Parabacteroides merdae: a rare Gram-negative anaerobic rod

Authors:
Aprill Park, Rebecca Kowalski, Sridha Gona, Aaron George, Stephen Kavic, Hugo Bonatti

Body of Abstract:
Introduction: Parabacteroides spp were previously listed under Bacteroides spp but refined microbiology testing led to the creation of the new genus with Parabacteroides distasonis being common than Parabacteroides merdae. They are Gram-negative obligate anaerobic rods and part of the normal microbiome. Isolation of the organisms within the microbiome has been associated with inflammatory bowel disease, metabolic syndrome, and most notable obesity. Parabacteroides merdae has been recovered from drained abscesses, wound infections, and blood. Resistance rates to clindamycin, cefoxitin, and moxifloxacin are high, which may lead to treatment failure.

Patients and Methods: Following an index case of a patient with perirectal abscess, in which Parabacteroides merdae was isolated, the institutional database of a rural hospital in the Appalachian region of Western Maryland was searched for all infections caused by the pathogen during a 4-year period. Also, all cases of Parabacteroides distasonis the more common relative were reviewed.

Results: Two patients (3 isolates) with surgical infections associated with Parabacteroides merdae were identified including a 50-year-old woman and a 74-year-old man. The morbidly obese female with poorly controlled DM underwent incision and drainage of an ischiorectal abscess which grew the pathogen together with Eggerthella lenta and Fusobacterium necrophorum and the male patient had an infected tunneled dialysis catheter, which was removed growing Parabacteroides merdae, Enterobacter cloacae and Peptostreptococcus anaerobius. Both infections were successfully managed with surgical intervention and antibiotics. Parabacteroides distasonis was isolated in 42 patients (54 isolates) and intraabdominal infections accounted for 74% of cases including diverticulitis in 11 and appendicitis in 9 patients. No sensitivity testing is done at our hospital for anaerobic bacteria such as Parabacteroides merdae or distasonis.

Conclusion: Parabacteroides merdae is an extremely rare pathogen and much less common than Parabacteroides distasonis. These infections are usually polymicrobial and whereas the exact role of these anaerobes is unknown, they should be considered true pathogens and not contaminants or innocent bystanders.