Leukotoxin and pyrogenic toxin Superantigen gene backgrounds in bloodstream and wound Staphylococcus aureus isolates from eastern region of China

Staphylococcus aureus (S. aureus) is a serious pathogen that causes various clinical infections with considerable morbidity and mortality due to its capability to produce different virulence factors [1]. Among these virulence factors, the bicomponent leukotoxins and the pyrogenic toxin superantigens (PTSAgs) have attracted great attention for their ability to destruct the membranes of host cells or regulate the immune responses by activating immune cells abnormally [2‐5].

In S. aureus, seven leukotoxins have been identified. Panton-Valentine leukocidin (PVL), gamma (γ)-hemolysin (HlgAB and HlgBC), leukotoxin ED (LukED), and leukotoxin AB/GH (LukAB/GH) are found in isolates associated with human infections [2, 3]. Leucocidin MFʹ (LukMFʹ) and leucocidin PQ (LukPQ) are only detected in strains from zoonotic infections [6, 7]. Except for pvl (encoding PVL), the data on the overall prevalence of leukotoxin family in clinical S. aureus isolates are very limited in China, especially that of the recently identified LukAB, which is the only leukotoxin known to enhance the survival of S. aureus [2, 3, 8].

Previous epidemiological data showed that lukED existence is widespread among S. aureus isolates [9], and this toxin has an important role in S. aureus bloodstream infection, impetigo and antibiotic-associated diarrhea [3, 10]. Said-Salim et al. [11], Boakes et al. [12] and Yu et al. [13] reported that the production of PVL differentiated from strain to strain, and this difference is associated with the severity of specific infections (such as skin and soft tissue infections, SSTIs). Then, is the expression pattern of lukED among clinical S. aureus isolates the same as that of PVL? Until now, no data can be used to clarify this question.

S. aureus can also secrete an array of pyrogenic toxin superantigens (PTSAgs), including toxic shock syndrome toxin-1 (TSST-1), staphylococcal enterotoxins (SEs), and SE-like toxins. PTSAgs are able to activate T-cells and antigen-presenting cells (APCs) to release proinflammatory cytokines, increase sensitivity to bacterial lipopolysaccharide (LPS) [14], and are associated with some diseases, such as toxic shock syndrome, food poisoning and allergic syndromes [15]. Study also indicates that PTSAgs can play a major role in the pathophysiological mechanism of sepsis [16]. Therefore, it is required to get better understanding of the PTSAg genes distribution in S. aureus isolates from clinical samples.

In this study, we conducted a retrospective study to determine the distribution of genes encoding leukotoxins and PTSAgs among clinical S. aureus isolates obtained from blood and wounds. Subsequently, we analyzed the genetic characteristics of these isolates, and the expression of lukED in some lukED-positive strains. Furthermore, the relationship between genetic backgrounds and the carriage of virulence genes, or the expression level of lukED was assessed.

Blood and wound S. aureus infections are common clinical diseases. Therefore, we investigated some major toxin (such as leukotoxin and PTSAg) genes existence for getting insight into the potential pathogenic ability of S. aureus from the two kinds of samples.

Although previous studies reported that each member of the leukotoxins has its distinct role in the pathogenesis of S. aureus by both in vitro and in vivo investigation [10, 31‐35], to the best of our knowledge, this is the first study on the overall prevalence of this toxin family among clinical S. aureus isolates in China. Our data showed the prevalent rates of hlgBC (94.9%) and lukED (81.4%) were similar to those of previous reports [33, 36‐41]. The lukAB, whose distribution is unknown due to lack of investigation in a large number of clinical strains, was carried by 67.8% of our isolates. There is a varying carriage of pvl among MRSA, ranging from 2.3 to 50.7% in China [42‐45]. In the present study, a relatively low prevalence of pvl-positive isolates (7.9%) was found, which was in agreement with our previous data (6.6%) [46].

