Notes on Enterobacteriaceae Pathogenicity and Enteric Infections

Overview: Enterobacteriaceae as pathogens

  • The Enterobacteriaceae are a family of Gram-negative rods that range from primary pathogens causing severe disease to commensals that colonize the human GI tract and cause disease under certain conditions (opportunists).
  • Clinically, focus is on the pathogens and their related disease states.
  • A majority of these organisms are considered normal flora in the GI tract, GU tract, and respiratory tract, with colonization varying by species.

Opportunistic Enteric Infections

  • Even though these Enterics are normal flora in respiratory, GU, and GI tracts, they can cause serious infection, especially in immunocompromised hosts.
  • Gram-negative rod sepsis and shock can result because all enterics possess endotoxins that can lead to septicemia, DIC, shock, and death.

Genus Klebsiella

  • K. pneumoniae: most common cause of bronchial pneumonia; virulence factor is the capsule; other opportunistic infections include UTI, wound infections, septicemia.
  • K. oxytoca: respiratory infections, UTI, wound infections.

Genus Enterobacter

  • E. cloacae (most commonly encountered): common opportunistic infections include UTI, wound infections, septicemia, and respiratory infections; often seen in long-term intensive care patients.

Genus Serratia

  • S. marcescens: part of human NF and found in nature (soil, water, wood); important cause of health-care-associated infections and may be antibiotic resistant; other infections include UTI, wound infections, septicemia, and respiratory infections that can be severe.

Genus Providencia

  • P. stuartii: common opportunistic infections include UTI, wound infections, septicemia; frequently a cause of UTI in patients with spinal cord paralysis or injury; highly resistant to antibiotics; treated with aminoglycosides.

Genus Proteus

  • P. mirabilis and P. vulgaris: part of NF and the environment; common opportunistic infections include UTI, wound infections, septicemia, meningitis.

Genus Citrobacter

  • C. freundii: common opportunistic infections include UTI, wound infections, septicemia.

Genus Morganella

  • M. morganii: common opportunistic infections include UTI, wound infections.

Serology of Enterics

  • Serology uses antigen typing to identify varying strains of Enterics (notably E. coli, Klebsiella, Shigella, Salmonella).
  • O antigen (somatic, heat-stable lipopolysaccharide) determines serogroup; examples: E. coli O157 (O = somatic group/serogroup).
  • H antigen (flagellar, heat-labile protein) determines serotype; example: h7 (serotype).
  • K antigen (capsular, heat-labile polysaccharide) often masks somatic antigens; capsule can be boiled away to remove capsule.

Serology of Enterics in Primary Pathogens

  • Salmonella: possesses O and H antigens; S. typhi also has capsular Vi antigen.
  • Shigella grouping is based on O antigens (A, B, C, D).
  • Escherichia serology is based on O, K, and H antigens; example: E. coli O157:H7.

Intestinal Infections: Definitions and Enteric Pathogens

  • Intestinal infection types are defined by stool characteristics, tissue invasion, site, and pathogens.
  • Secretory or watery diarrhea: copious, watery stools, no blood or pus; invasion site: small intestine; pathogens include Vibrio cholerae and ETEC.
  • Dysentery: scant volume diarrhea with blood and pus; invasion site: large intestine; pathogens include Shigella species; sometimes parasites.
  • Hemorrhagic colitis: copious, watery diarrhea with no pus due to invasion of large intestine; small intestine involvement may be limited; EHEC (e.g., E. coli O157:H7) is a classic example.
  • Example pathogens and associated patterns:
    • Small intestine: Vibrio cholerae, E. coli (ETEC) -> secretory/watery diarrhea.
    • Large intestine: Shigella species -> bloody, mucus-containing stools with pus in some cases; Salmonella or Campylobacter can also involve colon.
    • EHEC: hemorrhagic colitis with potential progression to HUS.

Escherichia coli (E. coli)

Opportunistic E. coli Infections

  • #1 cause of community-acquired urinary tract infections (~80 ext{%}) and a major portion of HAIs in UTI.
  • Virulence factor: ability to adhere to epithelial cells of the urinary tract.
  • Hemolysin may be a virulence factor (E. coli is beta-hemolytic on SBA).
  • Meningitis: second most common cause of neonatal meningitis (after Group B Strep).
  • Other infections: wound infections, appendicitis, cholecystitis, septicemia, pneumonia, peritonitis.
  • Treatment: initially sensitive to many gram-negative antibiotics (ampicillin, TMP-SMX, cephalosporins); resistance develops rapidly; always perform susceptibility testing.

