Modulation of Staphylococcus aureus spreading by water

Staphylococcus aureus is known to form giant colonies when cultured on soft TSA medium in a process called colony spreading, which was first reported by Kaito and Sekimizu1. When cultured on TSA medium that contains 0.24% (TSA-0.24) agar, S. aureus is capable of expanding its colony at a speed of approximately 100 μm per minute, although the bacteria are unable to spread when the agar concentration is increased to 1.5%1. The ability to move at this speed is amazing, since the organism lacks flagellum to allow itself to move actively on the agar surface. They also showed that mutations in the dltABCD operon and tagO gene, which are involved in adding D-Ala to teichoic acid2 and the synthesis of teichoic acid3, respectively, disable the spreading ability of S. aureus, indicating the importance of teichoic acid in spreading1. Additionally, S. aureus spreading requires the synthesis of phenol-soluble modulins (PSMs), which are cytolytic toxins that have surfactant properties4,5,6,7; among the 8 PSMs produced by S. aureus, PSMα3 is particularly important for spreading6. Earlier studies also found that mutations in the genes encoding proteins in the Agr quorum-sensing system prevent spreading7,8. However, the lack of ability to spread could be reverted if PSMs are added to the culture medium6. Since the transcription of the psm genes is activated by the quorum sensing activator, AgrA9, the results indicated that the lack of PSMs expression contributes to the inability of the quorum-sensing mutants to spread. Additionally, cell wall-associated factors such as fibronectin binding proteins and clumping factors, which promote biofilm formation, antagonize colony spreading of S. aureus10. Spreading was also found to be inhibited by the secretion of δ-hemolysin11 but stimulated by bovine serum albumin and high density lipoprotein in the serum12. Swarming bacteria are found to extract water from agar medium13,14. Although a study suggested that Escherichia coli uses lipopolysaccharides (LPS) as osmolytes to extract water from agar15, the mechanisms involved in water extraction are not completely understood. However, the combination of water extraction from agar medium and the use of biosurfactants to facilitate swarming is well documented in B. subtilis16. This study shows that although S. aureus cannot move actively, the bacterium uses a mechanism similar to that of B. subtilis to extract water from agar medium and expresses biosurfactants to weaken the water surface tension to facilitate colony spreading.