Bacteremia with Bacteroides pyogenes after a Cat Bite

Abstract

Animal bite wounds are often infected with bacteria from the animal's oral flora. We report what we believe to be the first case of bacteremia with Bacteroides pyogenes resulting from an infected cat bite.

CASE REPORT

A 60-year-old man was bitten by his cat in his left wrist. Three days later, he consulted the emergency room as the hand was red and swollen, corresponding to a large circular area around the wound. A sample for culture was obtained from the wound, and the patient started treatment with oral dicloxacillin, 1 g four times a day. Eight days after the bite, the patient was admitted to the hospital because of thoracic pain and general malaise. His medical history included prior heroin addiction but also lung cancer which had been treated surgically 2 years earlier with left pneumonectomy and adjuvant chemotherapy. The wrist was still showing signs of inflammation with a 2- by 4-cm² area around the bite wound covered with pus. The left forearm was red, swollen, and warm. The patient was not febrile (37.3°C). The white blood cell count showed neutrophilic leukocytosis (13.8 × 10⁹/liter). The C-reactive protein level was 42 mg/liter (reference interval, <10 mg/liter).

Pasteurella multocida, susceptible to penicillin, was recovered from the sample obtained in the emergency room 5 days earlier. An additional wound swab was obtained from the patient upon admission to the hospital along with two sets of blood cultures (40 ml total), each set including an aerobic and an anaerobic bottle (Bactec Plus aerobic/F and anaerobic/F; Becton Dickinson Diagnostic Instrument Systems, Franklin Lakes, NJ). At this time, intravenous treatment with benzylpenicillin, 1 million units four times a day, was initiated and treatment with oral dicloxacillin was discontinued. Growth was detected in one of the anaerobic bottles after 3 days of incubation. Gram staining revealed a Gram-negative rod. Material was subcultured according to routine practice on different media (9). Forty-eight hours later, growth was observed under anaerobic conditions only. Colonies were large (>2 mm), circular, dome shaped, mucoid, and beige. Growth was not inhibited by kanamycin, vancomycin, bile, or colistin. The isolate was identified by 16S rRNA gene sequencing (MicroSeq 500 system; Perkin-Elmer, Applied Biosystems Division, Foster City, CA), as Bacteroides pyogenes/Bacteroides suis with a 99.80% match for both species. The consensus sequence (493 bp) was also compared with the EzTaxon server (4). The highest sequence similarity (99.8%) was obtained with Bacteroides pyogenes strain JCM 6294 (GenBank accession no. AB200229). The second best match was with the Bacteroides tectus strain JCM 10003 (GenBank accession no. AB200228), with a sequence similarity of 99.6%.

To identify the blood culture isolate further and to try to differentiate between B. pyogenes and B. suis/B. tectus, we examined the phenotypic characteristics of the blood culture isolate using commercially available substrates and diagnostic tablets/disks (Statens Serum Institut Diagnostica, Copenhagen, Denmark, and Rosco Diagnostica, Taastrup, Denmark) (Table 1). Using the identification table in a recent paper by Sakamoto et al. (10), the isolate was identified as B. pyogenes. The isolate differed from B. tectus and B. suis by producing β-glucosidase and by fermenting mannose, respectively (Table 1). The biochemical characteristics were also determined by the API Rapid ID 32A anaerobe identification system (bioMérieux, Marcy l'Etoile, France) according to manufacturer's instructions. The positive reactions were α-galactosidase, β- galactosidase, β-galactosidase 6-phosphate, α-glucosidase, N-acetyl-β-glucosaminidase, alkaline phosphatase, leucyl glycine arylamidase, alanine arylamidase, α-fucosidase, and glutamyl glutamic acid arylamidase. As B. pyogenes, B. tectus, and B. suis are not in the API database, the system was unable to identify the isolate to the species level. The closest match was Bacteroides capillosus (recently reassigned to Pseudoflavonifractor capillosus), with a 66.2% score.

Table 1.

Phenotypic characteristics of Bacteroides pyogenes, B. suis, and B. tectus

Characteristic	Our result for B. pyogenes	Result from the work of Sakamoto et al. (10) for:
		B. pyogenes	B. suis	B. tectus
Catalase	−	−	−	−
Indole	−	−	−	−
β-Glucosidase	+	+	+	−
Acid production from
Glucose	+	+	+	+
Cellobiose	−	−	+	−
Mannose	+	+	−	+
Sucrose	−	+	+	−
Sorbitol	−	−	+	−

The swab from the wound obtained upon admission revealed growth of Pasteurella multocida. After the initial handling, the swab was stored at 5°C in Stuart transport medium. When Bacteroides pyogenes was recovered from the blood culture, the swab was reexamined and cultured under anaerobic conditions and an identical isolate was retrieved. 16S rRNA gene sequencing confirmed identity match with the blood isolate.

