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  the story of fusidic acid can be likened to the proverbial saying  the stone the builders rejected has become the cornerstone . 
fusidic acid has been used in europe as an antistaphylococcal agent since the 1960s . with the increasing frequency of methicillin resistant staphylococcus aureus ( mrsa ) worldwide ,
fusidic acid , with favorable pharmacokinetics and pharmacodynamics , has the potential to fill this niche .
it is available in intravenous , oral , and topical preparations and is widely distributed through the body , including areas such as bone , joint fluid , prostate , and abscesses , when given parenterally .
furthermore , it has excellent bioavailability and is active against both methicillin susceptible and resistant staphylococcus and does not show cross - resistance with other antibiotics .  
there have been reports of rapidly increasing fusidic acid resistance in s. aureus in the last decade from centers in countries where it is routinely used [ 57 ] .
the mechanisms of resistance to fusidic acid have been ascribed to proteins encoded by a variety of genes ( e.g. , fusa and fusb ) [ 8 , 9 ] .
these proteins mediate resistance by ( i ) alteration of elongation factor ( chromosomally mediated ) , ( ii ) altering permeability ( plasmid mediated ) , ( iii ) inactivation of enzymes , and ( iv ) efflux of fusidic acid [ 8 , 10 ] .
interestingly , the epidemiology of fusidic acid resistance has been well studied and attributed to inappropriate usage as monotherapy and indiscriminate prescription practices [ 2 , 11 ] .  
a confounding factor in determining resistance rates of fusidic acid is that there are differing standardized minimum inhibitory concentration ( mic ) break points used to classify s. aureus as fusidic acid resistant .
some authors have proposed that isolates with mic  1.0  g / ml are susceptible ( s ) and those with mic  2.0  g / ml are resistant ( r ) while others have proposed an mic  0.5  g / ml as the susceptible breakpoint [ 10 , 12 ] .
most recently jones et al . compared broth dilution , etest mic , and disk diffusion , and they proposed an mic  4.0  g / ml as the interpretive break point for resistance and 1.0  g / ml for susceptibility . for disk diffusion testing , eucast has set the 10  g fusidic acid zone size for resistance at 24  mm , while skov et al .
recently proposed 18  mm for resistance and 21  mm as susceptible interpretive break points [ 10 , 12 ] .  
fusidic acid is a common therapy in saudi arabia for s. aureus infection , but in at least one location mrsa fusidic acid resistance rates approach 96% . in this context ,
our study sought to determine the correlation of disk diffusion zone size and etest mic using different published criteria , in a setting of high s. aureus resistance in riyadh , saudi arabia .
these results will help inform the appropriateness of using only disk diffusion to determine s. aureus susceptibility to fusidic acid .
one hundred and sixty s. aureus clinical specimens consecutively collected from january 1 , 2009 to february 28 , 2009 by the clinical microbiology lab at king khalid university hospital , riyadh , saudi arabia were studied .
s. aureus was identified by colony morphology and the presence of -hemolysis and confirmed by gram stain , and positive catalase and staphaurexplus ( murex biotech ltd , dartford , united kingdom ) reactions .   of the original 160 s. aureus
isolates collected , 122 had complete data for both etest mic and disk diffusion susceptibility testing and were included in the study .
these 122 isolates were recovered from 103 patients with either colonization ( nasal swabs ) or probable infection .
ninety - two patients provided single specimens that grew s. aureus , while 11 patients had multiple specimens ( n = 27 ) with s. aureus accounting for the remaining 30 isolates . since bacterial isolates from the same patient were recovered from independent clinical samples submitted to the microbiology laboratory and received separate susceptibility testing , they were included in the study . isolates from presumed infections were recovered from soft tissue including pus , joint fluid , and blood samples .
this study was approved by the institutional review board of the college of medicine , king saud university .  
mic to fusidic acid were determined using etests ( biomrieux , ab biodisk , solna , sweden ) on mueller - hinton agar incubated for 24  h. disk diffusion zone sizes were determined by direct colony suspension to 0.5 mcfarland , the suspension inoculated to mueller - hinton plates with 10  g fusidic acid disks ( biomerieux , ab biodisk , solna , sweden ) , and the plate was read after incubation for 1620  h at 35c .
a colony suspension equivalent to 0.5 mcfarland was inoculated to mueller - hinton agar with a 30  g cefoxitin disk(oxoid , basingstoke , uk ) and interpreted after 1620  h. mrsa was identified using a breakpoint of 21  mm zone size for cefoxitin disks .
for the oxacillin etests ( ab biodisk , solna , sweden ) , a 0.5 mcfarland direct colony suspension was inoculated to mueller - hinton plates with 2.0% nacl and interpreted after 24  hr incubation .
an isolate with an mic  4.0  g / ml was considered oxacillin resistant .  
categorical data were compared using fisher 's exact test using prism ( graphpad software , la jolla , ca ) . a p value of < 0.05 was considered significant .
the patient population was 42% ( 50/119 ) female and 58% ( 69/119 ) male with no gender listed for three patients .
there were 26.2% ( 32/122 ) isolates from nasal swabs , and 73.8% ( 90/122 ) isolated recovered from presumed sites of infection .
the rate of fusidic acid resistance among all s. aureus isolates was 36.9% ( 45/122 ) using an mic break point of 4.0  g / ml and 40.2% ( 49/122 ) using a break point of 2.0  g / ml for resistance determination ( table 1 ) . using a breakpoint 2.0  g / ml for resistance determination , fusidic acid resistance was significantly higher amongst mrsa isolates at 80.4% ( 37/46 ) compared with mssa 15.8% ( 12/76 ) ( table 1 , p < 0.001 ) .
