With 2 new cases, infections in healthcare workers in the DRC have now reached 27.
The outbreak total has reached 305, with 189 deaths and 60 suspected cases.
The researchers estimate that 670,000 infections are caused by select antibiotic-resistant bacteria in a year.
Two of the latest confirmed cases involve a mother and her 2-week-old baby.
Reasons for sharing antibiotics included recipients experiening similar symptoms, saving money, and avoiding another doctor's visit.
In other developments, Uganda said it will start vaccinating health workers in high-risk areas and two global health experts pressed for more high-level involvement in ending the outbreak, including committment from the United States.
Investigators were't able to determine exactly how the water contaminated the lettuce, which may have occurred by irrigation or the dilution of chemical to protect crops.
The DRC health ministry reports 6 cases today, and deaths have climbed to 180.
Three were hospital contacts in a hospital in Dammam, and two were household contacts in Riyadh.