The Enterobacteriaceae
Common characteristics ★★★
The Enterobacteriaceae are a large, heterogeneous group of Gram-negative rods whose natural habitat is the intestinal tract of humans and animals. 51 genera are defined. Three matter here: Escherichia (normal flora that can turn pathogen), and Shigella and Salmonella (pathogens).
| Feature | Detail |
|---|---|
| Morphology | Gram-negative rods · no endospores · most form pili and peritrichous flagella |
| Culture | Facultative anaerobes; grow readily on simple media. Colonies circular, convex, smooth; some form haemolytic zones on blood agar; liquid media become turbid |
| Biochemical reactions | Active and diverse — the key measure to isolate and identify Enterobacteriaceae. Catalase (+), oxidase (−) |
| Resistance | Relatively weak — 60 °C for 30 min; chlorine compounds used for water |
⭐ The lactose fermentation test — the single most useful rule
| Organism | Lactose |
|---|---|
| Pathogenic enteric bacilli — Shigella, Salmonella | Negative |
| Non-pathogenic enteric bacilli — E. coli as normal flora | Positive |
SS agar (Salmonella–Shigella agar) exploits this. Its ingredients are nutrient agar · lactose · bile salts (inhibitor) · sodium citrate (inhibitor) · sodium thiosulfate · 10% ferric citrate · 1% neutral red (indicator) · 1% brilliant green (inhibitor). The inhibitors suppress Gram-positives and Proteus; the indicator turns red in acid and yellow in alkali. A colourless colony is a non-lactose fermenter — Shigella or Salmonella.
Ask why lactose in particular, and the design of the whole medium becomes obvious.
A stool sample contains overwhelmingly more E. coli than pathogen. You cannot look for the pathogen directly; you must make it stand out. E. coli ferments lactose, producing acid, turning neutral red — a pink or red colony. Shigella and Salmonella cannot, so they stay colourless against a field of pink.
The bile salts and brilliant green remove the Gram-positives that would clutter the plate; the thiosulfate and ferric citrate add a second readout, blackening colonies that produce H₂S — which separates Salmonella (H₂S +/−) from Shigella (H₂S −).
So a single plate answers three questions at once: is it an enteric rod, does it ferment lactose, does it make H₂S. That is why the lecture calls biochemical reactions the key measure.
- Describe the common morphology of Enterobacteriaceae. → Gram-negative rods, no endospores, most with pili and peritrichous flagella
- Catalase and oxidase? → Catalase positive, oxidase negative
- What does the lactose fermentation test show? → Pathogenic enteric bacilli are negative; non-pathogenic are positive
- What is a colourless colony on SS agar? → A non-lactose fermenter — Shigella or Salmonella
- Resistance? → Relatively weak — killed at 60 °C in 30 min, and by the chlorine compounds used in water treatment
The antigens ★★★
| Antigen | Nature | Antibody class |
|---|---|---|
| O antigen | LPS — heat-stable somatic antigen | IgM |
| H antigen | Flagella | IgG |
| Capsular antigens | Vi antigen and K antigen |
In E. coli the numbers are large — more than 170 O types, more than 50 H types and more than 100 K types. A particular combination is a serotype: E. coli O157:H7 is the O antigen 157 with the H antigen 7.
Unit 2 §4 already gave the reason: the O polysaccharide of LPS carries species specificity. Serotyping is reading that molecule.
- Name the antigens and what they are. → O (LPS, somatic, heat-stable, IgM) · H (flagellar, IgG) · capsular Vi and K
- How many O, H and K types has E. coli? → More than 170 O, more than 50 H, more than 100 K
- What does O157:H7 mean? → O antigen type 157 with H antigen type 7 — a serotype
Escherichia coli ★★★
| Property | Detail |
|---|---|
| Morphology | Peritrichous flagella |
| Culture | Grows well on non-selective media — circular, smooth, colourless colonies 2–3 mm in diameter in 18 h on nutrient agar |
| Biochemistry | Most strains ferment lactose and glucose with production of acid and gas. H₂S test negative. IMViC reaction: + + − − (indole, methyl red, Voges–Proskauer, citrate utilisation) |
| On EMB agar | Purple to black colonies with a green metallic sheen |
| On SS agar | Pink colony |
Virulence factors
| Group | Factors |
|---|---|
| 1 · Adhesins — facilitate specific adherence to the urinary and intestinal tract | Colonisation factor antigens (CFA/I, II, III) · aggregative adherence fimbriae (AAF I/III) · bundle-forming pili (Bfp) |
| 2 · Exotoxins | Shiga toxins (Stx) · heat-stable enterotoxins (STa, STb) · heat-labile enterotoxins (LT-I, LT-II) · haemolysin |
Diseases
| Group | Diseases |
|---|---|
| Extraintestinal infections — opportunistic E. coli | Pyogenic infection: sepsis (septicaemia), neonatal meningitis · urinary tract infection (UTI) by uropathogenic E. coli (UPEC) — acute cystitis, pyelonephritis |
| Intestinal infections — pathogenic E. coli | Gastroenteritis — the five classes in §4 |
- What is the IMViC reaction of E. coli? → + + − − (indole, methyl red, Voges–Proskauer, citrate)
- How does E. coli appear on EMB and SS agar? → Purple to black with a green metallic sheen on EMB; pink on SS
- Name the two groups of E. coli virulence factors. → Adhesins (CFA, AAF, Bfp) and exotoxins (Shiga toxin, ST, LT, haemolysin)
- Name the extraintestinal diseases. → Sepsis, neonatal meningitis, and UTI (acute cystitis, pyelonephritis) caused by uropathogenic E. coli
⭐ The five pathogenic E. coli ★★★
Five classes of E. coli that cause diarrhoeal disease are now recognised. The deck gives them their own summary table, which means it expects this to be reproduced — and it is exactly Section III shape.
