# Drug Susceptibility of 33 Reference Strains of Slowly Growing Mycobacteria to 19 Antimicrobial Agents

Authors: Hui Pang; Yi Jiang; Kanglin Wan
Journal: BioMed Research International
 (2017)

---

## Abstract

Objectives. Slowly growing mycobacteria (SGM) are prevalent worldwide and cause an extensive spectrum of diseases.Methods. In this study, the antimicrobial susceptibility of 33 reference strains of SGM to 19 antimicrobial agents was tested using a modified microdilution method.Results. Cefmetazole (32/33) and azithromycin (32/33) exhibited the highest antimicrobial activity, and dapsone (9/33) exhibited the lowest activity against the tested strains. Cefoxitin (30/33), cefoperazone (28/33), and cefepime (28/33) were effective against a high proportion of strains, and macrolides were also highly effective as well as offering the benefit of convenient oral administration to patients. Linezolid (27/33), meropenem (26/33), sulfamethoxazole (26/33), and tigecycline (25/33) showed the highest activity; clofazimine (20/33) and doxycycline (18/33) showed intermediate activity; and rifapentine (13/33), rifabutin (13/33), and minocycline (11/33) showed low antimicrobial activity, closely followed by thioacetazone (10/33) and pasiniazid (10/33), against the tested organisms. According to their susceptibility profiles, the slowly growing speciesMycobacterium avium andMycobacterium simiae were the least susceptible to the tested drugs, whereasMycobacterium intracellulare,Mycobacterium asiaticum,Mycobacterium scrofulaceum,Mycobacterium szulgai,Mycobacterium branderi, andMycobacterium holsaticum were the most susceptible.Conclusions. In summary, cephalosporins and macrolides, particularly cefmetazole, azithromycin, clarithromycin, and roxithromycin, showed good antimicrobial activity against the reference strains of SGM.


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## Body

## 1. Introduction

Slowly growing mycobacteria (SGM) species are ubiquitous organisms that are widely distributed in the environment [1], not only in tap water, soil, dust, and food products but also in domestic and wild animals [2]. SGM form colonies visible to the naked eye in more than 7 days on subculture media [3]. SGM comprise some common species, such as theMycobacterium avium complex (Mycobacterium avium,Mycobacterium intracellulare, andMycobacterium chimaera),Mycobacterium kansasii,Mycobacterium haemophilum,Mycobacterium marinum, andMycobacterium ulcerans, in addition to some less common pathogens, such asMycobacterium scrofulaceum,Mycobacterium simiae,Mycobacterium malmoense andMycobacterium xenopi.Mycobacterium xenopi is largely distributed in Canada and northern Europe [4]. Slowly growing species were the first nontuberculous mycobacteria (NTM) to be recognized as causing chronic lung disease [4, 5], which may bring about diverse infections from minor sicknesses to serious widespread disorders [6].At present, standard therapeutic strategies to treat SGM infections are lacking. In this study, 19 new antimicrobial agents were tested against 33 reference SGM pathogens using a modified broth microdilution method with the aim of identifying optimal schemes according to the Clinical Laboratory Standards Institute (CLSI) (USA) [7, 8] and World Health Organization (WHO) [9] guidelines.


