macrolide , lincosamide and streptogramin b ( mlsb ) antibiotics are structurally different with a same mechanism of action .
these drugs , especially clindamycin , are alternative drugs for some staphylococcus aureus infections such as skin and soft tissue infections particularly in penicillin - allergic patients ( 1 , 2 ) .
clindamycin resistance in s. aureus strains may be either constitutive ( cmlsb ) or inducible ( imlsb ) .
relapse of infection in a patient with s. aureus endocarditis was reported in 1976 ( 3 ) .
detection of isolates with inducible clindamycin resistance is a problematic issue for the routine diagnosis , because they are erythromycin resistant and clindamycin susceptible in vitro and in the condition that they are not place adjacent to each other . in these cases , in vivo treatment with clindamycin may lead to clinical therapeutic failures ( 4 , 5 ) .
the aim of this study was to determine the incidence of inducible clindamycin resistance in s. aureus isolates collected from clinical specimens in this geographic area .
totally 162 s. aureus isolates were collected from clinical specimens of patients admitted to three university hospitals with 462 , 420 and 300 bed capacities in kerman , south - east of iran , from march 2011 to february 2012 .
confirmation of s. aureus isolates and detection of methicillin resistant s. aureus ( mrsa ) were performed by phenotypic and genotypic methods as previously described ( 6 ) . only one isolate per patient
this test was performed by placing an erythromycin ( 15 g ) disk ( mast , group ltd .
, merseyside , uk ) at a distance of 15 to 20 mm from clindamycin ( 2 g ) disk ( mast , group ltd . ,
merseyside , uk ) on a mueller - hinton agar plate inoculated with 0.5 mcfarland standard bacterial suspensions . following overnight incubation at 37c , flattening of the clindamycin zone adjacent to the erythromycin disk ( d shaped ) indicated inducible clindamycin resistance ( 7 ) .
from 162 s. aureus isolates tested for determination of inducible clindamycin resistance , 70 ( 43.2% ) were mssa and 92 ( 56.8% ) isolates were mrsa ( table 1 ) .
sensitivity to both erythromycin and clindamycin was significantly higher in mrsa compared to mssa isolates .
resistance to methicillin , erythromycin and clindamycin was observed in 92 ( 56.8% ) , 75 ( 46.29% ) and 46 ( 28.4% ) of the isolates , respectively .
inducible resistance to clindamycin was determined in 14 ( 8.64% ) isolates ( d - test positive , figure 1 ) .
table 2 shows the distribution of d - test positive s. aureus isolates in clinical specimens . among those , 11 ( 78.57% )
abbreviations : mssa , methicillin susceptible s. aureus ; mrsa , methicillin resistant s. aureus ; ery , erythromycin ; cli , clindamycin ; s , sensitive , r , resistant ; cmlsb , constitutive macrolide lincosamide and streptogramin b ; imlsb , inducible macrolide lincosamide and streptogramin b. data are presented as no . ( % ) .
flattening of the clindamycin zone adjacent to the erythromycin disk produces a d shape .  
( % ) .   including : blood , skin and soft tissue , cerebrospinal fluid .
clindamycin is an effective antibiotic agent used in the treatment of skin and soft - tissue infections caused by s. aureus isolates .
this antibiotic has good tissue penetration , potential antitoxin effects , and also accumulates in abscesses ( 8) .
however , resistance to clindamycin may develop in s. aureus isolates with inducible phenotype and these isolates have a high rate of spontaneous mutation which would enable these isolates to develop constitutive resistance to clindamycin during therapeutic process ( 4 ) . a positive
d - test indicates the existence of inducible resistance to clindamycin . in this study 8.64% of 162
s. aureus isolates were d - test positive that is comparable with the findings of the studies by rahbar and hajia ( 9.7% ) ( 9 ) and sedighi et al .
the prevalence of inducible and constitutive clindamycin resistance differs in different geographic areas ( 12 ) .   in our study
high number ( 46.29% ) of s. aureus isolates were resistant to erythromycin . among those , 14 ( 18.66% ) isolates had positive and 15 ( 20% ) had negative d - test results and 46 ( 61.33% ) were constitutive clindamycin resistance . in this study the prevalence of inducible and constitutive resistance to clindamycin in mrsa ( 11.95% and 47.8% respectively ) is higher than those in mssa ( 4.28% and 2.85% , respectively ) .
the results of this study revealed that inducible resistance to clindamycin in s. aureus isolates is relatively high in this region .
therefore , d - test should be performed to prevent treatment failures of s. aureus infections caused by the agents that are resistant to erythromycin and sensitive to clindamycin .
 background : clindamycin is a frequently used antimicrobial therapeutic medicine used for the treatment of skin and soft tissue infections caused by staphylococcus aureus strains . resistance to this antibiotic is either constitutive or inducible . 
 constitutive resistance to clindamycin could be detected by standard susceptibility testing methods . 
 inducible clindamycin resistance could not be detected by in vitro routine tests . 
 this type of resistance can be identified by d-test.objectives:the outbreak of inducible resistance to clindamycin in methicillin resistant and - susceptible s. aureus isolates were investigated in this study.materials and methods : totally 162 s. aureus isolates were evaluated for inducible clindamycin resistance by d - test in accordance with clinical and laboratory standards institute ( clsi ) guidelines.results:inducible clindamycin resistance was detected in 8.64% of s. aureus isolates . 
 inducible and constitutive resistance to clindamycin was found to be higher in methicillin resistant s. aureus ( 11.95% and 47.8% respectively ) compared to methicillin susceptible s. aureus ( 4.28% and 2.85% respectively ) isolates.conclusions:our results showed that inducible resistance to clindamycin in s. aureus isolates is relatively higher in this region . 
 therefore , d - test should be performed to prevent treatment failures against infections caused by s. aureus , which are resistant to erythromycin and the sensitive ones against clindamycin . 