Antimicrobial susceptibilities of the blood culture isolate of B. pyogenes to six different antimicrobial agents were determined by the Etest gradient method (bioMérieux, Lyon, France) on brucella blood agar supplemented with hemin and vitamin K (Becton Dickinson GmbH, BD Diagnostics, Heidelberg, Germany) according to manufacturer's instructions. A test with a nitrocefin disk for the detection of β-lactamase production was negative (Cefinase; Becton Dickinson, Sparks, MD). The isolate was susceptible to penicillin, piperacillin-tazobactam, meropenem, metronidazole, and clindamycin according to the interpretive categories proposed by the CLSI (M11-A7) for anaerobic bacteria (5). Bacteroides pyogenes has previously been shown to be highly susceptible to penicillin (3).

The patient needed surgery because the tissue surrounding the wound was necrotic and had to be excised. Several revisions of the hand and wrist were necessary. The healing process was facilitated with vacuum-assisted closure treatment before the patient could be discharged 9 days later. The antimicrobial treatment consisted of intravenous benzylpenicillin for 9 days followed by oral phenoxymethylpenicillin for another 11 days. The patient was seen 3 weeks later in the outpatient clinic. The wound had healed, and there were apparently no functional sequelae.

Animal bite wounds are often infected with bacteria from the animal's oral flora. The majority of bite wound infections are found to be mixed infections (11). While Pasteurella spp. have been shown to be the most frequent isolate from both dog bites (50%) and cat bites (75%) (11), anaerobic bacteria such as Bacteroides spp. may also play an important role in bite wound infections and can be isolated from approximately 40% of the infected wounds (7).

B. pyogenes and B. tectus are two very closely related species within the Bacteroides genus. Both species have been isolated from the oral flora of dogs (6), while B. tectus has been isolated from cats as well (8). B. tectus has been reported to be transmitted to humans through dog and cat bites (1, 6, 11). However, wound infections caused by B. pyogenes in humans after cat bites have, to our knowledge, not been reported before.

B. pyogenes and B. suis were proposed by Benno et al. in 1983 (2), while B. tectus was proposed by Love et al. in 1986 (8). The gene sequence similarity between B. pyogenes and B. suis has been shown by Sakamoto et al. (10) to be 100% for the hsp60 gene and 100% for the 16S rRNA gene. Sakamoto et al. investigated the physiological and biochemical characteristics of B. pyogenes, B. suis, and B. tectus and found that the three strains were phenotypically similar while the cellular fatty acid composition and the DNA G+C content varied only slightly. Sakamoto et al. concluded that B. suis and B. tectus are heterotypic synonyms of B. pyogenes (10). We found that only a few biochemical reactions separated B. pyogenes from B. suis/B. tectus. Thus, the differentiation between these three Bacteroides spp. is difficult.

To summarize, in this case report we have described what we believe to be the first case of bacteremia with Bacteroides pyogenes resulting from an infected cat bite. We would like to emphasize that since anaerobic bacteria play an important role in infected animal bite wounds, samples from the bite wounds should be cultured under anaerobic as well as aerobic conditions. Our case report also demonstrates the importance of species identification in cases of serious infections with Bacteroides spp., since suspected Bacteroides spp. from, e.g., blood cultures might easily be mistaken as originating from the gastrointestinal tract. In our case, species identification convincingly pointed toward origin from the cat bite. Although 16S rRNA gene sequencing is a powerful identification tool, even 16S rRNA gene sequencing fails to correctly differentiate between species when gene sequences are similar. We used biochemical reactions to assist us in separating B. pyogenes from B. suis and B. tectus and found the phenotypic characteristics to be compatible with B. pyogenes. However, by strict phenotypic identification alone according to the Manual of Clinical Microbiology (9) or by the API Rapid ID 32A anaerobe identification system, the isolate could very well be misidentified as Pseudoflavonifractor capillosus. Even so, P. capillosus has, to our knowledge, not been associated with animal bite wound infections.