Molecular Epidemiology of Penicillin-Susceptible Bacteremia in Australia and Reliability of Diagnostic Phenotypic Susceptibility Methods to Detect Penicillin Susceptibility.

BACKGROUND

Defined by the emergence of antibiotic resistant strains, is a priority bacterial species with high antibiotic resistance. However, a rise in the prevalence of penicillin-susceptible (PSSA) bloodstream infections has recently been observed worldwide, including in Australia, where the proportion of methicillin-susceptible causing bacteremia identified phenotypically as penicillin-susceptible has increased by over 35%, from 17.5% in 2013 to 23.7% in 2020.

OBJECTIVES

To determine the population structure of PSSA causing community- and hospital-onset bacteremia in Australia and to evaluate routine phenotypic antimicrobial susceptibility methods to reliably confirm penicillin resistance on -positive initially classified as penicillin-susceptible by the Vitek 2 automated microbiology system.

RESULTS

Whole genome sequencing on 470 PSSA collected in the 2020 Australian Group on Antimicrobial Resistance Australian Sepsis Outcome Programme identified 84 multilocus sequence types (STs), of which 79 (463 isolates) were grouped into 22 clonal complexes (CCs). The dominant CCs included CC5 (31.9%), CC97 (10.2%), CC45 (10.0%), CC15 (8.7%), and CC188 (4.9%). Many of the CCs had multiple STs and types and, based on the immune evasion cluster type, isolates within a CC could be classified into different strains harboring a range of virulence and resistance genes. Phylogenetic analyses of the isolates showed most CCs were represented by one clade. The gene was identified in 45 (9.6%) PSSA. Although multiclonal, approximately 50% of -positive PSSA were from CC15 and were found to be genetically distant from the -negative CC15 PSSA. The broth microdilution, Etest and cefinase, performed poorly; however, when the appearance of the zone edge was considered; as per the EUCAST and CLSI criteria, disc diffusion detected 100% of -positive PSSA.

CONCLUSIONS

In Australia, PSSA bacteremia is not caused by the expansion of a single clone. Approximately 10% of classified as penicillin-susceptible by the Vitek 2 harbored . Consequently, we recommend that confirmation of Vitek 2 PSSA be performed using an alternative method, such as disc diffusion with careful interpretation of the zone edge.