Population Structure of Listeria monocytogenes Serotype 4b Isolates from Sporadic Human Listeriosis Cases in the United States from 2003 to 2008

Abstract

c Listeria monocytogenes can cause severe food-borne disease (listeriosis). Numerous outbreaks have involved three serotype 4b epidemic clones (ECs): ECI, ECII, and ECIa. However, little is known about the population structure of L. monocytogenes serotype 4b from sporadic listeriosis in the United States, even though most cases of human listeriosis are in fact sporadic. Here we analyzed 136 serotype 4b isolates from sporadic cases in the United States, 2003 to 2008, utilizing multiple tools including multilocus genotyping, pulsed-field gel electrophoresis, and sequence analysis of the inlAB locus. ECI, ECII, and ECIa were frequently encountered (32, 17, and 7%, respectively). However, annually 30 to 68% of isolates were outside these ECs, and several novel clonal groups were identified. An estimated 33 and 17% of the isolates, mostly among the ECs, were resistant to cadmium and arsenic, respectively, but resistance to benzalkonium chloride was uncommon (3%) among the sporadic isolates. The frequency of clonal groups fluctuated within the 6-year study period, without consistent trends. However, on several occasions, temporal clusters of isolates with indistinguishable genotypes were detected, suggesting the possibility of hidden multistate outbreaks. Our analysis suggests a complex population structure of serotype 4b L. monocytogenes from sporadic disease, with important contributions by ECs and several novel clonal groups. Continuous monitoring will be needed to assess long-term trends in clonality patterns and population structure of L. monocytogenes from sporadic listeriosis.

Novel Cadmium Resistance Determinant in Listeria monocytogenes

“…The gene cadA4 was also detected in another strain from this outbreak (20) and in additional strains (14), including in 29% of serotype 4b isolates from sporadic human listeriosis cases in the United States from 2005 to 2008 (8). The majority of these cadA4-harboring isolates were members of the same clonal complex as Scott A (CC2, also previously referred to as epidemic clone [EC]Ia or ECIV) (8).…”

“…The gene cadA4 was also detected in another strain from this outbreak (20) and in additional strains (14), including in 29% of serotype 4b isolates from sporadic human listeriosis cases in the United States from 2005 to 2008 (8). The majority of these cadA4-harboring isolates were members of the same clonal complex as Scott A (CC2, also previously referred to as epidemic clone [EC]Ia or ECIV) (8). An unusual attribute of cadA4 is that it was associated with growth at 35 g/ml cadmium chloride but not at 70 g/ml, whereas strains harboring cadA1 to cadA3 grew at cadmium concentrations up to and often exceeding 140 g/ml (8,12).…”

“…In particular, heavy metals in the environment can exert long-term selective pressure on bacteria (7), as heavy metals do not degrade over time. It is worthy of note that several major foodborne outbreaks of listeriosis have involved cadmium-resistant L. monocytogenes strains (8). Interestingly, cadmium resistance in L. monocytogenes has been found to occur more commonly among strains repeatedly isolated from foods than from those encountered only sporadically (9).…”

Characterization of epidemic clones of Listeria monocytogenes serotype 4b isolated from humans and meat products in Brazil

“…This change leads to a deletion of 35 amino acids in the ActA protein, that represent two proline-rich regions (PRR) required for binding to the WASP (Wiskott-Aldrich syndrome protein) and Mena proteins, essential steps to triggering actin-based cell mobility in the host cell cytoplasm. Nevertheless, in vitro and in vivo assays showed that the invasion and multiplication ability of bacteria apparently is not affected by this deletion [27,32,34].…”

“…A recent work evaluated the presence of major ECs in 4b serotype isolates from sporadic cases of listeriosis in the United States (2003 to 2008), identifying ECI, ECII and ECIa, in 32%, 17% and 7% of total isolates, respectively, with an overall prevalence of 57% of these clonal groups [32]. Similarly, another recent study in India identified ECI in most 4b-serotype isolates by multi-virulence-locus sequence typing technique (MVLST), a multi-locus sequence type (MLST) variation [33].…”