# Observations on emergence of mannitol-use-deficient Staphylococcus aureus

## TL;DR

A new Staphylococcus aureus clone emerged after 2020, showing a pink color on agar and producing virulence factors longer than usual.

## Contribution

Discovery of a new S. aureus clone with altered virulence factor production and a distinct pink phenotype on mannitol salt agar.

## Key findings

- The new S. aureus clone lacks the gene for mannitol-1-phosphate dehydrogenase and appears hyper-pink on agar.
- This clone produces virulence factors into the stationary growth phase, unlike typical S. aureus.
- The hyper-pink clone was found in patients with moderate to severe atopic dermatitis.

## Abstract

Staphylococcus aureus clones emerge and recede in roughly 10-year intervals unless virulence factors impose clonal continuation. This study investigates a newly emergent clone. Prior to 2020, all of 322 clinical isolates tested grew, as expected, yellow colonies on mannitol salt agar. After 2020, 10% of clinical S. aureus isolates grew as intensely pink colonies (hyper-pink, not pale red as seen in coagulase-negative staphylococci). The hyper-pink S. aureus lacked the gene mannitol-1-phosphate dehydrogenase within the mannitol fermentation operon. The organisms were methicillin-sensitive. S. aureus usually expresses cell-surface virulence factors during exponential growth and secretes toxins for a brief time in the post-exponential phase, but not in the stationary phase. The hyper-pink organisms expressed both cell-surface and secreted virulence factors well into the stationary phase. S. aureus, which is usually yellow on mannitol salt, showed the virulence factor production profile of hyper-pink organisms when the media pH was maintained at 8.0. Comparison of hyper-pink organism culture fluids with those of USA300 and USA400 S. aureus strains for interaction with epithelial cells revealed similar harmful pro-inflammatory properties. Known global regulatory systems in the hyper-pink organisms were intact, including the two-component system SrrA/B, which senses both oxygen and pH. This newly emergent clone appears to have been selected after 2020 by its inability to ferment mannitol. The clone was cultured from the skin of patients with moderate to severe atopic dermatitis.

A novel clone of Staphylococcus aureus emerged after 2020, unusually, with production of cell-surface and secreted virulence factors into late stationary growth phase. This newly emerging clone may be confused with coagulase-negative staphylococci because it has a strong pink phenotype on mannitol salt agar, whereas S. aureus usually forms bright yellow colonies on this medium.