The purpose of our study is to examine the effects of a second antibiotic, daptomycin, in combination with a beta lactam antibiotic on treating bloodstream infection caused by methicillin-susceptible S. aureus.

When patients have a blood infection with methicillin-susceptible S. aureus, they will all be treated with the usual standard of care, including abeta-lactam antibiotic, have their blood tested daily for the presence of the infection until they test negative, as well as an ultrasound of the heart to rule out an infection of the heart valves, as well as have any catheters removed (if possible) to ensure adequate control of the infection.

This study involves adding a second antibiotic, daptomycin, to the treatment regimen of half of the patients enrolled in this study. Daptomycin is approved by Health Canada for treating blood infections due to Staphylococcus aureus. It is currently not used in combination with a beta-lactam for these infections because there is insufficient evidence of benefit to justify the cost.

The study participants will receive an additional daily iv therapy which will consist of either daptomycin or a placebo for a total of five days while admitted to the hospital. During this time, the participants will continue to have their blood drawn to assess for the presence of bacteria. The participants' liver and muscle enzymes will also be measured on the first and fifth day of therapy to test for potential side effects of the daptomycin.

Inclusion Criteria:

• Patient has a methicillin susceptible Staphylococcus aureus bacteremia.

Exclusion Criteria:

• Underlying terminal illness
• Significant contraindication to beta-lactam therapy (i.e. severe allergy, severe adverse drug reaction)
• Expected death before 5 days
• Microbiologically confirmed need for concomitant VANCOMYCIN or DAPTOMYCIN

We will make this available under the right conditions including the appropriate data sharing agreements and ethics applications under Quebec law.