Enterotoxigenic potential of Staphylococcus intermedius

Abstract

Staphylococcal food poisoning (SFP) caused by enterotoxigenic staphylococci is one of the main food-borne diseases. In contrast to Staphylococcus aureus, a systematic screening for the enterotoxins has not yet been performed on the genomic level for the coagulase-positive species S. intermedius. Therefore, the enterotoxigenic potential of 281 different veterinary (canine, n = 247; equine, n = 23; feline, n = 9; other, n = 2) and 11 human isolates of S. intermedius was tested by using a multiplex PCR DNA-enzyme immunoassay system targeting the staphylococcal enterotoxin genes sea, seb, sec, sed, and see. Molecular results were compared by in vitro testing of enterotoxin production by two immunoassays. A total of 33 (11.3%) S. intermedius isolates, including 31 (12.6%) canine isolates, 1 equine isolate, and 1 human isolate, tested positive for the sec gene. In vitro production of the respective enterotoxins was detected in 30 (90.9%) of these isolates by using immunological tests. In contrast, none of 65 veterinary specimen-derived isolates additionally tested and comprising 13 (sub)species of coagulase-negative staphylococci were found to be enterotoxigenic. This study shows on both molecular and immunological levels that a substantial number of S. intermedius isolates harbor the potential for enterotoxin production. Since evidence for noninvasive zoonotic transmission of S. intermedius from animal hosts to humans has been documented, an enterotoxigenic role of this microorganism in SFP via contamination of food products may be assumed.

FIG. 1

Agarose gel electrophoresis pattern showing PCR-amplified products in multiplex PCR for the SE genes for S. aureus control strains and animal-derived sec-positive S. intermedius isolates. Lanes M1 and M2, DNA molecular size markers (1-kb/100-bp DNA ladder); lane 1, positive control with multiplex PCR of all enterotoxin genes tested (sea, seb, sec, sed, and see); lane 2, SEC-positive S. aureus ATCC 19095 with amplification product of sec gene; lane 3, negative control strain S. aureus Cowan 1; lane 4, S. epidermidis negative control strain DSM 20044; lanes 5 to 11, representatives of sec-PCR-positive S. intermedius isolates. Sizes are marked in base pairs on the left.

FIG. 2

Alignment of sequences coding for SEC including reference sequences of S. aureus subtypes, S. intermedius canine subtype, and clinical sec-positive isolates derived from canine, equine, and human specimens. The EMBL accession numbers of the sec nucleotide sequences used for the alignment are as follows: sec₁, X05815; sec₃, X51661; sec_canine, U91526. For sec₂, a sequence was used as reported by Bohach and Schlievert (21). The sequences of the three S. aureus subtypes were merged, since there were no differences in the aligned region as showed here. Dots indicate identity.