Access Type

Open Access Thesis

Date of Award

January 2015

Degree Type


Degree Name



Nutrition and Food Science

First Advisor

Yifan Zhang


A total of 138 Listeria isolates, including 58 Listeria welshimeri, 44 Listeria monocytogenes, and 36 Listeria innocua, from retail meat were characterized by antimicrobial susceptibility tests against nine antimicrobials. In addition, the 44 L. monocytogenes were analyzed by serotype identification using PCR and genotyping using pulsed-filed gel electrophoresis (PFGE). Resistance to one or two antimicrobials was observed in 32 Listeria (23.2%). No multidrug resistance was identified. Tetracycline resistance was the most common resistance phenotype and identified in 22 Listeria isolates. Low prevalence of resistance to ciprofloxacin, erythromycin, gentamicin, and vancomycin was also detected. L. innocua demonstrated the highest overall prevalence of antimicrobial resistance, being 36.1%, followed by 34.1% in L. monocytogenes, and 6.9% in L. welshimeri. Serotypes 1/2a, 1/2b, and 4b were identified in 19, 23, and one L. monocytogenes, respectively. One isolate was untypeable. Fifteen L. monocytogenes were antimicrobial resistant (12 of 1/2b, 2 of 1/2a, and 1 of serotype untypeable). A diverse population of L. monocytogenes was identified as evidenced by multiple PFGE patterns in the 44 isolates. The data indicate that Listeria contamination is common in retail meat. Although antimicrobial resistance still occurs at low prevalence, multiple Listeria species can serve as reservoir of antimicrobial resistance. Variation of antimicrobial susceptibilities in L. monocytogenes serotypes may exist.