From a food safety perspective, under what conditions should a plastic or wood cutting board be replaced?  I have a plastic one that developed a crack that goes through both sides of the board.  I presume a crack like this would indicate it needs to be replaced since it could harbor bacteria.   Please provide a reputable reference with your answer. 
The USDA suggests that "all plastic and wooden cutting boards wear out over time. Once cutting boards become excessively worn or develop hard-to-clean grooves, they should be discarded."  Other advice they offer for avoiding food contamination from cutting boards:      Avoid Cross-Contamination The Meat and Poultry Hotline says that consumers may use wood or a nonporous surface for cutting raw meat and   poultry. However, consider using one cutting board for fresh produce   and bread and a separate one for raw meat, poultry, and seafood. This   will prevent bacteria on a cutting board that is used for raw meat,   poultry, or seafood from contaminating a food that requires no further   cooking.      Cleaning Cutting Boards To keep all cutting boards clean, the Hotline recommends washing them with hot, soapy water after each use;   then rinse with clear water and air dry or pat dry with clean paper   towels. Nonporous acrylic, plastic, or glass boards and solid wood   boards can be washed in a dishwasher (laminated boards may crack and   split).      Both wooden and plastic cutting boards can be sanitized with a   solution of 1 tablespoon of unscented, liquid chlorine bleach per   gallon of water. Flood the surface with the bleach solution and allow   it to stand for several minutes. Rinse with clear water and air dry or   pat dry with clean paper towels.  