Gram Negative Rods - Enterobacteriaceae (Coliforms)

Question 1

Features of enterobacteriaceae

Answer 1

1. Oxidase negative
2. Facultative anaerobes
3. Part of gut flora
4. Grows on selective MacConkey agar

Question 2

Classification of enterobacteriaceae

Answer 2

• MacConkey agar inhibits certain bacteria & produces colonies w 2 colours depending on their ability to acidify lactose
• Lactose Fermenters (produce acid - pink/red colonies)
  1. Escherichia coli
  2. Klebsiella
• Non-lactose Fermenters (pale colourless/yellow colonies)
  1. Salmonella enterica (S. Typhi, Paratyphi, Enteritidis, Typhimurium etc)
  2. Shigella (S. sonnei, dysenteriae, flexneri, boydii)
  3. Proteus

Question 3

Virulence factors of E. coli (2)

Answer 3

1. Adhesins (enable them to be uropathogenic), fimbrae, K antigen - not found in normal fecal flora
2. Enterotoxins produced by certain strains eg ETEC

Question 4

Clinical presentations of E. coli (4)

Answer 4

1. UTI (most common cause)
   - common in women due to proximity of short urethra to anus
   - unusual in men unless they have an anatomical disturbance
   - urethral valves, congenital abnormalities, stones, enlarged prostate, cystocele (outpouching of bladder wall), tumour, catheters, disruption of flow at bile ducts eg biliary stones, tumour - disturbs normal flow/poor drainage
2. Diarrheal Disease (major killer)
   - most strains do not cause disease (normal flora) unless they have acquired virulence determinants
   - Enteropathogenic E. coli (EPEC) - infantile GE
   - Enterotoxigenic E. coli (ETEC) - in LDC, travellers diarrhea
   - Enteroinvasive E. coli (EIEC) - invades colonic wall, similar to shigella dysentery
   - Enterohemorrhagic E. coli (EHEC)/Verocytotoxin-producing E.coli (VTEC)/Shiga toxin producing E. coli (STEC) - hemorrhagic colitis (bloody diarrhea), complicated by hemolytic uremic syndrome (acute renal failure, microangiopathic hemolytic anemia, thrombocytopenia)
   - Enteroaggregative E. coli (EAggEC), diffusely adherent E. coli (DAEC)
3. Neonatal Infections (septicemia, meningitis)
4. Sepsis assoc w GIT, biliary systems, post-operatively
   - disease blocks a tube - bacteria cannot drain away - sepsis
   - disease damages GIT epithelium - allows bowel flora to invade bloodstream

Question 5

Diagnosis of E. coli (3)

Answer 5

1. Toxin tests for shiga toxin
2. PCR - detects genes encoding toxins & invasion factors
3. Serology - antibody test for O157 - O antigen is part of bact cell wall, H antigen is part of flagellum

Question 6

Clinical presentations of Klebsiella

Answer 6

Normal colonic flora

1. UTI
2. Severe community-acquired infection - liver abscess, Friedlander’s pneumonia (pneum w abscesses)

• impt cause of HAI, freq multi resistant

Question 7

Transmission of salmonella enterica

Answer 7

Not normal flora

1. Faecal oral - from humans & other animals, contaminated food, poultry, eggs, milk & milk products, reptiles
   - in enteric fever: primarily man
   - in acute gastroenteritis: poor food prep/storage & inadequate cooking

Question 8

Classification of salmonella enterica (3)

Answer 8

1. Enteric Fever group (typhoid, paratyphoid fever) - Salmonella Typhi, Salmonella Paratyphi A, B & C
2. Acute gastroenteritis group (salmonella food poisoning) - Salmonella Enteritidis, Salmonella Typhimurium
3. Carrier state

Question 9

Replication of microbes in enteric fever

Answer 9

• faecal oral - bacteria penetrate ileal mucosa - multiply in mesenteric lymph nodes - enter bloodstream, replicate within macrophages
• incubation period ~10-14 days
• 2nd invasion of bloodstream - clinical symptoms

