Changes in antimicrobial resistance of Staphylococcus aureus in bovine quarter milk samples from southern Germany between 2012 and 2022.

The objective of this study was to describe the in vitro resistance of Staphylococcus aureus from bovine quarter milk samples obtained by the udder health laboratory of the Bavarian Animal Health Services between 2012 and 2022. All S. aureus samples were tested for β-lactamase production and only forwarded to further microbroth susceptibility testing either if the β-lactamase result was positive or upon explicit request by the submitter. The growth of most S. aureus isolates was inhibited at the lowest evaluated minimum inhibitory concentration (MIC) of tested antimicrobials, with the MIC50 and MIC90 (the MIC where 50% and 90% of isolates were inhibited by the tested antibiotics, respectively) mostly beneath the respective breakpoint. On average, about one-fourth (24%, n = 5,718) of tested isolates was resistant to erythromycin. However, the prevalence of resistant isolates dropped from 53% (n = 1,018) in 2012 to 8% (n = 113) in 2022. The second highest prevalence of in vitro resistance was to penicillin (17%, of all isolates tested for β-lactamase production, n = 28,069). Less than 14% of isolates were resistant to the remaining assessed antimicrobial agents (cefoperazone, pirlimycin, kanamycin-cefalexin, marbofloxacin, amoxicillin-clavulanate, cefquinome, or cefazolin, respectively). Over the years, 4% (n = 959) of the S. aureus isolates selected for microbroth susceptibility testing (and 0.8% (n = 1,392) of all submitted S. aureus isolates) were methicillin-resistant S. aureus, and 5% (n = 1,162) of S. aureus isolates were multidrug resistant. However, there was an overall trend toward fewer resistant isolates. These findings are consistent with those of several European monitoring programs that reported a slight decrease of antimicrobial resistance (AMR) of bovine S. aureus in countries where antibiotic use in veterinary medicine was reduced. Notably, isolates of clinical mastitis cases were consistently less likely to express in vitro resistance than isolates obtained from milk of healthy cows or subclinical mastitis cases. In conclusion, AMR of S. aureus was decreasing and penicillin should remain the first-choice antimicrobial in the attempt of treating S. aureus intramammary infections in Bavaria.

Karell J ，Petzl W ，Gangl A ，Huber-Schlenstedt R ，Sorge US ... -  《-》

Comparison of phenotypic and genotypic antimicrobial resistance patterns associated with Staphylococcus aureus mastitis in German and Danish dairy cows.

Staphylococcus aureus is one of the most common pathogens associated with bovine mastitis in Germany and Denmark. Successful therapy is strongly linked to the susceptibility of the pathogen to the administered antimicrobial. An increase in resistant pathogens in human and veterinary medicine has become a concern worldwide and hampers therapy due to reduced susceptibility. In the present study, susceptibility testing was performed for 85 and 93 S. aureus isolates originating from mastitis cases on 12 German and 8 Danish dairy farms, respectively. Phenotypic examination was performed by detection of minimal inhibitory concentration (MIC) values using the broth microdilution method, followed by genotypic investigations of the blaZ and mecA resistance genes via PCR. The tested antimicrobials were the most frequently used β-lactams in German and Danish dairy farms, including cefquinome, cefoperazone, cephapirin, penicillin, oxacillin, cloxacillin, amoxicillin-clavulanic acid, and cephalexin-kanamycin. Special attention was paid to varying therapy concepts because, in Germany, third- and fourth-generation cephalosporins have been predominantly used in mastitis therapy, whereas in Denmark, restrictive use of penicillin is followed by a general avoidance of cephalosporins. Differences in MIC values were mainly based on determined MIC90 values (MIC at which 90% of isolates are inhibited). In general, Danish S. aureus isolates were inhibited at comparatively lower MIC90 values than S. aureus isolated from German dairy farms for most β-lactams. No differences were observed regarding cefquinome, because both German and Danish isolates exhibited MIC50 and MIC90 values of 0.5 and 1 µg/mL, respectively. In contrast, the MIC90 for penicillin against German and Danish S. aureus were 0.5 and ≤0.06 µg/mL, respectively. Resistance genes (blaZ, mecA) were only detected in German S. aureus isolates on 3 dairy farms in Germany. A total of 5 isolates tested positive for both blaZ and mecA, whereas 1 isolate carried the blaZ resistance gene only. A direct correlation between frequently used antimicrobials and reduced susceptibility could not be determined based on results of the present study. In addition to further research to determine factors associated with resistance development, we emphasize the urgent need for internationally standardized clinical breakpoints to assess resistance situations more accurately.

Bolte J ，Zhang Y ，Wente N ，Mahmmod YS ，Svennesen L ，Krömker V ... -  《-》

Antimicrobial resistance of Streptococcus dysgalactiae, Streptococcus agalactiae, and Streptococcus canis in quarter milk samples from Bavaria, Southern Germany, between 2012 and 2022.

