Phenotypic detection of inducible clindamycin resistance among Staphylococcus aureus isolates by using the lower limit of recommended inter-disk distance

Context: Clindamycin is one of the important alternative antibiotics in the therapy of Staphylococcus aureus, particularly in methicillin-resistant S. aureus (MRSA) infections. Inducible clindamycin resistance (iMLS B - inducible Macrolide-Lincosamide-Streptogramin B resistance) is a critical factor in antimicrobial susceptibility testing. Aims: To know the rate of inducible clindamycin resistance among clinical isolates of Staphylococcus aureus in our hospital by Disk approximation test (D-test) using the average recommended inter-disk distance and comparing the results with that of D-test using the lower limit of recommended inter-disk distance. Materials and Methods: A total of 51 erythromycin-resistant and clindamycin-susceptible S. aureus isolates were subjected to disk approximation testing with 21 +/- 1 mm and 15 mm edge-to-edge distance between the clindamycin and erythromycin disks. Statistical Methods: Z-test levels. Results: Among 51 erythromycin-resistant and clindamycin-susceptible S. aureus isolates, 25 (49%) were recorded as inducible clindamycin resistant by D-test with 21 +/- 1 mm edge-to-edge distance between the clindamycin and erythromycin disks. When we re-tested all the 51 strains by D-test with 15 mm inter-disk distance, we identified 14% more iMLS B strains previously reported as D-test negative. Z-test for MRSA indicates that 15 mm edge-to-edge distance has significant advantage. Conclusions: Since the incidence of inducible clindamycin resistance is high (63% in our study), accurate identification of inducible clindamycin resistance is important to prevent therapeutic failure in infections caused by these strains. We suggest the use of D-test with 15 mm edge-to-edge inter-disk distance for detecting iMLS B .     

Inducible clindamycin resistance in Staphylococcus aureus isolated from clinical samples

Purpose: Clindamycin is commonly used in the treatment of erythromycin resistant Staphylococcus aureus causing skin and soft tissue infections. In vitro routine tests for clindamycin susceptibility may fail to detect inducible clindamycin resistance due to erm genes resulting in treatment failure, thus necessitating the need to detect such resistance by a simple D test on routine basis. Materials and Method: 247 Staphylococcus aureus isolates were subjected to routine antibiotic susceptibility testing including oxacillin (1μg) by Kirby Bauer disc diffusion method. Inducible clindamycin resistance was detected by D test, as per CLSI guidelines on erythromycin resistant isolates. Results: 36 (14.5%) isolates showed inducible clindamycin resistance, nine (3.6%) showed constitutive resistance while remaining 35 (14.1%) showed MS phenotype. Inducible resistance and MS phenotype were found to be higher in MRSA as compared to MSSA (27.6%, 24.3% and 1.6%, 4% respectively). Conclusion: Study showed that D test should be used as a mandatory method in routine disc diffusion testing to detect inducible clindamycin resistance.     

Detection of various types of resistance patterns and their correlation with minimal inhibitory concentrations against clindamycin among methicillin-resistant Staphylococcus aureus isolates