## 2. Materials and Methods

### 2.1. Reference Strains

Thirty-three international reference SGM strains were purchased from Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) and the American Type Culture Collection (ATCC), includingMycobacterium avium,Mycobacterium intracellulare,Mycobacterium shimoidei,Mycobacterium farcinogenes, andMycobacterium simiae (Table 1). These strains were cultured at the appropriate temperatures.Table 1
The MIC (μg/mL) of 19 antimicrobial agents against 33 reference slowly growing mycobacteria.
(a)
Sp. (international code)
FOX
CFP
CMZ
FEP
RPT
RBT
AZM
CLR
ROX
THI
Mycobacterium avium (DSM44133)
>256
128
>256
>256
4
0.25
4
0.5
1
>256
Mycobacterium intracellulare (ATCC13950)
0.5
8
1
4
<0.03
<0.03
<0.03
0.13
<0.03
<0.25
Mycobacterium shimoidei(ATCC27962)
8
16
4
>256
2
0.06
0.13
<0.03
0.06
>256
Mycobacterium farcinogenes (ATCC35753)
4
16
2
<0.25
<0.03
<0.03
0.13
<0.03
<0.03
2
Mycobacterium simiae (ATCC25275)
>256
256
256
128
16
4
16
1
4
256
Mycobacterium asiaticum (ATCC25276)
2
<0.25
<0.25
1
4
<0.03
0.5
<0.03
0.13
1
Mycobacterium scrofulaceum (ATCC19981)
64
64
32
32
<0.03
<0.03
2
0.06
0.13
<0.25
Mycobacterium szulgai (ATCC35799)
4
2
2
1
<0.03
<0.03
0.13
<0.03
<0.03
256
Mycobacterium africanum (ATCC35711)
64
64
8
256
16
32
32
32
32
>256
Mycobacterium alvei (DSM44176)
16
64
0.5
<0.25
1
0.13
0.06
0.06
<0.03
256
Mycobacterium branderi (ATCC51788)
4
16
1
16
32
0.25
0.25
<0.03
<0.03
>256
Mycobacterium celatum (ATCC44243)
128
128
64
64
32
0.5
1
0.06
0.25
128
Mycobacterium chimaera (DSM44623)
64
32
16
64
16
16
8
0.5
0.5
64
Mycobacterium cosmeticum (DSM44829)
64
>256
8
256
>32
32
2
0.25
1
>256
Mycobacterium duvalii (ATCC43910)
<0.25
16
1
0.5
0.06
<0.03
0.5
0.06
0.13
8
Mycobacterium elephantis (DSM44368)
8
256
2
4
8
2
8
1
4
32
Mycobacterium hassiacum (DSM44199)
128
>256
16
256
32
0.5
<0.03
<0.03
<0.03
>256
Mycobacterium hiberniae (DSM44241)
1
2
<0.25
16
0.25
<0.03
2
0.06
0.13
64
Mycobacterium holsaticum (DSM44478)
1
32
<0.25
0.5
4
1
<0.03
<0.03
<0.03
256
Mycobacterium houstonense (DSM44676)
32
32
64
32
64
>32
32
8
2
64
Mycobacterium kubicae (DSM44627)
64
16
0.5
16
64
>32
2
<0.25
16
256
Mycobacterium lentiflavum (DSM44418)
2
4
4
0.13
0.5
>32
8
2
32
>256
Mycobacterium mageritense (DSM44476)
8
1
0.5
8
32
>32
8
4
16
64
Mycobacterium nonchromogenicum (DSM44164)
2
<0.25
4
<0.25
1
>32
8
2
<0.03
2
Mycobacterium palustre(DSM44572)
<0.25
<0.25
0.5
<0.25
0.06
>32
<0.03
0.5
0.06
4
Mycobacterium parascrofulaceum (DSM44648)
<0.25
<0.25
4
0.25
0.06
8
1
16
<0.03
64
Mycobacterium senuense (DSM44999)
0.5
<0.25
2
<0.25
<0.03
32
1
<0.25
0.06
2
Mycobacterium seoulense (DSM44998)
<0.25
<0.25
4
<0.25
0.5
>32
2
32
<0.03
128
Mycobacterium thermoresistibile (DSM44167)
8
0.25
4
2
2
>32
2
1
1
2
Mycobacterium triplex (DSM44626)
0.25
0.5
<0.25
1
4
>32
1
<0.25
0.13
0.5
Mycobacterium vanbaalenii (DSM7251)
8
0.25
0.5
1
0.06
>32
<0.25
64
1
256
Mycobacterium murale (DSM44340)
