What is the classification of all medically relevant bacteria?

Classification of Medically Relevant Bacteria

Primary Classification Systems

Bacteria are fundamentally classified by Gram stain reaction (Gram-positive vs. Gram-negative), which reflects critical differences in cell wall structure that determine inherent antimicrobial susceptibility patterns. 1

Gram Stain Classification

The Gram stain, developed in 1884, remains the cornerstone of bacterial identification and separates bacteria into four primary morphological groups 2:

• Gram-positive cocci - thick peptidoglycan layer fused to cytoplasmic membrane 1
• Gram-negative cocci - thin peptidoglycan with outer lipopolysaccharide membrane 1
• Gram-positive rods (bacilli) - thick peptidoglycan structure 2
• Gram-negative rods (bacilli) - complex outer membrane with porins and periplasmic space containing β-lactamases 1

The cell wall composition directly determines antibiotic susceptibility: Gram-positive bacteria have peptidoglycan as the major structural component, while Gram-negative bacteria possess a hydrophobic lipopolysaccharide capsule surrounding a lipoprotein-phospholipid membrane with a thin peptidoglycan layer between outer and inner membranes 1.

Clinical Pathogen Classification

For infectious disease management, bacteria are classified by their clinical significance and resistance patterns 1:

Major Respiratory Pathogens

• Streptococcus pneumoniae (most common) 1
• Haemophilus influenzae 1
• Moraxella catarrhalis 1

Typical Endocarditis Pathogens

• Viridans streptococci - most common in native valve endocarditis 1
• Staphylococcus aureus - both community-acquired and healthcare-associated, now considered major criterion regardless of source 1
• Streptococcus bovis 1
• Enterococci - community-acquired without primary focus 1
• HACEK group - Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens, Kingella species 1

Intra-abdominal Infection Pathogens

Organisms requiring susceptibility testing due to resistance potential 1:

• Pseudomonas aeruginosa 1
• Proteus species 1
• Acinetobacter species 1
• Staphylococcus aureus (including MRSA) 1
• Enterobacteriaceae (including ESBL-producers) 1
• Bacteroides fragilis group (anaerobic) 1

Skin and Soft Tissue Infection Pathogens

Classified by infection type 1:

• Type 1 (polymicrobial) - mixed aerobic and anaerobic bacteria 1
• Type 2 (monomicrobial) - β-hemolytic streptococci or community-acquired MRSA 1
• Type 3 (monomicrobial) - various pathogenic bacilli 1

Metabolic and Oxygen Requirement Classification

Bacteria are characterized by their metabolic requirements 3:

• Aerobic bacteria - require oxygen 3
• Facultatively anaerobic bacteria - can survive with or without oxygen 3
• Obligate anaerobes - cannot tolerate oxygen 3

Resistance-Based Classification (WHO AWaRe Framework)

The WHO classifies 257 antibiotics into Access, Watch, and Reserve groups based on resistance potential, which indirectly classifies bacteria by their susceptibility patterns 3:

Access Group Targets

Bacteria susceptible to narrow-spectrum agents with low resistance levels 3:

• Common community-acquired pathogens
• First-line treatment targets

Watch Group Targets

Bacteria with higher resistance potential requiring broader-spectrum agents 3:

• Healthcare-associated pathogens
• Organisms with emerging resistance

Reserve Group Targets

Multidrug-resistant organisms requiring last-resort antibiotics 3:

• ESBL-producing Enterobacteriaceae 1
• Carbapenem-resistant organisms 1
• Methicillin-resistant S. aureus (MRSA) 1
• Vancomycin-resistant Enterococcus (VRE) 3

Epidemiological Classification

Community-Acquired vs. Healthcare-Associated

Bacteria must be classified by acquisition setting, as this determines resistance patterns and empiric therapy selection 1:

Community-Acquired Pathogens

• Generally more susceptible to narrow-spectrum agents 1
• Lower rates of multidrug resistance 1
• Examples: community-acquired S. aureus, S. pneumoniae 1

Healthcare-Associated Pathogens

Require institution-specific antibiograms for classification 1:

• ≥20% resistant Pseudomonas aeruginosa 1
• ESBL-producing Enterobacteriaceae 1
• Multidrug-resistant Gram-negative bacilli 1
• MRSA 1

Colonization vs. Infection

Screening specimens detect colonizing bacteria (carriers) and must be classified separately from diagnostic specimens that identify causative pathogens 1:

• Colonization specimens - rhinopharynx, rectum, skin, vagina for surveillance 1
• Diagnostic specimens - from infection sites for clinical diagnosis 1

Susceptibility Classification

Bacteria are categorized as susceptible, intermediate, or resistant based on laboratory MIC breakpoints, though this in vitro classification does not always predict clinical outcomes 4:

• Susceptible - likely inhibited by usual achievable antibiotic concentrations 3, 4
• Intermediate - may respond if antibiotic concentrated at infection site or dosage increased 3, 4
• Resistant - unlikely to respond even with maximum doses 3, 4

Critical Caveat

β-lactams can eradicate "nonsusceptible" strains from lungs despite in vitro resistance because achievable tissue concentrations exceed MIC breakpoints 4. Penicillin nonsusceptibility does not significantly impair efficacy in pneumococcal pneumonia 4.

Practical Clinical Algorithm

When classifying bacteria for treatment decisions:

1. Determine Gram stain and morphology - establishes inherent antibiotic susceptibility patterns 1, 2
2. Identify acquisition setting - community vs. healthcare-associated determines resistance likelihood 1
3. Assess local resistance patterns - use institution-specific antibiograms 1
4. Consider infection site - tissue penetration affects clinical susceptibility despite in vitro resistance 4
5. Obtain cultures before antibiotics - particularly in patients with prior antibiotic exposure who harbor resistant pathogens 1

Common pitfall: Relying solely on in vitro susceptibility without considering achievable antibiotic concentrations at the infection site, which can lead to unnecessarily broad-spectrum therapy 4.