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Authors Chen D, Song Q, Xu Z, Zhang D
Received 1 February 2018
Accepted for publication 15 February 2018
Published 13 April 2018 Volume 2018:11 Pages 531—538
DOI https://doi.org/10.2147/IDR.S164278
Checked for plagiarism Yes
Review by Single-blind
Peer reviewers approved by Dr Andrew Yee
Peer reviewer comments 2
Editor who approved publication: Dr Sahil Khanna
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: Staphylococcus aureus ,
staphylococcal enterotoxin A, multilocus sequence typing, phage, SEA
production capacity