What’s New From the CLSI Subcommittee on Antimicrobial Susceptibility Testing

ContagionContagion, February 2021 (Vol. 06, No. 01)
Volume 06
Issue 01

The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antimicrobial Susceptibility Testing (AST) held its midyear meeting virtually on several dates in September. In addition to the new and revised breakpoints voted on and approved at the January 2020 meeting, the committee members discussed and approved several more at the midyear meeting.

New Breakpoints

Lefamulin for S aureus, S pneumoniae, and H influenzae

Pleuromutilins are a class of antibacterial drugs derived from the naturally-occurring antibiotic pleuromutilin that act as inhibitors of bacterial protein synthesis via interactions with the ribosomal peptidyl transferase of target pathogens.1 In 2019, the US Food and Drug Administration (FDA) approved lefamulin, the first pleuromutilin for systemic use with both oral and intravenous administration. It is indicated for the treatment of adults with community-acquired bacterial pneumonia (CABP) resulting from infection with the following respiratory pathogens: Streptococcus pneumoniae, methicillin-susceptible Staphylococcus aureus (MSSA), Haemophilus influenzae, Legionella pneumophila, Mycoplasma pneumoniae,and Chlamydophila pneumoniae.2 The approved dose is 150 mg every 12 hours intravenous infusion over 60 minutes for 5 to 7 days, and 600 mg orally every 12 hours for 5 days. Clinical andin vitrostudies oflefamulin have demonstrated minimal resistance development and little cross-resistance with other antibiotic classes commonly used for CABP, including macrolides, fluoroquinolones, vancomycin, and tetracyclines.

Among the targeted organisms, MSSA, H influenzae, and S pneumoniae were granted susceptibility breakpoints by the FDA. The drug manufacturer requested CLSI approval of these breakpoints, as well as the susceptibility breakpoint for methicillin-resistant S aureus (MRSA). The AST committee reviewed the pharmacokinetic (PK) and pharmacodynamic (PD) target attainment analysis and clinical efficacy studies.

Data from lung infection studies in neutropenic mice and PK studies in humans presented target attainments above 90% for all proposed organisms.3 They also showed low affinity of lefamulin for both major plasma binding proteins, resulting in rapid tissue distribution.4 Phase 3 trials established the general efficacy and safety of lefamulin in patients with CABP per FDA guidance. Additionally, lefamulin exhibited comparable success rates to moxifloxacin, the standard of care for pathogens consistent with CABP, and efficacy against community-acquired MRSA pneumonia at a 2- to 3-fold lower dose compared with linezolid and vancomycin.3,4

From the data presented, the AST subcommittee approved the FDA-proposed breakpoints and decided that an intermediate susceptibility category was not justified at this time due to the lack of resistant isolates. The manufacturer and AST agreed that resistant isolates should continue to be monitored in the future. Motions were passed to accept the S aureus (MSSA and MRSA), S pneumoniae, and H influenzae minimum inhibitory concentration (MIC) and disk diffusion breakpoints. A final motion was passed for a comment to be added to the lefamulin breakpoints that they should not be reported for cerebrospinal fluid or urinary tract isolates.

Revised Breakpoints

Azithromycin against Shigella species

Azithromycin is an azalide and part of a subclass of macrolide antibiotics with both bactericidal and bacteriostatic activities. Its mechanism of action is the inhibition of the translocation step of protein synthesis by reversibly binding to the 50S subunit of the 70S ribosome.5 Azithromycin is used to treat Shigella species (spp) infections, though clinical outcomes are uncertain.6 In the 30th edition of CLSI’s M100: Performance Standards for Antimicrobial Susceptibility Testing (M100-S30), there is a singular investigational (ie, not FDA approved for use in the United States) azithromycin breakpoint for Enterobacterales (for Salmonella enterica serovar Typhi only), whereas S sonnei and S flexneri have separate epidemiological cutoff values (ECVs).7 An increased incidence of non–wild type (NWT) Shigella spp has been noted in clinical isolates, based on the 2015 ECV of 32 µg/mL or more for Shigella sonnei and 16 µg/mL or more for Shigella flexneri,according to the National Antimicrobial Resistance Monitoring System.7 The Centers for Disease Control and Prevention (CDC) National Center for Emerging and Zoonotic Infectious Diseases proposed to establish a singular unified breakpoint for Shigella spp, as clinical laboratories are not performing susceptibility testing for azithromycin due to the lack of established breakpoints.

Results of several studies have found poor clinical outcomes when Shigella infections, specifically those caused by NWT, were treated with azithromycin.10,11 In one such study in Dhaka, Bangladesh, MICs ranged from 4 to greater than 256 µg/mL for S sonnei, 1 to greater than 256 µg/mL for S flexneri, and 1 to 4µg/mL for other Shigella spp.6 The results found high rates of azithromycin resistance in the Shigella tested, notably higher in S sonnei, leading to poor clinical outcomes when the patients were treated with azithromycin.6 According to data from the California Department of Public Health presented at the CLSI midyear meeting, patients whose NWT Shigella spp infections were treated with azithromycin had worse outcomes of longer duration of diarrheal symptoms and increased length of hospital stay compared with patients treated with another appropriate antibiotic.8

These data support the need to establish unified breakpoints for azithromycin against Shigella spp, which the CDC proposed. The proposed MIC (susceptible (S)≤ 8 µg/mL, intermediate (I)= 16 µg/mL, resistant (R)≥ 32 µg/mL)and disk diffusion (S≥16 mm, I = 11-15 mm, R≤ 10 mm) breakpoints differ slightly from the previously established Enterobacterales breakpoint from the M100-S30 document. The breakpoints proposal passed as written by the subcommittee. Along with the proposed breakpoints, it was noted that MIC testing is recommended, as the disk diffusion may, in some cases, be too hazy and difficult to measure.

