Comparison of subtypes of Listeria monocytogenes isolates from naturally contaminated watershed samples with and without a selective secondary enrichment.

Lisa Gorski,Samarpita Walker,Diana Carychao,Michael B Cooley,Robert E Mandrell,Anita S Liang,Kimberly M Nguyen,Jessica Govoni

doi:10.1371/journal.pone.0092467

Lisa Gorski, Samarpita Walker + Show 6 more

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https://doi.org/10.1371/journal.pone.0092467

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Abstract

Two enrichment methods for Listeria monocytogenes using Immuno Magnetic Separation (IMS) were tested to determine if they selected the same subtypes of isolates. Both methods used a non-selective primary enrichment and one included subculture in Fraser Broth, while the other involved direct plating of IMS beads. Sixty-two naturally contaminated watershed samples from the Central California Coast were used as a source of L. monocytogenes, and subtype diversity was measured by serotype and Multiple Number Variable Tandem Repeat Analysis (MLVA). Three different serotypes were detected from both methods with serotype 4b strains making up 87% of the isolates, serotype 1/2a making up 8%, and serotype 1/2b making up 5%. The data suggest that serotype 1/2a strains were more likely to be isolated from the Fraser Broth culture method. Sixty-two different MLVA types were detected and the more common MLVA types were detected by both culture methods. Forty-three MLVA types were detected only from one culture method or the other, while 19 types were detected from both culture methods. The most common MLVA type-12 was detected in 33 of the 62 water samples, and represented 31% of the isolates from both culture methods. This limited study provides evidence that using both enrichment culture methods allowed for detection of a greater diversity of isolates among the samples than the use of one method alone, and that a wide diversity of L. monocytogenes strains exist in this watershed.

Highlights

Listeria monocytogenes is a gram-positive facultatively intracellular, foodborne pathogen that survives in nature as a saprophyte.Between 2009–2011 there were 1651 cases of listeriosis in the United States with a fatality rate of 21%, with the majority of cases in elderly and immunocompromised individuals, and 14% of the cases pregnancy-related
Analysis (MLVA) is a faster throughput method that measures the number of tandem repeats present in a number of different loci on the chromosome, and several Multiple Number Variable Tandem Repeat Analysis (MLVA) methods for L. monocytogenes have been developed in the last few years [15,16,17]
The index for the individual methods and both together were similar and ranged from 0.875–0.887, indicating that both methods reflected a wide diversity of MLVA types in the Bacteria in nature and in foods often are stressed [24,25]

Summary

Introduction

Listeria monocytogenes is a gram-positive facultatively intracellular, foodborne pathogen that survives in nature as a saprophyte. Several methods are used for the enrichment and isolation of Listeria monocytogenes Many of these methods utilize selective compounds such as dyes and antibiotics to favor L. monocytogenes over competing microbiota. Studies indicate that these enrichment methods show bias in the types of Listeria spp. and/or L. monocytogenes strains that are isolated as a result of different strains outcompeting others in selective enrichment cultures [8,9,10,11]. Analysis (MLVA) is a faster throughput method that measures the number of tandem repeats present in a number of different loci on the chromosome, and several MLVA methods for L. monocytogenes have been developed in the last few years [15,16,17]. Clearing underneath colonies could be discerned by holding the plates up to a light to determine transparency

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Discussion