Emerging Bacterial Infections

Intro: Emerging Infections - New Bugs on Block

• Emerging Infections: Newly identified in a population or those with rapidly ↑ incidence or geographic spread.
• Re-emerging Infections: Previously controlled infections now reappearing or ↑ in incidence (e.g., Diphtheria).
• Key Factors Driving Emergence:
  • Microbial adaptation (e.g., genetic mutations, antimicrobial resistance)
  • Human demographics & behavior (e.g., urbanization, international travel)
  • Ecological changes (e.g., deforestation, climate change)
  • Breakdown in public health measures (e.g., ↓ surveillance, ↓ vaccination rates)

⭐ Antimicrobial resistance (AMR) is a primary engine for the rise of both emerging and re-emerging bacterial infections globally, complicating treatment and control efforts significantly.

Gram-Positive Threats - Super Staph & C.diff Clout

and Gram-positive rods (Clostridioides difficile) microscopy)

1. MRSA (Methicillin-Resistant S. aureus)

• CA-MRSA vs. HA-MRSA:

  Feature	CA-MRSA	HA-MRSA
  Virulence	PVL+ (SSTIs, necro. pneumonia)	PVL often -
  SCCmec	IV, V	I, II, III
  Rx Sensitivity	Better to non-β-lactams	Multi-drug resistant

• PVL (Panton-Valentine Leukocidin): CA-MRSA virulence, pore-forming.
• Treatment (severe): Vancomycin, Linezolid, Daptomycin.

2. Hypervirulent Clostridioides difficile

• Strain: NAP1/BI/027 - ↑ FQ resistance, ↑ toxin production.
• Toxins:
  • Toxin A (enterotoxin)
  • Toxin B (cytotoxin, potent)
  • Binary toxin (CDT) - assoc. ↑ severity.
• Clinical: Severe diarrhea, pseudomembranous colitis.
• Diagnosis: NAAT for toxin genes.
• Management (initial episode): Oral Vanco (125mg QID)/Fidaxo (200mg BID). FMT (recurrent).

⭐ > NAP1/BI/027 C. difficile: ↑ toxins A & B production → severe disease, ↑ mortality.

Gram-Negative Threats - Resistance Rampage

• Carbapenem-Resistant Enterobacteriaceae (CRE): Significant global health threat.
  • Key Species: Klebsiella pneumoniae, E. coli.
  • Mechanisms (Carbapenemases): 📌 King New Ox (KPC, NDM, OXA)
    • KPC (Klebsiella pneumoniae carbapenemase): Class A Serine β-lactamase.
    • NDM-1 (New Delhi metallo-β-lactamase): Class B Metallo-β-lactamase.
    • OXA-48 (Oxacillinase-48 type): Class D Serine β-lactamase.
  • Detection:
    • Phenotypic: Modified Hodge Test (MHT), Carba NP test, mCIM (modified Carbapenem Inactivation Method).
    • Genotypic: PCR for resistance genes (e.g., blaKPC, blaNDM, blaOXA-48).
  • Treatment: Challenging; often requiring last-resort drugs (e.g., colistin, tigecycline) or newer agents (ceftazidime-avibactam). Associated with ↑ mortality.
• MDR Acinetobacter baumannii & P. aeruginosa: Also significant carbapenem-resistant threats, complicating treatment.

⭐ NDM-1 confers resistance to almost all β-lactams, including carbapenems, except aztreonam (variable) and newer β-lactam/β-lactamase inhibitor combinations.

Zoonotic & Vector-Borne - Ticks & Tricks

• Lyme Disease (Borrelia burgdorferi)
  • Vector: Ixodes ticks (deer tick)
  • Features: Stage 1: Erythema migrans (bull's-eye rash); Stage 2: Bell's palsy, AV block; Stage 3: Arthritis, encephalopathy
  • Dx: Serology (ELISA, Western Blot)
  • Rx: Doxycycline; Amoxicillin (<8 yrs); Ceftriaxone (severe)
• Ehrlichiosis (Ehrlichia chaffeensis - HME) & Anaplasmosis (Anaplasma phagocytophilum - HGA)
  • Vectors: Ticks (Lone Star - Ehrlichia; Ixodes - Anaplasma)
  • Features: Fever, myalgia, headache; Morulae in WBCs (monocytes - HME; granulocytes - HGA)
  • Dx: PCR, Serology, Blood smear (morulae)
  • Rx: Doxycycline
• Scrub Typhus (Orientia tsutsugamushi)
  • Vector: Trombiculid mites (chiggers, larval stage)
  • Features: Eschar (often pathognomonic), fever, rash. Severe: ARDS, multi-organ dysfunction
  • Dx: Serology (Weil-Felix OX-K positive), IFA, PCR
  • Rx: Doxycycline, Azithromycin

  ⭐ Orientia tsutsugamushi (Scrub Typhus) is a key cause of acute undifferentiated fever in India, frequently presenting with an eschar.

High‑Yield Points - ⚡ Biggest Takeaways

• Melioidosis, caused by Burkholderia pseudomallei, shows "safety pin" appearance on Gram stain; endemic in SE Asia.
• Capnocytophaga canimorsus from dog bites causes severe sepsis in asplenic patients.
• Elizabethkingia meningoseptica causes neonatal meningitis and is often multidrug-resistant.
• CRE (e.g., NDM-1 Klebsiella) pose a significant threat due to extensive antibiotic resistance.
• Whipple's disease (Tropheryma whipplei) presents with PAS-positive macrophages on duodenal biopsy.
• Cat Scratch Disease (Bartonella henselae) causes regional lymphadenopathy; bacillary angiomatosis in immunocompromised individuals.