A small but clinically significant portion of Staphylococcus aureus isolates with reduced vancomycin susceptibility also demonstrated reduced susceptibility to telavancin and daptomycin. This is according to data published in the European Journal of Clinical Microbiology & Infectious Diseases.

The study characterized S aureus isolates from a large tertiary care academic medical center: 50 identified as vancomycin susceptible and 250 identified as reduced vancomycin susceptibility. Researchers noted that 51.8% of all samples were also resistant to methicillin, but there were no statistically significant differences in the distribution of methicillin resistance across samples that were vancomycin susceptible and those that demonstrated reduced susceptibility.

All of the vancomycin-susceptible isolates were also susceptible to telavancin, whereas 6.4% of isolates with reduced vancomycin susceptibility were nonsusceptible to telavancin. Of the S aureus isolates that demonstrated reduced vancomycin susceptibility, 3.6% were nonsusceptible to daptomycin. Further, 1.2% of the isolates with reduced vancomycin susceptibility demonstrated nonsusceptibility to both telavancin and daptomycin. Further tests of both the S aureus isolates with reduced vancomycin susceptibility and isolates with vancomycin susceptibility against other classes of antimicrobials found no significant differences in susceptibility except for ciprofloxacin and moxifloxacin. In addition, 15 isolates (approximately 6%) that demonstrated reduced vancomycin susceptibility also demonstrated a high-level of resistance to mupirocin.

Molecular characterization of the isolates revealed that SCCmec types II and IV represented over half of the S aureus isolates that showed reduced susceptibility to vancomycin, whereas 12% of the vancomycin-susceptible isolates were SCCmec type II. Investigators also detected 16 distinct strain types in this collection of isolates and found that carriage of the tst-1 gene, which encodes the Staphylococcus toxic shock syndrome super-antigen, was low at only 5.4%.

The study is limited in that all isolates came from a single center, but the molecular characterization did show that they were not all from a single clone. The study also does not contain more recent data from 2016 to 2019. In addition, most isolates were stored at -80 °C for long periods of time. This can affect the susceptibility profiles of isolates, because of the possible selective pressure maintained during recovery and passaging in an attempt to retain the original isolate phenotype.

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According to investigators, this study demonstrated that rare isolates with reduced in vitro susceptibility to vancomycin may also test as nonsusceptible to telavancin and daptomycin. They recommend that laboratories perform antimicrobial susceptibility testing for these agents if they will be used for patient treatment, especially in S aureus with reduced susceptibility to vancomycin. Investigators also reported that in this cohort, S aureus isolates with reduced susceptibility to vancomycin were more likely than isolates with vancomycin susceptibility to carry the mupA gene, thus leading to high levels of resistance. The presence of high levels of resistance can complicate patient care and lead to the need for additional decolonization strategies. Therefore, “further studies are needed to determine if [S aureus with reduced susceptibility to vancomycin] isolates are more likely to also carry the mupA gene as compared with [vancomycin-susceptible] isolates.”

Reference

McMullen AR, Lainhart W, Wallace MA, Shupe A, Burnham CD. Evaluation of telavancin susceptibility in isolates of Staphylococcus aureus with reduced susceptibility to vancomycin [published online August 24, 2019]. Eur J Clin Microbiol Infect Dis. doi:10.1007/s10096-019-03683-z