0.5
<0.25
0.25
0.25
0.25
>32
<0.25
8
<0.03
2
Mycobacterium gordonae (ATCC14470)
4
0.5
16
0.5
1
>32
8
8
16
128
(b)
Sp. (international code)
DOX
MIN
TIG
MEM
CLO
SMZ
PASI
LNZ
DAP
Mycobacterium avium (DSM44133)
>256
64
32
>256
16
32
32
32
>256
Mycobacterium intracellulare (ATCC13950)
<0.25
<0.25
0.5
0.5
0.13
<0.25
<0.25
<0.06
<0.25
Mycobacterium shimoidei(ATCC27962)
256
64
1
>256
0.13
4
0.5
0.5
64
Mycobacterium farcinogenes (ATCC35753)
0.25
8
<0.03
<0.25
<0.03
1
16
0.25
4
Mycobacterium simiae (ATCC25275)
>256
16
16
>256
<0.03
16
16
32
16
Mycobacterium asiaticum (ATCC25276)
<0.25
0.5
<0.03
<0.25
<0.03
<0.25
2
0.13
<0.25
Mycobacterium scrofulaceum (ATCC19981)
0.25
2
0.5
32
<0.03
16
1
1
2
Mycobacterium szulgai (ATCC35799)
<0.25
<0.25
0.13
0.5
<0.03
<0.25
1
0.13
<0.25
Mycobacterium africanum (ATCC35711)
4
8
0.25
4
32
256
>256
16
4
Mycobacterium alvei (DSM44176)
<0.25
32
<0.03
<0.25
0.5
4
8
16
64
Mycobacterium branderi (ATCC51788)
<0.25
0.5
0.06
0.5
<0.03
0.5
1
<0.06
<0.25
Mycobacterium celatum (ATCC44243)
2
0.5
4
64
<0.03
2
0.5
16
<0.25
Mycobacterium chimaera (DSM44623)
8
32
4
32
0.5
64
>256
16
64
Mycobacterium cosmeticum (DSM44829)
2
128
1
8
32
256
>256
32
256
Mycobacterium duvalii (ATCC43910)
<0.25
16
<0.03
2
1
4
0.5
0.25
8
Mycobacterium elephantis (DSM44368)
<0.25
0.5
<0.03
8
0.13
<0.25
0.5
0.5
0.5
Mycobacterium hassiacum (DSM44199)
<0.25
8
<0.03
256
0.13
8
8
4
16
Mycobacterium hiberniae (DSM44241)
1
8
0.13
4
<0.03
1
8
1
4
Mycobacterium holsaticum (DSM44478)
<0.25
1
0.25
16
0.13
<0.25
1
0.5
0.5
Mycobacterium houstonense (DSM44676)
64
64
16
16
32
256
64
32
256
Mycobacterium kubicae (DSM44627)
>256
16
2
2
32
>256
16
32
>256
Mycobacterium lentiflavum (DSM44418)
>256
128
4
4
4
128
128
4
128
Mycobacterium mageritense (DSM44476)
>256
128
8
8
32
8
128
32
8
Mycobacterium nonchromogenicum (DSM44164)
128
128
16
16
2
4
128
2
4
Mycobacterium palustre(DSM44572)
2
32
0.06
<0.25
0.25
8
32
0.25
8
Mycobacterium parascrofulaceum(DSM44648)
64
32
0.03
<0.25
2
32
32
2
32
Mycobacterium senuense (DSM44999)
1
4
1
1
0.5
4
4
0.5
4
Mycobacterium seoulense (DSM44998)
2
128
8
8
4
8
128
4
8
Mycobacterium thermoresistibile (DSM44167)
256
256
2
2
1
8
256
1
8
Mycobacterium triplex (DSM44626)
256
0.5
0.06
<0.25
0.13
0.5
0.5
0.13
0.5
Mycobacterium vanbaalenii (DSM7251)
8
4
8
8
2
4
4
2
4
Mycobacterium murale (DSM44340)
256
16
0.06
<0.25
0.5
8
16
0.5
8
Mycobacterium gordonae (ATCC14470)
>256
16
16
16
0.5
256
16
0.5
256
Note
1. FOX: cefoxitin; CFP: cefoperazone; CMZ: cefmetazole; FEP: cefepime; RPT: rifapentine; RBT: rifabutin; AZM: azithromycin; CLR: clarithromycin; ROX: roxithromycin; THI: thioacetazone; DOX: doxycycline; MIN: minocycline; TIG: tigecycline; MEM: meropenem; CLO: clofazimine; SMZ: sulfamethoxazole; PASI: pasiniazid; LNZ: linezolid; DAP: dapsone.
Note
2. Bold numbers indicate drug susceptibility. Numbers in bold and cursive indicate intermediate drug susceptibility.


