What is the microbiology definition of sepsis?

Microbiology Definition of Sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection, with infection being a sine qua non (essential component) of sepsis. 1, 2

Causative Microorganisms

• More than 90% of sepsis cases are caused by bacteria, with Gram-negative and Gram-positive organisms occurring with approximately equal frequency 2
• Common bacterial pathogens include Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus pyogenes 3
• Fungi, particularly Candida species, are responsible for a significant minority of cases 2, 3
• Viruses can cause sepsis in vulnerable patients (e.g., neonates, infants, and immunocompromised individuals), though viral sepsis is less commonly diagnosed 4

Pathophysiological Mechanisms

• Sepsis involves the recognition of pathogen-associated molecular patterns (PAMPs) by the host immune system, triggering inflammatory signaling pathways 2
• The dysregulated host response leads to profound circulatory, cellular, and metabolic abnormalities 5
• Microcirculatory dysfunction causes tissue hypoperfusion, leading to organ dysfunction 5
• Cellular and metabolic abnormalities include altered cellular metabolism resulting in lactate accumulation 5
• The inflammatory response depends on both the causative pathogen and host factors (genetic characteristics and co-existing illnesses) 1

Clinical Identification and Diagnosis

• Organ dysfunction can be clinically identified by an increase in the Sequential Organ Failure Assessment (SOFA) score of ≥2 points 1
• Quick SOFA (qSOFA) criteria for rapid identification outside ICU settings include:
  • Respiratory rate ≥22 breaths/min
  • Altered mental status (Glasgow Coma Scale score ≤13)
  • Systolic blood pressure ≤100 mmHg 1
• Microbiological diagnosis is essential for effective antimicrobial therapy, with blood cultures being the cornerstone of diagnosis 2
• Distinguishing colonization from infection is a key challenge in interpreting microbiological findings 2

Septic Shock Definition

• Septic shock is defined as a subset of sepsis with particularly profound circulatory, cellular, and metabolic abnormalities 1, 5
• Clinically identified by:
  • Vasopressor requirement to maintain mean arterial pressure ≥65 mmHg
  • Serum lactate level >2 mmol/L (>18 mg/dL) despite adequate volume resuscitation 5

Evolution of Sepsis Definitions

• Previous definitions (pre-2016) focused heavily on inflammation and used Systemic Inflammatory Response Syndrome (SIRS) criteria 1
• SIRS was defined as at least two of:
  • Temperature >38°C or <36°C
  • Heart rate >90 beats/minute
  • Respiratory rate >20 breaths/minute or PaCO2 <32 mmHg
  • White blood cell count >12,000/mm³ or <4,000/mm³, or >10% immature forms 2
• Current definition (Sepsis-3) emphasizes organ dysfunction rather than inflammatory response alone 2

Clinical Implications and Pitfalls

• Early recognition is crucial for improving outcomes 1, 6
• Failing to recognize perfusion abnormalities despite normal blood pressure in patients receiving vasopressors is a common pitfall 5
• Not measuring lactate levels, which are essential for diagnosis according to current definitions, is a significant oversight 5
• Confusing septic shock with other forms of distributive shock can lead to inappropriate management 5
• Special consideration needed for immunocompromised and elderly patients who may present with attenuated inflammatory responses 1

Diagnostic Challenges

• Up to 42% of sepsis presentations are culture negative, suggesting possible non-bacterial causes 4
• Prior antibiotic therapy may lead to negative cultures despite ongoing infection 2
• Correctly identifying unfamiliar organisms and determining the significance of mixed culture results present additional challenges 2
• Interpreting the importance of organisms normally of low virulence requires clinical correlation 2

Understanding the microbiology of sepsis is essential for appropriate antimicrobial therapy, which significantly impacts patient survival compared to inadequate or inappropriate treatment 2.