mycobacterium abscessus is the most pathogenic and chemotherapy - resistant rapidly growing mycobacteria ( rgm ) .
it accounts for 80% of lung disease caused by rgm and is the second most common rgm species present in extrapulmonary disease ( following m. fortuitum ) ( 1 - 4 ) .
m. abscessus is resistant to all antituberculosis drugs including isoniazid , rifampin , ethambutol , and pyrazinamide .
m. abscessus is considered to be susceptible to amikacin , cefoxitin , and clarithromycin , and moderately susceptible to imipenem ( 1 - 4 ) . however , m. abscessus is not an easy organism to treat , and long - term treatment with multiple antimicrobial agents is usually required .
some studies have reported that in vitro susceptibilities to several antimicrobial agents are correlated with clinical responses to treatment in rgm infections ( 5 , 6 ) .
in addition , all clinically significant isolates should be tested against selected antimicrobial agents ( 1 , 2 ) .
in korea , m. abscessus is the second most common etiology of lung disease caused by nontuberculous mycobacteria ( ntm ) ( following the m. avium - intracellulare complex ) .
it accounts for 21 to 33% of ntm lung disease in korea ( 7 - 9 ) ; however , in vitro antimicrobial susceptibility test results for this organism have never been investigated in korean population .
therefore , we examined the in vitro susceptibilities of m. abscessus isolates from patient respiratory specimens that displayed m. abscessus lung disease .
from july 2005 to december 2006 , 74 nonduplicate isolates of m. abscessus ( one isolate per patient ) recovered from respiratory clinical samples were collected for use in this study .
ntm species identification was performed using the polymerase chain reaction - restriction fragment length polymorphism ( pcr - rflp ) method based on the rpob gene ( 10 ) .
all patients met the diagnostic criteria recommended by the american thoracic society for m. abscessus lung disease ( 11 ) .
antimicrobial susceptibility testing was performed at the korean institute of tuberculosis according to the guidelines set forth by the national committee for clinical laboratory standards ( nccls ) ( 12 , 13 ) .
we tested eight antimicrobial agents against m. abscessus isolates : amikacin ( sigma , st .
louis , mo , u.s.a . ) , cefoxitin ( sigma ) , imipenem ( msd , seoul , korea ) , tobramycin ( sigma ) , clarithromycin ( hanmi , seoul , korea ) , ciprofloxacin ( ildong , seoul , korea ) , moxifloxacin ( bayer , seoul , korea ) , and doxycycline ( sigma ) .
minimal inhibitory concentrations ( mics ) of all tested drugs were determined by the broth microdilution method and interpreted according to the guidelines described by the nccls document m24-a in 2003 ( 12 ) .
the mic of moxifloxacin for this rgm has not yet been described by the nccls . in its place
, we followed the recommendation for aerobic organisms in nccls m100-s11 in 2001 ( 13 ) .
the colonies present on the culture medium were transferred to 15-ml tubes of 7h9 broth ( becton dickinson , piscataway , nj , u.s.a . ) with 0.02% tween 80 ( junsei chemical , tokyo , japan ) .
the suspension was mixed vigorously on a vortex mixer for 15 to 20 sec until the turbidity matched the density of a 0.5 mcfarland standard .
the suspension was then diluted a few times following consideration of the final inoculum concentrations in the well .
serial 2-fold dilutions of antimicrobial agents were prepared and added across the 96-well plates ( becton dickinson ) .
the concentrations of antimicrobial agents in the wells were determined based on the nccls recommendation ( 12 , 13 ) . the final inoculum suspension ( 150 l )
was transferred to the wells of a microtiter tray containing 150 l of each antimicrobial type and dilution .
the inoculated tray was covered and incubated at 30 for over 3 days in room air .
endpoint mics of all the antimicrobial drugs were read as the first well in which no growth occurred .
staphylococcus aureus atcc 29213 and mycobacterium peregrinum atcc 700686 were also used for quality control purposes .
the susceptibility categories of all antimicrobial agents were determined according to the breakpoints recommended by the nccls and are presented in table 1 ( 12 , 13 ) .
the results of antimicrobial susceptibility tests are presented in table 2 . of the parenteral antibiotics ,
amikacin ( 99% , 73/74 ) and cefoxitin ( 99% , 73/74 ) were active against most m. abscessus isolates .
amikacin demonstrated better in vitro activity against m. abscessus than tobramycin ( 36% , 27/74 ) .
imipenem ( 55% , 36/74 ) had in vitro activity against a moderate number of isolates .
of the oral antibiotics , clarithromycin ( 91% , 67/74 ) was active against the majority of isolates .
moxifloxacin ( 73% , 54/74 ) had better in vitro activity than ciprofloxacin ( 57% , 42/74 ) .
doxycycline was the least active , inhibiting only 7% ( 5/74 ) of the m. abscessus isolates .