| Class | Site | Disease | Pathogenesis |
|---|---|---|---|
| ETEC — enterotoxigenic | Small intestine | Traveller's and infant diarrhoea; watery diarrhoea without blood; self-limiting (3–4 days) | Plasmid-mediated LT and ST enterotoxins, stimulating hypersecretion of water and electrolytes. Also CFA I/II/III. LT is similar to Vibrio cholerae enterotoxin, destroyed at 65 °C for 30 min; ST is stable at 100 °C for 20 min |
| EIEC — enteroinvasive | Large intestine | Diarrhoea in older children and adults; dysentery-like, with mucus and blood; severe inflammation and fever | Plasmid-mediated invasion and destruction of colonic epithelial cells. Shigella-like; does NOT produce enterotoxin |
| EPEC — enteropathogenic | Small intestine | Infant diarrhoea; watery diarrhoea | Plasmid-mediated adherence and destruction of epithelial cells via bundle-forming pili; “attaching and effacing” — destruction of microvilli. Does not produce LT or ST |
| EHEC — enterohaemorrhagic | Large intestine | Mild diarrhoea through to haemorrhagic colitis with bloody diarrhoea; haemolytic uraemic syndrome (HUS) — haemolytic anaemia, thrombocytopenia and kidney failure | Bacteriophage-mediated toxin. Represented by serotype O157:H7; produces Stx-I and Stx-II (Shiga toxin) but not LT or ST, which disrupts protein synthesis and selectively destroys renal endothelial cells |
| EAEC — enteroaggregative | Small intestine | Persistent watery diarrhoea in young children | Plasmid-mediated aggregative adherence — adheres via pili, aggregating in a “stacked brick” arrangement; produces enterotoxin causing mucosal damage and secretion of large amounts of mucus |
Four features of EHEC hang together, and each explains the next.
Its toxin is phage-encoded, not plasmid-encoded like the other four. That is Unit 4's lysogenic conversion: an ordinary E. coli became lethal because a bacteriophage infected it and brought the Shiga toxin gene.
Shiga toxin disrupts protein synthesis and selectively destroys renal endothelial cells. Hence haemolytic uraemic syndrome — red cells shredded through damaged vessels (haemolytic anaemia), platelets consumed there (thrombocytopenia), and glomeruli destroyed (kidney failure). Three findings, one mechanism.
It is the reason antibiotics are contraindicated in suspected O157:H7. Killing the organism lyses it and releases more toxin, and the risk of HUS rises. This is the same principle as endotoxin release in Unit 6 §3.
Which is also the deck's message in “the best treatment is fluid and electrolyte replacement”.
Laboratory diagnosis: specimens — urine, blood, stool; smears and stains; isolation and identification on differential media, then biochemical reactions for genus/species and serological tests for species/serotype. Prevention is preventing faecal contamination of food and water; the best treatment is fluid and electrolyte replacement, with antibiotics guided by susceptibility testing.
