St. Louis-area community health centers, a large chunk of the healthcare safety net that serves the uninsured and underinsured, have been strained by a major boost in business during the past year. The uptick comes as jobless rates spike and more people lose their employer-based health insurance. For the growing group that depends on the health centers, officials said, the increased demand could mean longer waits to get appointments. The financial strain could reverse some of the recent progress at St. Louis area sites toward keeping wait times down, they added.