Virulence and resistance determinants of German Staphylococcus aureus ST398 isolates from nonhuman sources

Abstract

A series of 100 Staphylococcus aureus isolates ascribed to sequence type 398 (ST398) and recovered from different sources (healthy carrier and diseased pigs, dust from pig farms, milk, and meat) in Germany were investigated for their virulence and antimicrobial resistance genetic background. Antimicrobial resistance was determined by the disk diffusion method. Virulence and resistance determinants (37 and 31 genes, respectively) were tested by PCR. Only two virulence profiles, including the accessory gene regulator agrI and three or four hemolysin-encoding genes, were detected. In contrast, 33 resistance profiles were distinguished (only 11 were shown by more than one isolate). Fifty-nine isolates were multiresistant (four or more antimicrobial classes), and 98 were methicillin resistant (mecA positive). All of the ST398 isolates showed resistance to tetracycline [encoded by tet(M) alone or together with tet(K) and/or tet(L)]. In addition, 98% were resistant to other antimicrobials, including macrolide-lincosamine-streptogramin B (70%, encoded by ermA, ermB, and ermC, alone or in combination), trimethoprim (65%, mostly due to dfrK and dfrG), kanamycin and gentamicin [29% and 14%, respectively, mainly related to aac(6')-Ie-aph(2″)-Ia and/or ant(4')-Ia but also to aph(3')-IIIa], chloramphenicol (9%, fexA or cfr), quinupristin-dalfopristin (9%), ciprofloxacin (8%), and trimethoprim-sulfamethoxazole (4%). The heterogeneity of the resistance profiles underlines the ability of the ST398 clone to acquire multiple antimicrobial resistance genes. However, the virulence gene content of the tested isolates was low. Continuous surveillance is needed to clarify whether its pathogenicity potential for animals and humans will increase over time.

Fig. 1.

Dendrogram showing the relatedness between virulence gene profiles (V-profile) for 30 exotoxin genes, 3 island markers, and the agr group of the ST398 and outgroup S. aureus isolates tested. The number of isolates is shown in parentheses when n > 1. Clusters are described in the text. The scale shows genetic similarity using Jaccard's coefficient (J). Dashed lines separate clusters/subclusters grouping ST398 isolates.

Fig. 2.

Dendrogram showing the relatedness between resistance gene profiles (R-profile) for 31 resistance genes or cassettes of the ST398 and outgroup S. aureus isolates tested. The scale shows genetic similarity using Jaccard's coefficient (J). Dashed lines separate clusters/subclusters.