Purpose: The macrolide lincosamide streptogramin B (MLS_B) family of antibiotics serves as an alternative for the treatment of skin and soft tissue infections caused by methicillin-resistant Staphylococcus aureus (MRSA). However, resistance to clindamycin too has emerged, which is of two types, inducible and constitutive. Therapeutic failure is common with inducible type of clindamycin resistance. This study was done to determine the various clindamycin resistance patterns in MRSA isolates and to compare them with minimal inhibitory concentration (MIC) of clindamycin. Materials and Methods: Fifty MRSA isolates were studied by disc approximation test (D test) to detect inducible iMLS_B resistance and MIC by agar dilution technique. Results: Of the 50 isolates, 34 were sensitive to both clindamycin and erythromycin. 16 isolates showed different sensitivity patterns; nine of these were positive for D zone indicating inducible iMLS_B resistance, five were positive for constitutive MLS_B resistance and two showed possible efflux mechanism for macrolide resistance. Out of the 34 sensitive isolates, 5 showed isolated colonies (subpopulation) inside the clindamycin-sensitive zone. When these sub-populations were tested further, two were constitutive MLS_B phenotypes, two were inducible iMLS_B and one was HD (hazy D zone), which is D⁺ with growth up to clindamycin disc (which is also considered as constitutive MLS_B phenotype). Seven isolates showed an MIC of ≥4 μg/ml to clindamycin in spite of being susceptible to both erythromycin and clindamycin by Kirby Bauer disc diffusion technique. Out of these seven isolates, five were those which grew as subpopulation inside the clindamycin-sensitive zone. Conclusion: Detection of iMLS_B resistance among MRSA helps to avoid treatment failure with clindamycin. Studying the subpopulation inside the clindamycin-sensitive zone raises the question of existence of hetero-resistance or some other mechanism, which needs further study.     

Prevalence of inducible clindamycin resistance in staphylococcal isolates at a Korean tertiary care hospital

Clindamycin resistance in Staphylococcus species can be either constitutive or inducible. Inducible resistance cannot be detected by the conventional antimicrobial susceptibility test. In this study, we determined the prevalence of inducible clindamycin resistance in staphylococcal isolates at a Korean tertiary care hospital. Between February and September 2004, 1,519 isolates of Staphylococcus aureus and 1,043 isolates of coagulase-negative staphylococci (CNS) were tested for inducible resistance by the D-zone test. Overall, 17% of MRSA, 84% of MSSA, 37% of MRCNS, and 70% of MSCNS were susceptible to clindamycin. Of the erythromycin non-susceptible, clindamycin-susceptible isolates, 32% of MRSA, 35% of MSSA, 90% of MRCNS, and 94% of MSCNS had inducible clindamycin resistance. Inducible clindamycin resistance in staphylococci was highly prevalent in Korea. This study indicates importance of the D-zone test in detecting inducible clindamycin resistance in staphylococci to aid in the optimal treatment of patients.     

Prevalence and mechanisms of erythromycin resistance in group A and group B Streptococcus: implications for reporting susceptibility results

Increased rates of erythromycin resistance among group B Streptococcus (GBS) and group A Streptococcus (GAS) have been reported. Cross-resistance to clindamycin may be present, depending on the mechanism of resistance. We determined the prevalence of macrolide-resistant determinants in GBS and GAS isolates to guide the laboratory reporting of erythromycin and clindamycin susceptibility. Susceptibilities were determined by the disk diffusion and broth microdilution methods. Inducible and constitutive resistance to clindamycin was determined by the double-disk diffusion method. The presence of the ermTR, ermB, and mefA genes was confirmed by PCR. Of the 338 GBS isolates, 55 (17%) were resistant to erythromycin, whereas 26 (8%) were resistant to clindamycin. The erm methylase gene was identified in 48 isolates, 22 of which had inducible resistance to clindamycin and 26 of which had constitutive resistance to clindamycin. The remaining seven resistant isolates had mefA. Of the 593 GAS isolates, 49 (8%) and 6 (1%) isolates were resistant to erythromycin and clindamycin, respectively. Erythromycin resistance was due to mefA in 33 isolates, whereas 14 isolates had erm-mediated resistance (9 isolates had inducible resistance and 5 isolates had constitutive resistance). In our population, erythromycin resistance in GAS was predominantly mediated by mefA and erythromycin resistance in GBS was predominantly mediated by erm. Regional differences in mechanisms of resistance need to be taken into consideration when deciding whether to report clindamycin susceptibility results on the basis of in vitro test results. Testing by the double-disk diffusion method would be an approach that could be used to address this issue, especially for GAS.     