### 2.2. Antimicrobial Agents

Nineteen chemicals were purchased from Sigma-Aldrich Company: cefoxitin (FOX), cefoperazone (CFP), cefmetazole (CMZ), cefepime (FEP), rifapentine (RPT), rifabutin (RBT), azithromycin (AZM), clarithromycin (CLR), roxithromycin (ROX), thioacetazone (THI), doxycycline (DOX), minocycline (MIN), tigecycline (TIG), meropenem (MEM), clofazimine (CLO), sulfamethoxazole (SMZ), pasiniazid (PASI), linezolid (LNZ), and dapsone (DAP). All of the antituberculous agents were freshly prepared.


### 2.3. Drug Susceptibility Test

SGM strains were incubated using Difco Middlebrook 7H10 Agar (BD company) with 5% oleic acid-albumin-dextrose-catalase (OADC) [8]. The drug sensitivity tests were performed using a cation-adjusted Mueller-Hinton (CAMH) broth microdilution method, with the addition of 5% OADC, according to the CLSI standard operating procedure [8]. All of the experiments were performed in 96-well microplates and repeated. The minimum inhibitory concentration (MIC) for each antibiotic for each strain was the mean of two experiments. Firstly, the bacterial suspensions were prepared as follows: bacterial inocula were adjusted with normal saline to a density of a 0.5 McFarland standard with an inoculum density of approximately 1 × 107 colony forming units (CFU)/mL; then 50 μL of the bacterial suspension was mixed with 10 mL of CAMH and 5% OADC broth for a 1 : 200 dilution. Secondly, 100 μL of CAMH and 5% OADC medium were added to each well of a 96-well microplate, with the exception of the first well of every row to which 180 μL of medium and a 20 μL drug dilution were added. The solution in the first well was successively diluted into subsequent wells, up to the 11th well. The 12th well in every row was used as a blank control. Finally, 100 μL of the bacterial dilution was added to all of the wells. The ultimate volume in each well was 200 μL. All of the 96-well microplates were sealed in a plastic bag and incubated at 37°C. The concentrations of sulfamethoxazole, dapsone, cefoxitin, cefmetazole, cefoperazone, cefepime, thioacetazone, pasiniazid, minocycline, doxycycline, tigecycline, and meropenem were 0.25–256 μg/mL; the concentrations of clarithromycin, azithromycin, roxithromycin, clofazimine, rifapentine, and rifabutin were 0.03–32 μg/mL; and the concentration of linezolid was 0.06–64 μg/mL. Two negative controls were applied: a no drug control (CAMH + OADC + bacteria) and a no bacteria control (barely CAMH and OADC) [10]. The MIC breakpoints of the drugs exhibiting susceptibility, moderate susceptibility, and resistance were assigned according to the CLSI [7, 8] and WHO [9] guidelines (Table 2).Table 2
The MIC (μg/mL) breakpoints of 19 antibacterial agents.
Susceptibility
Intermediate susceptibility
Resistance
Cefoxitin
≤16
32–64
≥128
Cefoperazone
≤16
32–64
≥128
Cefmetazole
≤16
32–64
≥128
Cefepime
≤16
32–64
≥128
Rifapentine
—
—
>1
Rifabutin
—
—
>2
Azithromycin
≤8
16
≥32
Clarithromycin
≤8
16
≥32
Roxithromycin
≤8
16
≥32
Thioacetazone
—
—
≥8
Doxycycline
≤1
2–4
≥8
Meropenem
≤4
8–16
≥32
Clofazimine
—
—
≥1
Sulfamethoxazole
≤38
—
≥76
Pasiniazid
—
—
≥2
Minocycline
≤1
2–4
≥8
Linezolid
≤8
16
≥32
Dapsone
—
—
≥4
Tigecycline
≤1
2–4
≥8


