Effectiveness of five-day-old 10% bleach in a student microbiology laboratory setting

In the student laboratory, 10% bleach (sodium hypochlorite) is used to disinfect benches before and after work, and when a spill occurs. Centers for Disease Control and Prevention (CDC) Guidelines recommend diluting bleach daily. In a student laboratory setting, the organisms in use are known and may include only selected standard bacteria. Diluting bleach daily is time consuming and wastes bleach. We hypothesized that 10% bleach stored in low density polyethylene (LDPE) wash bottles would maintain sufficient chlorine concentration to be effective against the organisms used in the student laboratory for five days, so that bleach could be diluted weekly instead of daily. Approximately 3 x 10(6) CFU of each bacterium were spotted to a laboratory bench surface in duplicate and allowed to air dry. One spot was individually cleaned with five-day old 10% bleach following the same protocol as student laboratories. The second spot was uncleaned and sampled as a control. Contact plates containing D/E Neutralizing agar were touched to the spots, incubated overnight at 35 degrees C and examined for growth. An uninoculated spot was also sampled as a background control. A total of 22 different organisms were tested, representing the major groups of organisms used in the student laboratories. All organisms tested were eliminated by the five-day old bleach. All uncleaned spots showed dense growth. The background control had no growth. Reducing the dilution of bleach to once a week rather than daily will save time and money, which can then be devoted to more teaching and curriculum responsibilities, while still maintaining laboratory safety.