Publikationsdatenbank

Genetic relatedness and antimicrobial susceptibility of porcine respiratory tract pathogens Streptococcus suis, Bordetella bronchiseptica and Pasteurella multocida (2019)

Art

Hochschulschrift

Autor

Niemann, Lisa (WE 7)

Quelle

Berlin, 2019 — 89 Seiten

Sprache

Englisch

Verweise

URL (Volltext): https://refubium.fu-berlin.de/handle/fub188/25562

Kontakt

Institut für Mikrobiologie und Tierseuchen

Robert-von-Ostertag-Str. 7-13
14163 Berlin
+49 30 838 51843 / 66949
mikrobiologie@vetmed.fu-berlin.de

Abstract / Zusammenfassung

Streptococcus suis is a Gram-positive bacterium and a commensal colonizing the porcine respiratory tract. However, some serotypes have a higher virulence and can cause pneumonia, meningitis, septicaemia, arthritis, endocarditis, polyserositis or abortion in pigs. Bordetella bronchiseptica and Pasteurella multocida are Gram-negative bacteria that can cause bronchopneumonia or atrophic rhinitis. Bacterial infections of the porcine respiratory tract are commonly treated with antimicrobials. In this thesis, bronchoalveolar lavage fluid samples (BALFs) of weaners of four to ten weeks of age and originating from 30 farrow-to-finish farms were investigated for the aforementioned pathogens. Furthermore, the influence of antimicrobial treatment during respiratory tract infection on such pathogens within a one-year period was monitored. The genetic relatedness of selected S. suis, B. bronchiseptica and P. multocida isolates on farm level and between different farms was investigated by macrorestriction with subsequent pulsed-field gel electrophoresis (PFGE). In addition, the antimicrobial susceptibility and resistance mechanisms of the pathogens were determined. However, a valid recommendation to the susceptibility of the pathogens to all examined antibiotics is sometimes difficult, since CLSI approved breakpoints are not available for all tested antimicrobial agents. This study revealed a great diversity of PFGE patterns among S. suis isolates. Single pigs can harbor S. suis isolates with completely different macrorestriction patterns, which also differ in their antimicrobial susceptibilities. B. bronchiseptica showed a greater homogeneity in their PFGE patterns and P. multocida displayed related and different PFGE patterns. One or two closely related B. bronchiseptica isolates with quite similar minimal inhibitory concentrations (MICs) existed over a one-year period within one farm. One or two P. multocida isolates with different PFGE patterns can exist on one farm at a certain time point, nevertheless most P. multocida isolates had similar MICs. Antimicrobial agents were prescribed 24 times for five to seven days in the farms of this project. Doxycycline was most frequently prescribed (n=14 times) in this study, followed by amoxicillin (n=8), amoxicillin/clavulanic acid (n=1) and tiamulin/florfenicol (n=1). S. suis isolates were frequently tetracycline-resistant, since they harbored resistance genes such as tet(O), tet(M), tet(L) or tet(W). S. suis exhibited higher MICs to tiamulin, was susceptible to florfenicol and to aminopenicillins alone and in combination with clavulanic acid. However, individual S. suis isolates showed distinctly higher MICs to penicillin. Therefore, three S. suis isolates, which were susceptible, intermediate or resistant to penicillin were subjected to whole genome sequencing. The analyses of the whole genome sequences revealed genes encoding five penicillin-binding proteins (PBPs) for all three isolates. The penicillin-resistant isolate had the most amino acid alterations in PBP2x, PBP2b and PBP2a, whereas the penicillin-intermediate isolate had the most amino acid substitutions only in PBP2x and PBP2b compared to the penicillin-susceptible isolate. However, none of the isolates exhibited amino acid alterations in the conserved motifs, which form the catalytic centre of the PBPs and interact with penicillin. Tetracyclines may have bacteriostatic effects on B. bronchiseptica, since these bacteria exhibit low doxycycline and tetracycline MIC90 values of 0.25 mg/L and 0.5 mg/L, respectively. The B. bronchiseptica isolates showed an overall reduced susceptibility to β-lactams except for amoxicillin/clavulanic acid. B. bronchiseptica isolates had a tiamulin MIC90 of ≥128 mg/L and were either susceptible or intermediate towards florfenicol. The P. multocida isolates were overall β-lactam- and florfenicol-susceptible. Some isolates showed tetracycline MICs of 1 mg/L (n=9) and 2 mg/L (n=3) that classified the respective isolates as intermediate and resistant, respectively. The tiamulin MIC90 was 32 mg/L. Regardless of the antimicrobial treatment on the farms, trimethoprim/sulfamethoxazole (SXT)-resistant P. multocida isolates were detected on three different farrow-to-finish farms. A closer examination of these isolates revealed their close genetic relatedness and the existence of a dfrA14-carrying, 6 kb plasmid. The dfrA14 gene mediates trimethoprim resistance and was detected in P. multocida for the first time. However, a similar plasmid with a nucleotide sequence identity of 99.9 % to that of P. multocida was already identified in Actinobacillus pleuropneumoniae. Hence, we suspect that a plasmid exchange of the pathogens within the common habitat of the porcine respiratory tract has taken place in the past. The MIC testing revealed a broad susceptibility of S. suis and P. multocida to florfenicol as well as an intermediate susceptibility of B. bronchiseptica, which might identify florfenicol as an effective therapeutic alternative to tetracyclines or β-lactams. Besides, the frequent use of β-lactams, tetracyclines and sulfonamides in veterinary medicine increases the selection pressure on certain pathogens as shown in this thesis. Tetracycline resistance genes seems to become conserved in the streptococcal genome due to the frequent use of doxycycline. Former fully β-lactam susceptible isolates, such as S. suis, accumulate amino acid alterations in their PBPs, which mediate a lower affinity to penicillin. The development of a SXT resistance plasmid and its presence in already two members of the family Pasteurellaceae, demonstrates the dissemination of such plasmids within this family.