Incidence of Inducible Clindamycin Resistance in Staphylococcus pseudintermedius from Dogs

Clindamycin is increasingly used to treat canine pyoderma. Eight of 608 Staphylococcus pseudintermedius isolates were positive for inducible clindamycin resistance by double-disk diffusion testing and PCR detection of ermB. Staphylococcus pseudintermedius isolates that are erythromycin resistant but clindamycin susceptible by in vitro antimicrobial susceptibility testing should be tested for inducible clindamycin resistance.     

Antimicrobial susceptibility and survey of macrolide resistance mechanisms among Streptococcus pyogenes isolated in Rio de Janeiro,Brazil

A total of 357 clinical Streptococcus pyogenes isolates collected between 1994 and 1999 in Rio de Janeiro city were tested for susceptibility to 10 antimicrobial drugs by agar-diffusion tests. All isolates were susceptible to penicillin, cephems, and vancomycin. High resistance rates were observed for tetracycline (43.1%) and trimethoprim/sulfamethoxazole (77.9%). Three isolates (0.8%) were resistant to erythromycin, and three exhibited intermediate susceptibility. Determination of the erythromycin MICs by the agar dilution method, showed 1.6% of erythromycin resistant isolates (the three erythromycin-resistant and the three erythromycin-intermediate isolates found by agar-diffusion test). Of the erythromycin-resistant isolates subjected to the double-disc diffusion test for erythromycin and clindamycin, three isolates expressed the iMLSB and three the M phenotype. The resistance phenotypes were confirmed by comparing the clindamycin MICs determined under normal testing conditions and those determined after induction by pre-growth in 0.06 microg/ml of erythromycin. Three ermTR and three mefA-containing isolates were detected by PCR. In strains belonging to the iMLSB phenotype, two clones were identified by PFGE following restriction with SmaI. M phenotype isolates could not be restricted with SmaI. Our results indicate a low rate of erythromycin resistance among S. pyogenes isolated in Rio de Janeiro, Brazil, and pointed to the presence of both resistance mechanisms found in streptococci.     

Macrolide resistance in beta-hemolytic streptococci: changes after the implementation of the separation of prescribing and dispensing of medications policy in Korea

This study evaluated the antimicrobial susceptibilities and macrolide resistance mechanisms of beta-hemolytic streptococci (BHS), and an additional objective was to assess the effects of 'the separation of prescribing and dispensing (SPD) of medications' on bacterial resistance rate and distribution of phenotypes and genotypes of erythromycin-resistant BHS by comparing the antimicrobial susceptibility data before (1990- 2000) and after the implementation of SPD at one tertiary care hospital in South Korea. Between the period of January 2001 and December 2002, the minimal inhibitory concentrations of six antimicrobials were determined for 249 clinical isolates of BHS. Resistance mechanisms of erythromycin-resistant (intermediate and resistant) isolates were studied by using the double disk test and PCR. Overall, the resistance rates to tetracycline, erythromycin, and clindamycin were 75.5%, 32.9%, and 32.5%, respectively. Sixty-seven (81.7%) of 82 erythromycin- resistant isolates expressed constitutive resistance to macrolide- lincosamide-streptogramin B antibiotics (a constitutive MLSB phenotype); 11 isolates (13.4%) expressed an M phenotype; and four isolates (4.9%) had an inducible MLSB resistance phenotype. erm(A) was found in isolates with constitutive/ inducible MLSB phenotypes, erm(B) with the constitutive/ inducible MLSB phenotype, and mef(A) with the M phenotype. We found that resistance rates to erythromycin and clindamycin among S. agalactiae, S. pyogenes, and group C streptococci isolates were still high after the implementation of the SPD policy in Korea, and that the constitutive MLSB resistance phenotype was dominant among erythromycin- resistant BHS in this Korean hospital.     