## 2.1. Reference Strains

Thirty-three international reference SGM strains were purchased from Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) and the American Type Culture Collection (ATCC), includingMycobacterium avium,Mycobacterium intracellulare,Mycobacterium shimoidei,Mycobacterium farcinogenes, andMycobacterium simiae (Table 1). These strains were cultured at the appropriate temperatures.Table 1
The MIC (μg/mL) of 19 antimicrobial agents against 33 reference slowly growing mycobacteria.
(a)
Sp. (international code)
FOX
CFP
CMZ
FEP
RPT
RBT
AZM
CLR
ROX
THI
Mycobacterium avium (DSM44133)
>256
128
>256
>256
4
0.25
4
0.5
1
>256
Mycobacterium intracellulare (ATCC13950)
0.5
8
1
4
<0.03
<0.03
<0.03
0.13
<0.03
<0.25
Mycobacterium shimoidei(ATCC27962)
8
16
4
>256
2
0.06
0.13
<0.03
0.06
>256
Mycobacterium farcinogenes (ATCC35753)
4
16
2
<0.25
<0.03
<0.03
0.13
<0.03
<0.03
2
Mycobacterium simiae (ATCC25275)
>256
256
256
128
16
4
16
1
4
256
Mycobacterium asiaticum (ATCC25276)
2
<0.25
<0.25
1
4
<0.03
0.5
<0.03
0.13
1
Mycobacterium scrofulaceum (ATCC19981)
64
64
32
32
<0.03
<0.03
2
0.06
0.13
<0.25
Mycobacterium szulgai (ATCC35799)
4
2
2
1
<0.03
<0.03
0.13
<0.03
<0.03
256
Mycobacterium africanum (ATCC35711)
64
64
8
256
16
32
32
32
32
>256
Mycobacterium alvei (DSM44176)
16
64
0.5
<0.25
1
0.13
0.06
0.06
<0.03
256
Mycobacterium branderi (ATCC51788)
4
16
1
16
32
0.25
0.25
<0.03
<0.03
>256
Mycobacterium celatum (ATCC44243)
128
128
64
64
32
0.5
1
0.06
0.25
128
Mycobacterium chimaera (DSM44623)
64
32
16
64
16
16
8
0.5
0.5
64
Mycobacterium cosmeticum (DSM44829)
64
>256
8
256
>32
32
2
0.25
1
>256
Mycobacterium duvalii (ATCC43910)
<0.25
16
1
0.5
0.06
<0.03
0.5
0.06
0.13
8
Mycobacterium elephantis (DSM44368)
8
256
2
4
8
2
8
1
4
32
Mycobacterium hassiacum (DSM44199)
128
>256
16
256
32
0.5
<0.03
<0.03
<0.03
>256
Mycobacterium hiberniae (DSM44241)
1
2
<0.25
16
0.25
<0.03
2
0.06
0.13
64
Mycobacterium holsaticum (DSM44478)
1
32
<0.25
0.5
4
1
<0.03
<0.03
<0.03
256
Mycobacterium houstonense (DSM44676)
32
32
64
32
64
>32
32
8
2
64
Mycobacterium kubicae (DSM44627)
64
16
0.5
16
64
>32
2
<0.25
16
256
Mycobacterium lentiflavum (DSM44418)
2
4
4
0.13
0.5
>32
8
2
32
>256
Mycobacterium mageritense (DSM44476)
8
1
0.5
8
32
>32
8
4
16
64
Mycobacterium nonchromogenicum (DSM44164)
2
<0.25
4
<0.25
1
>32
8
2
<0.03
2
Mycobacterium palustre(DSM44572)
<0.25
<0.25
0.5
<0.25
0.06
>32
<0.03
0.5
0.06
4
Mycobacterium parascrofulaceum (DSM44648)
<0.25
<0.25
4
0.25
0.06
8
1
16
<0.03
64
Mycobacterium senuense (DSM44999)
0.5
<0.25
2
<0.25
<0.03
32
1
<0.25
0.06
2
Mycobacterium seoulense (DSM44998)
<0.25
<0.25
4
<0.25
0.5
>32
2
32
<0.03
128
Mycobacterium thermoresistibile (DSM44167)
8
0.25
4
2
2
>32
2
1
1
2
Mycobacterium triplex (DSM44626)
0.25
0.5
<0.25
1
4
>32
1
<0.25
0.13
0.5
Mycobacterium vanbaalenii (DSM7251)
8
0.25
0.5
1
0.06
>32
<0.25
64
1
256
Mycobacterium murale (DSM44340)
0.5
<0.25
0.25
0.25
0.25
>32
<0.25
8
<0.03
2
Mycobacterium gordonae (ATCC14470)
4
0.5
16
0.5
1
>32
8
8
16
128
(b)
Sp. (international code)
DOX
MIN
TIG
MEM
CLO
SMZ
PASI
LNZ
DAP
Mycobacterium avium (DSM44133)
>256
64
32
>256
16
32
32
32
>256
Mycobacterium intracellulare (ATCC13950)
<0.25
<0.25
0.5
0.5
0.13
<0.25
<0.25
<0.06
<0.25
Mycobacterium shimoidei(ATCC27962)
256
64
1
>256
0.13
4
0.5
0.5
64
Mycobacterium farcinogenes (ATCC35753)
0.25
8
<0.03
<0.25
<0.03
1
16
0.25
4
Mycobacterium simiae (ATCC25275)
>256
16
16
>256
<0.03
16
16
32
16
Mycobacterium asiaticum (ATCC25276)
<0.25
0.5
<0.03
<0.25
<0.03
<0.25
2
0.13
<0.25
Mycobacterium scrofulaceum (ATCC19981)
0.25
2
0.5
32
<0.03
16
1
1
2
Mycobacterium szulgai (ATCC35799)
<0.25
<0.25
0.13
0.5
<0.03
<0.25
1
0.13
<0.25
Mycobacterium africanum (ATCC35711)
4
8
0.25
4
32
256
>256
16
4
Mycobacterium alvei (DSM44176)
<0.25
32
<0.03
<0.25
0.5
4
8
16
64
Mycobacterium branderi (ATCC51788)
<0.25
0.5
0.06
0.5
<0.03
0.5
1
<0.06
<0.25
Mycobacterium celatum (ATCC44243)
2
0.5
4
64
<0.03
2
0.5
16
<0.25
Mycobacterium chimaera (DSM44623)
8
32
4
32
0.5
64
>256
16
64
Mycobacterium cosmeticum (DSM44829)
2
128
1
8
32
256
>256
32
256
Mycobacterium duvalii (ATCC43910)
<0.25
16
<0.03
2
1
4
0.5
0.25
8
Mycobacterium elephantis (DSM44368)
<0.25
0.5
<0.03
8
0.13
<0.25
0.5
0.5
0.5
Mycobacterium hassiacum (DSM44199)
<0.25
8
<0.03
256
0.13
8
8
4
16
Mycobacterium hiberniae (DSM44241)
1
8
0.13
4
<0.03
1
8
1
4
Mycobacterium holsaticum (DSM44478)
<0.25
1
0.25
16
0.13
<0.25
1
0.5
0.5
Mycobacterium houstonense (DSM44676)
64
64
16
16
32
256
64
32
256
Mycobacterium kubicae (DSM44627)
>256
16
2
2
32
>256
16
32
>256
Mycobacterium lentiflavum (DSM44418)
>256
128
4
4
4
128
128
4
128
Mycobacterium mageritense (DSM44476)
>256
128
8
8
32
8
128
32
8
Mycobacterium nonchromogenicum (DSM44164)
128
128
16
16
2
4
128
2
4
Mycobacterium palustre(DSM44572)
2
32
0.06
<0.25
0.25
8
32
0.25
8
Mycobacterium parascrofulaceum(DSM44648)
64
32
0.03
<0.25
2
32
32
2
32
Mycobacterium senuense (DSM44999)
1
4
1
1
0.5
4
4
0.5
4
Mycobacterium seoulense (DSM44998)
2
128
8
8
4
8
128
4
8
Mycobacterium thermoresistibile (DSM44167)
256
256
2
2
1
8
256
1
8
Mycobacterium triplex (DSM44626)
256
0.5
0.06
<0.25
0.13
0.5
0.5
0.13
0.5
Mycobacterium vanbaalenii (DSM7251)
8
4
8
8
2
4
4
2
4
Mycobacterium murale (DSM44340)
256
16
0.06
<0.25
0.5
8
16
0.5
8
Mycobacterium gordonae (ATCC14470)
>256
16
16
16
0.5
256
16
0.5
256
Note
1. FOX: cefoxitin; CFP: cefoperazone; CMZ: cefmetazole; FEP: cefepime; RPT: rifapentine; RBT: rifabutin; AZM: azithromycin; CLR: clarithromycin; ROX: roxithromycin; THI: thioacetazone; DOX: doxycycline; MIN: minocycline; TIG: tigecycline; MEM: meropenem; CLO: clofazimine; SMZ: sulfamethoxazole; PASI: pasiniazid; LNZ: linezolid; DAP: dapsone.
Note
2. Bold numbers indicate drug susceptibility. Numbers in bold and cursive indicate intermediate drug susceptibility.


