Bacteroides fragilis: A Case Study of Bacteremia and Septic Arthritis

Clinical Laboratory Science, Summer 2009 by Martin, Tareese, Aziz, Hassan

ABSTRACT

A 67-year-old African-American male presented with nausea, vomiting, diarrhea, fever, and knee pain. Four sets of blood cultures were collected and resulted in the growth of Bacteroides fragilis in all anaerobic bottles. Later, a fluid and tissue sample from the patients knee grew the same species of bacteria. The patient was placed on intravenous antibiotics to fight the infection.

ABBREVIATIONS: CT = computerized tomography; NIDDK = National Institute of Diabetes and Digestive and Kidney Diseases; PMNs = polymorphonuclear neutrophils; BUN = blood urea nitrogen; CBC = complete blood count; BAP = blood agar plate; BBE = Bacteroides bile esculin; Ref = Reference; Adm = Admission; C = critical result; NA = not applicable; BCC = blood cultures collected

INDEX TERMS: Bacteroides fragilis; bacteremia; septic arthritis; anaerobic infection; gout

Clin Lab Sci 2009;22(3):131

CASE HISTORY

A 67-year-old African-American male was admitted to a local hospital two days after seeing his primary physician with the chief complaint of pain in his knee. Since this patient had a history of severe tophaceous gout, his primary physician administered a shot of Depo-Medrol to relieve some of the pain caused by the gout flare. The patient had a history of hypertension and chronic renal insufficiency as well. Besides pain in his left knee, he also was experiencing symptoms of nausea, vomiting, diarrhea, fever and some chills for about a week. After a few more days of not feeling well, the patient was advised to go to the hospital. After his initial blood work came back abnormal, he was admitted to the hospital. On admission, he was diagnosed with anion gap metabolic acidosis which correlates with the laboratory results in Table 1. The severe anion gap quickly improved after admission, as depicted by the laboratory results from Day 1 in Table 2. Also striking were laboratory results correlating with acute renal failure.

As evident in Table 2, an elevated white blood cell count, as well as a shift in the white cell differential and a high sedimentation rate, most likely encouraged the collection of blood cultures on this patient. Four sets of blood cultures were drawn over two consecutive days. As a result, the laboratory isolated Bacteroides fragilis from the anaerobic bottles of all four sets. The patient s symptoms and laboratory results were consistent with bacteremia as well. Moreover, the patients knee was still very painful and swollen, so a physician aspirated synovial fluid. The fluid from his left knee was thick and opaque with a green tint and a foul odor. The purulence and odor gave the doctor the impression that the patient had an infection superimposed over the gout. The synovial fluid and a tissue sample, collected during surgery on the knee the next morning, went to the laboratory for culture. The fluid and tissue also grew B.fragilis, therefore the patient was diagnosed with septic arthritis as well.

CHARACTERISTICS OF Bacteroides fragilis

Bacteroides fragilL� is an obligate anaerobe which will appear as a gram negative bacillus on a gram stain. It is part of the normal flora of the human gastrointestinal tract. Bacteroides species comprise about 30% of the bacterial population in the lower intestine (1). Moreover, B. fragilL� predominantly colonizes the lower left colon (2) . Bacteroides species are considered commensal organisms because they benefit their hosts in numerous ways, such as assisting in the digestion of complex carbohydrates and development of the immune system, as well as contributing to the biotransformation of bile salts and vitamin synthesis. However, like many indigenous microflora, Bacteroides species can become an opportunistic pathogen once released into sterile areas. In fact, B.fragilL� is the most commonly isolated organism in anaerobic infections (1).

Although this organism is an anaerobe, it can tolerate oxygen and even grow in the presence of nanomolar concentrations of oxygen. Studies have attributed the aerotolerance and oxidative stress response of B. fragilis to enzymes which detoxify and protect the bacterium from oxygen radicals. These detoxification enzymes include catalase and superoxide dismutase (3). Interestingly, there are also reports of certain genes for metabolic enzymes on B.fragilL� that are actually stimulated by oxygen exposure. One such gene is involved in starch utilization (1). One thing is certain: its aerotolerance allows its survival in a spreading infection and contributes to its virulence (3).

Other potent virulence factors include its complex polysaccharide capsule, fimbriae, adhesions, enterotoxin, and proteolytic enzymes. The capsule of B. fragilis mediates resistance to death by both complement and phagocytosis, and it initiates the host immune response known as abscess formation. Although abscess formation is an attempt to isolate an infectious organism, it can ultimately lead to further spread of the infection if left untreated. Also, B. fragilis may possess peritrichous fimbriae and lectin-like adhesions. These cell surface structures are involved in the adherence of the organism to tissues, hence initiating its destruction. B.fragilL� enterotoxin in some strains may destroy tight junctions in intestinal epithelium, resulting in diarrhea (3).