Significance of MRSA strains in community associated skin and soft tissue infections

The study was conducted to determine the antibiotic susceptibility profile of community-associated methicillin resistant Staphylococcus aureus (CAMRSA) strains isolated from infections. S. aureus strains were isolated from clinical specimens using the standard procedures. CDC definition was used to classify CAMRSA. Antibiotic susceptibility test was done using Kirby-Bauer disk diffusion method. Double disk diffusion method (D-test) was used to detect inducible macrolide, lincosamide and streptogramin_B resistance (inducible MLS_B resistance). A total of 83 CAMRSA were isolated from abscesses and other skin infections in persons without known risk factors for MRSA infection. All CAMRSA were susceptible to vancomycin. Out of 83 CAMRSA, 13 (15.65%) were D-test positive (inducible MLS_B positive) and 6 (7.23%) were positive for constitutive MLS_B resistance. Eight strains (9.63%) were resistant to tetracycline and 26 (31.32%) strains were resistant to erythromycin. Increased rate of inducible clindamycin resistance among CAMRSA indicates the importance of identification of such strains by D test to avoid treatment failure when clindamycin is used.     

Antibiotic susceptibility in prostate-derived Propionibacterium acnes isolates

The aim of this study was to determine antibiotic susceptibility of Propionibacterium acnes isolates from prostate. Prostate‐derived P. acnes isolates (n = 24, Umeå & Örebro, Sweden, 2007–2010) and a panel of control strains (n = 25, Sweden) collected from skin and deep infections were assessed for resistance to penicillin G, piperacillin–tazobactam, imipenem, gentamicin, azithromycin, erythromycin, vancomycin, ciprofloxacin, moxifloxacin, tetracycline, tigecycline, fusidic acid, clindamycin, rifampicin, linezolid, daptomycin, trimethoprim–sulfamethoxazole, and metronidazole. In addition, the isolates were tested for inducible clindamycin resistance. All prostate derived P. acnes isolates displayed wild‐type distribution of MIC‐values, without evidence of acquired resistance. In the reference panel, 5 of 25 isolates had acquired macrolide resistance with cross‐resistance to azithromycin, clindamycin, and erythromycin. In addition, one of these isolates was resistant to tetracycline.     

Emergence of macrolide resistant Streptococcus pyogenes strains in pediatric patients in France

A total of 206 recent throat isolates of Streptococcus pyogenes collected between 2002 and 2004 from children were tested for their susceptibility to penicillin, amoxycillin, erythromycin, clarythromycin and clindamycin. The erythromycin resistant isolates were further studied for their genetic mechanism of resistance by means of PCR. In all, 14.5% of the strains were erythromycin resistant and 13.5 and 1% expressed the constitutive MLS(B) and M resistance phenotypes and harbored the ermB and mef A genes respectively.     

Molecular characterization and antibiotic susceptibility of Staphylococcus aureus from a multidisciplinary hospital in Romania

From 2004 to 2005, 60%-72% of invasive Staphylococcus aureus isolates from Romanian hospitals were resistant to methicillin (methicillin-resistant S. aureus [MRSA]), the highest frequency for any European nation. Few reports, however, have addressed the molecular characteristics of S. aureus in Romania. In this study, we utilized spa typing, multilocus sequence typing, staphylococcal cassette chromosome mec (SCCmec) typing, dru typing, pulsed-field gel electrophoresis, and detection of virulence factors to characterize 146 S. aureus strains isolated from 2004 to 2005 at the Clinic County Hospital in Bra?ov. Antibiotic susceptibility patterns for all MRSA isolates and patient demographic data were also obtained. Fifty-six strains (38.4%) were determined to be MRSA by susceptibility testing and SCCmec typing. All MRSA strains were resistant to beta-lactams and tetracycline, but susceptible to nitrofurans, vancomycin, and clindamycin, with inducible clindamycin resistance in 23/28 clindamycin-sensitive/erythromycin-resistant isolates. Molecular typing identified 15 clonal backgrounds (CC 1, 5, 8, 8/239, 9, 15, 20, 22, 25, 30, 45, 80, 97, 101, and 121), only 4 of which were associated with MRSA (CC 1, 8/239, 30, and 80). Spa types 35 (t127, CC 1) and 351 (t030, CC 8/239) accounted for 27.4% and 21.9% of all S. aureus strains, respectively, and 19.6% and 57.1% of all MRSA strains. Both hospital-associated (SCCmec type III) and community-associated (SCCmec type IV) elements were identified within MRSA strains, whereas Panton-Valentine leukocidin was detected in 10 MRSA and 12 methicillin-sensitive S. aureus strains. These results demonstrate the presence of various endemic S. aureus clones within the Clinic County Hospital in Bra?ov, suggestive of ongoing nosocomial and community transmission.     