## 2.2. Antimicrobial Agents

Nineteen chemicals were purchased from Sigma-Aldrich Company: cefoxitin (FOX), cefoperazone (CFP), cefmetazole (CMZ), cefepime (FEP), rifapentine (RPT), rifabutin (RBT), azithromycin (AZM), clarithromycin (CLR), roxithromycin (ROX), thioacetazone (THI), doxycycline (DOX), minocycline (MIN), tigecycline (TIG), meropenem (MEM), clofazimine (CLO), sulfamethoxazole (SMZ), pasiniazid (PASI), linezolid (LNZ), and dapsone (DAP). All of the antituberculous agents were freshly prepared.


## 2.3. Drug Susceptibility Test

SGM strains were incubated using Difco Middlebrook 7H10 Agar (BD company) with 5% oleic acid-albumin-dextrose-catalase (OADC) [8]. The drug sensitivity tests were performed using a cation-adjusted Mueller-Hinton (CAMH) broth microdilution method, with the addition of 5% OADC, according to the CLSI standard operating procedure [8]. All of the experiments were performed in 96-well microplates and repeated. The minimum inhibitory concentration (MIC) for each antibiotic for each strain was the mean of two experiments. Firstly, the bacterial suspensions were prepared as follows: bacterial inocula were adjusted with normal saline to a density of a 0.5 McFarland standard with an inoculum density of approximately 1 × 107 colony forming units (CFU)/mL; then 50 μL of the bacterial suspension was mixed with 10 mL of CAMH and 5% OADC broth for a 1 : 200 dilution. Secondly, 100 μL of CAMH and 5% OADC medium were added to each well of a 96-well microplate, with the exception of the first well of every row to which 180 μL of medium and a 20 μL drug dilution were added. The solution in the first well was successively diluted into subsequent wells, up to the 11th well. The 12th well in every row was used as a blank control. Finally, 100 μL of the bacterial dilution was added to all of the wells. The ultimate volume in each well was 200 μL. All of the 96-well microplates were sealed in a plastic bag and incubated at 37°C. The concentrations of sulfamethoxazole, dapsone, cefoxitin, cefmetazole, cefoperazone, cefepime, thioacetazone, pasiniazid, minocycline, doxycycline, tigecycline, and meropenem were 0.25–256 μg/mL; the concentrations of clarithromycin, azithromycin, roxithromycin, clofazimine, rifapentine, and rifabutin were 0.03–32 μg/mL; and the concentration of linezolid was 0.06–64 μg/mL. Two negative controls were applied: a no drug control (CAMH + OADC + bacteria) and a no bacteria control (barely CAMH and OADC) [10]. The MIC breakpoints of the drugs exhibiting susceptibility, moderate susceptibility, and resistance were assigned according to the CLSI [7, 8] and WHO [9] guidelines (Table 2).Table 2
The MIC (μg/mL) breakpoints of 19 antibacterial agents.
Susceptibility
Intermediate susceptibility
Resistance
Cefoxitin
≤16
32–64
≥128
Cefoperazone
≤16
32–64
≥128
Cefmetazole
≤16
32–64
≥128
Cefepime
≤16
32–64
≥128
Rifapentine
—
—
>1
Rifabutin
—
—
>2
Azithromycin
≤8
16
≥32
Clarithromycin
≤8
16
≥32
Roxithromycin
≤8
16
≥32
Thioacetazone
—
—
≥8
Doxycycline
≤1
2–4
≥8
Meropenem
≤4
8–16
≥32
Clofazimine
—
—
≥1
Sulfamethoxazole
≤38
—
≥76
Pasiniazid
—
—
≥2
Minocycline
≤1
2–4
≥8
Linezolid
≤8
16
≥32
Dapsone
—
—
≥4
Tigecycline
≤1
2–4
≥8


