The Anaerobes Flashcards
Q: What are anaerobes and why are they clinically significant?
A: Anaerobes are organisms that grow without oxygen. They’re significant in medicine due to their role in infections such as abscesses, dental diseases, and deep tissue infections. Many are part of normal flora but become pathogenic when displaced.
Classification of Anaerobes
Anaerobes are classified as:
Obligate anaerobes: Cannot survive in oxygen.
Aerotolerant anaerobes: Tolerate oxygen but don’t use it.
Facultative anaerobes: Can use oxygen but grow without it.
What are the key features of anaerobic bacteria?
Not a uniform group: vary in Gram stain, spore formation, oxygen tolerance, and metabolism
Found in environment and normal human flora
Most infections are endogenous; exceptions include spore-formers
What are essential principles for isolating anaerobes from clinical samples?
Use fresh samples or store in low oxygen conditions
Avoid contamination with normal flora
Use pre-reduced media
Store inoculated plates in nitrogen if delayed
What types of media are used for anaerobes?
Non-selective: Blood agar with vitamin K & haemin, thioglycolate broth, cooked meat broth
Selective:
PEA-BA (inhibits facultative anaerobes)
Colistin/naladixic acid (general selective)
Kanamycin/vancomycin (selects Gram-negative anaerobes)
BE/gentamicin (for bile-resistant Bacteroides)
Metronidazole disc can indicate presence of anaerobes (zone + smell)
What are the three broad groups of anaerobic rods?
Anaerobic Gram-positive spore-forming rods
Anaerobic Gram-positive non-spore-forming rods
Anaerobic Gram-negative rods
(Note: According to Bergey’s Manual, anaerobic cocci are grouped with rods)
What systems are used for incubating anaerobic cultures?
Anaerobic glove box (rarely used)
Anaerobic jars with H₂ gas packs + palladium catalyst
Evacuation/replacement with H₂, N₂, CO₂ (safer & faster)
Use redox indicators like methylene blue:
Blue = oxidised (oxygen present)
Clear = reduced (anaerobic)
How do you identify anaerobic bacteria in the lab?
Preliminary Test:
Better growth in anaerobic conditions
Sensitive to metronidazole
Test growth with/without air
Cellular Morphology (Gram stain):
Gram-positive rods, spore-forming → Clostridium
Gram-positive rods, non-spore-forming → Actinomyces, Lactobacillus, Propionibacterium
Gram-positive cocci → Peptostreptococcus
Gram-negative rods, large/robust → Bacteroides
Gram-negative rods, fine/slim → Fusobacterium
Gram-negative cocci → Veillonella (most common)
What genera are most commonly found among anaerobic Gram-positive non-spore-forming rods and cocci?
Gram-positive cocci (GPC):
Peptostreptococcus (most significant)
Gram-positive rods (GPR):
Actinomyces, Lactobacillus, Propionibacterium
Traits:
Obligate/facultative anaerobes
Non-spore-forming
Rarely motile
What are the taxonomic and clinical features of anaerobic Gram-positive rods and cocci?
Taxonomy:
Previously based on phenotypes; now being revised with genomics
Moles % G+C separates anaerobes into:
Low G+C: Clostridium, Lactobacillus, Peptostreptococcus, Peptococcus
High G+C: Actinomyces, Propionibacterium, Bifidobacterium, Mobiluncus
Clinical relevance:
Ubiquitous flora of mucosa (oral, urogenital, gut)
Cause abscesses and skin infections
Often slow-growing → may require >1 week anaerobic incubation for detection
What are the key features of the genus Actinomyces?
Anaerobic (better growth in anaerobic conditions)
Differentiated from aerobic actinomycetes (e.g. Nocardia)
Filamentous, branching Gram-positive bacilli (GPB)
Found in oral cavity, tonsils, and dental plaque
A. humiferus is found in soil
May be confused with fungal infections (mycoses) due to morphology
What is actinomycosis and how does it present?
Chronic granulomatous disease
Forms abscesses and draining sinus tracts
Often polymicrobial with other anaerobes
Common in humans (head and neck), and cattle (A. bovis)
A. bovis causes “lumpy jaw” in cattle from trauma/rough feed
What are common human infections caused by Actinomyces species?