Practical disk diffusion method for detection of inducible clindamycin resistance in Staphylococcus aureus and coagulase-negative staphylococci

Resistance to macrolides in staphylococci may be due to active efflux (encoded by msrA) or ribosomal target modification (macrolide-lincosamide-streptogramin B [MLSB] resistance; usually encoded by ermA or ermC). MLSB resistance is either constitutive or inducible following exposure to a macrolide. Induction tests utilize closely approximated erythromycin and clindamycin disks; the flattening of the clindamycin zone adjacent to the erythromycin disk indicates inducible MLSB resistance. The present study reassessed the reliability of placing erythromycin and clindamycin disks in adjacent positions (26 to 28 mm apart) in a standard disk dispenser, compared to distances of 15 or 20 mm. A group of 130 clinical isolates of Staphylococcus aureus and 100 isolates of erythromycin-resistant coagulase-negative staphylococci (CNS) were examined by disk approximation; all CNS isolates and a subset of S. aureus isolates were examined by PCR for ermA, ermC, and msrA. Of 114 erythromycin-resistant S. aureus isolates, 39 demonstrated constitutive resistance to clindamycin, while 33 showed inducible resistance by disk approximation at all three distances. Only one isolate failed to clearly demonstrate induction at 26 mm. Of 82 erythromycin-resistant CNS isolates that contained ermA or ermC, 57 demonstrated constitutive clindamycin resistance, and 25 demonstrated inducible resistance, at 20 and 26 mm. None of the 42 S. aureus isolates or 18 CNS isolates containing only msrA and none of the erythromycin-susceptible isolates yielded positive disk approximation tests. Simple placement of erythromycin and clindamycin disks at a distance achieved with a standard disk dispenser allowed detection of 97% of S. aureus strains and 100% of CNS strains with inducible MLSB resistance in this study.     

Detection of multiple macrolide- and lincosamide-resistant strains of Streptococcus pyogenes from patients in the Boston area

Macrolide (including erythromycin and azithromycin) and lincosamide (including clindamycin) antibiotics are recommended for treatment of penicillin-allergic patients with Streptococcus pyogenes pharyngitis. Resistance to erythromycin in S. pyogenes can be as high as 48% in specific populations in the United States. Macrolide and lincosamide resistance in S. pyogenes is mediated by several different genes. Expression of the erm(A) or erm(B) genes causes resistance to erythromycin and inducible or constitutive resistance to clindamycin, respectively, whereas expression of the mef(A) gene leads to resistance to erythromycin but not clindamycin. We studied the resistance of S. pyogenes to erythromycin and clindamycin at an urban tertiary-care hospital. Of 196 sequential isolates from throat cultures, 15 (7.7%) were resistant to erythromycin. Three of these were also constitutively resistant to clindamycin and had the erm(B) gene. Five of the erythromycin-resistant isolates were resistant to clindamycin upon induction with erythromycin and had the erm(A) gene. The remaining seven erythromycin-resistant isolates were susceptible to clindamycin even upon induction with erythromycin and had the mef(A) gene. Pulsed-field gel electrophoresis analysis and emm typing demonstrated that the erythromycin-resistant S. pyogenes comprised multiple strains. These results demonstrate that multiple mechanisms of resistance to macrolide and lincosamide antibiotics are present in S. pyogenes strains in the United States.     

