The Difco/BBL Gram stain kit (BD, Franklin Lakes, NJ) was used for performing the Gram stain procedure. The kit consists of 4 bottles of 1X solutions of crystal violet, Gram’s iodine, acetone-alcohol and safranin. The microscope slides, for use in this study, were prepared using fresh cultures of Staphylococcus aureus strain 6538 (a Gram-positive coccus; ATCC, Manassas, VA) and Klebsiella pneumoniae strain 13882 (a Gram-negative bacillus; ATCC) that had been maintained and grown for 24 h at 37°C on tryptic soy agar plates (BD). These BSL-2 microbes were selected because they are medically important pathogens and relevant to the participants. Smears of each bacterium were made onto separate slides and these were heat-fixed just prior to their use by the study participants. The kit reagents and the microscope slide preparations were examined before being used in the study to ensure their performance accuracy and to provide the expected staining outcomes. The Gram stain procedure was performed following the protocol described in a standard microbiology laboratory manual .