m. abscessus isolates have been determined to be uniformly resistant to standard antituberculous agents . for pulmonary diseases caused by m. abscessus , it is recommended that susceptible oral antibiotics ( including clarithromycin or azithromycin ) be administered in combination with parenteral medications ( amikacin , cefoxitin , or imipenem ) ( 1 , 2 ) . due to its variable in vitro susceptibilities to some drugs , antibiotic susceptibility testing of all clinically significant isolates is recommended , although the correlation between in vitro susceptibility results for m. abscessus and clinical response for specific antimicrobial agents has not been established ( 1 , 2 ) . our results in this study demonstrated that the resistance rates of m. abscessus isolates to various antibiotics are high .
in addition , ciprofoxacin , moxifloxacin , and imipenem exhibited moderate activity ( > 50% susceptibility ) , but the susceptibility of m. abscessus to doxycyline was poor .
traditionally , amikacin was the most active agent against rgm species . according to a study by swenson et al .
, 95% of m. abscessus , 99% of m. fortuitum , and 88% of m. chelonae isolates were inhibited by this agent with a mic of 16 g / ml ( 5 ) .
intravenous amikacin is administered at a dose of 10 to 15 mg / kg daily to adult patients with normal renal function .
this dose elicits peak serum levels in the low 20 mg / ml range . in this study ,
99% of m. abscessus isolates were susceptible to amikacin and 36% were susceptible to tobramycin .
this agent is administered at a dose of up to 12 g / day intravenously in divided doses . in this study ,
this agent has been reported to be moderately susceptible against m. abscessus ( 14 ) .
this study demonstrated that imipenem inhibits 55% of m. abscessus isolates at the breakpoint of < 8 g / ml and 88% of isolates at the intermediate breakpoint of < 16 g / ml . therefore , imipenem may be useful in clinical treatment regimens for m. abscessus , especially in situations when cefoxitin can not be used due to adverse effects .
clarithromycin is representative of this class and has been considered to be the most effective ( 12 ) . in this study , 91% of m. abscessus
brown et al . reported that the newer macrolides generally show better activity against rgm species , and among them , clarithromycin is the strongest ( 15 ) . oral fluoroquinolones have also been used in the treatment of rgm diseases .
ciprofloxacin is the class representative of the older fluoroquinolones ( i.e. , ciprofloxacin , ofloxacin , and levofloxacin ) . in the current study , we tested ciprofloxacin and moxifloxacin ( newer 8-methoxyfluoroquinolone ) , and both antimicrobials showed high activities , with 57 and 73% , respectively , of m. abscessus isolates susceptible . even though fluoroquinolone can not be used as a single - drug therapy due to the risk of developing mutational resistance ( 16 ) , both ciprofloxacin and moxifloxacin could be used as alternative oral agents during combination treatment for m. abscessus lung disease .
doxycycline is a member of the tetracycline antibiotics and has shown poor activity against m. abscessus and m. chelonae in previous studies . in the current study , only 21% of m. abscessus isolates were susceptible or intermediate and approximately 80% were resistant to this agent .
doxycycline was the least active agent among all those tested in this study . in this study
, ntm species identification was performed using the pcr - rflp method based on the rpob gene .
several genes such as 16s rrna , hsp65 , and rpob gene have been used to identify ntm to the species level . however , some gene target is often limited by the lack of sequence divergence among closely related mycobacterium species .
for example , 16s rrna internal transcribed spacer assay can not differentiate m. abscessus from m. massiliense or m. bolletii , although this assay has proven to be valuable to distinguish m. abscessus and m. chelonae ( 17 ) .
accurate identification of isolated ntm species is important since distinguishing these species is clinically relevant . in conclusion ,
the variations in susceptibility within m. abscessus isolates to currently available antimicrobials suggest that the antimicrobial susceptibility testing of all clinically significant m. abscessus isolates be needed .
in addition , our results offer clinicians choices for empirical treatment when m. abscessus lung disease is diagnosed and the in vitro susceptibilities are not available .
 mycobacterium abscessus is the second most common etiology of pulmonary disease caused by nontuberculous mycobacteria in korea . 
 although antimicrobial susceptibility tests are important for appropriate patient management in m. abscessus lung disease , the tests have never been investigated in korea . 
 seventy - four isolates of m. abscessus recovered from patient respiratory samples were tested against eight antimicrobial agents following the guidelines set forth by the national committee for clinical laboratory standards . 
 of the parenteral antibiotics , amikacin ( 99% , 73/74 ) and cefoxitin ( 99% , 73/74 ) were active against most isolates . 
 imipenem ( 55% , 36/66 ) and tobramycin ( 36% , 27/74 ) had activity against moderate number of isolates . 
 of the oral antibiotics , clarithromycin ( 91% , 67/74 ) was active against the majority of isolates . 
 moxifloxacin ( 73% , 54/74 ) and ciprofloxacin ( 57% , 42/74 ) had activity against a moderate number of isolates . 
 doxycycline was the least active , inhibiting only 7% ( 5/74 ) of isolates . in conclusion , 
 the variations in susceptibility within m. abscessus isolates to currently available antimicrobials suggest that the antimicrobial susceptibilities of any clinically significant m. abscessus isolate be needed individually . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION