Susceptibility of S. aureus to antimicrobials
The result of the antimicrobial susceptibility of all 89 S. aureus isolates were summarized in Table 2. Overall, S. aureus isolates showed high resistance rates against multiple antimicrobials, including penicillin (93.3%, 83/89), ampicillin (92.1%, 82/89), followed by tetracycline (57.3%, 51/89), cefazolin (47.2%, 42/89), doxycycline (44.9%, 40/89), erythromycin (41.6%, 37/89), azithromycin (40.4%, 36/89), clarithromycin (38.3%, 35/89), clindamycin (38.2%, 34/89), kanamycin (34.8%, 31/89), chloramphenicol (31.5%, 28/89), tobramycin (30.3%, 27/89). Moderate resistance can be found from ciprofloxacin (29.2%, 26/89), cefoxitin (28.1%, 25/89), trimethoprim (28.1%, 25/89), norfloxacin (27.0%, 24/89), enrofloxacin (27.0%, 24/89), enoxacin (25.8%, 23/89) and trimethoprim-sulfamethoxazole (20.2%, 18/89). On the contrary, the isolates indicated significantly low resistance to amikacin (4.5%, 4/89), ofloxacin (9.0%, 8/89), cephalothin (10.1%, 9/89), gentamicin (12.4%, 11/89) and imipenem (15.7%, 14/89).
All pig-associated isolates showed resistance to penicillin and ampicillin, and majority of them was resistant to azithromycin (60.0%, 15/25), doxycycline (60.0%, 15/25), clindamycin (60.0%, 15/25), erythromycin (56.0%, 14/25), tetracycline (56.0%, 14/25), chloramphenicol (52.0%, 13/25), norfloxacin (52.0%, 13/25), enrofloxacin (52.0%, 13/25) and trimethoprim (52.0%, 13/25). Meanwhile, most of them were susceptible or intermediately susceptible to amikacin (100.0%, 25/25) and cephalothin (96.0%, 24/25). All cattle-associated isolates showed resistance to penicillin, ampicillin, and susceptibility or intermediate susceptibility to cefoxitin, amikacin, ofloxacin, cephalothin, gentamicin, chloramphenicol, enrofloxacin and cefazolin. Meanwhile, most isolates were susceptible to other antimicrobials. All goat-associated isolates showed resistance to penicillin and ampicillin and (intermediate) susceptibility to cefoxitin, imipenem, enoxacin, gentamicin, clindamycin, norfloxacin, enrofloxacin, enoxacin, and trimethprim-sulfamethoxazole. More than 80% isolates were (intermediately) susceptible to cephalothin, amikacin, clarithromycin, chloramphenical, ciprofloxacin, ofloxacin and trimethoprim. All rabbit-associated isolates were susceptible or intermediately susceptible to 4 tested aminoglycosides (kanamycin, gentamicin, amikacin and tobramycin), 5 fluoroquinolones (norfloxacin, ciprofloxacin, enrofloxacin, ofloxacin, enoxacin), chloramphenicol and doxycycline. Most strains were (intermediate) susceptibility to azithromycin (81.3%, 13/16), trimethoprim-sulfamethoxazole (87.5%, 14/16) and trimethoprim (81.3%, 13/16). Nevertheless, more than half of them were resistant to penicillin (62.5%, 10/16) and ampicillin (62.5%, 10/16). All chicken-associated isolates showed resistance to penicillin and more than half of them was resistant to ampicillin (96.9%, 31/32), tetracycline (65.6%, 21/32), doxycycline (59.4%, 19/32) and cefazolin (53.1%, 17/32). Meanwhile, most of them were more (intermediately) susceptible to cephalothin (93.8%, 30/32), imipenem (90.6%, 29/32), gentamicin (90.6%, 29/32), amikacin (90.6%, 29/32), ofloxacin (90.6%, 29/32) and trimethoprim-sulfamethoxazole (87.5%, 28/32).
All MRSA strains were resistant to penicillin, ampicillin, erythromycin, azithromycin, clarithromycin, tetracycline, doxycycline, chloramphenicol, clindamycin, trimethoprim-sulfamethoxazole and trimethoprim. Three pig-associated MRSA indicated resistance to kanamycin, tobramycin, norfloxacin, ciprofloxacin, enrofloxacin and enoxacin (Table S2).
Among the 89 organisms, 66 (74.2%) isolates exhibited varying degree of MDR. The S. aureus isolates that exhibited MDR were composed of 19 (76.0%) strains from pigs, 3 (50.0%) strains from cattle, 7 (70.0%) strains from goats, 10 (62.5%) strains from rabbits, and 27 (84.4%) strains from chicken. Notably, the isolates (80.7%) from pigs and chicken exhibited substantially higher MDR rate than those (62.5%) from herbivores. Twenty-two (24.7%) isolates were resistant to 7 or more chasses of antimicrobials; among them, 13 were isolated from pigs and 9 were acquired from chicken, the ratios in their total quantities were 52.0% and 28.1%, respectively. Three isolates from pigs were found to be resistance to 10 classes of drugs.
Four MRSA strains had high MDR. The chicken-associated MRSA was resistant to 8 classes of drugs. However, 2 pig-associated isolates were resistant to 9, and 1 was resistant to 10 classes of antimicrobials.
Profile of staphylococcal SE/SEl/TSST–1 genes
A total of 20 staphylococcal toxin genes were amplified in S. aureus isolates. The results were shown in Table 4, Table S3 and Fig. 1. Nineteen genes, except seq, were detected in this study. The most prevalent gene was sei (52, 58.4%), followed by tst–1 (50, 56.2%), seg (46, 51.7%), selj (35, 39.3%), seo (34, 38.2%), sem (32, 36.0%), sek (29, 32.6%), sel (24, 27.0%), sec (21, 23.4%) and sen (19, 21.3%). The prevalence rates of 5 genes were between 10% and 20%, namely, seh (13, 14.6%), ser (12, 13.5%), ses (11, 12.4%), set (10, 11.2%)and sed (9, 10.1%). However, sea and seb were amplified from 4 (4.5%) and 5 (5.6%) strains, respectively. Only 1 strain obtained from dairy cattle was positive for gene see.
The toxin patterns were variable, from 1 to 10 except for one rabbit-associated strain that no toxin gene was amplified (Table 5, Table S3). Four isolates only had 1 gene. More than 1 gene were detect in 8, 13, 18, 12, 9, 15 and 5 isolates, and they were positive for 2, 3, 4, 5, 6, 7 and 8 toxin genes. Two pig-associated isolates and 1 rabbit-associated isolate harbored 9 toxin genes. Ten toxin genes were observed from 1 strain isolated from a pig.