Genome Tracing Follows Livestock MRSA From Pig Farms to Ready-to-Eat Sausage

Methicillin-resistant Staphylococcus aureus from pig farms has been tracked, genome by genome, all the way into finished pork products sold for direct consumption. In a study of four commercial pig fattening farms in northeastern Germany, researchers used whole-genome sequencing and core genome multilocus sequence typing (cgMLST) to show that livestock-associated MRSA of Sequence Type 398, the dominant livestock lineage in Europe, survived the entire seven-day shelf life of a spreadable raw fermented sausage. The work, published in Frontiers in Microbiology, is among the first to trace LA-MRSA across a complete farm-to-fork chain with genomic resolution, and it also uncovered a surprise: a human-associated MRSA lineage that appears to have entered the food chain at the slaughterhouse rather than on the farm.

The research team, led by scientists at Neubrandenburg University of Applied Sciences with collaborators at the University of Zurich, LMU Munich and the German Federal Institute for Risk Assessment, sampled four fattening farms in the Ludwigslust-Parchim district of Mecklenburg-Western Pomerania during May to July 2016. The design prioritized complete traceability over statistical representativeness: farms were chosen because they could support pig-to-product continuity, not through random sampling. Nasal swabs from up to nine animals per pen were pooled, individual animals were followed via ear tags and ham-stamp numbering applied before slaughter, and environmental sock swabs plus dead pest animals, including flies, a spider and a mouse, were collected at each site. Sampling was performed under strict biosecurity conditions to prevent cross-contamination between farms, with the sampler avoiding contact with other pig holdings for at least 48 hours before each visit.

MRSA was detected on three of the four farms. Every pooled nasal swab from the positive farms was positive, both at the farm level (seven of seven pools) and again when the same animals were sampled at the slaughterhouse (eight of eight pools). Environmental sock swabs, quarantine-pen swabs and scalding water all came back negative, but the band saw blade used during slaughter of farm 1 animals and the cutting knife used on farm 3 carcasses each yielded MRSA. In minced meat prepared from the positive farms, six of twelve samples were positive before freezing and two of twelve after roughly two months of storage at minus 18 degrees Celsius and thawing. All nasal swabs taken from the three project staff members at the start and end of the project were negative.

The genomic analysis was the heart of the study. All 34 recovered strains, 33 MRSA isolates and one mecA-negative S. aureus, were sequenced and compared using a 1,861-locus S. aureus cgMLST scheme. Twenty-six isolates belonged to ST398, the livestock-associated lineage typically carrying spa type t034 or t011 with SCCmec type V. Multiple transmission clusters of eight alleles or fewer emerged within this clade. Isolates from different pigs on the same farm differed by as little as four to six alleles, and isolates from pigs sampled on the farm versus at slaughter were separated by only one to eight alleles, indicating that the strains colonizing animals in their pens were the same strains arriving at the slaughter plant.

One of the most striking findings concerned farm pests. A fly captured at farm 3 carried an ST398 strain just one allele away from a pig nasal swab isolate from the same farm, and a fly at farm 4 clustered within the farm 4 subclade at seven alleles from on-farm and at-slaughter pig isolates. A mouse from farm 3 was also MRSA-positive. The authors note that a cross-sectional design cannot determine whether flies were passive recipients of environmental contamination or active participants in within-farm transmission, but the genomic clustering points to a shared, recent source and supports the idea that synanthropic pests act as biosecurity-relevant carriers. This aligns with earlier work showing house flies and stable flies on a Danish pig farm carrying viable LA-MRSA acquired through passive environmental exposure, and with an Austrian cross-sectional study detecting ST398 MRSA in 53.2 percent of house flies and 19.1 percent of stable flies on MRSA-positive piglet-producing holdings.

The fate of MRSA in the final products depended sharply on processing intensity. The team manufactured four products spanning a gradient: Hackepeter, a seasoned raw minced meat with no preservation steps; Frische Zwiebelmettwurst, a spreadable raw sausage fermented at 7 degrees Celsius for 48 hours; a cervelat-type sliceable raw sausage fermented at 20 degrees Celsius for 96 hours; and Lyoner, a cooked sausage heat-treated at 75 degrees Celsius for 45 minutes. MRSA was not detected in any Hackepeter sample (0 of 9) or any Lyoner sample (0 of 12). In contrast, LA-MRSA ST398 was recovered from six of twelve Frische Zwiebelmettwurst samples through the full seven-day shelf life, and a single ST398 isolate of type t011 with SCCmec V appeared in one cervelat-type sausage after fermentation. The authors note that the absence of MRSA in Hackepeter, despite its detection in the minced meat used for its preparation, could not be explained in this study; possible factors such as competitive exclusion by lactic acid bacteria, dilution below the detection limit, or sampling variability remain speculative.

