Whatman 7141-104 Cellulose Nitrate Membrane Sterile Filter with Grid, 47mm Diameter, 0.45 Micron (Pack of 100) 0.45m cellulose nitrate membrane filter with grid, sterile, 47mm Gridded surface (3.1mm intervals) make filters ideal for colony counting Special non-toxic ink used is completely free from bacterial growth inhibitors Performance improvements include very narrow pore size distribution and low levels of extractables Has a noticeably improved flexibility; is made to tolerate abuse