Comparative activity of tedizolid against clinical isolates of linezolid-resistant coagulase-negative staphylococci and methicillin-resistant Staphylococcus aureus

Tedizolid phosphate is an antibiotic from the oxazolidinone group that was recently approved for the treatment of acute bacterial skin and soft-tissue infections caused by Gram-positive cocci.¹ This second-generation drug has higher activity in vitro than its analogue linezolid.^2–4 Although the breakpoints for susceptibility to tedizolid against Staphylococcus were recently established,⁵ its activity against linezolid-resistant (LR) isolates has not been well studied. Our objective was to evaluate in vitro the susceptibility to tedizolid of methicillin- and linezolid-resistant coagulase-negative Staphylococcus (LR-CNS) and methicillin-resistant Staphylococcus aureus (MRSA). Our secondary objective was to compare the susceptibility of our isolates to other antibiotics used as an alternative in MRSA infections.

Susceptibility in vitro to tedizolid was studied in 22 LR-CNS and 33 MRSA isolates in the Department of Microbiology at Hospital Universitario San Cecilio in Granada, Spain, from 2007. Nine of the LR-CNS isolates were recovered from a previous study conducted by Sorlozano et al.⁶ Identification was performed using MALDI-TOF (Bruker Daltonics^®, Germany) and all isolates underwent a study of the minimum inhibitory concentration (MIC) against tedizolid, linezolid, vancomycin, ceftaroline and daptomycin through the disk diffusion method with a gradient strip (Liofilchem^®, Italy). All LR isolates also underwent a study of the MIC against erythromycin, clindamycin, chloramphenicol and streptogramin A, and the different profiles of resistance linked to different mechanisms of resistance to linezolid were evaluated⁷: point mutations in the V domain of 23S ribosomal RNA (rrn gene), presence of the cfr gene or mutations in the genes that encode the L3 and L4 proteins of the 50S ribosomal subunit. All isolates were seeded in Mueller-Hinton agar with a 0.5 McFarland inoculum and incubated for 24 h at 37 °C. Methicillin resistance was confirmed using 5 μg cefoxitin. The MIC of the different antibiotics was interpreted according to the EUCAST recommendations (v. 6.0). All redundant isolates with a period less than 5 days for a single patient were excluded. In addition, a clonality study was conducted using pulsed-field gel electrophoresis (PFGE)⁶ on the isolates that had signs of epidemiological outbreak: isolates that were grouped in time and space.⁸

A total of 55 isolates from 30 men (54.5%) and 25 women (45.5%), from different locations, were analysed: 29 blood culture samples (13 S. aureus, 7 Staphylococcus epidermidis and 9 Staphylococcus hominis), 15 respiratory samples (15 S. aureus) and 11 samples from exudates (5 S. aureus and 6 S. epidermidis).

The epidemiological relationship of the patients infected with LR-CNS was as follows: 9 of the patients infected with S. hominis between 2007 and 2008 were admitted to the intensive care unit (n = 8) or the emergency department (n = 1). All other patients infected with LR-CNS (S. epidermidis, n = 13) were admitted to different departments between 2009 and 2015: general surgery (n = 2, 2009–2013), internal medicine (n = 3, 2010–2015), gastroenterology (n = 1, 2009), vascular medicine (n = 2, 2009–2014), nephrology (n = 3, 2011–2015), anaesthesia (n = 1, 2010) and emergency medicine (n = 1, 2013). The clonality study for the S. hominis isolates that had signs of epidemiological outbreak confirmed the presence of 2 different clones: clone A (n = 8) and clone B (n = 1).⁶ All other isolates had no signs of epidemiological outbreak.

All isolates were resistant to methicillin. The susceptibility profile for the isolates is presented in Table 1. Tedizolid proved to have higher activity in vitro compared to linezolid. The MIC₉₀ for tedizolid versus LR-CNS and MRSA was lower (8 and 0.5 mg/l) compared to that observed for linezolid (>256 mg/l and 2 mg/l). In addition, intervals were estimated for the MIC of tedizolid versus LR-CNS and MRSA from 1 to 8 mg/l and from 0.25 to 0.5 mg/l, respectively, compared to linezolid (MIC of 8 to >256 mg/l for LR-CNS and 1 to 3 mg/l for MRSA). We found resistance to tedizolid in all LR-CNS (MIC > 0.5 mg/l, the EUCAST breakpoint to consider them resistant). In addition, 4 MRSAs had a MIC of 0.5 mg/l against tedizolid. No resistance to vancomycin, ceftaroline or daptomycin was found in any of the isolates that we studied.

Table 1.

Summary of the in vitro activity of different antibiotics in 22 LR-CNS and 33 MRSA isolates.

MIC (mg/l)				Percentage of resistance^a
Antibiotics	Range	50%	90%
LR-CNS (22)
Linezolid	8 to >256	48	>256	100
Tedizolid	1–8	4	8	100
Vancomycin	0.5–2	1	1.5	0
Ceftaroline	0.125–0.75	0.5	0.75	0
Daptomycin	0.19–0.38	0.19	0.38	0
MRSA (33)
Linezolid	1–3	2	2	0
Tedizolid	0.25–0.5	0.38	0.5	0
Vancomycin	0.38–1	0.5	1	0
Ceftaroline	0.25–1	1	1	0
Daptomycin	0.125–1.5	0.38	0.5	0

MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus; LR-CNS, methicillin- and linezolid-resistant coagulase-negative Staphylococcus.

a

According to the breakpoint points established by EUCAST (v 6.0).

The resistance phenotypes of the LR-CNS isolates were consistent with the presence of a single strain carrying the cfr gene (cross-resistance to linezolid, amphenicols, lincosamides and streptogramin A, as well as susceptibility to macrolides), with a MIC to linezolid higher than 256 mg/l and a MIC to tedizolid higher than 3 mg/l. In addition, we found another strain consistent with the presence of mutations in the V domain of 23S ribosomal RNA (resistance to linezolid and susceptibility to amphenicols, lincosamides, streptogramin A and to macrolides), with a MIC for linezolid and tedizolid of 32 and 6 mg/l, respectively. All other isolates had phenotypes consistent with the coexistence of multiple mechanisms of resistance.

Tedizolid is presented as a highly potent drug, and in line with other series, we present data showing a MIC₉₀ 4 to 16 times lower compared to linezolid.⁹ Although our results suggest that this antibiotic has higher activity in vitro, as in other studies conducted in LR Staphylococcus,^10,11 the MIC of tedizolid often exceeds the breakpoint established by EUCAST. In our study, the MIC₅₀ and MIC₉₀versus LR-CNS were 4 and 8 mg/l, respectively, and all were resistant.

Although some studies have demonstrated greater susceptibility to tedizolid in LR isolates carrying the cfr gene,¹² we were unable to demonstrate this. However, we present results that demonstrate high resistance to tedizolid in isolates that often have phenotypic profiles of resistance consistent with the presence of multiple mechanisms of resistance to linezolid. In light of these facts, the existence of a new plasmid (optrA) conferring resistance to linezolid and to amphenicols was recently reported.¹³ Therefore, new studies are needed that provide definitive data on the efficacy of tedizolid in LR isolates, which often possess multiple mechanisms of resistance to this drug.