Infection of a Hip Prosthesis by Actinomyces naeslundii

Abstract

We present the case of a 77-year-old woman who developed an Actinomyces naeslundii infection of a hip prosthesis. The isolate grew well aerobically with 5% CO₂. Possible diagnostic problems may arise in the microbiological laboratory because aerobic growth is not sufficiently accounted for in some of the traditional identification schemes and commercial test kits. Therefore, besides presenting an unusual pathogen in this setting, this report focuses on possible diagnostic problems in the microbiological laboratory.

CASE REPORT

We report a case of an Actinomyces naeslundii infection of a hip prosthesis in a 77-year-old woman. In 1992, the patient underwent a total arthroplasty of the right hip. Due to loosening of the prosthesis, a replacement was necessary in November 1998. Perioperatively, three 1.5-g doses of cefuroxime were given parenterally, every 12 h, with the first dose given 1 h prior to surgery. Initially, a considerable response with good mobilization was achieved. Early in April 1999, severe motion-dependent pain was observed. There were no signs of infection, and an X ray of the hip joint was judged normal. On April 17, the patient had to be hospitalized due to progressive pain. The C-reactive protein level in the patient was 123 mg/liter (normal, <10.0 mg/liter), and the leukocyte count was 10,100/mm³ (normal, 4,000 to 10,000/mm³). Puncture yielded a purulent fluid. In a Gram stain, many leukocytes but no microorganisms could be seen. After 2 days of incubation at 37°C, a few tiny colonies grew on sheep blood agar incubated aerobically with 5% CO₂ as well as anaerobically; these bacteria were eventually identified as A. naeslundii. When antimicrobial susceptibilities were known, therapy with amoxicillin with clavulanic acid was replaced with cefuroxime plus rifampin on April 29 (16). The clinical response was good, laboratory parameters returned to normal, and, subsequently, therapy was stopped on June 8. On June 22, the C-reactive protein level had reached 11 mg/liter. At the present time, the clinical course has been excellent, and therefore, the hip has not been punctured again.

A. naeslundii infection is known in classical settings of actinomycosis in the head and the abdomen (2). The infection of a hip prosthesis presented here is unique. According to a Medline search, A. naeslundii has not been described as an infectious agent in infections of prostheses. However, a serious case of osteomyelitis with A. naeslundii after a foot injury has been reported (14). Two reports of Actinomyces israelii being the causative agent in late hip infection with possible hematogenous spread have been published (11, 13).

Since the infection in the patient reported herein occurred only 6 months after hip surgery, a hematogenous spread from the patient's own bacterial flora could be postulated (10). Dental work on a molar tooth was performed early in 1999. Routine antibiotic prophylaxis is not recommended for such procedures (1).

The bacterial isolate was identified by conventional methods (15) combined with gas-liquid chromatography of volatile and nonvolatile fatty acids from prereduced, anaerobically sterilized chopped-meat broth containing carbohydrates (9). MICs were determined by the E-test procedure (AB Biodisk, Solna, Sweden) with an inoculum corresponding to McFarland standard 0.5 on Mueller-Hinton agar with 5% sheep blood incubated at 37°C in 5% CO₂ for 24 h (8). The results are summarized in Table 1.

TABLE 1.

Microbiological characteristics of the A. naeslundii strain isolated from our patient

Bacteriological or biochemical test	Result
Gram stain	Gram-positive, slender, slightly irregular pleomorphic rods (0.8 to 1.0 μm wide, 1.5 to 5.0 μm long)
Catalase test	Negative
Acid production from glucose, sucrose, maltose (CTA medium)	Positive
Acid production from xylose,
mannitole (CTA medium)	Negative
Urease (Christensen)	Positive (after 72 h)
Nitrate reduction	Positive
Esculin hydrolysis	Positive
Fatty acid production	Major product, succinic acid
MICs (mg/liter)
Penicillin G	0.016
Amoxicillin	0.047 (not influenced by clavulanic acid)
Cefuroxime	0.19
Erythromycin	0.032
Clindamycin	0.094
Rifampin	<0.002
Ciprofloxacin	6.0
Vancomycin	0.75

Today, many laboratories rely on commercial identification kits rather than traditional media. Therefore, the following kits were challenged with our A. naeslundii isolate: RapID ANA II, RapID CB Plus (both from Remel, Lenexa, Kans.), API 20A, and API CORYNE 2.0 (both from BioMérieux, La-Balme-les-Grottes, France). Enzymatic activities were further determined by the API ZYM kit (BioMérieux). The results are presented in Table 2. The RapID ANA II and API 20A anaerobe identification kits performed well, with the exception of the negative test for urea degradation in the RapID ANA II. Enzymatic activity in the API ZYM kit corresponded to the data published by Brander and Jousimies-Somer (3). Problems were encountered with the API CORYNE, version 2.0, which was in fact used in a first attempt because the strain grew as well in 5% CO₂ as it did in an anaerobic atmosphere. Subsequent testing in RapID CB Plus, the only commercial alternative to API CORYNE for aerobically growing gram-positive rods, resulted in an excellent identification. For differentiation of A. naeslundii from A. israelii, urease production is a critical test; it could be detected only after 48 h of incubation in the API 20A kit, after 4 h in the RapID CB Plus kit, and after 3 days in Chistensen's medium.

TABLE 2.

Identification of the A. naeslundii strain by commercial systems

System	Results
RapID ANA II profile	071670a (urease negative!): A. israelii, 89.1%; A. naeslundii, 7.1%; A. viscosus, 3.7%
API 20 A profile	66347602a: A. naeslundii, 99.8% (T value, 0.57)
API CORYNE 2.0	3440121a: unacceptable profile
RapID CB Plus	3744570a: A. naeslundii, 99.99% (bioscore, 1/9)
API ZYM reactions (≥grade 3 color intensity)	Positive wells 6, 13, 14, and 17 (leucine aminopeptidase, α-galactosidase, β-galactosidase, β-glucosidase); all other enzymes negative

^aNumerical profile obtained. 

Problems of correct recognition of many Actinomyces spp. are posed by their ability to grow aerobically to some extent and by the fact that aerobic growth is not taken into account in some of the traditional identification schemes used in clinical laboratories (5) nor in commercial identification kits (6, 7). As early as 1977, in the VPI Anaerobe Laboratory Manual (9), some strains of A. israelii, A. naeslundii, and Actinomyces viscosus were described as being able to grow as well aerobically with CO₂ as anaerobically. This was also pointed out by Schaal in the 1986 Bergey's Manual of Systematic Bacteriology (12). Therefore, these Actinomyces spp. should be included in tables and commercial systems used for the identification of aerobically isolated gram-positive rods. They are dealt with, for example, in the identification scheme of von Graevenitz and Funke (15). In the 1999 edition of the Manual of Clinical Microbiology (4), there is an introductory algorithm that may be helpful in avoiding erroneous identifications.

ADDENDUM IN PROOF

A crude antigen of the A. naeslundii isolate from the patient was prepared as described previously (R. Zbinden, A. Hany, R. Lüthy, D. Conen, and I. Heiner, APMIS 106:547–552, 1998) and was used to determine the serological response by complement fixation. A serum sample collected on 31 August 1999 revealed a titer of 1:80 that sustains an immunological reaction against the causative A. naeslundii.