a suspension of microorganisms includes a detectable indicator material , such as a food coloring , for ensuring that a biological indicator receives a preselected population of the microorganisms . a microorganism delivery system delivers the suspension with the indicator material from a tank to a plurality of needles where it is delivered to a carrier material , such as filter , chromatography , or blotter paper , or glass fiber passing beneath the needles or micropipetted onto non - absorptive or non - penetrable carriers , such as plastic polymers or stainless steel coupons . in the event of a malfunction in the system , such as a blockage in one of the needles or intermittent delivery of the microorganism suspension , the absence of color on the impregnated carrier material indicates the absence of microorganisms . the impregnated or otherwise treated carrier material may be evaluated visually or using an automated detector system which signals an alarm in the event that absences in the color or other indicator material are detected .