Postenrichment population differentials using buffered Listeria enrichment broth: implications of the presence of Listeria innocua on Listeria monocytogenes in food test samples.
The recovery of low levels of Listeria monocytogenes from foods is complicated by the presence of competing microorganisms. Nonpathogenic species of Listeria pose a particular problem because variation in growth rate during the enrichment step can produce more colonies of these nontarget cells on selective and/or differential media, resulting in a preferential recovery of nonpathogens, especially Listeria innocua. To gauge the extent of this statistical barrier to pathogen recovery, 10 isolates each of L. monocytogenes and L. innocua were propagated together from approximately equal initial levels using the current U. S. Food and Drug Administration's enrichment procedure. In the 100 isolate pairs, an average 1.3-log decrease was found in the 48-h enrichment L. monocytogenes population when L. innocua was present. In 98 of the 100 isolate pairs, L. innocua reached higher levels at 48 h than did L. monocytogenes, with a difference of 0.2 to 2.4 log CFU/ml. The significance of these population differences was apparent by an increase in the difficulty of isolating L. monocytogenes by the streak plating method. L. monocytogenes went completely undetected in 18 of 30 enrichment cultures even after colony isolation was attempted on Oxoid chromogenic Listeria agar. This finding suggests that although both Listeria species were present on the plate, the population differential between them restricted L. monocytogenes to areas of the plate with confluent growth and that isolated individual colonies were only L. innocua.
U.S. Food and Drug Administration, Arkansas Regional Laboratory, 3900 NCTR Road, Building 26, Jefferson, Arkansas 72079, USA.
Ashley L
AL
Keys
Rachel C
RC
Dailey
Anthony D
AD
Hitchins
R Derike
RD
Smiley
D000362:Agar; D002021:Buffers; D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D003470:Culture Media; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D008087:Listeria / Q000302:isolation & purification; D008089:Listeria monocytogenes / Q000254:growth & development / Q000302:isolation & purification*; D014481:United States
D002021:Buffers; D003470:Culture Media; D000362:Agar