Question 10

Clinical presentations of enteric fever

Answer 10

• fever, anorexia, epistaxis, cough, headache, abdominal pain & tenderness
• also may have constipation/diarrhea, acute psychosis, bradycardia, enlarged (palpable) liver & spleen, rose spots (rash)
• gall bladder excretes infected bile into gut - 2nd invasion of intestinal wall - inflammation - typhoid ulcers in Peyer’s patches - severe hemorrhage, intestinal perforation
• some become chronic carriers despite recovery due to chronic gall bladder infection - fecal excretion/chronic excretion in urine - potential sources of infection!! (esp in poor sanitation)

Question 11

Diagnosis of enteric fever (2)

Answer 11

1. Culture - blood (early), stool/urine (later)

2. Serology - Widal test, but not that reliable/optimal

Question 12

Prevention of enteric fever (2)

Answer 12

1. Public health (sanitation, carrier detection, education of food handlers, sewage disposal)
2. Vaccination (oral or IM)

Question 13

Clinical presentations of acute gastroenteritis (2)

Answer 13

1. Salmonella food poisoning - usually self limiting
   - diarrhea, abdominal pain, vomiting, fever
   - may also present with bacteremia, focal infections eg bone, UTI
2. Invasive infections
   - atheromatous plaques inside arteries, implanted prosthesis, osteomyelitis (esp in sickle cell), meningitis (esp in neonates)
   - HIV/SLE patients - food poisoning, salmonella may produce an invasive disseminated infection

Question 14

Diagnosis of acute gastroenteritis

Answer 14

Culture - stool, blood

Question 15

Prevention of acute gastroenteritis (3)

Answer 15

1. Clean food, water, sewage (careful food prep, adequate cooking, no cross-contamination)
2. Detect & treat carriers
3. Vaccination & health precautions

Question 16

Transmission of shigella

Answer 16

• Faecal-oral

- person to person via fingers, fomites, flies

Question 17

Clinical presentations of shigella (3)

Answer 17

1. Dysentery (frequent passage of blood stained mucopurulent stools)
   - most strongly assoc with S. dysenteriae, may be complicated by hemolytic uremic syndrome
2. Mild diarrheal disease
   - outbreaks in institutions, commonly by S. sonnei
3. attaches to & invades terminal ileum/colon - inflammation & ulceration, rarely bacteremia

Question 18

Features & clinical presentations of proteus (2)

Answer 18

1. Swarms on the surface of blood agar, very motile
2. UTI
3. Renal calculi/stones (produces urease - splits urea into CO2 & NH3 - alkalinization of urine - precipitation - stone formation)

Question 19

Enterobacteriaceae as a cause of hospital acquired infections

Answer 19

1. GIT surgery - release of bacteria leading to wound inf, peritonitis, abdominal & pelvic abscesses - use prophylactic antibiotics
2. Prolonged hospitalization - broad spectrum antibiotics favour selection & survival of resistant strains of coliforms - urinary tract, wound, lung infections, invasion of bloodstream leading to septicaemia
3. Resistance - hospital acquired bacteria are freq multi resistant

Question 20

Treatment of enterobacteriaceae (5)

Answer 20

1. Ampicillin, amoxicillin (oral, IV) - but may strains are now resistant (produce beta lactamase)
2. Cephalosporins (eg Ceftriaxone) - but selects for a new generation of extended spectrum beta lactamases
3. Alternatives: co-amoxiclav, piptazo (piperacillin + tazobactam), aminoglycosides (gentamicin, amikacin, TDM req due to toxicity), co-trimoxazole (commonly for UTI), quinolones (emerging resistance), carbapenems (imipenem, for very resistant strains)
4. Usually given a combination of antibiotics until susceptibility results are available
5. Rehydration (in diarrheal disease)