Post CABG ( coronary artery bypass graft ) surgical site infection by Nocardia

Surgical site infection is the second most common nosocomial infection. Wound infection is an important cause of morbidity and occasional mortality after coronary artery bypass graft surgery (CABG). Common organisms encountered are Staphylococcus aureus, Staphylococcus epidermidis, Enterococcal species, Streptococcus pneumonia, and Pseudomonas aeruginosa [1]. In certain cases of delayed CABG wound healing, Candida species and occasionally rapidly growing atypical Mycobacterial species are isolated. Only one case of Nocardia asteroides mediastinitis complicating CABG surgery has been reported in the literature available by Susan Verghese et al [4]. Nocardia occurs worldwide and the annual incidence is 0.375 cases per 100000 persons as estimated by three continentsNorth America, Europe and Australia. This case report may be following transcutaneous inoculation during CABG surgery from the hospital environment or solution. The rarity of Nocardia causing sternal wound infection after CABG surgery prompted reporting of this case.


Introduction
Surgical site infection is the second most common nosocomial infection.Wound infection is an important cause of morbidity and occasional mortality after coronary artery bypass graft surgery (CABG).Common organisms encountered are Staphylococcus aureus, Staphylococcus epidermidis, Enterococcal species, Streptococcus pneumonia, and Pseudomonas aeruginosa [1].In certain cases of delayed CABG wound healing, Candida species and occasionally rapidly growing atypical Mycobacterial species are isolated.Only one case of Nocardia asteroides mediastinitis complicating CABG surgery has been reported in the literature available by Susan Verghese et al [4].Nocardia occurs worldwide and the annual incidence is 0.375 cases per 100000 persons as estimated by three continents-North America, Europe and Australia.This case report may be following transcutaneous inoculation during CABG surgery from the hospital environment or solution.The rarity of Nocardia causing sternal wound infection after CABG surgery prompted reporting of this case.

Case report
A 65 year old male patient reported to Surgical OPD, with history of incomplete healing of wound on the Chest wall for more than 2months duration after CABG surgery.He underwent surgery 2 ½ months ago.He is a known diabetic and a hypertensive under good control.On examination, wound dehiscence was noticed at the lower end of the CABG surgical scar with scanty discharge.The discharge was sent to Microbiology lab for Culture and sensitivity.
The sample was inoculated on to MacConkey agar and blood agar, and incubated aerobically at 37 degree Celsius overnight.A gram stained smear was also prepared from the sample and observed under the microscope.It showed gram positive branching filaments.The smear was completely decolorized with alcohol and re-stained with modified Zeihl-Nelson's stain using 1% sulfuric acid as decolourizer and observed under the microscope.It showed pink acid fast branching thin filaments and tentatively identified as Nocardia species.
The culture plates did not show any growth after overnight incubation and they were further incubated for another day.Next day when the plates were examined, MacConkey's agar showed no growth but blood agar showed growth of white, small, dry colonies (Figure 1).Culture smear made showed gram positive thin branching filaments.Modified Zeihl-Neelson's stain using 1% sulfuric acid as decolourizer showed pink acid fast filamentous branching bacteria (Figure 2).The bacteria grown were identified as Nocardia species.The biochemical reactions showed following results.It failed to hydrolyse casein, xanthine and tyrosine and Correspondence: M. Shenoy E-mail: manjunathshenoy61@gmail.com Figure 1.Showing dry white colonies on blood agar.was urease negative.It was identified as Nocardia asteroids.The sensitivity of the organism was done on Mueller-Hinton blood agar.It was sensitive to cotrimaxazole, penicillin, amoxicillin, gentamycin, erythromycin, amikacin, ciprofloxacin and ofloxacin.A repeat sample collected from the patient to confirm also yielded the growth of same organism.The patient was s t a r t e d o n t r i m e t h o p r i m ( 2 0 m g m / k g ) a n d sulfamethoxazole (100mgm per kg) each day in two divided doses for 15 days and later the dosage reduced to 5mgm per kg and 25mgm per kg respectively for 4 months.After 15 days of treatment the wound showed signs of healing and after a month it was completely healed.

Discussion
Post-operative wound infections like sternal wound infections, donor site infections and mediastinitis form serious complications after cardiac surgery.Softah et al have reported post CABG wound infection due to Staphylococcus aureus in 33% of their patients and Staphylococcus epidermidis in 31% of their patients and mixed infections in 11.7% of patients [2].
Preeti N. Malani et al from Michigan have reported that Candida species are an uncommon cause of sternal wound infections.Mediastinitis due to Candida is a rare but serious complication that usually occurs in the first few days to weeks after the surgical procedure and is associated with a mortality rate of >50%.They report on a series of 11 patients who developed deep sternal wound infections due to Candida albicans.These infections were characterized by a chronic, indolent, and recurrent course [3].
Susan Verghese et al have reported a case of Nocardia asteroides mediastinitis complicating coronary artery bypass surgery [4].
Thaler F et al have reported mediastinitis due to Nocardia asteroids after cardiac transplantation [5].