Abstract
Staphylococcus chromogenes is a common skin commensal in cattle and has been identified as a frequent cause of bovine mastitis and intramammary infections. We have developed a seven locus Multilocus Sequence Typing (MLST) scheme for typing S. chromogenes. Sequence-based typing systems, such as MLST, have application in studies of genetic diversity, population structure, and epidemiology, including studies of strain variation as a factor in pathogenicity or host adaptation. The S. chromogenes scheme was tested on 120 isolates collected from three geographic locations, Vermont and Washington State in the United States and Belgium. A total of 46 sequence types (STs) were identified with most of the STs being location specific. The utility of the typing scheme is indicated by a discrimination power of 95.6% for all isolates and greater than 90% for isolates from each of the three locations. Phylogenetic analysis placed 39 of the 46 STs into single core group consistent with a common genetic lineage; the STs in this group differ by less than 0.5% at the nucleotide sequence level. Most of the diversification in this lineage group can be attributed to mutation; recombination plays a limited role. This lineage group includes two clusters of single nucleotide variants in starburst configurations indicative of recent clonal expansion; nearly 50% of the isolates sampled in this study are in these two clusters. The remaining seven STs were set apart from the core group by having alleles with highly variable sequences at one or more loci. Recombination had a higher impact than mutation in the diversification of these outlier STs. Alleles with hypervariable sequences were detected at five of the seven loci used in the MLST scheme; the average sequence distances between the hypervariable alleles and the common core alleles ranged from 12 to 34 nucleotides. The extent of these sequence differences suggests the hypervariable alleles may be remnants of an ancestral genotype.
Highlights
Staphylococcus chromogenes was first recognized by Devriese et al [1] as one of two subspecies of Staphylococcus hyicus, and was subsequently elevated to a novel species based on chemical, physiological and DNA-DNA re-association binding experiments [2]
The Multilocus Sequence Typing (MLST) scheme described in this paper provides a tool for the differentiation and identification of strains within S. chromogenes
With a power of discrimination between strain types exceeding 90% in geographically localized populations and greater than 95% overall, this MLST scheme has potential for use in epidemiological investigations of pathologies associated with this species and the ecological relationships between microbe and host
Summary
Staphylococcus chromogenes was first recognized by Devriese et al [1] as one of two subspecies of Staphylococcus hyicus, and was subsequently elevated to a novel species based on chemical, physiological and DNA-DNA re-association binding experiments [2]. S. chromogenes is most commonly identified as a skin commensal and opportunistic mammary pathogen in cattle, sheep, goats and milking buffalo It is a frequent cause of bovine mastitis [5], and reported as a skin pathogen of pigs and goats and as a cause of caprine mastitis [6,7,8]. The organism has been identified from extra-mammary skin swabs of cattle, including udder skin, teat apex, and streak canal [15,16,17,18]; compared to some other nonaureus Staphylococcus species, S. chromogenes is less commonly isolated from environmental sites in surveys of dairy farm environmental sources (e.g. barn air, surfaces and bedding) [19]. S. chromogenes has been isolated infrequently from nasal swabs of humans in close contact with cattle [9]
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