Susceptibility of strains of Streptococcus agalactiae to macrolides and lincosamides, phenotype patterns and resistance genes

The Group B streptococcus ( Streptococcus agalactiae ) is a pathogen of increasing importance in human disease. We therefore studied the susceptibility of clinical isolates of S. agalactiae to penicillin G, erythromycin, azithromycin and clindamycin using National Committee for Clinical Laboratory Standards methodology, and we also determined the phenotypes of macrolide-lincosamide susceptibility and the resistance genes implicated in a group of selected isolates of the different phenotypes. We used 221 isolates collected between 1997 and 1999 in two Health Authority Areas in Móstoles and Granada, Spain. The minimal concentration for 90% inhibition (MIC₉₀) for penicillin G was 0.12 mg/L and all the isolates tested were susceptible. One hundred and eighty-five (83.7%) were susceptible to erythromycin and azithromycin and 191 (86.4%) were susceptible to miocamycin and clindamycin. Twenty-three isolates (10.4%) had a constitutive MLS_B phenotype, seven (3.2%) an inducible phenotype, and six (2.7%) an M phenotype. All except one of the MLS_B phenotype isolates tested ( n  = 23) carried erm genes; in two strains with the mef (A) gene, all the M phenotype ( n  = 6) isolates tested carried mef genes, while erm and mef (A) genes were absent in all the macrolide-lincosamide-susceptible ( n  = 12) isolates tested. In our environment, resistance to macrolide and lincosamide in S. agalactiae was present in 10–16% of the isolates. The majority of resistant strains had the MLS_B phenotype.     

Emergence ofStreptococcus pyogenes strains resistant to erythromycin in Gipuzkoa, Spain

The aim of this study was to determine the evolution of resistance to macrolides and other antibiotics in strains ofStreptococcus pyogenes isolated in the province of Gipuzkoa, Spain. During the period 1984–1996, all 2561 strains ofStreptococcus pyogenes studied showed full susceptibility to penicillin. Until 1990, only 1.2% ofStreptococcus pyogenes isolates were resistant to erythromycin. Since then, resistance to erythromycin increased every year until 1995, when 34.8% (87/250) ofStreptococcus pyogenes strains were found to be resistant. In 1996 the rate of resistance to erythromycin was 17.8% (75/422). During the study period, 96.1% (246/256) of theStreptococcus pyogenes isolates resistant to erythromycin were susceptible to clindamycin. Of the remaining erythromycin-resistantStreptococcus pyogenes strains, resistance to clindamycin was constitutive in seven strains and inducible in three. When investigated by the polymerase chain reaction (PCR), allStreptococcus pyogenes strains resistant to erythromycin and susceptible to clindamycin showed the 1.4 kb fragment of themefA gene, recently described as the novel macrolide-efflux-resistance determinant. The most frequent T-agglutination patterns amongStreptococcus pyogenes resistant to erythromycin were T4 and T8,25. The emergence and rapid spread of erythromycin-resistantStreptococcus pyogenes in Gipuzkoa and its relationship to the presence of themefA gene are described.     

苏州地区耐甲氧西林金黄色葡萄球菌的临床分布特点及耐药性分析

目的 探讨苏州高新区人民医院耐甲氧西林金黄色葡萄球菌(MRSA)的临床分布特点及其耐药性,为临床治疗葡萄球菌属感染提供正确选药依据.方法 对2015年1月至2017年8月苏州高新区人民医院临床各科送检的各类标本中分离鉴定出的MRSA,采用MIC法进行药敏试验,并探讨其临床分布与耐药特点.结果 分离出的170株金黄色葡萄球菌中,MRSA有40株,占23.5%,以呼吸科病房为最多(20株),占50.0%、其次是重症监护病房(ICU)、儿科、神经内科;多重耐药的MRSA对所有β-内酰胺类高度耐药,对大环内酯类耐药率为67.5%;对克林霉素类耐药性较高,其中红霉素诱导的克林霉素耐药率为47.5%,提示该药在苏州地区已不能够用于MRSA 感染的治疗.对氨基糖苷类、氟喹诺酮类耐药率较低(耐药率<30%),磺胺甲噁唑、甲氧苄啶及利福平具有良好的活性,尚未发现耐糖肽类和利萘唑胺的菌株.结论苏州地区近3年来MRSA菌株检出率较低;对常用抗菌素呈多重耐药,尤其是对大环内酯-克林霉素抗菌药物耐药率高.因此应加强对MRSA耐药率的监测,合理选抗菌药物,从而减少MRSA 的产生和传播.     

