Identification and antimicrobial susceptibility of referred isolates in Victoria, Australia 2009-2019.

Nocardia is an opportunistic pathogen that can cause significant morbidity and mortality, particularly in the immunocompromised host. Antimicrobial susceptibility profiles vary across spp. and vary within Australia as well as worldwide. Knowledge of local susceptibility patterns is important in informing appropriate empiric antimicrobial therapy. This is the largest study to date in Australia that correlates antimicrobial susceptibility profiles with molecular identification of species. It is the first study that examines isolates from multiple institutions across the state of Victoria, Australia. To investigate the species distribution and antibiotic susceptibility of spp. isolates referred to the Mycobacterial Reference Laboratory (MRL) in Victoria, Australia from 2009 to 2019. We conducted a retrospective review of spp. isolates which were identified using molecular sequencing. Antimicrobial susceptibility testing was performed using standardized broth microdilution method with Sensititre RAPMYCO1 plates. Species distribution and antibiotic susceptibility profiles were analysed. In total, 414 isolates were identified to 27 species levels, the majority originating from the respiratory tract (=336, 81.2 %). (=147, 35.5 %) was the most frequently isolated, followed by (=75, 18.1 %). Species distribution varied by isolate source, with and found more commonly from sterile sites. Linezolid and amikacin had the highest proportion of susceptible isolates (100 and 99% respectively), while low susceptibility rates were detected for ceftriaxone (59 %) and imipenem (41 %). Susceptibility to trimethoprim sulfamethoxazole varied by species (0-100 %). This is the largest study to date in Australia of species distribution and antimicrobial susceptibility patterns. and were more likely to be isolated from sterile sites, while and were more likely to be isolated from skin and soft tissue. First line therapeutic antimicrobial recommendations by local guidelines were not necessarily reflective of the susceptibility of isolates in this study, with high susceptibility detected for linezolid and amikacin, but poor susceptibility demonstrated for ceftriaxone and imipenem. Profiles for trimethoprim-sulfamethoxazole varied across different species, warranting ongoing susceptibility testing for targeted clinical use.