this high rate of resistance is consistent with previously published data from this institution on fusidic acid resistance amongst mrsa strains . using the same breakpoint of 2.0  g / ml , rates of fusidic acid resistance were lower for isolates recovered from nasal swabs , 18.8% , ( 6/32 ) compared with isolates recovered from other sites , 47.7% ( 43/90 ) ( p < 0.0058 ) .  
regression analysis of a semilog scatterplot revealed a strong correlation ( 83.8% ) between etest determined mic and disk diffusion susceptibility methods ( figure 1 ) .
disk diffusion criteria using zone sizes of 21  mm as susceptible and 18  mm as resistant produced no isolates in the intermediate range for disk diffusion interpretation .
in fact , the smallest zone size to fall in the susceptible range was 25  mm , and the largest zone size in the resistant category was 15  mm , offering a clear dichotomy between susceptible and resistant organisms using the disk diffusion criteria proposed by skov et al .
these disk diffusion results also correlate when the eucast zone size cutoff of 24  mm is applied , with no discrepant isolates comparing the two disk diffusion criteria .
thus , applying either of these disk diffusion criteria to the data gives a fusidic acid resistance rate of 41.0% ( 50/122 ) ( table 2 ) .  
mic criteria were applied ( mic of 0.5  g / ml = s and 2.0  g / ml = r ) the correlation between either disk diffusion cut - offs and etest mic was 100% for susceptible ( 70/70 ) and resistant ( 50/50 ) mic criteria .
two of these isolates with mic = 1.0  g / ml were susceptible by disk diffusion , while one isolate with an mic = 1.0  g / ml was resistant by disk diffusion using either the skov et al . or eucast criteria ( table 2 ) . when the jones et al
. criteria were applied ( mic  1.0  g / ml = s and mic  4.0  g / ml = r ) there was 99% ( 72/73 ) correlation with disk diffusion for susceptible strains .
one isolate with an mic = 1.0  g / ml was classified as resistant by disk diffusion testing ( 12  mm ) but was susceptible when applying the jones et al .
there were four strains in the intermediate range ( mic = 2.0  g / ml ) that were resistant by disk diffusion testing ( zones sizes from 11  mm13  mm ) ( table 2 ) .
for these isolates disk diffusion results best correlated with the application of the skov et al .
careful studies of broth dilution , etest mic determination , and disk diffusion have demonstrated excellent correlation in measuring s. aureus resistance to fusidic acid , but these studies have led to slightly different interpretive criteria for classifying resistance [ 10 , 12 , 13 ] . since broth dilution is not frequently performed in most clinical microbiology laboratories because of its laborious nature , we sought to determine the correlation between etest mic and disk diffusion for measurement of s. aureus resistance to fusidic acid in isolates from an academic hospital in riyadh , saudi arabia .
we analyzed s. aureus recovered from both nasal swabs and potential sites of infection , and our study set contained both msra and mssa isolates .
consistent with previous studies , our data further confirm the strong correlation between disk diffusion and mic regardless of the interpretative criteria used [ 10 , 12 , 13 , 16 ] .  
a high percentage of s. aureus isolates in this study were resistant to fusidic acid ( 36.7%41% ) regardless of which mic or disk diffusion breakpoint criteria was applied , suggesting our data is relevant to areas where fusidic acid resistance is frequently encountered .
however , the isolates were collected over a short - time period and in some cases from the same patient , so it is likely some isolates were clonal .
therefore , the applicability of these results to other institutions depends in part on the local , circulating s. aureus strains . only isolates with mic of either 1.0  g / ml or 2.0  g / ml ( 7/123 , 6% ) gave discrepant results compared with disk diffusion testing .
one isolate with an mic of 1.0 g / ml considered susceptible by the jones et al . criteria and intermediate by the skov et al .
this is the only isolate that also gives a discrepant result between disk diffusion and mic if the mic eucast criteria are applied wherein an mic of 1.0  g / ml is considered susceptible .  
we did not perform broth dilution in this study because it is not a standard procedure in most clinical microbiology laboratories while etest and disk diffusion testing are widespread .
importantly , we identified that disk diffusion testing results correlated well regardless of the mic interpretive criteria applied : eucast ( one discrepancy ) , the criteria of skov et al .
this suggests disk diffusion , regardless of whether the eucast or breakpoints proposed by skov et al .
are applied , is both a cost effective and reliable way to perform initial susceptibility testing of s. aureus to fusidic acid in areas where resistant organisms are frequently encountered .
fusidic acid is a common therapy for staphylococcal infections in saudi arabia , but reports have suggested high rates of resistance among clinical isolates . susceptibility testing of s. aureus to fusidic acid is further complicated by the lack of consensus on mean inhibitory concentrations ( mic ) and disk diffusion cutoffs to determine resistance . 
 the purpose of this study was to determine the correlation between disk diffusion and etest determined mic susceptibility results in clinical isolates of s. aureus from a large academic hospital in riyadh , saudi arabia . 
 our data demonstrate excellent correlation between etest determined mic and disk diffusion susceptibility data , using either previously proposed zone sizes of 21  mm as susceptible and 18  mm as resistant or the eucast recommended zone size of 24  mm for resistance , in an area with relatively high rates of fusidic acid resistance .