E. coli as a Primary Pathogen: Four Diarrheagenic E. coli Groups

  • 1) ENTEROPATHOGENIC E. coli (EPEC)
  • 2) ENTEROTOXIGENIC E. coli (ETEC)
  • 3) ENTEROINVASIVE E. coli (EIEC)
  • 4) ENTEROHEMORRHAGIC E. coli (EHEC)
ENTEROPATHOGENIC E. coli (EPEC)
  • Associated with childhood diarrhea in developing countries and hospital-acquired diarrhea in infants.
  • May be life-threatening due to dehydration.
  • Symptoms: diarrhea, malaise, vomiting, fever, mucus in stools (no blood).
  • Produces enterotoxin.
ENTEROINVASIVE E. coli (EIEC)
  • Penetrates epithelial cells and multiplies within them; plasmid-mediated.
  • Incubation: 12−24exth12-24 ext{ h}.
  • Dysenteric illness with fever, abdominal cramping and diarrhea with scant stool containing blood & pus.
  • Infection is rarely foodborne; seen in Eastern Europe, SE Asia, NA; similar to Shigella infections.
ENTEROTOXIGENIC E. coli (ETEC)
  • Most common cause of traveler’s diarrhea and a significant cause of infant diarrhea in developing countries; rarely seen in the U.S. or other developed countries.
  • Source: food and water; incubation: 1−2extdays1-2 ext{ days}; symptoms: watery diarrhea, fever, vomiting, chills, headache; usually resolves in 5−10extdays5-10 ext{ days}; treatment: supportive.
  • Produces two distinct enterotoxins that act on intestinal mucosa to cause fluid outpouring and diarrhea:
    • Heat-labile toxin (LT): interferes with transport of Na+ and Cl− ions, causing net movement of electrolytes and water from mucosa to lumen (similar to cholera toxin).
    • Heat-stable toxin (ST): similar effects.
  • Toxin genes are plasmid-borne and may be transferred to other E. coli.
ENTEROHEMORRHAGIC E. coli (EHEC) – O157:H7
  • Causes hemorrhagic colitis and can lead to HUS and TTP (thrombotic thrombocytopenic purpura).
  • Estimated 10,000−20,00010{,}000-20{,}000 cases per year in the U.S.
  • Reservoir: cattle GI tract; beef contamination during slaughter; undercooked contaminated meat; also linked to contaminated apple cider from cows; rare outbreaks linked to produce.
  • Most serious in children.
  • Symptoms: abdominal pain, watery diarrhea progressing to grossly bloody diarrhea; little or no fever.
  • Virulence factors: Shiga-like toxins (Verotoxin I and II) carried by bacteriophages; verotoxins are responsible for cytotoxic effects.

Sequelae of EHEC: Hemolytic Uremic Syndrome (HUS) and TTP

  • HUS occurs in about ildetilde 5 ext{%} of infected children; leading cause of acute kidney failure in children.
  • Primary symptoms: hematuria and hemolytic anemia; renal damage can cause long-term problems, with ~1/3 having abnormal kidney function years later and some requiring long-term dialysis; ~8% have other lifelong complications (hypertension, seizures, blindness, paralysis).
  • TTP: rapidly fatal with CNS impairment and widespread fibrin/platelet clots.

Prevention and Lab Diagnosis for EHEC

  • Prevention: eradicate organism in cattle/beef; ensure beef is well cooked (internal temperature about 155 extcircC155^\ ext{circ} C); roast beef cooking guidelines noted as 130 extcircC130^\ ext{circ} C for 2 hours in some contexts.
  • Antibiotics: have no effect and may precipitate kidney complications; management is supportive; avoid antidiarrheal agents such as loperamide.
  • Laboratory diagnosis: more than 80 ext{%} of EHEC causing hemorrhagic colitis are serotype O157:H7O157:H7; other serotypes like O26O26 reported elsewhere; these serotypes often appear as sorbitol fermentation negative on MacConkey (SMAC) plates; O157:H7O157:H7 colonies are colorless on SMAC (sorbitol-negative).

Serology of Escherichia coli (Expanded)

  • E. coli serology is based on O (somatic), K (capsular), and H (flagellar) antigens.
  • There are approximately 162162 O antigens, 100100 K antigens, and 5252 H antigens.
  • Certain serogroups/serotypes are more frequently associated with specific GI diseases:
    • ENTEROTOXIGENIC (ETEC): common serogroups include 06,015,0124,0136,0143,0145,014706, 015, 0124, 0136, 0143, 0145, 0147.
    • ENTEROPATHOGENIC (EPEC): common serogroups include 06,08,025,0111,0119,014206, 08, 025, 0111, 0119, 0142.
    • ENTEROINVASIVE (EIEC): common serogroups include 028,0112,0115,0124,0136,0143,0145,0147028, 0112, 0115, 0124, 0136, 0143, 0145, 0147.
    • HEMORRHAGIC COLITIS: 0157:H70157:H7.

