Phenotypes and Genotypes of Old and Contemporary Porcine Strains Indicate a Temporal Change in the S. aureus Population Structure in Pigs

Abstract

Introduction Staphylococcus aureus sequence type ST398 has recently gained attention due to the spread of methicillin-resistant strains among people exposed to livestock. The aim of this study was to explore temporal changes in the population structure of S. aureus in pigs over the last 40 years with particular reference to the occurrence of ST398.MethodsWe analysed a unique collection of 91 porcine strains isolated in six countries between 1973 and 2009 using a biotyping scheme described in the 1970's in combination with spa typing and multi-locus sequence typing (MLST). The collection comprised 32 historical isolates from 1973–1974 (n = 19) and from 1991–2003 (n = 13), and 59 contemporary isolates from 2004–2009. The latter isolates represented the most common MLST types (ST1, ST9, ST97 and ST433) and spa types isolated from pigs in Europe.Results and Discussion S. aureus sequence type ST398 was not found among old isolates from the 1970's or from 1991–2003, suggesting that this lineage was absent or present at low frequencies in pigs in the past. This hypothesis is supported by the observed association of ST398 with the ovine ecovar, which was not described in pigs by studies carried out in the 1970's. In addition, various phenotypic and genotypic differences were observed between old and contemporary isolates. Some biotypes commonly reported in pigs in the 1970's were either absent (human ecovar) or rare (biotype A) among contemporary isolates. Nine clonal lineages found among old porcine isolates are occasionally reported in pigs today (ST8, ST30, ST97, ST387, ST1092, ST2468) or have never been described in this animal host (ST12, ST133, ST1343). These results indicate that the population structure of porcine S. aureus has changed over the last 40 years and confirm the current theory that S. aureus ST398 does not originate from pigs.

Livestock-Associated MRSA and Its Current Evolution

“…However, it must have been circulating in pigs earlier, given its high prevalence in some countries, although it is difficult to estimate how long it has been circulating. In their recent study, Espinosa-Gongora et al [71] found that ST398 was absent in collections of pig isolates recovered between 1973 and 2003. The authors suggested that CC398 was either absent or present at low frequencies in pigs in the past and confirmed the current theory that S. aureus ST398 did not originate in pigs.…”

“…This clonal complex has been associated with pigs for quite some time and is found in historic collections dating back to the 1970s as well as in current collections [71]. MRSA ST97 was first discovered in Italy in pigs [3].…”

“…Since then, LA-MRSA ST9 isolates have been detected in pigs [48] and poultry [69] from Germany and in retail meat from the UK [70]. However, a recent study shows that LA-MRSA ST9 was present in some European countries before the emergence of CC398 [71].…”

“…It was one of the most frequent STs in a study in Minnesota in the USA [73] and was identified as one of the most common MSSA strain in pigs in a study comparing it with a historic collection of MSSA strains [71]. Although the reasons for the differences seen between Asia and the rest of the world remain obscure, it is quite possible that SCCmec was acquired only by Asian strains and not by the European ones.…”

“…According to EspinosaGongora et al [71], the first European CC9 isolates were discovered between 1973 and 2009 and carried diverse spa types (t337, t526, t899, t1334, t2498, t3446). The European isolates found recently carry mainly spa types t337 [48], t1430 [48,69], t1939 [70], and t4794 [3].…”