Background. The USA300 methicillin resistant Staphylococcus aureus (MRSA) genetic background has rapidly emerged as the predominant cause of community-associated S. aureus infections in the U.S. However, epidemiologic characteristics of S. aureus household transmission are poorly understood.Methods. We performed a cross-sectional study of adults and children with S. aureus skin infections and their household contacts in Los Angeles and Chicago. Subjects were surveyed for S. aureus colonization of the nares, oropharynx, and inguinal region and risk factors for S. aureus disease. All isolates underwent genetic typing.Results. We enrolled 1162 persons (350 index patients and 812 household members). The most common infection isolate characteristic was ST8/SCCmec IV, PVL1 MRSA (USA300) (53%). S. aureus colonized 40% (137/350) of index patients and 50% (405/812) of household contacts. A nares-only survey would have missed 48% of S. aureus and 51% of MRSA colonized persons. Sixty-five percent of households had .1 S. aureus genetic background identified and 26% of MRSA isolates in household contacts were discordant with the index patients' infecting MRSA strain type. Factors independently associated (P , .05) with the index strain type colonizing household contacts were recent skin infection, recent cephalexin use, and USA300 genetic background.Conclusions. In our study population, USA300 MRSA appeared more transmissible among household members compared with other S. aureus genetic backgrounds. Strain distribution was complex; .1 S. aureus genetic background was present in many households. S. aureus decolonization strategies may need to address extra-nasal colonization and the consequences of eradicating S. aureus genetic backgrounds infrequently associated with infection.

In our longitudinal study, patients with a S. aureus skin infection were more likely to suffer a recurrence if household fomites were MRSA contaminated. Interventions to prevent recurrence may be enhanced by decontamination of household fomites.

c Methicillin-resistant Staphylococcus aureus (MRSA) has become a common cause of skin infections and invasive infections in community dwellers in the United States since the late 1990s. Isolates characterized as USA300 by pulsed-field gel electrophoresis (PFGE) are the predominant strain type in these infections. USA100 and USA500 strains commonly cause health care-associated infections. We compared PFGE with a number of other methods of genotyping in a sample of 149 clinical MRSA isolates from the University of Chicago Medical Center. The 5 USA500 isolates yielded 3 spa types and 2 multilocus sequence types (MLSTs). Among the 24 USA100 isolates, 21 (88%) were of spa type t002, 19 (79%) were of ST5, 2 carried arcA and opp3, and 1 was Panton-Valentine leukocidin positive (PVL ؉ ). Among the 102 USA300 isolates, 96 (94%) were of ST8 and 94 (92%) were of spa type t008. The combination of traits that provided the best sensitivity (98%), specificity (97%), positive predictive value (PPV) (99%), and negative predictive value (NPV) (95%) for identifying USA300 isolates were the presence of the arcA gene and the presence of the PVL genes (area under the curve, 0.980; 95% confidence interval [CI], 0.955 to 1.0). PFGE did not delineate a homogeneous group of MRSA genetic backgrounds, as documented for other typing methods, particularly for USA500 and USA100 pulsotypes. Documenting the presence of arcA and PVL genes by PCR was an efficient and accurate means of identifying USA300 in a collection of MRSA isolates in which USA300 is common. None of the tested genotyping methods provided an accurate means of identifying the next most common PFGE-based backgrounds, USA100 and USA500.

Sequence type 398 (ST398) Staphylococcus aureus, frequently carried by livestock, has caused severe human infections and often carries transmissible antibiotic resistance genes. Among methicillin-susceptible S. aureus isolates colonizing Dallas County Jail detainees, 13.2% were ST398, spa type t571, and were genetically similar to human colonization isolates from New York, Chicago, and the Dominican Republic. Sequence type 398 (ST398) methicillin-resistant Staphylococcus aureus (MRSA) is often transmitted among livestock and animal handlers but is uncommonly associated with human disease in the United States (1). We unexpectedly identified ST398/t571 methicillin-sensitive S. aureus (MSSA) as the most common genetic background among MSSA isolates colonizing detainees in the Dallas County Jail. In this study, we compared the pulsed-field gel electrophoresis (PFGE) pulsotypes of isolates from the Dallas jail and from a single patient in Chicago with those of strains isolated in previous studies.Recognized first in Europe (2-9), ST398 MRSA has been isolated in the United States (10), Canada (11), and Latin America (12). ST398 methicillin-susceptible S. aureus (MSSA) isolates have been recovered from pigs (13), humans (14-17), and retail meat (18). Asymptomatic human colonization and infection with ST398 MSSA are rare in the United In January 2009, we screened the anterior nares and the hands of 928 detainees, housed in 68 divisions, called tanks, in the Dallas County Jail, for S. aureus carriage. Tanks had a capacity of 24 to 36 detainees. Detainees can interact with others within a single tank but have little interaction with those in other tanks. The study was approved by the Institutional Review Boards at the University of Chicago Medical Center and the University of Texas-Southwestern Medical Center. Recovered MSSA isolates were genotyped from subjects housed in a stratified random sample of 26 of the 68 tanks; all MRSA isolates were genotyped.In a separate study of S. aureus colonization among patients and household contacts (24), we also identified one patient in Chicago with ST398/t571 MSSA colonization.Multilocus sequence typing (MLST) (25), spa typing (26), PCR for the Panton-Valentine leukocidin (PVL) genes (27), and PFGE using Cfr9I digestion (22, 28) were performed. Antimicrobial susceptibilities were determined by automated testing (29).Among the 345 subjects in the 26 selected tanks, 110 (31.9%) carried MSSA. Of 158 MSSA isolates identified, 152 were available, and 34 MLST types were identified. Twenty isolates (13%) were ST398, the most common ST; all lacked PVL genes. Of the 345 subjects, the 16 (4.6%) who carried the 20 ST398 MSSA isolates were more likely than carriers of other MSSA genetic backgrounds to be female (P ϭ 0.0009) and older (P ϭ 0.01) ( Table 1).Among the 20 ST398 isolates, 18 were susceptible to ciprofloxacin, gentamicin, rifampin, and trimethoprim-sulfamethoxazole and resistant to clindamycin and erythromycin. The two other isolates differed in that one was susceptible to clindamyc...

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