Linezolid Susceptibility as a Surrogate to Predict Tedizolid Susceptibility

Tedizolid is an oxazolidinone antibiotic FDA-indicated for the treatment of acute bacterial skin and soft tissue infections of the following pathogens: S aureus (including MRSA), Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus anginosus, and Enterococcus faecalis.9 The FDA currently recognizes the CLSI M100 susceptibility interpretive criteria for tedizolid. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) has also published breakpoints for tedizolid.7,10 However, they have included a note for MIC breakpoints of Staphylococcus spp and Streptococcus groups A, B, C, and G, stating: “Isolates susceptible to linezolid can be reported as susceptible to tedizolid.”10

Linezolid was the first approved antibiotic of the oxazolidinone class and has an FDA-approved indication for acute bacterial skin and soft tissue infections of the same tedizolid-susceptible strains in addition to its other indications.11 AST’s objective was to evaluate the validity of linezolid use as a surrogate for predicting tedizolid susceptibility against FDA-approved species as proposed by EUCAST. Surveillance isolates from the Surveillance of Tedizolid Activity and Resistance Program were tested with broth microdilution (BMD) methodology per CLSI guidelines and then analyzed with scattergram plots for surrogacy. Notably, there was no analysis for predicting the resistance of tedizolid.

The data for each of the 5 organisms presented confirmed that linezolid susceptibility MIC breakpoints predicted the susceptibility of tedizolid for S aureus (n = 21,969), S pyogenes (n = 2035), S agalactiae (n = 1556), S anginosus group (n = 408), and E faecalis (n = 2986). Moreover, no organisms with MIC susceptibility to linezolid but resistance to tedizolid were identified. A motion was thus passed to add footnotes to M100 tables 2C, 2D, 2H-1, and 2H-2, stating that pathogens considered susceptible to linezolid by MIC can also be considered susceptible to tedizolid.

Hirsch is an appointed adviser to the CLSI Subcommittee on AST. She is also an associate professor at the University of Minnesota College of Pharmacy and runs the Hirsch Laboratory in the Experimental & Clinical Pharmacology Department, where Bixby and Billmeyer are a graduate student and a pharmacy student, respectively.


1. Paukner S, Riedl R. Pleuromutilins: potent drugs for resistant bugs–mode of action and resistance. Cold Spring Harb Perspect Med. 2017;7(1):a027110. doi:10.1101/cshperspect.a027110

2. Xenleta. Prescribing information.Nabriva Therapeutics US, Inc; 2019. Accessed November 28, 2020. https://www.accessdata.fda.gov/drugsatfda_docs/label/2019/211672s000,211673s000lbl.pdf

3. Wicha WW, Craig WA, Andes D. In vivo pharmacodynamics of lefamulin, the first systemic pleuromutilin for human use, in a neutropenic murine thigh infection model. J Antimicrob Chemother. 2019;74(suppl 3):iii5-iii10. doi:10.1093/jac/dkz085

4. File TM, Goldberg L, Das A, et al. Efficacy and safety of intravenous-to-oral lefamulin, a pleuromutilin antibiotic, for the treatment of community-acquired bacterial pneumonia: the phase iii lefamulin evaluation against pneumonia (LEAP 1) trial. Clin Infect Dis. 2019;69(11):1856-1867. doi:10.1093/cid/ciz090

5. Azithromycin. National Center for Biotechnology Information. Accessed November 30, 2020. https://pubchem.ncbi.nlm.nih.gov/compound/447043

6. Houpt ER, Ferdous T, Ara R, et al. Clinical outcomes of drug-resistant shigellosis treated with azithromycin in Bangladesh. Clin Infect Dis. Published online April 2, 2020. doi:10.1093/cid/ciaa363

7. CLSI. M-100: Performance Standards for Antimicrobial Susceptibility Testing.30th ed. Clinical Laboratory Standards Institute; 2020.

8. California Department of Public Health. Clinical and microbiologic outcomes in California of patients infected with azithromycin NWT Shigella: analysis to support the establishment of azithromycin clinical breakpoints. Presented at: Clinical and Laboratory Standards Institute Midyear Meeting; September 29, 2020; Virtual.

9. Sivextro. Prescribing information. Merck & Co., Inc.; 2020. Accessed January 4, 2021. https://www.merck.com/product/usa/pi_circulars/s/sivextro/sivextro_pi.pdf

10. The European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters. Version 11.0, 2021. http://www.eucast.org

11. Zyvox. Prescribing information. Pharmacia & Upjohn Company LLC; 2020. Accessed January 4, 2021. http://labeling.pfizer.com/showlabeling.aspx?id=649

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