Genus Salmonella

  • Salmonella species are named after the original isolations (e.g., Salmonella choleraesuis).
  • Two species: S.entericaS. enterica and S.bongoriS. bongori (the latter rarely isolated);
  • Within S.entericaS. enterica there are subspecies, and serotype names are used for identification.
  • Three classes of disease:
    • Gastroenteritis
    • Enteric fevers (typhoid and paratyphoid)
    • Septicemia (primary, with no other focus) with a potential asymptomatic carrier state.

Sources and Transmission

  • Primary sources are contaminated food and water.
  • Common routes include undercooked poultry (large pieces may not reach high enough internal temperatures), eggs and egg products, and seasonal peaks in summer and around Thanksgiving (late fall).

Prevention and Treatment

  • Prevention: thorough cooking, proper hygiene, washing knives, pans, and surfaces used for poultry; avoid wooden cutting boards; isolate infected patients; public health recalls as needed.
  • Treatment: oral or IV rehydration; antibiotics are not routinely recommended and may prolong carriage; antibiotics do not shorten disease duration and may promote carrier state.
  • Virulence factors: ability to invade and toxin production.

Typhoid Fever (Salmonella enterica serotype Typhi)

  • Source: humans are the only reservoir, especially food handlers; human fecal contamination of food or water.
  • Incubation: 7−14extdays7-14 ext{ days} after ingestion.
  • Week 1: organisms reach the small intestine, attach to ileum/jejunum, penetrate mucosa, spread to connective tissue and regional lymph nodes; phagocytosed by macrophages but not killed; then enter the bloodstream via macrophage transport, reach liver and spleen, replicate in macrophages, and are released into blood causing fever.
  • Symptoms in Week 1: high fever (up to 104extF104^ ext{F}), malaise, lethargy, dull headache, constipation, vomiting.
  • Week 2 and later: invasion of gallbladder, kidney, and Peyer’s patches; diaphoresis, diarrhea, rose spots (blanching, rose-colored spots around the umbilical area); possible death in untreated cases (up to ~10 ext{%}).

Typhoid/Paratyphoid Treatment and Prevention

  • Typhoid treatment: chloramphenicol for several weeks due to intracellular multiplication; difficult to treat.
  • Prevention: proper water treatment and sewage disposal; pasteurization of milk; exclusion of chronic carriers as food handlers; a killed vaccine is available but of limited value; oral vaccine (live attenuated) exists but not as effective as hoped; isolate infected individuals.
  • Paratyphoid fever (paratyphi): clinical features are similar but usually milder; caused by S. enteritidis serotypes A, B, and C (paratyphi C).

Salmonella: Carrier State and Serology Significance

  • Carrier state: about 4 ext{%} become chronic carriers; higher rate with S. Typhi; organisms in biliary tree/gallbladder; can be shed in stool for years; antibiotics may promote carriage.
  • Typhoid Mary is the famous historical carrier; sometimes only gallbladder removal resolves carriage.
  • Serological grouping: Group B (e.g., S. enteritidis, S. typhimurium) is the most common serotype associated with food infection; Group D (S. Typhi) is the most pathogenic group (often Vi antigen positive).
  • Serotyping is critical for tracking epidemics and sources; labs must report Salmonella isolates to public health authorities for outbreak investigation.

Laboratory Testing for Salmonella

  • Specimens: stool (with transport media), rectal swabs, blood.
  • Media: non-lactose fermenting (NLF) on selective-differential media such as BG, SS, XLD, HE; include broth such as selenite or GN broth.
  • Characteristic reactions: TSI = K/A with H2S production; LIA = K/K with H2S.
  • Definitive ID: API, Vitek, MALDI-TOF; confirm serology; report to health department for outbreak tracing.

Genus Shigella

  • Group A: Shigella dysenteriae
  • Group B: Shigella flexneri
  • Group C: Shigella boydii
  • Group D: Shigella sonnei
  • Virulence correlates with invasiveness; cytotoxins including Shiga toxin produced by Shigella dysenteriae type 1; Shiga-like cytotoxins in other species, though less potent.

Shigellosis (Bacillary Dysentery)

  • Humans are the only natural reservoir; contaminated food/water spreads infection.
  • Most common in young children (including daycare settings), travelers, immunocompromised, and men who have sex with men.
  • Inoculum factor: only about 100 organisms or fewer can cause infection.
  • Incubation: 1−7extdays1-7 ext{ days}.
  • Shigella is highly resistant to gastric acidity and can survive in acidic environments.
  • Pathogenesis: invades colonic epithelial cells, multiplies within them, spreads cell-to-cell, causing cell death and ulcer formation with an inflammatory response.
  • Symptoms: abdominal cramps, fever, diarrhea with scant bloody/mucous stools containing numerous PMNs.

