Determinants of Staphylococcus aureus nasal carriage

Abstract

Nasal carriage of Staphylococcus aureus has been identified as a risk factor for community-acquired and nosocomial infections. We screened 230 donors of diverse ethnic and socioeconomic backgrounds and identified 62 (27%) whose nasal secretions were colonized by S. aureus. In 18 donors in whom the various regions of the nasal luminal surface were separately sampled, the predominant region of S. aureus colonization was the moist squamous epithelium on the septum adjacent to the nasal ostium. Nasal fluid from carriers was defective in killing endogenous S. aureus and nasal carrier isolates of S. aureus but not a laboratory S. aureus strain. Transmission electron microscopy revealed that S. aureus isolates incubated in nasal fluid from carriers for 2 h at 37 degrees C were less damaged than those incubated in noncarrier fluid and were coated with an electron-dense layer. Compared with that from healthy donors and patients with acute rhinitis, nasal fluid from carriers contained elevated concentrations of the neutrophil-derived defensins human neutrophil peptides 1 to 3 (47- and 4-fold increases, respectively), indicative of a neutrophil-mediated inflammatory host response to S. aureus colonization. The concentration of the inducible epithelial antimicrobial peptide human beta-defensin 2 was also highly elevated compared to that in healthy donors, in whom the level was below the detection limit, or patients with acute rhinitis (sixfold increase). Thus, nasal carriage of S. aureus takes hold in nasal fluid that is permissive for colonization and induces a local inflammatory response that fails to clear the colonizing bacteria.

FIG. 1

S. aureus predominantly colonizes the moist squamous epithelium adjacent to the nasal ostium. The right nasal vestibule of each donor was swabbed in five distinct regions, A to E; in addition, bacteria were quantified from three clipped nose hairs (region F). The swab specimens were plated on TSA–5% sheep blood, and S. aureus CFU (A) and coagulase-negative staphylococcus CFU (B) were quantified for each region (18 donors). The majority (>60%) of S. aureus bacteria were isolated from region B (asterisk). Error bars represent the SEM.

FIG. 2

Nasal fluid from carriers supports the growth of endogenous S. aureus. Nasal fluid from Staphylococcus carriers was tested in a CFU microassay to measure the growth of endogenous bacteria. Error bars represent the SEM.

FIG. 3

Nasal fluid from carriers supports the growth of S. aureus isolates. Nasal fluids from S. aureus carriers (C; solid circles) and healthy noncarriers (NC; open triangles) were tested in CFU microassays with an isolate of S. aureus from the donor's own nose (A); an isolate of S. aureus from another, randomly selected carrier (B); and a laboratory strain of S. aureus (C) (eight donors for each condition, with four or five replicates per condition). Each point represents the log₁₀ CFU per milliliter at 24 h. Broken lines indicate the mean and SEM. Compared to noncarrier fluid, carrier fluid permitted more rapid growth of isolates derived from either the same (P = 0.00016) or a different (P = 0.002) carrier. In contrast, the difference in the growth of the laboratory strain of S. aureus in carrier versus noncarrier fluids did not reach significance (P = 0.059).

FIG. 4

Ultrastructural effects of carrier and noncarrier fluids on S. aureus. S. aureus was incubated with unmanipulated noncarrier fluid (A, overview; B, detail), carrier fluid (C and D), or HBSS-TSB control (E and F) for 2 h at 37°C. Note the effects of noncarrier fluid, including membrane ghosts of S. aureus (A, arrow), the denudation of peptidoglycan (B, thin arrow), and membrane rupture (B, thick arrow). The appearance of intact peptidoglycan is shown by the arrow in panel F. Arrows in panels C (carrier fluid) and E (control) indicate clefts of actively dividing cells, which are less prominent or absent in noncarrier fluid (A). S. aureus exposed to carrier fluid was not damaged (C), and an additional, possibly protective layer surrounds peptidoglycan (D, arrow). Overview bar, ∼500 nm; detail bar, ∼100 nm.

FIG. 5

Concentrations of neutrophil-derived and epithelial defensins are elevated in carrier nasal fluid. Concentrations (Conc) of the antimicrobial peptides HNP-1 to HNP-3 (A) and HBD-2 (B) are shown for carriers, noncarriers (normal, healthy donors), and patients with acute rhinitis. Levels of HNP-1 to HNP-3 were significantly higher in carriers than in acute rhinitis donors (P = 0.029) and noncarriers (P = 0.009). Similarly, the level of HBD-2 was higher in carriers than in rhinitis donors (P = 0.044). Noncarriers had an HBD-2 concentration below the limit of detection (<0.125 ng/ml). Error bars represent the SEM.