Perfusionist-Transmitted Bacterial Mediastinitis in a Heart Transplant Recipient

Abstract

We report the case of a 56-year-old man who developed bacterial mediastinitis with methicillin-resistant Staphylococcus aureus after undergoing heart transplantation. He had a history of insulin-dependent diabetes mellitus and prior cardiac surgery. To find the source of nosocomial infection, we cultured nasal swab specimens from all hospital personnel involved in this operation. We used antibiotic sensitivity profiling and pulsed-field gel electrophoresis to subtype the involved microorganism. The S. aureus isolates from the patient and the perfusionist were identical to each other and were different from the strains previously found in our hospital. It is almost certain that the S. aureus mediastinitis in this patient was transmitted from the perfusionist. We recommend obtaining cultures from hospital staff members when there is an outbreak of staphylococcal infection.

Bacterial mediastinitis after open-heart surgery is rare, ranging from 0.4% to 5%, but its occurrence may seriously affect a patient's postoperative course. The mortality rate ranges from 7% to 80%. ¹ When mediastinitis occurs in immunocompromised patients after heart transplantation, the treatment is even more problematic. Its incidence, clinical presentation, risk factors, microbiology, methods of treatment, and outcome have been well described in the literature. However, the mode of transmission has seldom been reported. We report a possible route of transmission to a heart transplant recipient who developed bacterial mediastinitis.

Case Report

A 56-year-old man with diabetes mellitus, hypertension, and coronary artery disease had refractory angina pectoris despite previous coronary artery bypass surgery and the presence of a patent internal mammary graft. He underwent orthotopic heart transplantation on 11 December 1998 with a standard mid-atrial anastomosis and an ischemic time of 155 minutes. The operation was technically uncomplicated, except for the prolonged duration of both cardiopulmonary bypass (219 minutes) and surgery (540 minutes) due to dense mediastinal adhesion. After induction therapy with antithymocyte globulin, the patient was maintained on triple immunosuppressive therapy. Immediately after surgery, the patient was noted to have an unstable sternum. One week after transplantation, he had erythema over the lower end of the sternal wound with a purulent discharge. We performed mediastinal débridement and soft tissue coverage with a pectoralis major muscle flap. Methicillin-resistant Staphylococcus aureus (MRSA) was identified as the responsible pathogen. A blood culture was also positive for MRSA. We discontinued prednisone administration and kept the serum cyclosporine level as low as possible. The serial endomyocardial biopsy showed no evidence of acute rejection. The patient was given intravenous vancomycin for 6 weeks and was discharged from the hospital 2 months after transplantation.

We sought to find the source of nosocomial infection by culturing nasal swab specimens from all hospital personnel involved in this operation. We used antibiotic sensitivity profiling and pulsed-field gel electrophoresis to subtype the microorganism. We identified S. aureus on the basis of colony morphology on trypticase soy agar supplemented with 5% sheep blood agar (BBL Microbiology Systems; Cockeysville, Md), Gram stain, and a positive staphylase test (Oxoid Limited; Basingstoke, UK). Susceptibility to various antibiotics was determined by disk diffusion method, as described by the National Committee for Clinical Laboratory Standards (NCCLS). ² The antimicrobial disks (BBL Microbiology Systems) included penicillin G (10 U), oxacillin (1 μg), cefazolin (30 μg), vancomycin (30 μg), gentamicin (10 μg), clindamycin (2 μg), erythromycin (15 μg), minocycline (30 μg), and trimethoprim (1.25 μg)-sulfamethoxazole (23.75 μg). All the antimicrobial susceptibility tests were performed by 1 investigator to avoid the interindividual variation of performance and interpretation.

Genomic DNA was prepared in agarose plugs by the method of Mlynarczyk and coworkers, ³ with some modification. For restriction endonuclease digestion, the DNA-containing agarose plug was incubated overnight at 25 °C with 250 μL of restriction buffer containing 20 U of SmaI (New England Biolabs; Beverly, Mass). After DNA digestion, the agarose plugs were incubated with 1 mL Tris-EDTA (TE) buffer at 37 °C for 1 hour. The plugs containing the restricted DNA were inserted into 1% agarose gels (BioRad Laboratories, Inc.; Hercules, Calif) in 0.5X Trisborate/EDTA (TBE) buffer, and restriction fragments were separated with use of a contour-clamped homogeneous electrical field system (CHEF-DRII; Bio-Rad Laboratories, Inc.). Electrophoresis was performed at 6 V/cm for 24 hours with pulse times of 1 to 35 seconds at 4 °C. The gels were stained with ethidium bromide and photographed. The result was that the S. aureus isolates from the patient and perfusionist were identical to each other and were different from the strains that had previously been found in our hospital.

When the patient was last seen in January 2001, he was doing well. He had a stable sternum with no infection.

Discussion

The incidence of mediastinal infection after conventional heart surgery through a median sternotomy is 0.4% to 5%. ¹ Baldwin and colleagues ⁴ reported that bacterial mediastinal abscess, or mediastinitis, developed in 2.5% of patients who underwent isolated heart transplantation. The predisposing factors included insulin-dependent diabetes mellitus, repeat operation for postoperative hemorrhage, S. aureus pneumonitis, and early cardiac allograft rejection. Most centers perform soft tissue coverage (using greater omentum or pectoralis muscle flaps), sternal débridement, or the 2 in combination. ⁵ With early intervention, the mortality rate of bacterial mediastinitis in heart transplant recipients becomes acceptable.

The microorganism of mediastinitis in heart transplant patients is similar to that reported in patients undergoing conventional surgery, with S. aureus predominating. However, the route of transmission is rarely reported. Staphylococcus aureus pneumonitis has been implicated as a possible source of hematogenous seeding of the mediastinum. ¹

During the past 20 years, reports of nosocomial infections with S. aureus have increased in frequency. ⁶ These infections are substantial causes of mortality and morbidity in hospitalized patients and require extensive antibiotic treatment. Once S. aureus has been introduced into a hospital, eradication is difficult or impossible. Accurate epidemiologic typing has become important for identifying clones of S. aureus in a hospital in order to reduce and control its spread. ⁷ For epidemiologic differentiation of S. aureus infection, many phenotypic and genotypic analyses have been applied. ⁸ These include antibiotyping, biotyping, phage typing, plasmid profile analysis, restriction fragment length polymorphism, pulsed-field gel electrophoresis, and polymerase chain reaction. Antibiotyping is the simplest method, but it is not easy to differentiate the various strains from their antibiotic susceptibility. Pulsed-field gel electrophoresis is the technique that has the highest power of resolution for the epidemiologic subtyping of bacterial isolates. ⁹

In our hospital, 60% to 70% of all S. aureus strains are MRSA. After the occurrence of MRSA in our patient, we obtained cultures from all staff members involved. Our results showed that the S. aureus isolates from the patient and perfusionist were identical to one another and were different from the strains that had been recognized previously in our hospital. It is almost certain that the S. aureus mediastinitis in this patient was transmitted from the perfusionist who manipulated the cannulas for cardiopulmonary bypass. It is possible that there was contamination of blood from a perfusion machine to the patient with subsequent bacteremia. The presumed MRSA carrier was treated with nasal mupirocin therapy.

Herein, we have described a possible route of transmission of Staphylococcus aureus leading to bacterial mediastinitis in a heart transplant recipient. We recommend that microbiologic cultures of all involved hospital personnel be taken when S. aureus is identified.