Distribution of HLER E. faecalis isolates and antimicrobial resistance
Comparative antimicrobial resistance of the 149 HLER E. faecalis isolates from bulk tank milk of four dairy companies is shown in Table 1. Although isolates from company D showed the highest prevalence of E. faecalis, the prevalence of HLER E. faecalis in company A (73.1%) and C (57.0%) was significantly higher than company D (33.9%) (P < 0.05). All HLER E. faecalis isolates showed high rates of resistance to TET (93.3%), followed by DOX (70.0%), and CHL (48.3%). In particular, the resistance against these three antimicrobials showed significant differences between dairy companies (P < 0.05). Resistance against AMP, CIP, PEN, RIF, and VAN was only 0.7–5.4% without significant differences between dairy companies.
Table 1
Antimicrobial resistance of 149 high-level erythromycin-resistant E. faecalis from bulk tank milk in dairy companies
Company (No. of farms) | No. of E. faecalis 1 | No. (%) of HLER 2 | No. (%) of antimicrobial resistant HLER isolates by company 3 |
AMP | CHL | CIP | DOX | PEN | RIF | TET | VAN |
A (106) | 52 | 38 (73.1)a | 0 (0.0) | 30 (78.9)a | 0 (0.0) | 27 (71.1)a,b | 1 (2.6) | 3 (7.9) | 37 (97.4)a | 0 (0.0) |
B (47) | 39 | 20 (51.3)a,b | 1 (5.0) | 13 (65.0)a,b | 0 (0.0) | 11 (55.0)b | 0 (0.0) | 0 (0.0) | 15 (75.0)b | 1 (5.0) |
C (120) | 86 | 49 (57.0)a | 0 (0.0) | 20 (41.0)b,c | 2 (4.1) | 29 (59.2)b | 0 (0.0) | 5 (10.2) | 45 (91.8)a,b | 0 (0.0) |
D (123) | 124 | 42 (33.9)b | 0 (0.0) | 9 (21.4)c | 0 (0.0) | 37 (88.1)a | 0 (0.0) | 0 (0.0) | 42 (100.0)a | 0 (0.0) |
Total (396) | 301 | 149 (49.5) | 1 (0.7) | 72 (48.3) | 2 (1.4) | 104 (70.0) | 1 (0.7) | 8 (5.4) | 139 (93.3) | 1 (0.7) |
Values within a column not having the same small subscripts are significantly different (P < 0.05).
1All E. faecalis were isolated from bulk tank milk samples collected each time in the summer and winter seasons by farms.
2HLER, high-level erythromycin-resistance.
3Abbreviation: AMP, Ampicillin; CHL, Chloramphenicol; CIP, Ciprofloxacin; DOX, Doxycycline; PEN, Penicillin; RIF, Rifampin; TET, Tetracycline; VAN, Vancomycin.
Distribution Of Mdr Patterns
The prevalence of MDR in the 149 HLER E. faecalis isolates is shown in Fig. 1. Although the prevalence of MDR in HLER E. faecalis showed no significant differences between dairy companies, HLER E. faecalis isolates from company D showed the highest MDR (100.0%), followed by company A (92.1%), C (85.7%), and B (85.0%). All MDR isolates were resistant against three to five antimicrobial classes. Interestingly, HLER E. faecalis isolates from company A (57.9%) and B (55.0%) showed significantly higher resistance against four antimicrobial classes, and isolates from company D (78.6%) showed the highest MDR against three classes (P < 0.05). MDR against five classes was only observed in HLER E. faecalis isolates from company A (7.9%) and C (10.2%).
Distribution Of Antimicrobial Resistance Genes And Transposon Genes
Distribution of antimicrobial resistance genes and transposon genes
The distribution of resistance genes and transposons in the 149 HLER E. faecalis isolates is shown in Table 2. In prevalence of macrolides resistance genes, 147 (98.7%) isolates carried ermB gene alone, and two isolates from company C carried both ermA and ermB genes. No isolates carried ermC, msrA, msrC, or mef genes. In the distribution of other resistance genes, 72 (48.3%) and 60 (40.3%) isolates carried both tetM and tetL genes, and tetM gene alone, respectively, which are related with tetracyclines resistance, and 38 (25.5%) isolates carried optrA gene, which is related with phenicols resistance. Of the aminoglycosides resistance genes, the prevalence of both aac(6′)Ie-aph(2″)-la and ant(6′)-Ia genes (43.0%) was the highest. Moreover, 104 (70.0%) isolates harbored Int-Tn gene carrying the Tn916/1545-like transposon. Although the distribution of ermB gene showed no significant differences between dairy companies, the prevalence of other resistance genes and transposons showed a significant difference between the dairy companies (P < 0.05).
