<P> The Anton test is used in the identification of L. monocytogenes; instillation of a culture into the conjunctival sac of a rabbit or guinea pig causes severe keratoconjunctivitis within 24 hours . </P> <P> Listeria species grow on media such as Mueller - Hinton agar . Identification is enhanced if the primary cultures are done on agar containing sheep blood, because the characteristic small zone of hemolysis can be observed around and under colonies . Isolation can be enhanced if the tissue is kept at 4 ° C for some days before inoculation into bacteriologic media . The organism is a facultative anaerobe and is catalase - positive and motile . Listeria produces acid, but not gas, in a variety of carbohydrates . The motility at room temperature and hemolysin production are primary findings that help differentiate listeria from coryneform bacteria . </P> <P> The methods for analysis of food are complex and time - consuming . The present U.S. FDA method, revised in September 1990, requires 24 and 48 hours of enrichment, followed by a variety of other tests . Total time to identification takes five to seven days, but the announcement of specific nonradiolabelled DNA probes should soon allow a simpler and faster confirmation of suspect isolates . </P> <P> Recombinant DNA technology may even permit two - to three - day positive analysis in the future . Currently, the FDA is collaborating in adapting its methodology to quantitate very low numbers of the organisms in foods . </P>

When listeria monocytogenes is growing in the absence of oxygen