Limitation in the detection of Listeria monocytogenes in food in the presence of competing Listeria innocua.
OBJECTIVE
Detectability of Listeria monocytogenes at 10(0) CFU per food sample in the presence of Listeria innocua using standard microbiological detection was evaluated and compared with the real-time PCR-based method.


RESULTS
Enrichment in half-Fraser broth followed by subculture in Fraser broth according to EN ISO 11290-1 was used. False-negative detection of 10(0) CFU L. monocytogenes was obtained in the presence of 10(1) CFU L. innocua per sample using the standard detection method in contrast to more than 10(5) CFU L. innocua per sample using real-time PCR. Identification of L. monocytogenes on the chromogenic medium by the standard procedure was impossible if L. innocua was able to overgrow L. monocytogenes by more than three orders of magnitude after the enrichment in model samples. These results were confirmed using naturally contaminated food samples.


CONCLUSIONS
Standard microbiological method was insufficient for the reliable detection of 10(0) CFU L. monocytogenes in the presence of more than 10(0) CFU of L. innocua per sample. On the other hand, if the growth of L. monocytogenes was sufficient to reach the concentration equal to the detection limit of PCR, the amount of the other microflora present in the food sample including L. innocua was not relevant for success of the PCR detection of L. monocytogenes.


CONCLUSIONS
After the enrichment, the PCR detection is more convenient than the standard one as PCR detection is not compromised by other present microflora.
Oravcová
K
K
Department of Microbiology and Molecular Biology, Food Research Institute, Bratislava, Slovakia.
Trncíková
T
T
Kuchta
T
T
Kaclíková
E
E
D000818:Animals; D015373:Bacterial Typing Techniques; D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D004269:DNA, Bacterial / Q000032:analysis; D003611:Dairy Products / Q000382:microbiology*; D005516:Food Microbiology; D008087:Listeria / Q000502:physiology; D008089:Listeria monocytogenes / Q000235:genetics / Q000302:isolation & purification* / Q000502:physiology; D020133:Reverse Transcriptase Polymerase Chain Reaction
D004269:DNA, Bacterial