Prevalence of erythromycin and clindamycin resistance among Streptococcus agalactiae isolates in Germany

The antimicrobial susceptibilities of 338 clinical Streptococcus agalactiae isolates from two geographical regions in Germany were determined by agar dilution. All isolates were susceptible to penicillin, cefotaxime and vancomycin. The overall frequencies of erythromycin and clindamycin resistance were 11% and 4.7%, respectively. Determination of resistance phenotypes among the 37 erythromycin-resistant isolates revealed constitutive and inducible MLS(B) resistance in 40.6% and 37.8% of isolates, respectively, and susceptibility to clindamycin in 21.6% of isolates. Only 14.3% of isolates with inducible MLS(B) resistance were identified as clindamycin-resistant by determination of clindamycin MICs. Pulsed-field gel electrophoresis suggested a clonal distribution pattern among the erythromycin-resistant isolates.     

Incidence of inducible clindamycin resistance in staphylococci: first results from Turkey

In total, 408 staphylococcal isolates were tested for inducible clindamycin resistance (ICR) by the disk-diffusion induction test (D-test). ICR was detected in 5.7% of 105 methicillin-resistant Staphylococcus aureus (MRSA) isolates, 3.6% of 111 methicillin-susceptible S. aureus isolates, 30.8% of 94 methicillin-resistant coagulase-negative staphylococcal (CoNS) isolates, and 11.2% of 98 methicillin-sensitive CoNS isolates. All MRSA isolates that were erythromycin-resistant and clindamycin-susceptible were positive by the D-test. The same results were obtained with an azithromycin instead of an erythromycin disk. All isolates were susceptible to quinupristin-dalfopristin. The cost-benefit of the d-test should be evaluated locally after determining the incidence of the different resistance phenotypes.     

A high frequency of macrolide-lincosamide-streptogramin resistance determinants in Staphylococcus aureus isolated in South Korea

Genes conferring resistance to one of the macrolide-lincosamide-streptogramin (MLS) antibiotics may confer cross-resistance to others, because they have similar effects on bacterial protein synthesis. In Korea, over 70% of Staphylococcus aureus isolates are methicillin-resistant and erythromycin-resistant methicillin-resistant S. aureus (MRSA) is also prevalent. We investigated the frequency of MLS resistance in erythromycin-resistant S. aureus isolates. A total of 682 isolates of S. aureus were collected in a nationwide antibiotic resistance survey. Susceptibility to erythromycin, clindamycin, and quinupristin/dalfopristin was tested by disk diffusion. In all, 37% of the methicillin-susceptible S. aureus (MSSA) and 97% of the MRSA isolates were resistant to at least one of the MLS antibiotics, whereas all were susceptible to quinupristin/dalfopristin. Out of 518 strains that were resistant to erythromycin, 60 clindamycin-susceptible (30 MSSA, 30 MRSA) and 44 clindamycin-resistant isolates (14 MSSA, 30 MRSA) were selected at random from these strains. Thirteen genes related to MLS resistance were detected in these isolates by PCR. Of the 104 MSSA and MRSA strains tested, 98 harbored one or more erm gene. The most common was erm(A), with erm(C) next. But, msr(A), lnu(A), and mef(A) were rare and no resistance to streptogramin A was encountered.