Horizontal Gene Transfers Link a Human MRSA Pathogen to Contagious Bovine Mastitis Bacteria Thomas Brody1*, Amarendra S. Yavatkar2, Yong Lin2, Jermaine Ross1, Alexander Kuzin1, Mukta Kundu 1 , Yang Fann 2 , Ward F. Odenwald1* 1 The Neural Cell-Fate Determinants Section, National Institutes of Health (NIH), Bethesda, Maryland, United States of America, 2 The Division of Intramural Research Information Technology Program, National Institute of Neurological Disorders and Stroke (NINDS), National Institutes of Health (NIH), Bethesda, Maryland, United States of America Abstract Background: Acquisition of virulence factors and antibiotic resistance by many clinically important bacteria can be traced to horizontal gene transfer (HGT) between related or evolutionarily distant microflora. Comparative genomic analysis has become an important tool for identifying HGT DNA in emerging pathogens. We have adapted the multi-genome alignment tool EvoPrinter to facilitate discovery of HGT DNA sequences within bacterial genomes and within their mobile genetic elements. Principal Findings: EvoPrinter analysis of 13 different Staphylococcus aureus genomes revealed that one of the human isolates, the hospital epidemic methicillin-resistant MRSA252 strain, uniquely shares multiple putative HGT DNA sequences with different causative agents of bovine mastitis that are not found in the other human S. aureus isolates. MRSA252 shares over 14 different DNA sequence blocks with the bovine mastitis ET3 S. aureus strain RF122, and many of the HGT DNAs encode virulence factors. EvoPrinter analysis of the MRSA252 chromosome also uncovered virulence-factor encoding HGT events with the genome of Listeria monocytogenes and a Staphylococcus saprophyticus associated plasmid. Both bacteria are also causal agents of contagious bovine mastitis. Conclusions: EvoPrinter analysis reveals that the human MRSA252 strain uniquely shares multiple DNA sequence blocks with different causative agents of bovine mastitis, suggesting that HGT events may be occurring between these pathogens. These findings have important implications with regard to animal husbandry practices that inadvertently enhance the contact of human and livestock bacterial pathogens. Citation: Brody T, Yavatkar AS, Lin Y, Ross J, Kuzin A, et al. (2008) Horizontal Gene Transfers Link a Human MRSA Pathogen to Contagious Bovine Mastitis Bacteria. PLoS ONE 3(8): e3074. doi:10.1371/journal.pone.0003074 Editor: Michael Hofreiter, Max Planck Institute for Evolutionary Anthropology, Germany Received April 4, 2008; Accepted August 7, 2008; Published August 27, 2008 This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. Funding: This research was supported by the Intramural Research Program of the NIH, NINDS. Competing Interests: The authors have declared that no competing interests exist. * E-mail: brodyt@ninds.nih.gov (TB); ward@codon.nih.gov (WFO) Introduction Staphylococcus aureus (S. aureus) infections in both man and domestic livestock present growing and formidable global challenges for human and animal health concerns. Methicillin- resistant S. aureus (MRSA) is now the leading cause of hospital- and community-acquired S. aureus infections [1,2]. Likewise, among dairy herds S. aureus is one of the major causal agents of contagious bovine mastitis [3,4]. A recent survey of mastitis outbreaks in Canadian dairy cows reported a total of 3,149 S. aureus infections in 106 farms over the course of a single year [5]. Comparative analysis of different S. aureus genomes has revealed that many strains have independently acquired genes from members of their surrounding microflora that confer antibiotic resistance and/or encode virulence factors [6,7,8]. Horizontal gene transfer (HGT) among bacteria and their mobile genetic elements (MGEs) is the primary mode for the spread of antibiotic resistance and virulence factors in clinically important pathogens [6,9,10]. Bacteriophage, plasmids, transposons and uptake of naked DNA have been shown to be involved in the movement of DNA between different bacteria [7,11,12,13,14]. HGT events have played a prominent role in the rapid acquisition of antibiotic resistance in S. aureus. Before the emergence of methicillin resistance in the early 1960s, a steady increase in penicillin resistance, mediated by plasmid transfer, was detected in hospital infections of S. aureus in both the UK and the USA [8,15]. Soon after methicillin was used in 1960, methicillin- resistant isolates were reported, and by 1967, multidrug-resistant MRSA was reported in numerous countries. Although the origins of the mecA gene (the principle component of methicillin resistance in staphylococci) are unknown, a mecA homologue (88% similarity) is ubiquitous in the antibiotic-susceptible S. sciuri, and may be a possible evolutionary precursor of the mecA gene of the MRSA strains. Transfer of mecA is mediated by the Staphylococcal Cassette Chromosome mec (SCCmec) [16]. Bacterial co-infections are likely to be a major point of origin for HGT events. For example, the gene encoding the biofilm- associated protein (Bap), a surface protein implicated in formation of aggregates of microorganisms, is present in S. aureus that have been isolated from chronic bovine mastitis infections, and is thought to be acquired by HGT, since it is shared by other causative agents of mastitis [17]. HGT has also been documented PLoS ONE | www.plosone.org 1 August 2008 | Volume 3 | Issue 8 | e3074