Colonization and cross-colonization of nursing home patients with trimethoprim-resistant gram-negative bacilli.

A prospective study of 67 patients in a nursing home and a subpopulation of 31 patients who were stratified according to functional level--a general measure of the amount of nursing care required by the patients--was performed. The goals of this study were (1) to determine whether antibiotic selection of resistant variants from within endogenous flora of hosts or horizontal transmission is more important in the development of colonization with trimethoprim-resistant gram-negative bacilli (TRGNB) in a nursing home; (2) to identify the mode(s) of transmission if horizontal transmission is more important; and (3) to identify risk factors for colonization and cross-colonization with TRGNB. Although a number of variables were associated with colonization, only a decreased functional level appeared to be independently associated with colonization. Isolates from the subpopulation were subtyped by restriction endonuclease digestion of cellular DNA, as were isolates from personnel of the nursing home. Results revealed that 16 of 21 staff members had 48 positive cultures for TRGNB. Of 25 typeable isolates, 12 from seven of the 21 staff members were identical to patient strains. Analysis of acquisition of new strains of TRGNB by members of the subpopulation showed that 67.5% were the result of cross-colonization. Our data are consistent with, but do not prove, the hypothesis that nurses' hands are the primary mode of transmission of TRGNB in this nursing home and suggest that most colonization could be prevented by interdicting horizontal transmission.