Genetic diversity of livestock-associated MRSA isolates obtained from piglets from farrowing until slaughter age on four farrow-to-finish farms

Abstract

During a previous longitudinal study, performed on four farrow-to-finish farms (A to D), samples were taken from twelve sows, their offspring, and the environment on various occasions over six months to study the MRSA presence. During the present study, a selection of the obtained MRSA isolates were typed by multiple-locus variable-number tandem-repeat analysis (MLVA), Pulsed Field Gel Electrophoresis (PFGE), spa typing, and SCCmec typing to study the genetic diversity of LA-MRSA isolates and to determine possible MRSA sources for pig(let)s. PFGE, spa typing, and SCCmec typing revealed the presence of one or few dominant genotype(s) per farm. In contrast, 212 MLVA types were detected on the four farms, forming one cluster on farm A, three on farm B, four on farm C and two on farm D. The genotype, found on farm A was unique for this farm. Farms B, C and D shared one cluster. In general, MLVA types from these clusters were isolated from piglets, sows, and the environment on various sampling events. Piglets carried MLVA types both related and unrelated to their mother sows' MLVA types at farrowing and onwards. In conclusion, molecular typing revealed that within a farm one or a few dominant strain(s) are widespread. Potential MRSA sources for piglets were mother sows, the environment and other piglets.

Figure 1

Minimum Spanning Tree of the farm A MLVA types. The MLVA types are indicated by numbers, according to origin (green: piglet, orange: sow and blue: wall). The dominant cluster A of the farm is indicated in a coloured sphere, whereas the non-dominant clusters are indicated in black spheres.

Figure 2

Dendrogram containing the pulsotypes obtained on the four farms. Per pulsotype, one isolate is shown as an example. Consecutive the dendrogram, pulsotype pattern, farm and number of isolates belonging to the pulsotype on the total number of typed isolates per farm are shown. For each example, the isolate origin, unit (NU: nursing unit, GU: growing unit, GU1: growing unit 1, FU: finishing unit, FU2: finishing unit 2), spa type, SCCmec type (NT type 3: mecA complex NT/ccr complec C) and MLVA cluster are given.

Figure 3

Minimum spanning tree of the farm B MLVA types. The MLVA types are indicated by numbers, according to origin (green: piglet, orange: sow and blue: wall). The dominant clusters B, G, and H of the farm are indicated in coloured spheres, whereas the non-dominant clusters are indicated in black spheres.

Figure 4

Minimum spanning tree of the farm C MLVA types. The MLVA types are indicated by numbers, according to origin (green: piglet, orange: sow and blue: wall). The dominant clusters B, C, D, and E of the farm are indicated in coloured spheres, whereas the non-dominant clusters are indicated in black spheres.

Figure 5

Minimum spanning tree of the farm D MLVA types. The MLVA types are indicated in numbers, according to origin (green: piglet, orange: sow and blue: wall). The dominant clusters B and D of the farm are indicated in coloured spheres, whereas the non-dominant clusters are indicated in black spheres.

Figure 6

Overview of some observations. A) Piglets carrying non-related MLVA types to their mother sow (sow 12 and her offspring sampled one day after farrowing on farm B); B) Overview of the MLVA types found in the three units of farm A (green: nursing unit, red: growing unit and blue: finishing unit); C) Piglets carrying related MLVA types to their mother sow (sow 9 and her offspring sampled 3, 6, and 20 days after farrowing on farm D). In addition, piglets of the same litter carried various related and unrelated MLVA types.

Figure 7

Distribution of the MLVA clusters, observed on the four farms, at each time point after farrowing. The cluster designation is shown in the bar. The non-dominant clusters and non-clustered MLVA types are given in the purple bar without indication. On farm A, no isolates were obtained within the hour and 17 days after farrowing, whereas on farm B 23 days after farrowing. The piglets of farm C were transported to the second site after a short stay in the growing unit of site 1. The animals received a promycin and amoxicillin treatment in both growing units. The pigs of farm D were moved from one finishing unit to another after a few weeks (h: hour, d: days, NU: nursing unit, GU: growing unit, FU: finishing unit).