Purpose: This study aims to characterize the wild-type staphylococcal enterotoxin A (SEA)-producing Staphylococcus aureus.
Materials and methods: We identified 29 wild-type sea-positive S. aureus isolates from dairy and meat samples, as well as from patients, measured the amount of SEA produced under favorable cultivation conditions using enzyme-linked immunosorbent assay and sea mRNA transcriptional level and investigated the phage type as well as genetic diversity by means of pulsed-field gel electrophoresis and multilocus sequence typing.
Results: Among 29 sea-positive isolates, 22 were from food sources (including one outbreak case) and seven from clinical patients. Five enterotoxin gene profiles, namely, sea (14 isolates), sea+sec (9 isolates), sea+seb (4 isolates), sea+seb+sec (1 isolate) and sea+seb+sed (1 isolate), were identified. Multilocus sequence typing generated sequence type (ST)1 (13 isolates), ST6 (5 isolates), ST59 (3 isolates), ST239 (3 isolates), ST5 (2 isolates), ST188 (2 isolate) and ST15 (1 isolate). The amount of SEA per 108 colony-forming unit (CFU) after 24 h of incubation was 1.1-33.5 (mean, 8.74; SD, 7.7) ng/108 CFU. The amount of SEA per hour incubation in the log growth phase was 0.1-12.0 (mean, 2.37; SD, 3.06) ng/108 CFU. Overall, 54.2% of SEA was produced in the log growth phase. Both the transcriptional level of sea mRNA and the amount of SEA in the log growth phase correlated well with the amount of SEA after 24 h of cultivation. Four isolates, namely, SA-212, SA-217, SA-340 and SA-341, were categorized to be of high SEA production (877-1,109 ng/mL, 24 h). The total amount of SEA was mainly based on the amount of SEA in 108 CFU, not the relatively fixed bacterial cell counts (21.1-43×108 CFU/mL). Seven isolates from patients all carried the ФMu3A phage, whereas 21 of the 22 isolates from the environmental sources all carried the ФSa3ms phage.
Conclusion: The present study exhibits varied SEA production capacity of the wild sea-positive S. aureus strains. An apparent boundary in phage types between strains from the clinical samples and strains from the environment was also identified.
Keywords: SEA production capacity; Staphylococcus aureus; multilocus sequence typing; phage; staphylococcal enterotoxin A.