The objective of this study was to analyze the in vitro antimicrobial resistance (AMR) of Streptococcusdysgalactiae, Strep. agalactiae, and Strep. canis over a 10-yr period from 2012 to 2022 against the most commonly used antimicrobial agents. For this purpose, all quarter milk samples (QMS) submitted to the milk laboratory of the Bavarian Animal Health Service were analyzed. Each QMS was tested using the California Mastitis Test and categorized as negative, subclinical, or clinical mastitis if the milk character was abnormal. Samples with Strep. dysgalactiae, Strep. agalactiae, or Strep. canis were included and a subset of isolates were further tested for in vitro AMR by breakpoint analysis with broth microdilution. Streptococcus dysgalactiae (61%, n = 65,750) was the most abundant pathogen among those 3 species, followed by Strep. agalactiae (28%, n = 30,486), and Strep. canis (11%, n = 11,336). All 3 species showed the highest resistance to the same 4 antimicrobial agents: erythromycin, marbofloxacin, pirlimycin, and cefalexin/kanamycin with varying degrees of resistance. Throughout the study period, Strep. dysgalactiae, Strep. agalactiae, and Strep. canis were largely susceptible to the remaining antimicrobial agents tested (penicillin, amoxicillin-clavulanate, oxacillin, cefazolin, cefoperazone, cefquinome). Only less than 14% of isolates of Strep. dysgalactiae and Strep. canis were resistant against any of the antimicrobials tested. Streptococcus agalactiae was the species with the highest percentage of resistant isolates. Although the percentage of resistant isolates from Strep. canis and Strep. dysgalactiae decreased, the percentage of resistant Strep. agalactiae isolates increased after 2017. In summary, most isolates were not resistant to the most commonly used antimicrobial agents for mastitis therapy, including β-lactam antibiotics and penicillin should remain the first-choice therapy against streptococcal mastitis.

Bechtold V ，Petzl W ，Huber-Schlenstedt R ，Gangl A ，Sorge US ... -  《-》

Different distribution of antimicrobial resistance genes and virulence profiles of Staphylococcus aureus strains isolated from clinical mastitis in six countries.

Staphylococcus aureus is recognized worldwide as one of the main contagious mastitis agents in cattle and can express a set of antimicrobial resistance genes and virulence-associated genes that explain the wide range of outcomes of intramammary infections. Staphylococcus aureus strains are heterogeneous: their different resistance and virulence patterns, associated with host-level factors and treatment factors, are related to the severity of infection. The aim of this study was to determine phenotypic antibiotic susceptibility, occurrence of selected antimicrobial resistance genes and other virulence genes in 93 S. aureus strains isolated from clinical mastitis in 6 countries: Argentina, Brazil, Germany, Italy, the United States (New York State), and South Africa. These isolates were tested against a total of 16 drugs (amoxicillin-clavulanate, ampicillin, cefazolin, cefoperazone, cefquinome, enrofloxacin, erythromycin, gentamicin, kanamycin, lincomycin, oxacillin, penicillin, rifampin, spiramycin, sulfamethoxazole/trimethoprim, tylosin) by minimum inhibitory concentration (MIC) assay, and examined for the presence of 6 antibiotic-resistance genes (blaZ, mecA, mecC, ermA, ermB, ermC) and 6 virulence-associated genes (scn, chp, sak, hla, hlb, sea) via PCR analysis. The phenotypic results of this study revealed the presence of 19.4% penicillin-resistant strains, whereas 22.6% of the strains were classified as having resistance (5.4%) or intermediate resistance (17.2%) to erythromycin. Most (96.8%) of the isolates were inhibited by cephalosporins, and all were susceptible to amoxicillin-clavulanate. Two strains (1 from Germany, 1 from Italy) were resistant to oxacillin and were positive for mecA. Among the other antimicrobial resistance genes, the most frequently detected was blaZ (46.2%), and 32.3% of the isolates were positive for erm genes: ermC (21.5%) and ermB (10.8%). The most prevalent virulence gene was hla (100%), followed by hlb (84.9%) and sea (65.6%). These results show a low prevalence of antibiotic multidrug resistance in S. aureus isolates, even if the detection of selected antimicrobial resistance genes did not always correspond with the occurrence of phenotypic antibiotic resistance; the immune evasion cluster gene prevalence was quite low in the samples analyzed.

Monistero V ，Barberio A ，Biscarini F ，Cremonesi P ，Castiglioni B ，Graber HU ，Bottini E ，Ceballos-Marquez A ，Kroemker V ，Petzer IM ，Pollera C ，Santisteban C ，Veiga Dos Santos M ，Bronzo V ，Piccinini R ，Re G ，Cocchi M ，Moroni P ... -  《-》

Antimicrobial resistance in Staphylococcus aureus, Streptococcus uberis and Streptococcus dysgalactiae from dairy cows with mastitis.

McDougall S ，Hussein H ，Petrovski K 《NEW ZEALAND VETERINARY JOURNAL》