What is a Facultative Anaerobe?
A facultative anaerobe is a microorganism that possesses the unique metabolic flexibility to grow and survive in both the presence and absence of oxygen, switching between aerobic respiration when oxygen is available and anaerobic metabolism when it is not. 1
Key Metabolic Characteristics
Facultative anaerobes can utilize oxygen for aerobic respiration when present, but can also switch to fermentation or anaerobic respiration pathways when oxygen is absent or limited. 1 This distinguishes them from strict aerobes (which require oxygen) and obligate anaerobes (which cannot tolerate oxygen).
These organisms are aerotolerant and anaerotolerant, meaning they can survive oxygen exposure and also thrive in oxygen-depleted environments. 1 This dual capability represents a significant selective advantage during infection.
Facultative anaerobes possess antioxidant enzyme systems that allow them to detoxify reactive oxygen species (ROS), enabling survival in oxygenated environments. 2
Clinical Significance in Infections
Facultative anaerobes are overrepresented among the most clinically important pathogens—8 of the 12 bacteria on the WHO antibiotic-resistant 'priority pathogens' list are facultative anaerobes. 1 This reflects their enhanced virulence potential.
The most common facultative anaerobic pathogen in clinical practice is Escherichia coli, which accounts for 71% of organisms isolated in complicated intra-abdominal infections. 3 Other important facultative anaerobes include Klebsiella species (14%), Enterobacter species (5%), and enterococci (12-23% combined). 3
Facultative anaerobes commonly cause infections in anatomically hypoxic sites including intra-abdominal abscesses, deep wounds, and the gastrointestinal tract. 1, 4 Their ability to consume oxygen creates localized hypoxia at infection sites, further favoring their growth. 1
Anatomic Distribution Patterns
Infections from the stomach, duodenum, biliary tract, and proximal small bowel contain both gram-positive and gram-negative aerobic and facultative organisms. 3
Distal small bowel infections contain gram-negative facultative and aerobic organisms with variable anaerobic density. 3
Colon-derived infections harbor both facultative and obligate anaerobic organisms, with facultative anaerobes playing a central role alongside strict anaerobes like Bacteroides fragilis. 3
Important Clinical Pitfalls
Antibiotic susceptibility testing for facultative anaerobes performed under standard aerobic laboratory conditions may not accurately reflect their behavior in anaerobic infection sites. 4 Studies show that 10 of 15 tested facultative anaerobic strains demonstrated significantly different antibiotic resistance patterns when tested under anaerobic versus aerobic conditions. 4
The oxygen-dependent metabolic flexibility of facultative anaerobes means they undergo gradual adaptation during the aerobic-to-anaerobic transition, with distinct reproducible stages of metabolic adjustment. 5 This adaptation affects their virulence and antibiotic susceptibility.
Empiric antibiotic regimens for infections involving facultative anaerobes must account for their presence in polymicrobial infections—over 50% of intra-abdominal abscesses are polymicrobial, with nearly 80% involving at least one anaerobic species alongside facultative organisms. 6