Follow-up of Staphylococcus aureus nasal carriage after 8 years: redefining the persistent carrier state

Abstract

Studies of Staphylococcus aureus nasal carriage have distinguished three carriage patterns: persistent, intermittent, and noncarriage. The criteria used to identify these carriage patterns have been inconsistent. In 1988 the S. aureus nasal carrier index, i.e., the proportion of nasal swab specimen cultures yielding S. aureus, was determined for 91 staff members of various departments of a large university hospital by obtaining weekly nasal swab specimens for culture over a 12-week period. Thirty-three (36%) persons had carrier indices of 0.80 or higher, 15 (17%) had indices between 0.1 and 0.7, and 43 (47%) had indices of zero. In 1995, 17 individuals with carrier indices of 0.80 or higher in 1988 were available for reexamination. For 12 (71%) of these individuals, S. aureus was again isolated from a single nasal swab, i.e., from each individual with a 1988 carrier index of 1.0 but from only half of those with indices below 1.0. Genotyping (by randomly amplified polymorphic DNA analysis and pulsed-field gel electrophoresis) of all S. aureus strains showed that strains isolated from only three individuals, all with 1988 carrier indices of 1.0, in 1988 and 1995 showed genetic similarity. In conclusion, persistent S. aureus nasal carriage is a unique characteristic of a fraction of the population, and the attribute "persistent" should be confined to those individuals for whom serial nasal swab specimen cultures consistently yield S. aureus.

FIG. 1

Distribution of S. aureus nasal carrier indices among 91 healthy adults repeatedly cultured over a 10- to 12-week period in 1988. The carrier index is defined as the proportion of cultures yielding S. aureus for a given individual.

FIG. 2

RAPD analysis with primer ERIC2 of S. aureus strains isolated in 1988 and 1995 from carriers 2, 11, 12, and 14. The arrows on the left and right identify the molecular length marker that is 600 bp long in the 100-bp ladder. The other fragments differ in size by multiples of 100 bp in length. Neg., negative control.

FIG. 3

PFGE of S. aureus strains isolated in 1988 and 1995 from carriers 2, 11, 12, and 14. The lane marked lambda contains bacteriophage lambda concatemers that differ in size by multiples of 50 kbp.

FIG. 4

Protein A gene PCR of S. aureus strains isolated in 1988 and 1995 from carriers 2, 11, 12, and 14. The arrows on the left and right identify the molecular length marker that is 600 bp long in the 100-bp ladder. The other fragments differ by multiples of 100 bp in length.

FIG. 5

Coagulase gene PCR of S. aureus strains isolated in 1988 and 1995 from carriers 2, 11, 12, and 14. The arrows on the left and right identify the molecular length marker that is 600 bp in the 100-bp ladder. The other fragments differ by multiples of 100 bp in length. Neg., negative control.