A. israelii: Head and neck actinomycosis with facial lumps
Urogenital infections, especially in association with intrauterine devices (IUDs)
Involved in polymicrobial pelvic inflammatory disease (PID)
How is Actinomyces isolated and identified in the lab?
Culture from pus containing “sulphur granules”
Gram stain: Filamentous GPB
Best grown anaerobically, but also grows in microaerophilic CO₂
Grows slowly – up to 2 weeks
Colony morphology: Heaped, lobulated “molar tooth” appearance
Not or only weakly acid-fast
Facultative anaerobes (mostly)
Volatile fatty acid (VFA) production via gas-liquid chromatography (GLC): Succinate and lactate
Species ID through biochemical tests (carbohydrate fermentation patterns)
What are the characteristics of Propionibacterium spp?
Part of normal skin and oral flora
Can be isolated from blood (5% significant, 50% contaminant)
Diphtheroid appearance
Aerotolerant anaerobe
Produces acetic and succinic acids
P. acnes: Associated with severe acne (Acne vulgaris), may involve cytokine-mediated inflammation
What are the characteristics of Lactobacillus spp?
Normal flora of the vagina, GI tract, and mouth
Straight, parallel Gram-positive rods (GPR), may form chains
Microaerophilic
Produces lactic acid
Rarely pathogenic
Used as probiotic for gut and vaginal health
Cultured on blood agar (BA)
Must be differentiated from Listeria: Catalase negative, non-motile
In vaginal Gram stain: Indicator of a healthy, infection-free state
What are the characteristics of Bifidobacterium spp?
Found in the GI tract and oral cavity
Gram-positive rods with bifurcated ends; diphtheroid appearance
Obligate anaerobe
Produces acetic and lactic acids
B. dentium: Only species considered pathogenic (linked to dental caries)
Used as a probiotic for gastrointestinal health
Grows on blood agar
What are the features of Eubacterium spp?
Found in GI and oral flora
Gram-positive coccobacilli (GPCB), pleomorphic
Obligate anaerobe
Produces acetic and butyric acids (varies among species)
Often isolated from polymicrobial abscesses
What are the characteristics of Mobiluncus spp?
Found in GI tract and genitourinary tract
Gram-positive, curved rods
Obligate anaerobes and motile
Produce succinic, lactic, and acetic acids
Associated with bacterial vaginosis:
Present in 50–65% of vaginosis cases
Rarely found (<10%) in healthy vaginal flora
Role as pathogen vs indicator of disrupted flora is still debated
What genera are included under anaerobic Gram-positive cocci, and where are they found?
Genera:
Peptostreptococcus (>15 species)
Peptococcus (only P. niger)
Anaerobic Streptococci (related to Clostridium)
Ruminococcus and Coprococcus (no clinical significance)
Normal flora: Skin, oropharynx, upper respiratory tract (URT), gastrointestinal tract (GIT), and urogenital tract
What infections are caused by anaerobic Gram-positive cocci?
Urogenital infections:
Endometritis (postpartum fever)
Tubo-ovarian abscesses
Polymicrobial pelvic inflammatory disease (PID)
Placental infections → associated with preterm birth
Head and neck infections:
Periodontitis
Chronic otitis media
Chronic sinusitis
Brain abscess
How are anaerobic Gram-positive cocci isolated and identified?
Common species: P. magnus, P. assaccharolyticus, P. anaerobius
Isolation: Similar to other anaerobes
No selective media for cocci
Use liquid media like thioglycolate broth
Difficult to differentiate from Gram-negative cocci or GPCB
P. anaerobius may resemble GPCB
Vancomycin susceptibility helps with differentiation
Identification is presumptive without GLC and VFA analysis
Commercial ID systems often unreliable
What are the key genera of anaerobic Gram-negative rods and cocci, and where are they found?
Rods: Bacteroides, Fusobacterium, Porphyromonas, Prevotella
Cocci: Veillonella, Megasphaera, Acidaminococcus
Habitat: Normal flora of the GIT, oral cavity, and urogenital tract
Many are part of polymicrobial abscesses
What is the clinical significance of anaerobic Gram-negative cocci?
Veillonella, Megasphaera, and Acidaminococcus are rarely pathogenic
Found in oral, GIT, and urogenital flora
May appear in mixed infections, but usually not primary pathogens
Isolation as for other anaerobes
GLC/VFA or WGS (whole genome sequencing) may be used for ID