- Name the five pathogenic E. coli. → ETEC, EIEC, EPEC, EHEC, EAEC
- Which causes traveller's diarrhoea, and how? → ETEC — plasmid-encoded LT and ST enterotoxins causing hypersecretion of water and electrolytes in the small intestine
- Which is Shigella-like, and what is missing? → EIEC — invades and destroys colonic epithelium, but produces no enterotoxin
- Which causes HUS, by what serotype and toxin? → EHEC, serotype O157:H7, Shiga toxins Stx-I and Stx-II (phage-encoded)
- What are the three features of HUS? → Haemolytic anaemia, thrombocytopenia, kidney failure
- What is the 'stacked brick' organism? → EAEC
- What is the best treatment for E. coli diarrhoea? → Fluid and electrolyte replacement
Shigella ★★★
Shigella are the agents of bacillary dysentery (shigellosis).
| Test | Result |
|---|---|
| Flagella | None |
| Motility | Negative |
| Glucose fermentation | Positive |
| Lactose fermentation | Negative — except S. sonnei |
| H₂S | Negative |
| Antigens | O and K — no H, because no flagella |
Virulence factors — three
| Factor | Detail |
|---|---|
| 1 · Invasiveness | Encoded by large extrachromosomal elements (plasmids). Induces endocytic uptake by M cells, epithelial cells and macrophages, then lyses the plasma membranes, resulting in intercellular bacterial spread. Does NOT enter the blood — infection is limited to the intestinal mucosa, and blood dissemination rarely occurs |
| 2 · Endotoxin | Damages the mucous membrane → necrosis, ulceration, bleeding: pus and blood in the stools. Stimulates the autonomic nerves of the bowel wall → abdominal cramps and tenesmus. Increases mucosal permeability → fever, shock, DIC |
| 3 · Exotoxin — Shiga toxin | Chromosomally encoded (unlike EHEC's, which is phage-encoded) · enterotoxic · inhibits protein synthesis |
Clinical findings
| Detail | |
|---|---|
| Source | Patients and carriers; humans are the primary reservoir |
| Route | Faecal–oral |
| Acute shigellosis | Diarrhoea, bloody stool more than ten times a day |
| Acute toxic shigellosis — in children | Mild or no gastrointestinal symptoms, but severe toxic symptoms: high fever, shock, toxic encephalopathy in infants — high mortality |
| Chronic shigellosis | More than 2 months, with atypical symptoms |
Immunity: circulating antibody in the blood gives no protection; intestinal sIgA is what protects. That follows directly from the organism staying in the mucosa and never entering the blood.
| Diagnosis | Detail |
|---|---|
| Sampling | Fresh stool, mucus flecks and rectal swabs; faecal leukocytes and some RBCs are seen |
| Isolation | SS agar, EMB agar → colourless, non-lactose-fermenting colonies, then Kligler's iron agar or triple sugar iron agar: Shigella gives an alkaline slant and an acid butt with no gas bubbles |
| Identification | Slide agglutination with antiserum for serogroup and serotype · PCR · ELISA |
| Treatment and control | Sanitary control of water, food and milk · isolation of patients and disinfection of excreta · detection of subclinical cases and carriers · antibiotics (e.g. ampicillin) for severe dysentery |
- What disease does Shigella cause, and by what route? → Bacillary dysentery (shigellosis), faecal–oral, humans the primary reservoir
- Give its biochemical profile. → Non-motile, no flagella; glucose +, lactose − (except S. sonnei), H₂S −; O and K antigens only
- Name its three virulence factors. → Plasmid-encoded invasiveness (does not enter the blood), endotoxin, and chromosomally encoded Shiga toxin
- What does the endotoxin cause? → Mucosal necrosis, ulceration and bleeding (pus and blood in stool); autonomic stimulation (cramps, tenesmus); increased permeability (fever, shock, DIC)
- What is acute toxic shigellosis? → In children — little or no GI upset but high fever, shock and toxic encephalopathy, with high mortality
- Why does circulating antibody not protect? → The organism stays in the intestinal mucosa; intestinal sIgA is what matters
Salmonella ★★★
| Test | Result |
|---|---|
| Flagella / motility | Flagellated; motility positive |
| Lactose fermentation | Negative |
| Glucose fermentation | Positive |
| H₂S | Positive or negative |
| Antigens | H · O · Vi (only a few serovars) |
Virulence factors
- Invasiveness — Vi (capsular) antigen and pili
- Endotoxin — fever, leukopenia, shock
- Enterotoxin (some strains) — like the ETEC enterotoxin
Transmission: the infection source is patients and carriers; the route is faecal–oral, by ingestion of contaminated food or water.
