection Molecular methods Antimicrobial susceptibility testing

Answer 10

Antibiotic associated diarrhoea(AAD) refers to diarrhea following antimicrobial administration Clostridium difficile is the most common identifiable cause Responsible for approximately 30% of cases Although Clostridium/Clostridioides difficile Infection (CDI)is endogenous in most cases, hospital outbreaks have clearly established that the environment can be the source as well Generation of spores from excretions promotes person-to person spread

Answer 11

When C. difficile becomes established in the colon of individuals with normal gut microbiota, its numbers are dwarfed by the other flora Introduction of antibiotics reduces the number of normal gut flora allowing growth of C. difficile This is in particular: ampicillin, cephalosporins, and clindamycin Most strains produce both toxins, the TcdA and TcdB Hypervirulent clones produce higher levels of both toxins and also secrete the C. difficile transferase (CDT) Responsible for more cases and more deaths

Answer 12

Colonic mucosa is studded with inflammatory plaques These may coalesce into an overlying “pseudomembrane”composed of fibrin, leukocytes, and necrotic colonic cells

Answer 13

Selective media used – However, direct detection of toxins in the stool has largely replaced culture for diagnostic purposes • Detection of its toxin has become routine – Immunoassays are used to demonstrate toxin TcdA and/or TcdB in stool – Nucleic acid-based test have also been developed

Answer 14

In AAD discontinuing the implicated antimicrobial often results in the resolution of clinical symptoms – Antibiotics for treatment: Metronidazole/vancomycin /Fidaxomicin • Fidaxomicin, achieves high levels in the bowel with results comparable to vancomycin – Fecal transplant – With all regimens relapses are common presumably due to the survival of the inert spores following a treatment course

Answer 15

– Helicobacter pylori is the etiologic agent – The organism is found in the stomachs of 30% to 50% of adults – The exact mode of transmission is not known, but is presumed to be person to person • Fecal–oral route • Contact with gastric secretions in some way – Colonization increases progressively with age – A declining prevalence in developed countries may be due to decreased transmission because of • Less crowding and frequent exposure to antimicrobial agents

Answer 16

1. Adherence to gastric mucosa Multiple outer membrane proteins which bind to the surface of gastric epithelial cells and certain erythrocyte antigens 2. Motility Flagella to swim to less acidic locale beneath the gastric mucus 3. Persistence in the hostile environment of the stomach and survival of the acid stomach: Urease Protease damage the mucosa – 70–80% of gastric ulcers – 90% of duodenal ulcers – Gastric carcinomas, intestinal lymphomas

Answer 17

Specimen - Gastric mucosa biopsy The most sensitive means of diagnosis is culture of the gastric mucosa Most sensitive indicator of cure following therapy The H. pylori urease activity in biopsies Noninvasive methods include serology and a urea breath test For the breath test, the patient ingests 13C- or 14C-labeled urea, from which the urease in the stomach produces products that appear as labeled CO2 in the breath Detection of antibody directed against H. pylori Because IgG or IgA remains elevated as long as the infection persists, these tests are valuable both for screening and for evaluation of therapy –

Answer 18

Bismuth salts (e.g, Pepto-Bismol) has antimicrobial activity Cure rates approaching 90% have been achieved with various combinations of bismuth salts and/or a protein pump inhibitor plus two antibiotics Clarithromycin plus either amoxicillin or metronidazole and metronidazole plus tetracycline have been effective Relapse rates are low, particularly when acid secretion is also controlled with the use of a proton pump inhibitor Prevention of H. pylori disease awaits further understanding of transmission and immune mechanisms Prophylactic treatment of asymptomatic persons colonized with H. pylori is not yet recommended

Answer 19

Most E. coli diarrhea are mild and self-limiting – Rehydration and supportive measures are the mainstays of therapy – EHEC with hemorrhagic colitis and HUS- supportive measures required, antimicrobials contraindicated – TMP-SMX or fluoroquinolones reduces the duration of diarrhea in ETEC, EIEC, and EPEC infection – Antimotility agents are not helpful and are contraindicated when EIEC or EHEC could be the etiologic agent

Answer 20

Traveler’s diarrhea incidence can be greatly reduced by - eating only cooked foods and peeled fruits - drinking hot or carbonated beverages Avoiding uncertain water, ice, salads, and raw vegetables is a wise precaution when traveling in developing countries • GIT infections caused by Campyl

Answer 21

C. jejuni is by far the most common – Primary reservoir is in animals – Bacteria are transmitted to humans by ingestion of contaminated food or by direct contact with pets • Mostly undercooked poultry, but outbreaks have been caused by contaminated rural water supplies and unpasteurized milk

