Fastidious Gram-Negative Rods and Coccobacilli
A study page for Haemophilus, Pasteurella, HACEK-like organisms, Eikenella, Moraxella-like organisms, and other source-driven Gram-negative coccobacilli.
Core idea
Fastidious Gram-Negative Rods and Coccobacilli gives students a place to organize the topic before memorizing organism lists, test names, or isolated exam facts.
Principle
Learn this topic by asking what the specimen, method, organism group, and clinical setting are trying to prove. Build the topic around practical study anchors and bench reasoning.
Study Organizer
| Anchor | What to learn first | Why it matters |
|---|---|---|
| Growth | chocolate agar, CO2, X/V factor, slow growth, or reference identification may be needed | Use this as a branch point for review, tables, and practice questions. |
| Source | respiratory tract, oral flora, animal bite, wound, blood culture, eye, or genital source | Use this as a branch point for review, tables, and practice questions. |
| Shape | small Gram-negative rods, coccobacilli, cocci, or pleomorphic forms can overlap | Use this as a branch point for review, tables, and practice questions. |
| Significance | sterile-site recovery, endocarditis context, and bite wounds change interpretation | Use this as a branch point for review, tables, and practice questions. |
Fastidious Gram-Negative Rods and Coccobacilli Detail Table
| Organism/group | Growth/source pattern | Clinical anchors | Student warning |
|---|---|---|---|
| Haemophilus influenzae | Respiratory tract colonizer; grows on chocolate agar with X and V factor support | Meningitis, epiglottitis, pneumonia, bacteremia, otitis, sinusitis, conjunctivitis, COPD exacerbation depending on strain and host | Encapsulated and nonencapsulated disease patterns differ |
| Haemophilus ducreyi | Fastidious organism tied to genital ulcer disease | Chancroid with painful genital ulcers and lymphadenitis | Not part of routine respiratory Haemophilus thinking |
| Other Haemophilus spp. | Upper respiratory/oral flora; variable X/V factor needs | Endocarditis, respiratory infection, bite or oral-flora-associated infection depending on species | Species and source matter more than the genus label alone |
| Aggregatibacter actinomycetemcomitans | Oral flora; slow-growing capnophilic Gram-negative coccobacillus | Periodontal disease, endocarditis, abscesses, bite/oral-source infection | Think HACEK/oral flora when blood cultures or endocarditis context fit |
| Actinobacillus spp. | Animal and oral flora association | Rare bite, wound, or invasive infection | Animal exposure can explain an unusual isolate |
| Kingella kingae | Oropharyngeal colonizer, especially children | Osteoarticular infection, bacteremia, endocarditis | Pediatric joint/bone context is a key anchor |
| Cardiobacterium hominis | Oral/upper respiratory flora | Endocarditis and rare invasive infection | Slow growth and blood culture context matter |
| Capnocytophaga spp. | Oral flora of humans, dogs, and cats; capnophilic growth | Periodontal infection, bite wound infection, sepsis in asplenia or immunocompromise, endocarditis | Dog/cat bite plus severe sepsis is a major clue |
| Dysgonomonas capnocytophagoides and related organisms | GI or oral-flora-associated unusual Gram-negative rods | Rare bacteremia, wounds, abscesses, or opportunistic infection | Clinical significance is highly source-dependent |
| Pasteurella multocida group | Cat/dog oral flora; animal bite or scratch exposure | Rapid-onset cellulitis, bite wound infection, abscess, respiratory or invasive disease in selected hosts | Animal exposure is usually the organizing clue |
| Mannheimia and related Pasteurellaceae | Animal-associated organisms, livestock or domestic animal exposure | Rare wound, respiratory, ocular, or invasive infection | Do not force routine Haemophilus logic |
| Eikenella corrodens | Human oral flora; may pit agar and can have bleach-like odor | Human bite, clenched-fist injury, oral-source abscess, endocarditis, head/neck infection | Often part of mixed oral flora; resistance assumptions can be tricky |
| Bergeyella zoohelcum | Dog and cat oral flora | Bite wound infection and rare invasive disease | Pet exposure matters |
| Moraxella catarrhalis | Upper respiratory colonizer; oxidase-positive coccobacillus | Otitis media, sinusitis, COPD exacerbation, lower respiratory infection | Beta-lactamase is common; source decides significance |
