Escherichia coli is the leading cause of community-onset gram-negative bloodstream infections. There has been a dramatic increase in the prevalence of extended-spectrum b-lactamases producing E. coli and K.pneumoniae in the community, which was considered to be exclusively a nosocomial pathogen in recent years. As a result, the treatment options for community-onset infections due to ESBL-producing E. coli or K.pneumoniae are limited and the initial empirical therapy is often ineffective and associated with increased mortality. Although there were some reports of the risk factors of community-onset ESBL producing E. coli in Spain, Korea, and Canada, few empirical data were available about China. Therefore, the investigators aim was to investigate the epidemiology, risk factors, and the hospital outcomes for patients with community-onset bacteremia caused by ESBL producing E. coli or K.pneumoniae in China.

Eligibility criteria:

1. Inclusion criteria:Blood cultures demonstrated E.coli or K.pneumoniae among nonhospitalized patients or within 48 h after hospitalization.
2. Exclusion criteria:Patients with hospitalization in the past 1 month.

Outcome measures:

1. Risk factors for community-onset bacteremia due to ESBL producing E. coli or K.pneumoniae
2. Microbial and clinical efficacy of commonly used antibiotics for community-onset bacteremia due to ESBL producing E. coli or K.pneumoniae

Inclusion Criteria:

• older than 18 years old
• community-onset bacteremia

Exclusion Criteria:

• recent hospitalization within one month in any hospital

All adult patients visited any of the 28 particitant hospitals during the study period were screened. Patients with community-onset bacteremia caused by E.coli or K. pneumoniae and without a recent hospitalization in any hospital will be included