Methicillin-Resistant Staphylococcus aureus Sequence Type (ST) 5 Isolates from Health Care and Agricultural Sources Adhere Equivalently to Human Keratinocytes

Abstract

Staphylococcus aureus is part of the nasal microbiome of many humans and has become a significant public health burden due to infections with antibiotic-resistant strains, including methicillin-resistant S. aureus (MRSA) strains. Several lineages of S. aureus, including MRSA, are found in livestock species and can be acquired by humans through contact with animals. These livestock-associated MRSA (LA-MRSA) isolates raise public health concerns because of the potential for livestock to act as reservoirs for MRSA outside the hospital setting. In the United States, swine harbor a mixed population of LA-MRSA isolates, with the sequence type 398 (ST398), ST9, and ST5 lineages being detected. LA-MRSA ST5 isolates are particularly concerning to the public health community because, unlike the isolates in the ST398 and ST9 lineages, isolates in the ST5 lineage are a significant cause of human disease in both the hospital and community settings globally. The ability of swine-associated LA-MRSA ST5 isolates to adhere to human keratinocytes in vitro was investigated, and the adherence genes harbored by these isolates were evaluated and compared to those in clinical MRSA ST5 isolates from humans with no swine contact. The two subsets of isolates adhered equivalently to human keratinocytes in vitro and contained an indistinguishable complement of adherence genes that possessed a high degree of sequence identity. Collectively, our data indicate that, unlike LA-MRSA ST398 isolates, LA-MRSA ST5 isolates do not exhibit a reduced genotypic or phenotypic capacity to adhere to human keratinocytes.IMPORTANCE Our data indicate that swine-associated livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) ST5 isolates are as capable of adhering to human skin and have the same genetic potential to adhere as clinical MRSA ST5 isolates from humans. This suggests that humans in contact with livestock have the potential to become colonized with LA-MRSA ST5 isolates; however, the genes that contribute to the persistence of S. aureus on human skin were absent in LA-MRSA ST5 isolates. The data presented here are important evidence in evaluating the potential risks that LA-MRSA ST5 isolates pose to humans who come into contact with livestock.

Keywords: LA-MRSA; Staphylococcus aureus; swine.

FIG 1

Capacity of swine-associated LA-MRSA ST5 isolates and clinical MRSA ST5 isolates from humans with no swine contact to adhere to human keratinocytes and swine skin biopsy specimens. (A) Number of CFU adhered per HEK cell for LA-MRSA ST5 isolates and clinical MRSA ST5 isolates, with each point representing the average for three technical replicates. (B) Number of MRSA CFU adhered per square millimeter of swine skin biopsy specimen for LA-MRSA ST5 and clinical MRSA ST5 isolates. *, P < 0.05.

FIG 2

Patterns of adherence of individual isolates to HEKs and swine skin biopsy specimens. Differential patterns of adherence to HEKs (A) and swine skin biopsy specimens (B) were noted between individual isolates. Differences in the adherence pattern between biological replicates of an individual isolate, such as the number of CFU per HEK cell for isolate ISU978 (A) or the number of CFU per square millimeter for isolate UMN38 (B), were also noted.

FIG 3

Percent nucleotide sequence identity of adherence-associated genes for isolates evaluated by the phenotypic assay. Each gene was compared to the gene in the reference isolate, Mu3. The percent identity of all screened genes was greater than 90%. The greatest variation was seen in the clfA gene. This was true for both LA-MRSA ST5 isolates and clinical MRSA ST5 isolates; however, a subset of LA-MRSA ST5 isolates (isolates ISU876, ISU949, ISU978, and ISU980) showed added variation due to nucleotide insertions and deletions. Genes not found in the draft genomes are depicted in black.

FIG 4

Representative image of the alignment of clfA from 19 LA-MRSA ST5 isolates and clfA from Mu3. In these 19 LA-MRSA ST5 isolates, the clfA gene showed a reduced percent nucleotide sequence identity to the clfA gene of Mu3. The clfA genes in the LA-MRSA ST5 isolates with a reduced identity possessed an insertion of 162 bp at position 2125 and a deletion of 24 bp at position 2799. The insertion and deletion did not result in a frameshift mutation, and the clfA genes in these isolates remained intact.