| Moraxella nonliquefaciens/lacunata-like organisms | Respiratory, eye, or mucosal flora depending on species | Eye infection, respiratory isolate, rare invasive disease | Do not overcall every Moraxella-like isolate |
| Neisseria elongata and N. weaveri | Elongate/coccobacillary Neisseria-like organisms; oral or animal exposure contexts | Bacteremia, endocarditis, dog-bite wound infection depending on species | They can break the "Neisseria are diplococci" memory shortcut |
Growth and Source Clues
| Clue | Points toward | How students should use it |
|---|---|---|
| Chocolate agar and X/V factor logic | Haemophilus spp. | Connect growth requirement to respiratory, CSF, blood, eye, or genital source |
| CO2 or capnophilic growth | Capnocytophaga, Aggregatibacter, Kingella, Cardiobacterium, some oral flora organisms | Think oral flora, endocarditis, bite, or deep abscess context |
| Cat or dog bite/scratch | Pasteurella, Capnocytophaga, Bergeyella, Neisseria weaveri-like organisms | Animal exposure can organize the whole differential |
| Human bite or clenched-fist injury | Eikenella and mixed oral flora | Do not miss oral-flora anaerobes and mixed infection context |
| Pitting or bleach-like odor | Eikenella teaching clue | Helpful only when source and Gram stain fit |
| Endocarditis or persistently positive blood cultures | HACEK-like organisms and oral flora Gram-negative rods | Slow growth and species-level ID can matter |
| Small pleomorphic Gram-negative coccobacilli in respiratory source | Haemophilus, Moraxella, or other respiratory flora | Clinical syndrome separates colonization from infection |
| Eye isolate with small Gram-negative coccobacilli | Haemophilus, Moraxella lacunata-like organisms, Pasteurellaceae in selected contexts | Specimen source changes the likely organism list |
Fastidious GNR AST Anchors
| Organism/group | Resistance or therapy idea | Reporting caution |
|---|---|---|
| Haemophilus influenzae | Beta-lactamase-mediated ampicillin resistance is common; beta-lactamase-negative ampicillin resistance is a special concept | Use organism-specific methods and current standards |
| Moraxella catarrhalis | Beta-lactamase production is common | Respiratory significance depends on syndrome and specimen quality |
| Pasteurella multocida group | Usually susceptible to several beta-lactams, but bite-wound therapy must cover mixed flora | Do not treat animal bites as single-organism problems |
| Capnocytophaga spp. | Resistance can vary; severe disease risk is high in asplenia or immunocompromise | Animal bite/sepsis context may require urgent clinical attention |
| Eikenella corrodens | Classic resistance traps include clindamycin and metronidazole inactivity | Mixed oral infections need source-aware coverage |
| HACEK-like organisms | Endocarditis context makes precise ID and susceptibility interpretation important | Slow growth and specialized methods may be required |
| Rare Pasteurellaceae or oral-flora rods | Breakpoints or validated methods may be limited | Avoid unsupported interpretations; reference methods may be needed |
Final susceptibility reporting depends on current CLSI/FDA guidance, validated test methods, and local lab policy.
Basic workflow
- Define the main question for fastidious gram-negative rods and coccobacilli.
- Separate screening clues from confirmatory information.
- Connect the topic to specimen quality, method limits, and clinical context.
- Use tables to compare similar organisms, methods, or syndromes.
- Circle back to tools or deep guides only after the core concept is clear.
How to read it
- Growth: chocolate agar, CO2, X/V factor, slow growth, or reference identification may be needed
- Source: respiratory tract, oral flora, animal bite, wound, blood culture, eye, or genital source
- Shape: small Gram-negative rods, coccobacilli, cocci, or pleomorphic forms can overlap
- Significance: sterile-site recovery, endocarditis context, and bite wounds change interpretation
High-yield trapCommon mistakes
- Memorizing names without knowing the organizing branch.
- Forgetting specimen source and patient context.
- Treating a screening clue as a final answer.
ShortcutStudent shortcut
For fastidious gram-negative rods and coccobacilli, learn the organizer first, then add exceptions.