Occurence and variability of norA and other main multidrug efflux pump determinants across the Staphylococcus genus

Background
Multidrug efflux pumps (MDR EPs) play a major role in the emergence of antimicrobial
resistance (AMR). NorA is one of the main native MDR EPs of Staphylococcus aureus,
contributing to reduced susceptibility towards fluoroquinolones and biocides[1,2]. In S.
aureus, its gene occurs in several allelic variants[3], but less is known about the occurrence
of norA and other MDR EPs across the Staphylococcus genus. In this work, we analyze the
presence of norA across staphylococci and identify other main MDR EP determinants in four
additional pathogenic staphylococcal species.
Methods
The norA nucleotide sequences from 61 Staphylococcus species were retrieved from public
databases and aligned. The corresponding predicted polypeptide sequences were also aligned
and the impact of possible residue substitutions on NorA activity was analyzed. Other MDR
EP genes were searched in public databases for Staphylococcus epidermidis, Staphylococcus
haemolyticus, Staphylococcus lugdunensis and Staphylococcus hominis.
Additionally, sequences homologues to S. aureus norB/C,
sdrM, lmrS, mepA and mdeA were screened by BLAST search in the complete genome of
representative strains of each species.
Results
The norA phylogenetic tree follows the evolutionary relations of the Staphylococcus genus
highlighting the presence of norA in the early branching of the genus. Comparative analyses
suggest a conservation of NorA function in staphylococci. Over 30 putative MDR EP genes
were identified for S. epidermidis, S. haemolyticus and S. lugdunensis. Homologues of S.
aureus norB/C and sdrM were identified in both S. epidermidis and S. hominis,
while mepA and mdeA were only detected in S. hominis and lmrS in S. epidermidis.
Conclusions
Our results suggest an overall conservation of NorA function across staphylococci, indicating
that norA is part of the staphylococcal core genome. The identification of main MDR EP
determinants in other staphylococcal species of clinical relevance opens new avenues for the
study of their impact on AMR.

Project BIOSAFE funded by FEDER through the Programa Operacional Factores de
Competitividade—COMPETE and by Fundação para a Ciência e a Tecnologia (FCT, Portugal)—
Grants LISBOA-01-0145-FEDER-030713, PTDC/CAL-EST/30713/2017, 2021.05063.BD and
UID/04413/2020