Use of DNA fingerprinting to assess tuberculosis infection control.

BACKGROUND: DNA fingerprinting establishes the genetic relatedness of Mycobacterium tuberculosis isolates and has become a powerful tool in tuberculosis epidemiology. OBJECTIVE: To use DNA fingerprinting to assess the efficacy of current tuberculosis infection-control practices. DESIGN: Retrospective molecular and descriptive epidemiologic study. SETTING: A 700-bed urban public hospital that follows the Centers for Disease Control and Prevention (CDC) guidelines for tuberculosis infection control. PATIENTS: 183 patients who had positive cultures for M. tuberculosis from 1 April 1995 to 31 March 1996. RESULTS: 173 of 183 M. tuberculosis isolates from the study period underwent DNA fingerprinting. Fingerprinting revealed that five isolates represented false-positive cultures and that 91 (54%) of the remaining 168 isolates were in 15 DNA fingerprinting clusters, which ranged in size from 2 to 29 isolates. Risk factors for clustering were birth in the United States, African-American ethnicity, homelessness, substance abuse, and male sex. Retrospective epidemiologic analysis of inpatient and outpatient visits by the 91 patients who had clustered isolates revealed only one possible instance of patient-to-patient transmission. CONCLUSIONS: The DNA fingerprinting of all M. tuberculosis isolates from a 1-year period revealed one possible instance of nosocomial transmission and five false-positive M. tuberculosis cultures. However, these results did not lead to changes in infection-control practices or in clinical care. The study findings do not support the use of DNA fingerprinting for nosocomial tuberculosis surveillance, but they suggest that compliance with the CDC tuberculosis infection-control guidelines may control patient-to-patient transmission in high-risk urban hospitals.

An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seeds.

An outbreak of Salmonella serotype stanley infections occurred in the United States and Finland in 1995. The outbreak was investigated through case-control studies in Arizona, Michigan, and Finland; by isolate subtyping; and by tracing and culturing of the implicated food. Alfalfa sprout consumption was the only exposure associated with S. stanley infections in Arizona (matched odds ratio [MOR] = 11.1; 95% confidence interval [CI], 1.4-513), Michigan (MOR = 5.5; CI, 1.6-23), and Finland (MOR undefined; CI, 4.9-infinity). US and Finnish patient isolates were a unique outbreak strain distinct from S. stanley isolates not linked to the outbreak. Alfalfa sprouts eaten by patients in 6 US states and Finland were traced to seed shipped by a Dutch shipper. Thus, it was concluded that alfalfa sprouts grown from contaminated seed caused an international outbreak of > or =242 S. stanley infections in > or =17 US states and Finland. This outbreak illustrates a new mechanism through which contamination of fresh produce can cause large, widely dispersed outbreaks.

Tuberculosis among Tibetan immigrants from India and Nepal in Minnesota, 1992-1995.

OBJECTIVE: To study screening outcomes among a group of Tibetan immigrants at high risk for developing active tuberculosis (TB) after arrival in Minnesota. DESIGN: Retrospective cohort study. PARTICIPANTS: A total of 191 Tibetan immigrants undergoing medical screening. MAIN OUTCOME MEASURES: Occurrence and treatment outcomes of active TB. SETTING: A health maintenance organization and a public TB clinic in Minneapolis, Minn. RESULTS: Positive (induration, > or =10 mm) tuberculin skin test results were documented in 98% of Tibetans, compared with 44% of Vietnamese, 10% of Hmong, and 51% of Russian refugees in Minnesota (P<.001 for each group). Sixteen active cases (8.4%) were confirmed by isolation of Mycobacterium tuberculosis; however, 5 (31%) were culture-negative on initial screening in Minnesota. Seven cases (44%) were diagnosed during initial screening efforts, and 9 cases (56%) were diagnosed a mean of 19 months (range, 10-27 months) after their initial medical evaluation. Of these 9 cases, 6 (38% of all Tibetan cases) had isolates resistant to 1 or more antituberculous drugs, and 3 (19% of all Tibetan cases) were multidrug resistant (MDR TB). All 3 MDR TB cases were culture-negative on initial screening; these cases constituted 75% of the MDR TB isolates in Minnesota in 1994. The presence of MDR TB was associated with a known history of active TB in Asia (P<.02). Any abnormality on chest radiograph noted either during the Immigration and Naturalization Service screening evaluation in India (relative risk [RR], 5.2; P=.006) or on arrival in Minnesota (RR, 6.8; P=.005) was associated with an increased risk of subsequent active TB. CONCLUSIONS: Tuberculosis infection is nearly universal among Tibetans settling in Minnesota. A single screening evaluation failed to detect the majority of TB cases among Tibetans. Even in the face of negative M tuberculosis cultures, persons with a history of active TB require particularly close follow-up.

Outbreak of Shigella flexneri linked to salad prepared at a central commissary in Michigan.

In August 1992, the Michigan Department of Public Health was notified of a cluster of persons with Shigella flexneri infections, all of whom had eaten at different outlets of a single restaurant chain. The chain prepared many foods at a central kitchen. A matched case-control study to determine risk factors for illness among patrons of the restaurant chain was undertaken. An inspection of the commissary and a review of commissary inspection and employee records were conducted. Of the 46 patients identified, 44 had eaten tossed salad versus 33 of the 71 controls (matched odds ratio = 56.9; 95 percent confidence interval 5.0, 648.1). Improper salad preparation techniques were used, and the salad preparation area had not been inspected in several years. Some salad preparers had been ill shortly before the outbreak but continued to work. The greater use of central kitchens could lead to larger outbreaks of illness related to improper food handling. Raw vegetables are a potential vehicle for transmission of shigellosis. Coordination of all agencies responsible for inspecting commissaries should be assured. Assuring restriction of ill food handlers will require management to take an active role in identification of ill employees and in the development of incentives to report illness.

Malignant transformation of human fibroblasts by a transfected N-ras oncogene.

The only ras oncogene as yet identified in cells from human fibrosarcomas is N-ras, but the relationship between N-ras oncogene expression and the malignant state of these cell lines is not known. To determine if expression of an N-ras oncogene causes human cells to become malignant, we transfected the N-ras oncogene from human leukemia cell line 8402, cloned into a high expression vector pSV N-ras, into MSU-1.1 cells, a nontumorigenic, infinite life span fibroblast cell strain with a normal morphology and a stable near-diploid karyotype. The transformants formed distinct foci composed of morphologically transformed cells. Cells from such foci expressed higher than normal levels of N-ras protein, exhibited growth factor independence, and formed large colonies in soft agar at a high frequency. Injection of progeny of these focus-derived cells s.c. into athymic mice resulted in progressively growing, invasive malignant tumors (round cell, spindle cell, or giant cell sarcomas) which reached a diameter of 6 mm in 3 to 4 weeks. Injection of focus-derived or tumor-derived cells i.v. resulted in tumors in various organs of the mice. The focus-derived cell strain tested, as well as the majority of the cells derived from the tumor it produced, exhibited the same near-diploid karyotype as the parental MSU-1.1 cells. Cells transfected with an N-ras oncogene that was expressed at a normal level formed only a single, indistinct focus, and cells from that focus were not malignant.