Table 2
Antimicrobial resistance genes in 149 high-level erythromycin-resistant E. faecalis from bulk tank milk in dairy companies
Antimicrobial resistance gene | No. (%) of isolates with antimicrobial resistance gene(s) by company 1 |
A | B | C | D | Total |
(n = 38)* | (n = 20) | (n = 49) | (n = 42) | (n = 149) |
Macrolides | | | | | |
ermA | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
ermB | 38 (100.0) | 20 (100.0) | 47 (95.9) | 42 (100.0) | 147 (98.7) |
ermC | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
ermA + ermB | 0 (0.0) | 0 (0.0) | 2 (4.1) | 0 (0.0) | 2 (1.3) |
msrA | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
msrC | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
mef | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
Tetracyclines | | | | | |
tetL | 0 (0.0) | 1 (5.0) | 2 (4.1) | 0 (0.0) | 3 (2.0) |
tetM | 23 (60.5)a | 2 (10.0)b | 27 (55.1)a | 8 (19.0)b | 60 (40.3) |
tetO | 0 (0.0) | 2 (10.0) | 0 (0.0) | 0 (0.0) | 2 (1.3) |
tetL + tetM | 12 (31.6)b | 10 (50.0)a,b | 16 (32.7)b | 34 (81.0)a | 72 (48.3) |
Phenicols | | | | | |
cfr | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
fexA | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
optrA | 17 (44.7)a | 6 (30.0)a,b | 10 (20.4)a,b | 5 (11.9)b | 38 (25.5) |
Aminoglycosides | | | | | |
aac(6′)Ie-aph(2″)-la | 2 (5.3) | 1 (5.0) | 4 (8.2) | 1 (2.4) | 8 (5.4) |
aph(2″)-Ib | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
aph(2″)-Ic | 0 (0.0) | 1 (5.0) | 1 (2.0) | 0 (0.0) | 2 (1.3) |
aph(2″)-Id | 0 (0.0) | 0 (0.0) | 0 (0.0) | 3 (7.1) | 3 (2.0) |
ant(3″)-Ia | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
ant(6′)-Ia | 6 (15.8) | 6 (30.0) | 11 (22.4) | 4 (9.5) | 27 (18.1) |
aac(6′)Ie-aph(2″)-la + ant(6′)-Ia | 11 (28.9)a,b | 6 (30.0)b | 18 (36.7)b | 29 (69.0)a | 64 (43.0) |
aph(2″)-Ic + ant(6′)-Ia | 1 (2.6) | 0 (0.0) | 1 (2.0) | 1 (2.4) | 3 (2.0) |
aph(2″)-Id + ant(6′)-Ia | 1 (2.6) | 2 (10.0) | 3 (6.1) | 0 (0.0) | 6 (4.0) |
aph(2″)-Ic + aph(2″)-Id + ant(6′)-Ia | 0 (0.0) | 0 (0.0) | 1 (2.0) | 0 (0.0) | 1 (0.7) |
Transposon | | | | | |
Int-Tn | 26 (68.4)a,b | 9 (45.0)b | 31 (63.3)b | 38 (90.5)a | 104 (70.0) |
tndX | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
* n = No. of high-level erythromycin resistant E. faecalis isolated from bulk tank milk by company.
1Values within a row not having the same small subscripts are significantly different (P < 0.05).
Distribution Of Virulence Genes
The distribution of virulence genes in the 149 HLER E. faecalis isolates is shown in Table 3. The most prevalent virulence gene was ace (99.3%), followed by cad1 and efaA (each 98.7%), gelE (83.9%), asa1 (67.1%), esp (13.4%), and cylA (8.1%). In particular, the prevalence of cylA, esp, and gelE genes was significantly differences between dairy companies (P < 0.05).
Table 3
Virulence genes in 149 high-level erythromycin-resistant E. faecalis from bulk tank milk in dairy companies
Company | No. (%) of isolates by company 1 |
ace | asa1 | cad1 | cylA | efaA | esp | gelE |
A (n = 38) * | 38 (100.0) | 27 (71.1) | 38 (100.0) | 6 (15.8)a | 38 (100.0) | 3 (7.9)a,b | 33 (86.8)a,b |
B (n = 20) | 19 (95.0) | 12 (60.0) | 20 (100.0) | 2 (10.0)a,b | 18 (90.0) | 2 (10.0)a,b | 15 (75.0)b |
C (n = 49) | 49 (100.0) | 30 (61.2) | 48 (98.0) | 4 (8.2)a,b | 49 (100.0) | 13 (26.5)a | 35 (71.4)b |
D (n = 42) | 42 (100.0) | 31 (73.8) | 41 (97.6) | 0 (0.0)b | 42 (100.0) | 2 (4.8)b | 42 (100.0)a |
Total (n = 149) | 148 (99.3) | 100 (67.1) | 147 (98.7) | 12 (8.1) | 147 (98.7) | 20 (13.4) | 125 (83.9) |
* n = No. of high-level erythromycin-resistant E. faecalis isolated from the dairy company. |
1Values within a column not having the same subscript letter differ significantly (P < 0.05). |