Resistencia a antimicrobianos en cepas de Mannheimia haemolytica aisladas de exudado nasal de bovinos productores de leche

Two hundred and one strains of M. haemolytica isolated from nasal exudate of dairy cattle were used, 123 strains from clini­cally healthy (CH) bovines and 78 from clinically ill (CI) bovines affected by pneumonia, obtained from a dairy complex in the Tizayuca region of the state of Hidalgo, Mexico. Strains were previously identified by conventional culture and biochemical tests, and serotyped by indirect haemagglutination. Disk diffusion test was performed to determine antimicrobial resistance to antibiotics, such as: ampicillin, gentamicin, ceftiofiur, penicillin, streptomycin, trimethoprim-sulfamethoxazole, tetracycline and erythromycin. Frequencies of higher antimicrobial resistance were: streptomycin (81.6%) and gentamicin (24.4%), all strains were susceptible to ampicillin and penicillin. Because of the high resistant strain frequency (81.6%) of M. haemolytica to streptomycin, obtained by Kirby-Bauer test, presence of the strA gene, which encodes the enzyme aminoglycoside- 3-phosphotransferase that provides resistance to streptomycin, PCR was performed by testing the presence of the strA gene. Of the 201 strains tested, 42.7% showed the gene str A, 17.4% of which was serotype A1, 1.4% serotype A6 and 23.8% non-typeable strains. Of the 78 CI strains and 123 CH strains, 80% and 18.7%, had the gene str A, respectively.