## 3. Results

The antimicrobial susceptibility profiles of the 33 SGM reference species to 19 antibacterial agents are presented in Table1. Cephalosporins including cefoxitin (30/33, 90.91%), cefoperazone (28/33, 84.85%), cefmetazole (32/33, 96.97%), and cefepime (28/33, 84.85%) exhibited high activity against the tested strains. Macrolide antibiotics including azithromycin (32/33, 96.97%), clarithromycin (30/33, 90.91%), and roxithromycin (31/33, 93.94%) were also effective against the SGM strains. Linezolid (27/33, 81.82%), meropenem (26/33, 78.79%), and sulfamethoxazole (26/33, 78.79%) showed similar levels of activity against the tested strains, and clofazimine (20/33, 60.61%) inhibited most of the SGM strains. The tetracyclines, doxycycline (18/33, 54.55%), minocycline (11/33, 33.33%), and tigecycline (25/33, 75.76%), exhibited different levels of activity against the SGM standard species, whereas rifapentine (13/33, 39.39%) and rifabutin (13/33, 39.39%) showed weak antimicrobial activity against the SGM, as did thioacetazone (10/33, 30.30%), pasiniazid (10/33, 30.30%), and dapsone (9/33, 27.27%).The drug susceptibility profiles of the tested organisms revealed thatMycobacterium avium andMycobacterium simiae were the least susceptible to the tested drugs, whereasMycobacterium intracellulare,Mycobacterium asiaticum,Mycobacterium scrofulaceum,Mycobacterium szulgai,Mycobacterium branderi, andMycobacterium holsaticum were the most susceptible (Table 3 and Figure 1). Among theMycobacterium avium complex,Mycobacterium avium was the most resistant to the tested drugs, whereasMycobacterium intracellulare was the most susceptible (Figure 2). Azithromycin was identified as the most effective antimicrobial agent against SGM species among the drugs tested, and dapsone was the least effective.Table 3
Susceptibility of 33 international standard slowly growing mycobacterial strains to 19 antibacterial agents.
Sp. (international code)
FOX
CFP
CMZ
FEP
RPT
RBT
AZM
CLR
ROX
THI
DOX
MIN
TIG
MEM
CLO
SMZ
PASI
LNZ
DAP
Susceptibility rate (%)
Mycobacterium avium (DSM44133)
-
-
-
-
-
+
+
+
+
-
-
-
-
-
-
+
-
-
-
26.32
Mycobacterium intracellulare (ATCC13950)
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
+
94.74
Mycobacterium shimoidei(ATCC27962)
+
+
+
-
-
+
+
+
+
-
-
-
+
-
+
+
+
+
-
63.16
Mycobacterium farcinogenes (ATCC35753)
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
-
+
-
84.21
Mycobacterium simiae (ATCC25275)
-
-
-
-
-
-
+
+
+
-
-
-
-
-
+
+
-
-
-
26.32
Mycobacterium asiaticum (ATCC25276)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
94.74
Mycobacterium scrofulaceum (ATCC19981)
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
94.74
Mycobacterium szulgai (ATCC35799)
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
94.74
Mycobacterium africanum (ATCC35711)
+
+
+
-
+
+
+
-
-
-
+
-
+
+
-
-
-
+
-
52.63
Mycobacterium alvei (DSM44176)
+
+
+
+
+
+
+
+
+
-
+
-
+
+
+
+
-
+
-
78.95
Mycobacterium branderi (ATCC51788)
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
94.74
Mycobacterium celatum (ATCC44243)
-
-
+
+
+
+
+
+
+
-
+
+
+
-
+
+
+
+
+
78.95
Mycobacterium chimaera (DSM44623)
+
+
+
+
+
+
+
+
+
-
-
-
+
-
+
-
-
+
-
63.16
Mycobacterium cosmeticum (DSM44829)
+
-
+
-
+
+
+
+
+
-
+
-
+
+
-
-
-
-
-
52.63
Mycobacterium duvalii (ATCC43910)
+
+
+
+
+
+
+
+
+
-
+
-
+
+
-
+
+
+
-
78.95
Mycobacterium elephantis (DSM44368)
+
-
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
89.47
Mycobacterium hassiacum (DSM44199)
-
-
+
-
+
+
+
+
+
-
+
-
+
-
+
+
-
+
-
57.89
Mycobacterium hiberniae (DSM44241)
+
+
+
+
+
+
+
+
+
-
+
-
+
+
+
+
-
+
-
78.95
Mycobacterium holsaticum (DSM44478)
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
94.74
Mycobacterium houstonense (DSM44676)
+
+
+
+
+
-
+
+
+
-
-
-
-
+
-
-
-
-
-
47.37
Mycobacterium kubicae (DSM44627)
+
+
+
+
+
-
+
+
+
-
-
-
+
+
-
-
-
-
-
52.63
Mycobacterium lentiflavum (DSM44418)
+
+
+
+
+
-
+
+
-
-
-
-
+
+
-
-
-
+
-
52.63
Mycobacterium mageritense (DSM44476)
+
+
+
+
+
-
+
+
+
-
-
-
-
+
-
+
-
-
-
52.63
Mycobacterium nonchromogenicum (DSM44164)
+
+
+
+
+
+
+
+
+
+
-
-
-
+
-
+
-
+
-
68.42
Mycobacterium palustre(DSM44572)
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
-
+
-
84.21
Mycobacterium parascrofulaceum (DSM44648)
+
+
+
+
+
+
+
+
+
-
-
-
+
+
-
+
-
+
-
68.42
Mycobacterium senuense (DSM44999)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
-
89.47
Mycobacterium seoulense (DSM44998)
+
+
+
+
+
-
+
-
+
-
+
-
-
+
-
+
-
+
-
57.89
Mycobacterium thermoresistibile (DSM44167)
+
+
+
+
+
-
+
+
+
+
-
-
+
+
-
+
-
+
-
68.42
Mycobacterium triplex (DSM44626)
+
+
+
+
+
-
+
+
+
+
-
+
+
+
+
+
+
+
+
89.47
Mycobacterium vanbaalenii (DSM7251)
+
+
+
+
+
+
+
-
+
-
-
+
-
+
-
+
-
+
-
63.16
Mycobacterium murale (DSM44340)
+
+
+
+
+
-
+
+
+
+
-
-
+
+
+
+
-
+
-
73.68
Mycobacterium gordonae (ATCC14470)
+
+
+
+
+
-
+
+
+
-
-
-
-
+
+
-
-
+
-
57.89
Note. “+” indicates sensitivity; “−” indicates resistance.Figure 1
The sensitivity profiles of 33 reference slowly growing mycobacteria to 19 antimicrobial agents.Figure 2
The sensitivity profiles of theMycobacterium avium complex to 19 antimicrobial agents.


