Nasal carriage of staphylococcus aureus and epidemiology of surgical-site infections in a Sudanese university hospital

Abstract

Surgical site infections (SSI) due to Staphylococcus aureus among 256 male and 158 female patients (mean age, 28 years) undergoing elective surgery at the Soba University Hospital (Khartoum, Sudan) were studied. During an 11-month study period all patients were analyzed for nasal carriage of S. aureus at the time of admission. Follow-up of the development of SSI proceeded until 4 weeks after the operations. In addition, nasal swabs were obtained periodically during the same period from 82 members of the staff. In order to discriminate autoinfection from cross infection, bacterial isolates were typed by random amplification of polymorphic DNA (RAPD), pulsed-field gel electrophoresis (PFGE) of DNA macrorestriction fragments, and restriction fragment length polymorphism analysis of the protein A and coagulase genes. Preoperative cultures revealed the presence of S. aureus in the noses of 98 patients (24%). The overall number of postsurgical wound infections in the entire group was 57 (14%), 24 of which were due to S. aureus. Only 6 of the 98 nasal S. aureus carriers suffered from wound infections by the same species. In these six cases the infecting strain could not be genetically discriminated from the nasal inhabitant, substantiating autoinfection. However, nasal carriage of S. aureus is not a significant risk factor for the development of SSI in this setting (6 of 98 patients with autoinfection versus 18 of 316 patients [414 - 98 patients] with cross infection; P = 0.81), most probably due to the fact that noncarriers are at a significant and relatively large risk for acquiring an independent S. aureus SSI. The other S. aureus strains causing SSI showed a high degree of genetic heterogeneity, demonstrating that it is not an epidemic strain that is causing the SSI. Among the staff personnel screened, 47.4% did not carry S. aureus in the nose at any time during the study period, whereas 13. 2% persistently carried a single strain in the nose. Another 39.5% could be classified as intermittent carriers. When strains derived from staff personnel were genetically typed, it was demonstrated that most of the strains represented genetic variants clearly differing from the isolates causing SSI. On the other hand, possible cross colonization among staff personnel and even cross infection from staff personnel to patients or from patient to patient were demonstrated in some cases, but epidemic spread of a single strain or a few clonally related strains of S. aureus could be excluded.

FIG. 1

Survey of RAPD data obtained for paired S. aureus strains. Each pair was derived from the nose and surgical wound of the same patient. Above the lanes, strain and patient numbers are given. The different panels display experimental data obtained by the application of different RAPD primers (ERIC2, RAPD7, and RAPD1). Data are summarized in Table 2. The arrow on the left identifies the molecular length marker that is 600 bp long in the 100-bp ladder. Adjacent markers differ by 100 bp.

FIG. 2

Survey of RAPD data obtained for S. aureus strains derived from the noses of staff personnel in a longitudinal fashion. Data shown were collected with primer RAPD1 and represent samples from four different persons (highlighted by lines). Note that for person 65 a clear shift in the nature of the colonizing strain can be observed: strain 2138 is clearly different from strain 2139. Data are summarized in Tables 3 and 4. The arrows on the left and on the right identify the molecular length marker that is 600 bp long in the 100-bp ladder. Adjacent markers differ by 100 bp.

FIG. 3

PFGE of DNA macrorestriction fragments of S. aureus strains that were isolated from nasal carriers among the staff personnel in the surgical ward of Soba Hospital. Numbers for individuals and strains are above the lanes. The lane marked lambda contains lambda concatemers that differ in size by multiples of 50 kbp. The fragments of 50 and 500 kbp are highlighted by arrows on the right. For several of the staff members (indicated by lines and boldface) multiple strains are included. All of the pairs are different, and this is reminiscent of strain shifts in intermittent carriers. Data are summarized in Table 4. Note that the patterns for strains 1971, 1977, 1991, 2660, 2002, 2656, and 2014 are identical. Also, strains 2658 and 2139 cannot be discriminated on the basis of PFGE.