Shigella Species and Antibiotic Treatment

E. coli and Shigella

• E. coli meningitis and respiratory tract infections require third-generation cephalosporins (e.g., ceftriaxone).
• Shigella species:
  • Contain four species (A, B, C, and D) that differ antigenically and biochemically.
  • Biochemistry: TSI K/A with no gas, urea negative, motility negative, and all ferment mannitol except S. dysenteriae.
  • Antigenic structure: differentiated into groups (A, B, C, and D) based on O antigen serotyping; K antigens may interfere with serotyping.
  • Virulence factors: Shiga toxin, outer membrane and secreted proteins, and attachment and penetration mechanisms.
  • Clinical significance: causes shigellosis or bacillary dysentery, transmitted via the fecal-oral route.

Enterobacteriaceae

• Phenotypic properties: growth on MacConkey's agar and Blood agar.
• Antigenic structure:
  • Heat-stable somatic O (lipopolysaccharide) antigens (>150).
  • Heat-labile K (capsular) antigens (>100).
  • Flagellar H antigens (>50).
  • Vi antigens in Salmonella serotype Typhi.
• Somatic O antigens: heat-stable polysaccharide part of LPS; variation from smooth to rough colonial forms accompanied by progressive loss of smooth O antigen.
• Flagellar H antigens: heat-labile.
• Envelope or capsule K antigens: overlay the surface O antigen and may block agglutination by O-specific antisera; destroyed by boiling for 15 minutes.

E. coli

• Normal inhabitant of the GI tract.
• Some strains cause gastroenteritis, urinary tract infections, and neonatal meningitis and septicemia.
• Biochemistry: most are motile, may be hemolytic on BA, TSI is A/A + gas, urea -, indole +, citrate -, and motility +.
• Antigenic structure: has O, H, and K antigens; K1 has a strong association with virulence, particularly meningitis in neonates.
• Adhesins (colonization factors): include pili or fimbriae and non-fimbrial factors involved in attachment; at least 21 different types.
• Virulence factors: capsule, iron capturing ability (enterochelin), and outer membrane proteins.

E. coli: Urinary Tract Infections (UTI)

• Clinical significance: leading cause of UTIs, which can lead to acute cystitis and pyelonephritis.
• The formation of E. coli biofilms inside bladder epithelial cells may contribute to recurrent UTIs.

E. coli Infections

• Neonatal meningitis: leading cause of neonatal meningitis and septicemia with a high mortality rate; usually caused by strains with the K1 capsular antigen.
• Gastroenteritis: several distinct types of E. coli are involved in different types of gastroenteritis, including enterotoxigenic, enteroinvasive, enteropathogenic, enteroaggregative, and Shiga toxin-producing E. coli.

Salmonella

• Clinical significance: causes enteric fevers (typhoid and paratyphoid) and gastroenteritis.
• In enteric fevers, Salmonella disseminates before multiplying, causing a low-grade fever and constipation; later, the bacteria re-enter the bloodstream, disseminating throughout the body.
• Diagnosis: blood cultures, stool cultures, and urine cultures; Widal test for antibodies against Salmonella typhi.
• Antimicrobial therapy: chloramphenicol usually; resistant strains have emerged, making antimicrobial susceptibility testing essential.

Citrobacter

• TSI: K/A or A/A, both + gas and H2S.
• Urea: usually +.
• Motility: +.
• Clinical significance: opportunistic pathogen causing urinary tract or respiratory tract infections, and occasionally wound infections, osteomyelitis, endocarditis, and meningitis.

Edwardsiella tarda

• TSI: K/A + gas and H2S.
• Urea: -.
• Citrate: -.
• Indole: +.
• Clinical significance: causes GI disease in tropical and subtropical countries.

Klebsiella

• Respiratory and GI tract, but potential pathogen in other areas.
• TSI: A/A + gas.
• Urea: +.
• Citrate: +.
• MR: -, VP: +.
• Motility: -.
• Has both O and K antigens.
• Virulence factors: capsule, adhesions, and iron capturing ability.
• Clinical significance: causes pneumonia, mostly in immunocompromised hosts; permanent lung damage is a frequent occurrence.

Enterobacter

• GI tract.
• TSI, LIA, and urea give variable results depending on the species.
• Citrate: +.
• Clinical significance: nosocomial infections, bacteremia in burn patients.

Serratia

• A free-living saprophyte.
• TSI: A/A or K/A, +/- gas (does not ferment lactose).
• LIA: usually K/K.
• Citrate: +.
• Motility: +.
• Urea: +/-.
• Has been found in RT and UT infections, and is resistant to many antimicrobics.

Proteus, Providencia, and Morganella

• GI tract infections.
• All motile, with Proteus swarming.
• PA: +.
• Lysine deamination.
• Urea: + for most, strongly + for Proteus.
• TSI: variable.
• Indole: -.