Identification of antibiotic resistance of the cattle pathogen Histophilus somni

Histophilus somni is a Gram-negative bacterium of the Pasteurellaceae family which is a component of the Bovine Respiratory Disease Complex and the pathogen, causing a multisystem disease - histophilosis. For the treatment of diseases caused by Pasteurellaceae bacteria, aminoglycosides, sulfonamides, beta-lactams, tetracyclines and macrolides are most often used, therefore the formation of resistance of H. somni to antibiotics of these groups can be expected. This work is the first study of antibiotic resistance of H. somni isolated from cattle in the Russian Federation. This work aimed at exploration of the antibiotic resistance of circulating H. somni strains by phenotypic and genotypic methods and the evaluation of the PCR method applicability for the prediction H. somni resistance to antimicrobial agents. We studied 18 cultures of H. somni , the causative agent of histophilosis, isolated in 2018-2019 from biological material (parenchymal organs, washes, sperm) of 145 animals of different breed, age, sex and physiological groups using microbiological method. The cultures were studied using the disk diffusion method for sensitivity to 13 antibiotics of aminoglycosides, beta-lactams, tetracyclines and sulfonamides classes. All obtained isolates were tested by PCR for the presence of genetic determinants of antibiotic resistance, most often found in H. somni : tetH (resistance to tetracyclines), blaOXA-2 (resistance to penicillins), aadA25 , strA , strB , aphA1 (resistance to aminoglycosides), sul2 (sulfonamide resistance). Resistance to aminoglycoside group was most prevalent, i.e., resistance to streptomycin was 50 %, and resistance to neomycin exceeded 40 %. Resistance genes aadA25 , strA , strB and aphA1 were found in the resistant samples. A total of 33 % isolates showed resistance to sulfonamides, all this samples were positive for the sul2 gene in PCR. The sensitivity to penicillins was quite high (~ 75 %), the sensitivity to beta-lactams approached 100 %. The sensitivity to antimicrobials of the tetracycline group was higher than 80 %. However, neither tetracyclines ( tetH ) nor penicillins ( blaOXA-2 ) resistance genes were identified during the study. Two isolates were multidrug resistant with resistance to aminoglycosides, beta-lactams and tetracyclines. Also, four samples were resistant to antimicrobial agents of two different groups, i.e., two samples were resistant to aminoglycosides and sulfonamides with strA , strB , aadA25 , aphA1 , and sul2 genes found, and two samples were resistant to aminoglycosides and beta-lactams with only aminoglycoside resistance genes aadA25 and strA identified. With the exception of samples resistant to tetracyclines and beta-lactams, in which the expected genes were not detected, all observed phenotypes of antimicrobial resistance were consistent with the PCR test results. The combination of genotypic and phenotypic methods for determining antibiotic resistance is necessary for understanding of the resistance mechanisms and increases the efficiency of antibiotic resistance monitoring programs.