The survival pattern tracks the physical chemistry of the products. Inhibitory thresholds for S. aureus, a water activity of roughly 0.83 to 0.86 and a pH of about 4.0 to 4.5, were not reached in either fermented sausage. At the end of shelf life, the Zwiebelmettwurst measured pH 5.15 to 5.47 and water activity 0.972 to 0.985, roughly one pH unit and more than 0.10 water activity units above the inhibitory thresholds. The cervelat-type process achieved greater acidification and drying, pH 4.69 to 5.23 and water activity 0.957 to 0.974, which may explain why only one sample was positive, and that sample came from the batch with the mildest values recorded for that product. The heat treatment of Lyoner, meanwhile, is consistent with published thermal inactivation kinetics for S. aureus and with a recent systematic review and meta-analysis of 34,103 meat samples from 98 studies, which quantified pooled MRSA prevalence at 3.72 percent across all meat categories, 2.84 percent in processed products, and effectively zero in cooked meat.

The second major lineage told a different story. Seven isolates of ST39, spa type t007 with SCCmec type II, were recovered exclusively at the slaughterhouse and processing stages, never from any farm-level or slaughterhouse nasal swab. All seven shared identical cgMLST profiles and sat roughly 1,645 alleles away from the ST398 hub, confirming an independent introduction. Their gene content reinforced the separation: every ST39 isolate carried the immune evasion cluster genes scn and sak plus the egc enterotoxin cluster and the enterotoxin-like gene selv, elements typically acquired by human-adapted S. aureus via the PhiSa3 prophage and absent from all 26 ST398 isolates. Antimicrobial resistance genes segregated just as cleanly, with ST39 uniquely carrying mecI, mecR1, an aminoglycoside resistance cassette, bleO, qacC and fosB, while tetracycline and trimethoprim resistance genes dominated the ST398 lineage. None of the 34 isolates carried classical staphylococcal enterotoxin genes or Panton-Valentine leukocidin genes.

Because ST39 strains turned up in minced meat from all three farms slaughtered at the shared facility, and simultaneously on a cutting knife sampled during operations, the slaughterhouse environment emerges as a plausible entry point for this human-adapted lineage. The authors caution that the direction of transfer between knife and meat cannot be determined, and that the broader slaughterhouse workforce was not screened, so an occupational exposure route remains a hypothesis rather than a demonstrated source. Elevated nasal MRSA carriage among slaughterhouse workers in contact with live pigs has been documented in European settings, which is consistent with such a route. A single mecA-negative S. aureus of ST15, recovered from a cutting knife used on farm 4 carcasses and separated from all other isolates by about 1,739 alleles, rounded out the collection.

The study’s limitations are clearly acknowledged: only four farms, a single region and seasonal window, one slaughterhouse, and pooled swabbing that precludes full individual-animal traceability, since pigs can carry multiple genetically distinct MRSA clones simultaneously. Within-farm transmission therefore remains a hypothesis rather than a proven chain of custody, and the transient absence of MRSA at the 50-hour sampling point in Zwiebelmettwurst cannot be interpreted without quantitative data. Still, the core conclusion stands on firm genomic ground: LA-MRSA ST398 can persist through fermentation into ready-to-eat pork at the end of its labeled shelf life, flies may help move strains around farms, and slaughterhouses may admit human-associated MRSA lineages into the meat chain. The authors argue that tracing individual strains, not just reporting prevalence, is the way to understand and interrupt farm-to-fork transmission, and they support continued inclusion of MRSA in coordinated European zoonosis monitoring alongside worker and equipment hygiene programs in pork processing.

Subject of Research: Genomic tracing of livestock-associated MRSA along the pork farm-to-fork chain and its survival in processed meat products

Article Title: Livestock-associated methicillin-resistant Staphylococcus aureus along the pork food chain: a study of four commercial pig fattening farms in north-east Germany and survival in processed meat products

Article References: Ebert, M., Rose, S., Biggel, M., & Fetsch, A. (2026). Livestock-associated methicillin-resistant Staphylococcus aureus along the pork food chain: a study of four commercial pig fattening farms in north-east Germany and survival in processed meat products. Frontiers in Microbiology, 17, 1928317. https://doi.org/10.3389/fmicb.2026.1928317

Image Credits: AI Generated

DOI: 10.3389/fmicb.2026.1928317

Keywords: LA-MRSA, ST398, MRSA, pork food chain, whole-genome sequencing, cgMLST, food safety, fermented sausage, pig farming, slaughterhouse, antimicrobial resistance, farm-to-fork