Methicillin-resistant Staphylococcus aureus (MRSA) is a notorious nosocomial pathogen prevalent in many countries. A study by the Korean Nationwide Surveillance of Antimicrobial Resistance (KONSAR) program showed that 68% of S. aureus isolates in 2003 were methicillin-resistant.1 Vancomycin has been used increasingly to treat MRSA infections. Dissemination of vancomycin-resistant enterococci was considered to be partly due to increased vancomycin use. Rapid increase in vancomycin resistance necessitates the restriction of vancomycin usage, as well as encourages treatment with older antimicrobial agents, such as trimethoprim-sulfamethoxazole and clindamycin. Clindamycin, a lincosamide antibiotic active against gram-positive microorganisms including staphylococci and streptococci, inhibits bacterial protein synthesis. It can be administered orally to treat mild infections in children or soft tissue infections.2-4 The clindamycin resistance mechanism is primarily due to ribosomal modification by methylases encoded by erm genes. Methylation of 23S rRNA decreases the affinity for clindamycin, all macrolides, and type B streptogramins (the MLS_B phenotype).5 Some of the enzymes are constitutively regulated, while others are inducibly regulated by translational attenuation of a mRNA leader sequence. In the absence of erythromycin, the mRNA is in an inactive conformation due to a sequestered Shine-Dalgarno sequence, preventing the efficient initiation of translation of erm transcripts.6

Constitutive resistance can be readily detected, but inducible resistance is not detectable by routine antimicrobial susceptibility tests.7,8 The Clinical and Laboratory Standards Institute (CLSI) recommends testing for inducible clindamycin resistance in isolates of staphylococci by using a D-zone test.9 This test is important for optimal treatment of patients, but the prevalence of inducible clindamycin resistance has not yet been reported in Korea. Aim of this study was to determine the prevalence of inducible clindamycin resistance in S. aureus and coagulase-negative staphylococci (CNS) isolated from patients in a tertiary care hospital in Korea.

Between February and September 2004, non-duplicate S. aureus and CNS were isolated from patients in a tertiary care university hospital in Korea. The species were identified by conventional methods using a coagulase tube, mannitol-salt agar, and DNase agar or by using the Vitek GPI card system (bioMerieux, Marcy l'Etoile, France). Antimicrobial susceptibilities were determined by the CLSI disk diffusion method.9 To detect inducible clindamycin resistance, the D-zone test was performed. A staphylococcal suspension equivalent to 0.5 McFarland turbidity was used to inoculate a Mueller-Hinton agar (MHA) plate. Then, 2-µg clindamycin and 15-µg erythromycin disks (Becton-Dickinson Microbiology Systems, Cockeysville, MA, USA) were placed 15 mm apart (margin to margin).10 After an 18-hour incubation at 35℃, a D shaped blunting of the clindamycin disk inhibition zone adjacent to the erythromycin disk was interpreted as positive.

Overall, the antimicrobial resistance rates of S. aureus and CNS in 2004 at the tertiary care hospital were 59% and 54% to oxacillin (data not shown). The resistance rates of MRSA and methicillin-susceptible Staphylococcus aureus (MSSA) were as follows: 77% and 4% to clindamycin, 90% and 26% to erythromycin, 22% and 1% to cotrimoxazole, 66% and 14% to tetracycline, and 84% and 10% to fluoroquinolone, respectively. Resistance rates of CNS are shown in Table 1.

The percentages of strains with constitutive and inducible clindamycin resistance were as follows: 79% and 4% of MRSA, 6% and 9% of MSSA, 33% and 30% of MRCNS, and 9% and 21% of methicillin-susceptible coagulase-negative staphylococci (MSCNS), respectively (Table 2). Of the erythromycin non-susceptible but clindamycin-susceptible isolates, 32% of MRSA, 35% of MSSA, 90% of MRCNS, and 94% of MSCNS were inducibly clindamycin resistant.

In the present study, the resistance rates of methicillin-susceptible staphylococci to clindamycin, erythromycin, tetracycline, and fluoroquinolone were much lower than those of methicillin-resistant isolates. However, in general, tetracyclines and fluoroquinolones are not recommended for the treatment of pediatric patients and pregnant women because of possible side effects. It is noteworthy that clindamycin susceptible rates were higher than those of erythromycin, regardless of methicillin susceptibility.

Clindamycin is indicated for the treatment of soft tissue infections, pediatric infections caused by staphylococci, or for patients allergic to β-lactam agents.2-4 Inducible clindamycin-resistant staphylococci show susceptible results in conventional susceptibility tests, but can be converted to a constitutively resistant phenotype during clindamycin treatment.11,12 As the resistance conversion may result in clindamycin treatment failure,13,14 detection of inducible clindamycin resistance is necessary.9 Inducible clindamycin resistance can be detected only by the D-zone test.7,8 When a D-zone test shows a distorted zone of inhibition around a clindamycin disk by erythromycin, the isolate is considered to be inducible clindamycin resistance.

Possible variations in the prevalence of constitutive and inducible clindamycin resistance have been reported depending on regional and bacterial species.11,14,15 In our study, the rate of inducible clindamycin resistance in erythromycin non-susceptible and clindamycin-susceptible staphylococcal isolates was 63% (data not shown); this was similar to 62% in Iowa11 and 56% in Maryland14 in the USA. Schreckenberger reported that inducible clindamycin resistance was more prevalent in MRSA.15 However, in our study; inducibly clindamycin-resistant strains were more prevalent in CNS (91%) than in MRSA (32%). Our data indicate that if the D-zone test is not performed, 32% of MRSA, 35% of MSSA, 90% of MRCNS, and 94% of MSCNS isolates with an erythromycin non-susceptible and clindamycin-susceptible pattern are mistakenly interpreted as clindamycin susceptible, possibly resulting in treatment failure.

Almer reported that inducible clindamycin resistance in CA-MRSA was relatively prevalent (28%).16 In this study, there were no significant differences between inpatient and outpatient incidences of inducible clindamycin resistance of staphylococci, except for MRSA. The higher prevalence of inducible clindamycin resistance in outpatient isolated MRSA is not clear, but it is possible that the isolates are, in reality, hospital-associated strains.

In summary, 32-35% of erythromycin non-susceptible and clindamycin-susceptible S. aureus and 90-94% of erythromycin non-susceptible and clindamycin-susceptible CNS showed inducible resistance to clindamycin. This study indicates the importance of the D-zone test to differentiate inducibly clindamycin-resistant isolates of staphylococci to facilitate the optimal treatment of patients.