## 4. Discussion

In this study, 19 antimicrobial susceptibility tests were performed against 33 SGM organisms by a Microplate Alamar Blue Assay. The current first-line drugs for the treatment of nontuberculous mycobacteria are capreomycin, clarithromycin, and rifampin. And the current second-line drugs for the treatment of nontuberculous mycobacteria are moxifloxacin, linezolid, amikacin, ciprofloxacin, ethambutol, isoniazid, rifabutin, streptomycin, and trimethoprim-sulfamethoxazole [7]. Our findings indicated that cephalosporins and macrolides, particularly cefmetazole, azithromycin, clarithromycin, and roxithromycin, showed effective antimicrobial activity against the tested strains.In recent studies [4, 11–15], cefoxitin and meropenem have been reported to show some activity againstMycobacterium abscessus,Mycobacterium chelonae, andMycobacterium fortuitum, whereasMycobacterium kansasii has been shown to be susceptible to clarithromycin and linezolid. Macrolides were active against isolates ofMycobacterium avium [12, 16, 17], and tigecycline has been demonstrated to exhibit high level antimicrobial activity against RGM in vitro [18]. In other studies,Mycobacterium kansasii was reported to be the most susceptible NTM species in vitro [19], andMycobacterium simiae was found to be resistant to clarithromycin, doxycycline, and sulfamethoxazole [20, 21]. However, few studies have tested the activity of cefoperazone, cefmetazole, and cefepime against SGM. In our study, cephalosporins were found to be effective antimicrobial agents and cefmetazole in particular was identified as a good candidate for the treatment of SGM infections. In previous research [15, 22], clarithromycin has been widely used as an antimicrobial agent to SGM, whereas azithromycin and roxithromycin have rarely been tested. Among the tetracyclines, tigecycline was found to be the most effective against SGM. Previous studies have reported thatMycobacterium kansasii was 100% resistant to doxycycline, andMycobacterium simiae isolates were 100% resistant to clarithromycin, doxycycline, and sulfamethoxazole.Mycobacterium avium andMycobacterium intracellulare are important members of the SGM. Macrolides and sulfamethoxazole are recognized as useful drugs againstMycobacterium avium andMycobacterium intracellulare, but rifapentine is ineffective againstMycobacterium avium.Mycobacterium chimaera, a recently described species distinct fromMycobacterium intracellulare, is regarded as less virulent thanMycobacterium intracellulare [23, 24], but neither rifapentine nor rifabutin was effective againstMycobacterium chimaera.Mycobacterium simiae was highly resistant to the tested drugs. It was first isolated from monkeys in 1965 and is now most frequently isolated from human respiratory specimens [25, 26], predominantly being reported in the southwest of the United States and Middle Eastern countries, including Israel and Iran [27].


## 5. Conclusions

Our findings present the drug susceptibility profiles of representative SGM species to a range of antimicrobial agents and provide insight into potentially effective therapeutic strategies. In the future, susceptibility testing of clinical isolates may help to tailor therapeutic strategies to individual patients. Combination therapy should also be explored as a means to increase the efficacy of drug treatment against SGM pathogens. Furthermore, the synergistic activity of some drugs will be analyzed, and drug susceptibility in vivo response must be performed in our recent research.



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*Source: 1584658-2017-04-20.xml*