Antimicrobial Resistance Distribution Differs Among Methicillin Resistant Staphylococcus aureus Sequence Type (ST) 5 Isolates From Health Care and Agricultural Sources

Abstract

Antimicrobial resistance (AMR) is an expanding public health concern and methicillin resistant Staphylococcus aureus (MRSA) is a notable example. Since the discovery of livestock associated MRSA (LA-MRSA), public health concerns have arisen surrounding the potential of LA-MRSA isolates to serve as a reservoir for AMR determinants. In this study, we compare swine associated LA-MRSA ST5 and human clinical MRSA ST5 isolates for phenotypic antimicrobial susceptibilities determined via broth microdilution and genotypic determinants of AMR using whole genome sequencing and comparative genomic analysis to identify AMR elements. Swine associated LA-MRSA ST5 isolates exhibited phenotypic resistance to fewer antibiotics than clinical MRSA ST5 isolates from humans with no swine contact. Distinct genomic AMR elements were harbored by each subgroup, with little overlap in shared AMR genes between swine associated LA-MRSA ST5 and clinical MRSA ST5 isolates. Our results demonstrate that phenotypic antimicrobial susceptibilities and genotypic determinants of AMR among swine associated LA-MRSA ST5 and clinical MRSA ST5 isolates are separate and distinct.

Keywords: LA-MRSA; Staphylococcus aureus; agriculture; antimicrobial resistance; mobile genetic elements; swine.

FIGURE 1

AMR index of isolates from clinical MRSA ST5 and swine associated LA-MRSA ST5. AMR index is defined as the proportion of the tested antibiotics to which an isolate is phenotypically resistant. (A) The AMR indexes determined for clinical isolates show a range of 0.21–0.59 with an average AMR index of 0.50. (B) The AMR indexes determined for swine associated LA-MRSA ST5 isolates. Swine associated isolates had AMR indexes with a wider range (0.14–0.66) and higher maximum AMR index; however, the average AMR index was 0.40, which was significantly less than that of clinical isolates (P < 0.0001).

FIGURE 2

Hierarchical cluster dendrogram displaying the relatedness of the MRSA ST5 isolates based on the presence or absence of AMR genetic elements. Isolates are grouped by average hierarchical clustering based on Euclidean Distance. AMR genetic elements are listed at the top, isolate names are listed to the right, and presence (blue) or absence (white) of genes is indicated.