In recent years, infections caused by multi-drug resistant MRSA have increased all over the world. MRSA strains resistance to various antimicrobials such as fluoroquinolones have led to use of glycopeptide antibiotics as the first and sometimes the only option2. With the reporting of glycopeptide resistance in MRSA infections, it has brought the use of antimicrobials such as linezolid, tigecycline and quinopristin / dalfopristin in treatment16,17.
In this study, a very high rate of fluoroquinolone resistance was found. 71% of 94 MRSA strains were resistant to ciprofloxacin and 64% to moxifloxacin. The fluoroquinolone resistance rate reported for MRSA strains in our country is between 33% and 85.9%; in other countries it ranges from 9.2–85%. Similar to this study, in a study in which Dündar et al.; investigated the antimicrobial susceptibility of S. aureus strains in a 3-year period (2005–2007) and reported ciprofloxacin resistance rates as 87%, 90% and 92%, respectively18.
Linezolid and tigecycline are reported to be highly effective in MRSA strains. Linezolid resistance has been reported to be less than 0.1% in various surveillance programs since linezolid resistance, which was first published in 200119–22. In this study, no resistance to linezolid was found among the MRSA strains. Similar results have been obtained in various studies, too. In a study conducted by Dizbay et al. in 2005 on 120 MRSA strains isolated from various clinical samples, all strains were found to be susceptible to linezolid23. In another study conducted with 1707 MRSA strains between 1997–1999, again, linezolid sensitivity was found to be 100%23. A study conducted in Korea retrospectively examined antibiotic susceptibility tests of a total of 22,067 MRSA isolates over 4 years, and only 110 (0.5%) were found to be resistant to linezolid24.
In various studies, MRSA strains were found to be highly susceptible to tigecycline and resistance was not reported. For example; Arslan et al. investigated tigecycline in 100 MRSA strains isolated from various clinical specimens and linezolid in 80 of them and found all strains susceptible to linezolid and tigecycline4. Similarly, Goff et al. found all strains susceptible to tigecycline and linezolid in a study they conducted between January 2004 and September 2005 on 879 MRSA strains26. Behera et al. found 21 MRSA strains isolated from a hospital in India to be 100% susceptible to tigecycline27. In a study conducted in Malesia, five isolates (5.6%) were found, tigecycline-resistant but they were not linezolid resistance in 90 MRSA28. In this study, 16 (17%) of the MRSA strains were found to be resistant to tigecycline. Similar to this study, in a study by Kaya et al. investigating the in-vitro activity of tigecycline and linezolid in 60 MRSA strains; while they found all strains susceptible to linezolid, they found resistance against tigecycline in 1 strain29. Hoban et al. reported tigecycline sensitivity as 98.9% in a study they conducted with 5348 MRSA strains in 200430. The lower rate of tigecycline resistance in various studies conducted in the past years may be attributed to the resistance of MRSA strains to this antibiotic over time.
In a review article published in 2020, quinupristin/dalfopristin resistance was found as 0.7% (0.3-1%) in MRSA strains31. Additionally, in some studies investigating the susceptibility of MRSA to quinupristin/dalfopristin abroad, the rate of resistance was reported to be between 0–31%13,32,33. Kim et al. did not find resistance to quinopristin/dalfopristin in any of 439 MRSA strains in Korea13. Baddour et al. found that all 512 MRSA strains in Saudi Arabia were susceptible to quinopristin/dalfopristine34. Luh et al. determined this rate as 31% in Taiwan32. In our country, Baysallar et al. and Yavuz et al. found the quinopristin/dalfopristin resistance to be 1% for MRSA strains and it was found as 2.3% by Tünger et al.35,36,37. Kılıç et al. found no resistance in MRSA strains in the study they conducted in 2001 and 2002 while they reported that they found 2% resistance in 20032. In this study, similar to various studies conducted in our country, quinopristin / dalfopristin resistance was found to be 2%.
Although macrolides and lincosamides are used effectively in MRSA infections, they cause problems in treatment due to MLSB resistance detected recently. Among 94 MRSA isolates included in this study, MLSB resistance was determined as 49% and iMLSB as 19%; no resistance was found in 30 MRSA strains (32%). These rates are similar to various studies conducted in our country. For example; In the study conducted by Doğruman et al. on 63 MRSA strains isolated from various clinical samples in Ankara between 2005 and 2006; 32 (50.8%) had cMLSB resistance, 13 (20.6%) had iMLSB resistance and 18 (28.6%) had no resistance14. In the strains included in the study, no strains resistant to erythromycin, susceptible to clindamycin but not inducible resistance (MS phenotype) were detected. In the study conducted by Azap et al. in Ankara, similar to the results of this study, cMLSB resistance was found as 45% and iMLSB resistance was found as 37%38.
Different resistance rates were found in the studies abroad examining MLSB resistance in MRSA infections. Otsuka et al. In Japan, reported that they found iMLSB resistance is 38.7%, cMLSB resistance is 61.3%; Fiebelkorn et al. reported that they found iMLSB resistance as 29.8% and cMLSB resistance as 34.2% in the USA39,40.