📘 Enterobacteriaceae & Unusual Pathogens

gram-negative rod lactose fermenter facultative anaerobe

Key virulence & syndromes

• UTI: ascending from fecal flora; adhesins (P pili), hemolysin.
  Diagnosis: ≥10⁵ CFU/mL, Gram-negative rods.
• Neonatal meningitis/sepsis: K1 capsule (anti-phagocytic); maternal flora.
• Gastroenteritis pathotypes:
  • ETEC (traveler’s): LT (adenylate cyclase) / ST (guanylate cyclase), colonization factors.
  • EPEC (infantile): attaching/effacing, actin rearrangement.
  • EIEC (inflammatory): invades colonic epithelium, shigella-like.
  • EAEC (aggregative): fimbriae, IL-8.
  • EHEC (O157:H7): Shiga-like toxins (verotoxin), inhibit 60S ribosome; sorbitol-negative; HUS risk; antibiotics may ↑ HUS.

encapsulated lactose fermenter oxidase negative

• Reservoir: colon, upper respiratory tract.
• Virulence: polysaccharide capsule (anti-phagocytic), endotoxin.
• Diseases: pneumonia (frequent abscesses), UTI (catheter-associated), septicemia.
• Treatment: 3rd-gen cephalosporin ± aminoglycoside; carbapenems for ESBL/KPC.

Klebsiella granulomatis (Donovanosis)

• Sexually transmitted → painless genital ulcers (beefy red).
• Diagnosis: Donovan bodies (Wright-Giemsa), not Gram stain.
• Treatment: doxycycline.

nonmotile non-lactose fermenter human reservoir only

• Species: S. sonnei (most common U.S.), S. flexneri, S. dysenteriae (Shiga toxin, most severe).
• Shiga toxin: A-B toxin, inhibits 60S ribosome → neurotoxic, cytotoxic, enterotoxic.
• Clinical: watery → bloody diarrhea, tenesmus, fever >38.3°C; rarely septicemia.
• Diagnosis: stool culture; nonmotile, H₂S negative.
• Treatment: fluid/electrolytes; antibiotics for severe (resistance common).

Yersinia pestis

bipolar staining coagulase positive

• Reservoir: rodents (prairie dogs, squirrels); southwestern U.S.
• Transmission: flea bite (sylvatic) or respiratory droplets (pneumonic).
• Virulence: F-1 envelope antigen, type III secretion system.
• Bubonic: fever, buboes; pneumonic: hemoptysis, highly contagious (bioterrorism).
• Treatment: aminoglycosides.

Yersinia enterocolitica

• Motile at 25°C, nonmotile at 37°C; cold growth.
• Transmission: unpasteurized milk, pork; northern climates.
• Disease: enterocolitis (febrile diarrhea, pseudoappendicitis), reactive arthritis.
• Treatment: supportive; fluoroquinolones for immunocompromised.

swarming motility urease positive non-lactose fermenter

• Reservoir: colon, environment.
• Diagnosis: swarming on blood agar; non-lactose fermenting.
• Treatment: fluoroquinolones, TMP-SMX, 3rd-gen cephalosporins; remove stones.

Salmonella typhi (typhoid fever)

• Human reservoir; fecal-oral; Vi capsule.
• Invades M cells → macrophages (survives intracellularly).
• Fever, headache, rose spots, constipation; complications: perforation.
• Diagnosis: blood, stool, Widal test (antibodies).
• Treatment: fluoroquinolones, 3rd-gen cephalosporins.

Non-typhi Salmonella (enteritidis, typhimurium)

• Reservoir: poultry, reptiles, domestic animals.
• Gastroenteritis (self-limiting), septicemia (elderly/young), osteomyelitis in sickle cell disease.
• Diagnosis: Hektoen agar; H₂S positive, motile.
• Antibiotics contraindicated for gastroenteritis; treat invasive disease.

Gram-variable catalase negative

• Bacterial vaginosis: thin, gray discharge, fishy odor (whiff test).
• Clue cells (epithelial cells covered with bacteria), pH >4.5.
• Treatment: metronidazole, clindamycin.

anaerobic Gram-negative rod capsule

• Reservoir: human colon (predominant anaerobe).
• Treatment: metronidazole, clindamycin, cefoxitin; surgical drainage.
• Resistance: penicillin, aminoglycosides; increasing clindamycin resistance.

Treponema pallidum (syphilis)

• Thin spirochete, not Gram-stained; obligate pathogen.
• Primary: painless chancre; secondary: maculopapular rash (palms/soles), condylomata lata; tertiary: gummas, aortitis, CNS.
• Diagnosis: serology (nontreponemal: VDRL/RPR; treponemal: FTA-ABS).
• Treatment: benzathine penicillin; Jarisch-Herxheimer reaction possible.

Borrelia burgdorferi (Lyme disease)

• Ixodes ticks; white-footed mice/deer reservoir.
• Stage 1: erythema migrans, flu-like; stage 2: neurologic, cardiac; stage 3: arthritis (immune complexes).
• Diagnosis: ELISA + Western blot.
• Treatment: doxycycline, amoxicillin; ceftriaxone for disseminated.

Leptospira interrogans

• Zoonotic: animal urine, contaminated water.
• Influenza-like symptoms, Weil disease (hepatorenal failure).
• Diagnosis: serology (MAT); dark-field microscopy (insensitive).
• Treatment: penicillin, doxycycline.

obligate intracellular elementary / reticulate bodies

• C. trachomatis: D–K (NGU, PID, inclusion conjunctivitis); L1–3 (lymphogranuloma venereum); A–C (trachoma).
• C. pneumoniae: atypical pneumonia (walking pneumonia), hoarseness.
• C. psittaci: psittacosis (parrot exposure), atypical pneumonia + hepatitis.
• Diagnosis: NAAT, cytoplasmic inclusions (Giemsa/iodine).
• Treatment: azithromycin, doxycycline; erythromycin for neonates.

• Treatment: doxycycline (even in children).
• Ehrlichia / Anaplasma: fever, leukopenia, thrombocytopenia, morulae (monocytes or granulocytes).
• Coxiella burnetii (Q fever): spore-like, inhalation; atypical pneumonia, hepatitis, endocarditis.

no cell wall sterols in membrane extracellular

• Walking pneumonia: persistent dry cough, pharyngitis; most common atypical pneumonia in young adults.
• Attachment via P1 protein, damage cilia; superantigen.
• Diagnosis: PCR/serology; cold agglutinins (65% positive); fried-egg colonies.
• Treatment: macrolides (no β-lactams).

Ureaplasma urealyticum

• Urethritis, prostatitis; urease positive → alkaline urine, calculi.
• Treatment: erythromycin, tetracycline.