Outbreak of S. haemolyticus nosocomial bloodstream infections in French NICU

Staphylococcus haemolyticus (SH) is an opportunistic pathogen associated with nosocomial infections, notably bacteraemia (NB) in neonates. The epidemiological trends and genetic diversity of these infections is largely unknown. Using the data from the French national network for surveillance of NB, we report an increase of SH NB in neonates between 2019 and 2023, representing 19.4 % to 28.7 % of NB and affecting mainly very low wight preterm neonates (= 1 500 g). Genomic and phylogenetic analyses of 490 strains isolated both in (n=380) our outside (n=110) the neonatal intensive care unit (NICU) settings of 25 hospitals reporting clusters of infections showed the predominance of two sequence types (STs), ST29 (65 % of strains) and ST25 in a lesser extent (14% of strains). The ST29 seemed particularly adapted to the NICU environment and was identified in almost all surveyed hospitals. In addition, sub-clades were identified within the ST29 corresponding to geographic clusters of strains indicating the capacity of ST29 to adapt to each NICU and cause outbreaks. In contrast, ST25 phylogeny suggested that it is a general nosocomial SH lineage, widely disseminated in the hospital environment, but less frequent in NICUs. Both STs showed a marked level of antibiotic resistance higher than that of other STs. To conclude, the present study highlights the spread of two successful and highly resistant lineages that have disseminated in French NICUs with the ST29 clade predominantly identified with a capacity to cause outbreaks.