⭐ The four clinical manifestations
| Form | Detail |
|---|---|
| 1 · Enteric fever | Typhoid — the severe systemic form, may be fatal. Incubation 10–14 days. Symptoms are non-specific: fever, anorexia, headache, myalgia and constipation, with rose spots. Complications: severe intestinal haemorrhage and intestinal perforation. Paratyphoid is less severe with a shorter course |
| 2 · Gastroenteritis (food poisoning) | The most common salmonellosis — 70%. Pathogens: S. typhimurium, S. enteritidis, S. cholerae-suis. Incubation 6–48 hours; fever and watery diarrhoea; self-limiting in 2–3 days |
| 3 · Septicaemia | S. cholerae-suis, S. typhimurium, S. enteritidis, S. schottmuelleri in children and low-immunity patients: fever, chill, meningitis, osteomyelitis. Intestinal manifestations are often absent and the bacteria cannot be isolated from faecal specimens |
| 4 · The prolonged carrier state | 1–5% of patients continue to excrete Salmonella for a year or more. The bacilli are most commonly present in the gallbladder |
⭐ Specimens in enteric fever — the classic examination point
| Time | Specimen |
|---|---|
| 1st week | Peripheral blood |
| 1st–3rd weeks | Bone marrow |
| 2nd week | Stool |
| 3rd week | Urine |
For gastroenteritis the specimens are stool and suspected food; for septicaemia, blood. Identification uses biochemical reactions (H₂S and others) on SS agar and serological agglutination tests.
A quantitative agglutination test for enteric fever, which detects a patient's antibodies to the specific O antigen of S. typhi and the H antigens of S. typhi, S. paratyphi A, B and C.
That four-row table looks arbitrary until you follow the organism.
Salmonella typhi is swallowed, invades through the small-intestinal wall, and goes to the lymphatics and the bloodstream — so in week 1 the patient has a bacteraemia and no diarrhoea, and you culture blood. (That is also why typhoid presents with constipation, which surprises students expecting a gut infection.)
It then multiplies in the reticuloendothelial system — hence bone marrow is positive longest, weeks 1–3, and is the most sensitive specimen.
It is excreted in bile back into the gut in week 2, so stool becomes positive, and reaches the kidney by week 3 for urine.
The bile point returns at the end: the chronic carrier harbours the organism in the gallbladder. Same anatomy, whole story.
Vaccines are available for typhoid fever, and typhoid and septicaemia are treated with antibiotics.
- Give Salmonella's biochemical profile and antigens. → Motile and flagellated; lactose −, glucose +, H₂S +/−; H, O and Vi antigens
- Name its three virulence factors. → Invasiveness (Vi capsular antigen and pili), endotoxin (fever, leukopenia, shock), enterotoxin in some strains
- Name the four clinical forms. → Enteric fever, gastroenteritis (most common, 70%), septicaemia, prolonged carrier state
- Describe typhoid. → Severe systemic form, incubation 10–14 days, non-specific fever, anorexia, headache, myalgia, constipation, rose spots; complications are intestinal haemorrhage and perforation
- Which specimen in which week of enteric fever? → Blood week 1 · bone marrow weeks 1–3 · stool week 2 · urine week 3
- Define the Widal test. → A quantitative agglutination test for enteric fever detecting antibodies to the O antigen of S. typhi and the H antigens of S. typhi and S. paratyphi A, B and C
- Where does the chronic carrier harbour the organism? → The gallbladder
Revision
⭐ The three genera side by side
| E. coli | Shigella | Salmonella | |
|---|---|---|---|
| Lactose | + | − (except S. sonnei) | − |
| Motility | + | − | + |
| H₂S | − | − | +/− |
| Antigens | O, H, K | O, K (no H) | H, O, Vi |
| Blood invasion | Yes, in extraintestinal disease | No — confined to mucosa | Yes — enteric fever |
| Key disease | UTI, sepsis, 5 diarrhoeal classes | Bacillary dysentery | Typhoid, food poisoning |
The five pathogenic E. coli, compressed
| Class | One-line answer |
|---|---|
| ETEC | Small intestine · traveller's/infant watery diarrhoea · LT and ST |
| EIEC | Large intestine · dysentery-like with blood and mucus · invasion, no enterotoxin |
| EPEC | Small intestine · infant watery diarrhoea · attaching and effacing, Bfp |
| EHEC | Large intestine · haemorrhagic colitis and HUS · O157:H7, Shiga toxin |
| EAEC | Small intestine · persistent watery diarrhoea · stacked brick adherence |
- Which lactose result marks a pathogen? → Negative — pathogenic enteric bacilli do not ferment lactose
- Name the five pathogenic E. coli with site and disease. → See the table above
- Compare Shigella and Salmonella invasion. → Shigella stays in the intestinal mucosa and does not enter the blood; Salmonella typhi invades and causes a bacteraemia
- Give the enteric-fever specimen sequence. → Blood (wk 1) · bone marrow (wks 1–3) · stool (wk 2) · urine (wk 3)
- Define the Widal test. → Quantitative agglutination for enteric fever, detecting antibody to the O antigen of S. typhi and H antigens of S. typhi and S. paratyphi A, B, C