Drug susceptibility profiles of MDR-TB strains
MDR was observed in 88 (26.0%; 88/339) isolates with 59 (67.0%; 59/88) resistant to SM, 28 (31.8%; 28/88) resistant to EMB, 6 (6.8%; 6/88) resistant to CAP, 11 (12.5%; 11/88) resistant to KAN, and 35 (39.8%; 35/88) resistant to OFX. In addition, 9 isolates (10.2%; 9/88) were identified as XDR strains (Table 3).
The results of MICs for MDR-TB isolates against six antibiotics were shown in Table 1. LFX had the highest resistance rate (50.0%; 44/88), followed by MFX (38.6%; 34/88). The resistance rate to LZD, CFZ, and DLM was 4.5% (4/88), 3.4% (3/88), and 4.5% (4/88), respectively, and the lowest resistance rate was observed in BDQ (2.3%; 2/88). In addition, out of the 44 LFX-resistant isolates, 33 (75%) were resistant to MFX, and 2 (66.7%) out of the 3 CFZ-resistant isolates were resistant to BDQ.
Table 3
Drug resistance profiles of MDR-TB isolates
Resistance profile
|
No. of isolates (n = 88)
|
Percentage (%)
|
SM
|
59
|
67.0
|
EMB
|
28
|
31.8
|
CAP
|
6
|
6.8
|
KAN
|
11
|
12.5
|
OFX
|
35
|
39.8
|
XDR
|
9
|
10.2
|
Mutations conferring antibiotices resistance
Among the 45 isolates resistant to FQs (44 isolates resistant to LFX, 34 isolates resistant to MFX, and 33 isolates cross resistant to LFX and MFX), the most prevalent resistance mutation of gyrA was the substitution in codon 94 (34/45, 75.6%), resulting in the amino acid substitution of Asp for 13 (13/45, 28.9%) Gly, 11 (11/45, 24.4%) Tyr, 6 (6/45, 13.3%) Ala, and 4 (4/45, 8.9%) Asn. In addition, the second most common substitution occurred in codon 90 with substitution of Ala-Val (8/45, 17.8%). The remaining substitutions were Gly88Cys (1/45, 2.2%), Asp89Asn (1/45, 2.2%), Ser91Pro (1/45, 2.2%). Notably, substitution Ser95Thr in gyrA known to be not associated with FQs resistance was found in all FQs resistance isolates. No nonsynonymous mutations were identified in the gyrB gene (Table 4).
A total of 3 strains were identified as CFZ resistant, sequence analysis revealed that all these strains harbored no nucleotide substitution in the rv1979c gene. And no mutation was observed in atpE gene of BDQ resistant isolates. All two CFZ-BDQ cross resistant strains had a mutation in the Rv0678 gene, one with Gln31Arg substitution and the other with Ser53Pro.
Of the four isolates resistant to LZD, two isolates carried mutations in rplC gene with amino acid substitution of Cys154Arg, while 23S rRNA and rplD gene seemed not to confer LZD resistance among these strains.
In addition, five candidate genes associated with DLM resistance was also analyzed in four DLM-resistant isolates. No mutations were observed in fbiA, fbiB, and fgd1 genes, while one isolate with the mutations in codon 318 of fbiC gene and two isolates in codon 81 of ddn gene.
Table 4
Mutations conferring antibiotices resistance
Gene
|
Nucleotide Substitution
|
Amino acid change
|
ATB drug
|
Number of isolates
|
gyrA
|
G262T
|
Gly88Cys
|
LFX
|
1
|
MXF
|
1
|
G265A
|
Asp89Asn
|
LFX
|
1
|
MXF
|
1
|
C269T
|
Ala90Val
|
LFX
|
8
|
MXF
|
6
|
T271C
|
Ser91Pro
|
LFX
|
1
|
MXF
|
1
|
G280T
|
Asp94Tyr
|
LFX
|
11
|
MXF
|
11
|
G280A
|
Asp94Asn
|
LFX
|
4
|
MXF
|
4
|
A281C
|
Asp94Ala
|
LFX
|
6
|
MXF
|
5
|
A281G
|
Asp94Gly
|
LFX
|
13
|
MXF
|
9
|
G284C
|
Ser95Thr
|
LFX
|
44
|
MXF
|
34
|
Rv0678
|
A152G
|
Gln31Arg
|
BDQ
|
1
|
CFZ
|
1
|
T157C
|
Ser53Pro
|
BDQ
|
1
|
CFZ
|
1
|
rplC
|
T460C
|
Cys154Arg
|
LZD
|
2
|
ddn
|
G241A
|
Gly81Ser
|
DMD
|
2
|
fbiC
|
G952A
|
Val318Ile
|
DMD
|
1
|