Regarding the PTSAg genes, Dramann et al. [5] has reviewed that approximately 80% of clinical S. aureus isolates carry an average of 5 to 6 genes, and the gene profiles varied remarkably among S. aureus strains. In this study, the overall positive rate of PTSAg genes was 72.9%, and a total of 59 PTSAg gene combinations were observed. However, due to only 13 PTSAg genes detected here, a much lower average carriage (mean, 3.3, 420/129) was found in the PTSAg gene-positive strains. A study from China showed sea was the most prevalent enterotoxin gene (41.53%) in S. aureus isolates from bacteraemia [47]. However, the positive rate of sea was only 15.9% in our blood strains, and had no significant difference between the strains from blood and wounds (15.9% versus 21.3%) (Table 1). This discrepancy is most likely caused by the difference in genetic backgrounds of strains [47, 48]. Although enterotoxin gene cluster egc has no connection with life-threatening infections, the possession of this operon may be conducive to the colonization of S. aureus and function in certain infections [49‐51]. The total prevalence rate of 17.5% for the intact egc in this study (Fig. 1) resembled the results observed by Xie et al. [52] and Chao et al. [53]. However, we found a significant difference (24.7% versus 10.2%, P = 0.011) of this gene cluster carriage in our isolates from blood and wounds (Fig. 1), which indicated this cluster might link to the origins of isolates. Usually, sed-sej is located on plasmid pIB485, and the coexistence of both genes has been reported in some studies [53, 54]. Here, the fixed combination was only detected in 4% isolates, and 10.7% (19/177) isolates possessed sej or sed (Fig. 1). This uncharacterized combination of toxin genes indicated the diversity of yet-undescribed variants of mobile genetic element (MGE).

Previous studies presented that CC clones of S. aureus often display different toxin gene patterns [48, 53, 55]. For example, the toxin locus of lukED was present in CC1, CC5 and CC7 etc., but completely absent from CC22, CC30 and CC398 etc. [53, 55]. In this study, the distribution of lukED in CC isolates was basically in line with the previous reports, except 1 isolate with CC22 and 1 isolate with CC30 (Fig. 1). Previous data indicated that the lukED is located on a mobile pathogenicity island, vSaβ [5]. Therefore, we speculated the isolates with CC22 and CC30 obtained the lukED through the horizontal transfer of the vSaβ. Apart from lukED negative, ST398 isolates also harbor fewer PTSAg genes [56, 57]. This phenomenon was confirmed by our data in Fig. 1. Previous data indicated that the egc cluster was a common feature of CC5 isolates [58, 59]. The same phenomenon was found in our isolates (Fig. 1). γ-hemolysin, a core genome-encoded leukotoxin, is highly conserved [3], and therefore can be detected in nearly all our S. aureus strains. Although lukAB is also located in the core genome, its locus is often disrupted by the insertion of a prophage [3]. This may explain the relatively lower prevalence of lukAB among each S. aureus lineage in this study, compared to that of γ-hemolysin-encoding gene hlgCB (Table 2, Fig. 1). Particular association had been observed between LukED-producing strains and agr II, as well as for TSST-1 and agr III isolates [21, 36]. However, our data only showed the correlation between lukED and agr II isolates. Besides, Fig. 1 displayed the PTSAg genes were preferable more common in agr II isolates. These carriage differences of toxin genes among isolates with different genetic backgrounds might be related to the heterogeneous nature of the infections and patients. In this study, the total number of isolates is not particularly large, which leaded to a relative small proportion (26.6%, 47/177) of MRSA strains and a few MRSA clones (Table 1 and Fig. 1). The asymmetrical distribution may influence the objective distribution of virulence determinants. This is one of the limitations of the present study.

The expression differences of immune evasion genes among strains may have vital influence for the pathogenesis of bacteria [55]. Previous studies exhibited that LukED plays an essential role in S. aureus infections [3, 10]. Therefore, we detected the transcription level of this leukotoxin in clinical isolates. The results of qRT-PCR revealed a marked strain-to-strain variation in lukED mRNA transcription, even in isolates with the same genetic characteristics. Consistent with the study on the production of PVL [12], we observed that agr types did not affect the lukED expressions significantly (Fig. 1). In addition, no remarkable associations were observed between major STs and the expression of this gene. Because only a limited number of ST or agr type isolates were included in this study, the different expressions of lukED attributed to various ST types or agr groups can’t be ruled out. And more comprehensive investigations of abundant isolates are needed to explore the association between lukED expression and different genetic background. In order to better verify the toxin’s role in bacterial pathogenesis, it is very important to study the relationship between the toxin expression level and the disease severity. In this study, we conducted a retrospective investigation designed only to understand the expression of lukED in S. aureus isolates from blood and wounds. If the correlation of the expression levels of lukED with the severity of infectious diseases is evaluated, it will provide a more convincing evidence to elucidate the function of LukED in the pathogenesis of S. aureus. This will be the research interest of the future study.

In summary, this work exhibits the prevalence of leukotoxin and partial PTSAg genes in clinical S. aureus isolates from blood and wound in eastern region hospitals of China. Genotypic analysis illustrates a high genetic diversity of these isolates, and certain toxin genes, such as lukED and the egc cluster, may be lineage specific. In particular, S. aureus isolates belonging to ST764 and ST5, as well as agr II, were likely to harbor more PTSAg genes, which may suggest an alarming situation of infected patients. The different expression of lukED was found in clinical isolates, however, the association of this difference with the genetic backgrounds of isolates needs to be further unraveled.