Shigella Serotyping and Epidemiology

  • Group A (dysenteriae type 1): most virulent; seen mainly in developing regions (Latin America, Africa, Asia); US isolation usually linked to travel to endemic areas.
  • Group D (sonnei): most common in the US and industrialized nations; common in children in daycare and among institutionalized individuals in crowded conditions.
  • Group B (flexneri): severe diarrhea in developing countries; recent US outbreaks linked to oral/anal sexual practices.
  • Group C (boydii): seen in India and nearby regions.

Virulence Factors, Treatment, and Prevention

  • Virulence: invasiveness and cytotoxins; Shiga toxin (Dysenteriae type 1) is highly potent and can cause neurological symptoms; Shiga-like toxins in other Shigella species.
  • Treatment: often self-limited with clinical recovery in 3-7 days; oral rehydration; antibiotics in severe cases (e.g., ampicillin, TMP-SMX, fluoroquinolones in older children/adults); resistance is common and increasing.
  • Prevention/Control: improved sanitation; isolation of infected patients.

Laboratory Diagnosis for Shigella

  • Specimen: stool (with transport media) and rectal swabs.
  • Media: NLF with selective-differential media such as XLD, SS, HE; enriched broths like selenite and GN broth may be used.
  • Reactions: TSI = K/A; LIA = K/A.
  • Definitive ID: API, Vitek, MALDI-TOF; confirm serology; report to health department for outbreak tracing.

Genus Yersinia

  • About 11 species; first recognized by Alexandre Yersin in 1894; reclassified from Pasteurella to Yersinia.
  • The most commonly isolated clinical species and its natural distribution: widely distributed in water bodies and found in animals (dogs, cats, rodents, rabbits, pigs, sheep, cattle).
  • Major disease in animals (diarrhea, pneumonia, spontaneous abortion) and in humans as gastroenteritis in some regions ( Scandinavia and parts of Europe) but not common in the U.K. or U.S.
  • Portal of entry: oral digestive route via incompletely cooked pork products, dairy products, contaminated water; also via contact with infected animals.
  • Pathogenesis: adheres to and penetrates ileum, causing mesenteric lymphadenitis.
  • Virulence factor: enterotoxin similar to ST of ETEC.

Yersinia enterocolitica: Treatment and Clinical Features

  • Treatment: often self-limiting; supportive care with rehydration (oral or IV); antibiotics (penicillin and cephalosporins) are effective in severe cases.
  • Symptoms: terminal ileitis, mesenteric lymphadenitis, acute enterocolitis; secondary symptoms include erythema nodosum and polyarthritis; can mimic appendicitis.
  • More common in children.

Laboratory Diagnosis for Yersinia enterocolitica

  • Specimen: stool (grows best at room temperature ~25extcircC25^ ext{circ} C or lower).
  • Plate stool to MacConkey or SS; look for clear, colorless colonies. On CIN agar, Yersinia forms distinctive colonies with a "bull's-eye" red center and colorless periphery.
  • Cold enrichment in phosphate-buffered saline; subculture to MacConkey at 25°C weekly for 3 weeks.
  • Biochemical profile: TSI = K/A, LIA = A/K, MR = +, urease = +, ODC = +; non-motile at 37°C, motile at 25°C.

Yersinia pestis: Plague

  • Causative agent of plague; historically caused massive mortality in the 14th century (1336–1350), killing about 25 million in Europe and a third of the world population.
  • Vector: rat flea Xenopsylla cheopis.
  • Reservoirs: via fleas that bite humans after feeding on infected rodents; transmission occurs through flea bite.
  • Epidemiologic forms:
    • URBAN PLAGUE: seen in western U.S.; fleas on prairie dogs, wood rats, skunks, rabbits, chipmunks.
    • SYLVATIC PLAGUE: fleas from rats; pandemics.
  • Treatment: streptomycin or chloramphenicol.
  • Prevention: control of rats/rodents; difficult to control sylvatic cycle.
  • Vaccine: killed whole cells or live attenuated; lifelong immunity after cure.
  • Laboratory diagnosis: safety-pin bipolar staining (bipolar staining) of coccobacilli; TSI: weak A/K.

Clinical Forms of Plague

  • Bubonic: most common; flea bite transmission; fever, buboes (swollen lymph nodes), N/V; if untreated, may progress to septicemia.
  • Pneumonic: inhalation of organisms from coughing patients; responsible for pandemics; >90 ext{%} fatal if untreated; fever, malaise, thin watery sputum with blood.
  • Septicemic: follows other forms; bloodstream infection with dissemination to tissues; 100% fatal if untreated.