Characteristics of methicillin-resistant Staphylococcus aureus from broiler farms in Germany are rather lineage- than source-specific

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) are a major concern for public health, and broiler farms are a potential source of MRSA isolates. In this study, a total of 56 MRSA isolates from 15 broiler farms from 4 different counties in Germany were characterised phenotypically and genotypically. Spa types, dru types, SCCmec types, and virulence genes as well as resistance genes were determined by using a DNA microarray or specific PCR assays. In addition, PFGE profiles of isolates were used for analysis of their epidemiological relatedness. While half of the isolates belonged to spa type t011, the other half was of spa types t1430 and t034. On 3 farms, more than 1 spa type was found. The most common dru type was dt10a (n = 19), followed by dt11a (n = 17). Susceptibility testing of all isolates by broth microdilution revealed 21 different resistance phenotypes and a wide range of resistance genes was present among the isolates. Up to 10 different resistance phenotypes were found on individual farms. Resistance to tetracyclines (n = 53), MLSB antibiotics (n = 49), trimethoprim (n = 38), and elevated MICs of tiamulin (n = 29) were most commonly observed. Microarray analysis detected genes for leucocidin (lukF/S), haemolysin gamma (hlgA), and other haemolysines in all isolates. In all t1430 isolates, the egc cluster comprising of genes encoding enterotoxin G, I, M, N, O, U, and/or Y was found. The splitstree analysis based on microarray and PCR gene profiles revealed that all CC9/SCCmec IV/t1430/dt10a isolates clustered apart from the other isolates. These findings confirm that genotypic patterns were specific for clonal lineages rather than for the origin of isolates from individual farms.

Keywords: MRSA; PCR; antimicrobial resistance; broiler; clonal complex.

Figure 1

Network graph illustrating similarity of the microarray and PCR gene profiles of all 56 MRSA isolates in a SplitsTree analysis. All t1430 isolates form one similarity cluster (marked with red circle). Colors of the isolates´ designation represent their origins from the same (same color) or different farm (different color).

Figure 2

All 56 isolates were typed by various typing methods and cluster analysis was performed with PFGE and array data. Susceptibility of the isolates to 32 antibiotic agents was analysed by broth microdilution. All t1430 isolates showed high similarity in all typing methods applied. Parentheses indicate that these isolates were classified as intermediate susceptible to the antimicrobial agent according to CLSI standards. Isolates listed as resistant to apramycin, enrofloxacin, spectinomycin or tiamulin showed MIC values of ≥128 µ g/mL, ≥4 µ g/mL, ≥512 µ g/mL, and ≥16 µ g/mL, respectively. An enrofloxacin MIC value of 1 µ g/mL is indicated as intermediate resistance.

Figure 3

Resistance genes of MRSA isolates as determined by microarray or PCR analyses. Isolates from different farms are separated by bold lines. Presence of genes is indicated as black, absence as white, and ambiguous genes as grey boxes.