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

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.

Figure 1. Categorical tree of MLVA types based on differences in VNTR copy number, and the numbers of isolates of each type and the serotypes represented in them.

BioNumerics assigns a value of −2 to loci yielding no product.

Figure 2. Minimum spanning tree with categorical data of all 62 MLVA types containing all 304 isolates.

Each circle is labeled with the MLVA type it represents. The size of the circle and number of divisions represent the number of isolates of that type. The blue and yellow segments represent isolates obtained by Direct plating or with Fraser broth, respectively. Heavy short connecting lines indicate a difference in a single locus; thinner, longer lines connect nodes with 2 and 3 locus differences; dotted lines connect nodes with 4 locus differences; a gray line connects nodes with 5 locus differences.