Most Common Organisms in Pericolonic Abscess
Pericolonic abscesses are predominantly polymicrobial infections caused by Escherichia coli (the single most common pathogen) and obligate anaerobes, particularly Bacteroides fragilis and other Bacteroides species, along with various streptococci and enterococci. 1
Primary Pathogens
Gram-Negative Facultative Organisms
- E. coli is by far the most commonly detected organism, isolated in 71% of complicated intra-abdominal infections including pericolonic abscesses 1
- Klebsiella species are found in 14% of cases 1
- Other gram-negative organisms include Proteus mirabilis (5%) and Enterobacter species (5%) 1
- Pseudomonas aeruginosa is present in 14% of cases, though this is more relevant in healthcare-associated infections 1
Obligate Anaerobes
Colon-derived intra-abdominal infections harbor both facultative and obligate anaerobic organisms 1:
- Bacteroides fragilis is present in 35% of cases 1
- Other Bacteroides species are even more prevalent, found in 71% of infections 1
- Clostridium species occur in 29% of cases 1
- Peptostreptococcus species are isolated in 17% 1
- Prevotella species in 12% 1
- Eubacterium species in 17% 1
- Fusobacterium species in 9% 1
Gram-Positive Aerobic Cocci
- Streptococcus species (particularly the S. milleri group) are commonly present in 38% of cases 1
- Enterococcus faecalis is found in 12% 1
- Other Enterococcus species collectively in 11% 1
- Staphylococcus aureus is relatively uncommon at 4% 1
Clinical Context and Pathophysiology
Colon-derived infections specifically harbor facultative and obligate anaerobic organisms because they reflect the normal colonic flora 1. When colonic contents contaminate the pericolonic space through perforation or microperforation, this polymicrobial mixture creates the abscess 1.
The polymicrobial nature of these infections is critical because organisms of the gastrointestinal flora (enteric gram-negative bacilli and Bacteroides fragilis group) predominate in intra-abdominal and pericolonic lesions 2. This reflects the fact that abscesses developing from introduction of normal flora into normally sterile body sites are characteristically polymicrobial 2.
Antimicrobial Coverage Implications
Based on this microbiology, empiric antibiotic therapy must cover 1:
- Enteric gram-negative aerobic and facultative bacilli (primarily E. coli)
- Obligate anaerobic bacilli (particularly Bacteroides fragilis group)
- Enteric gram-positive streptococci
Empiric coverage of Enterococcus is NOT necessary in community-acquired pericolonic abscesses despite its presence in 12-23% of cultures 1. This is a common pitfall—clinicians often over-treat enterococcus when it functions more as a colonizer than primary pathogen in these infections.
Important Caveats
Ampicillin-sulbactam should be avoided due to high rates of E. coli resistance 1. Similarly, cefotetan and clindamycin are not recommended because of increasing Bacteroides fragilis group resistance 1.
In regions with high prevalence of extended-spectrum β-lactamase (ESBL)-producing E. coli and Klebsiella, local resistance patterns must guide empiric therapy selection 1.