Answer 22

``` – Oral ingestion – Colonization of the intestinal mucosa – Adherence to enterocytes is facilitated by action of the flagellum – Entrance into cells in endocytotic vacuoles • Once inside, they move in association with the cell’s microtubule structure – Injury mechanisms include the • Cytotoxic cytolethal distending toxin (CDT) • Action of lipooligosaccharides (LOS) released in outer membrane vesicles – The intestinal pathology is that of an invasive pathogen with acute inflammation, crypt abscesses, and occasional seeding of the bloodstream GIT infections caused by Campylobacter • Diagnosis: – Stool samples – Buffered glycerol saline, is toxic to enteric campylobacters and should therefore be avoided – Incubate at 420C- C. fetus @ 370C • Enteric Campy lobacter require a microaerophilic and capnophilic environment ```

Answer 23

``` – Stool samples – Buffered glycerol saline, is toxic to enteric campylobacters and should therefore be avoided – Incubate at 420C- C. fetus @ 370C • Enteric Campy lobacter require a microaerophilic and capnophilic environment ``` ``` Diagnosis – Microscopic Morphology • Campy lobacter spp. are motile, curved, non spore-forming, oxidase positive Gramnegative rods • May appear as long spirals or S’ or seagullwing shapes • On Gram-stained smears, these organisms stain poorly • They exhibit a characteristic darting motility – Colony Morphology • Moist, runny looking, and spreading • Usually nonhemolytic • Some are round and raised and others may be flat • C. fetus subsp. f etus produces smooth, convex, translucent colonies • Most do not produce pigment ```

Answer 24

– Antimicrobial susceptibility testing not routinely performed – Most patients recover without antimicrobial intervention – Drugs of choice for treating intestinal campylobacteriosis are azithromycin and erythromycin – Ciprofloxacin and other quinolones can also be used

Answer 25

``` • Outbreaks usually involve rich meat dishes such as stews, soups, or gravies that have been kept warm for a number of hours before consumption ``` ``` Two types of food poisoning – Type A-relatively mild and self-limited GI illness – Type C- more serious but rarely seen disease • Follows the ingestion of enterotoxin-producing strains • Spores of some strains are heat-resistant and can convert to the vegetative form and multiply when food is not refrigerated or is rewarmed • After ingestion, the enterotoxin is released into the upper GIT, causing a fluid outpouring in which the ileum is most severely involved ```

Answer 26

``` caused by enterotoxin linked to sporulation • After an 8- to 30-hour incubation period, the patient experiences diarrhea and cramping abdominal pain for about 24 hours • Other than fluid replacement, therapy is usually unnecessary ```

Answer 27

``` Type C - enteritis necroticans is associated with strains that produce β-toxin – less comonly, α-toxin • After at least 5 to 6 hours, symptoms begin as acute onset of severe abdominal pain and diarrhea, which is often bloody, and may be accompanied by vomiting • Necrotic inflammation>>bowel perforation • Fatal, treatment is required ```

Answer 28

• Gram-positive bacilli • They are aerobic or facultative anaerobic bacilli that form endospores • Catalase positive • B. cereus food poisoning 2 forms: – Emetic (vomiting) – Diarrhoeal GIT infections caused by Bacillus cereus • Culture of suspected food or stools may be done to quantify and isolate B. cereus • More than 105 cereus cells per gram of food/stools with absence of other pathogens confirms the diagnosis • Most food poisoning cases caused by B. cereus do not require antimicrobial treatment • B. cereus is resistant to penicillin and all of the other βlactam antibiotics except for the carbapenems

Answer 29

``` Diarrheal: 8-16hrs Very common Ocassional 12-24 hrs Meat products ``` ``` Emetic: 1-5hrs Fairly common Very common 6-24hrs Fried or boiled rice ```

Answer 30

``` Staphylococcal food poisoning is one of the most common foodborne illnesses in the world – Short incubation period – Characteristically, the food is moist and rich (e.g, red meat, poultry, creamy dishes) – Contaminated by a preparer who is a carrier – If the food is left unrefrigerated for hours between preparation and serving, the staphylococci are able to multiply and produce enterotoxin in the food – S. aureus enterotoxins stimulate gastrointestinal symptoms (primarily vomiting) – Once formed, these toxins are quite stable • They retain activity even after boiling or exposure to gastric and jejunal enzymes • ```

Answer 31

Patients symptoms will guide Supportive Hand washing during, after and before eating