There are currently around 2600 known
Salmonella serotypes with a complex nature. According to the World Health Organization’s global
Salmonella monitoring system,
S. Enteritidis and
S. Typhimurium are the most prevalent bacteria responsible for foodborne disease (
19,
20). This study showed that the 111 strains of
Salmonella presented 17 serotypes, indicating that the serotypes in Chinese hospitals are diverse and abundant, with a major share of
S. Typhimurium,
S. Typhi, and
S. Enteritidis, accounting for 68.47% of them (76/111). Also,
S. Typhimurium and
S. Enteritidis accounted for 28.83 and 18.02% of the serotypes, respectively, close to the rates reported in a previous study by Xu et al. (
21). However,
S. Typhi was undetected in Xu et al.’s study, but in this study,
S. Typhi accounted for 21.62% of the serotypes. Compared to
Salmonella serotypes in other regions of China, the proportion of
S. Typhi in Weifang People’s Hospital is higher than the hospitals in other regions (
21,
22). It can be concluded that serotypes are different in different regions of China and among patients with severe
Salmonella infection.
Virulence is the root cause of
Salmonella infection, elicited by the interaction of several virulence genes. SPI mainly encodes
Salmonella virulence genes on the chromosome (
23). More than 20 types of SPI have been discovered so far. SPI1 is required for
Salmonella to invade host non-phagocytes (
24), and SPI2 primarily regulates
Salmonella multiplication in phagocytes and epithelial cells. SPI3 and SPI4 aid
Salmonella survival and adherence to the surface of polarized cells, while SPI5 encodes an effector protein released via the type III secretion system encoded by SPI1 and SPI2 (
25). Also, SPI6 encodes type VI secret system-related proteins (
26). These virulence factors aid
Salmonella invasion, reproduction, virulence, and transmission in a complex environment (
27).
This study selected virulence genes from SPI1 to SPI6 as target genes for PCR amplification. The carrying rates of
hilA,
ssaB,
sseC,
marT,
siiE,
pipB,
sopB, and
pagN were 100%, and the carrying rates of
invA,
misL, and
siiD were found to be 98.2%, 97.30%, and 97.30%, respectively. This study showed that the carrying rate of SPI1-6 genes was relatively high. Fabrega and Vila discovered that several virulence genes were involved in the expression. The higher virulence gene carrying rate translates into greater potential pathogenicity of
Salmonella (
28), implying that
Salmonella harboring in these virulence genes is highly pathogenic.
In this experiment, five of the six drug resistance genes were detected, among which
sul2 and
blaTEM had the highest detection rates. The
sul genes are found in plasmids and are associated with ubiquitous and long-known sulfonamide resistance Gram-negative bacteria (
29). The detection results of the antibiotic resistance genes of the 111 strains of
Salmonella revealed that the detection rate of the sulfonamide resistance gene
sul2 was 68.47%, which was similar to the rate reported by Adesiji et al. (
30). Due to the high detection rate of
sul2, sulfonamides should be used with caution to prevent the spread of antibiotic resistance and the formation of multidrug-resistant strains. A very important factor of
Salmonellaβ-lactam drug resistance is
β-lactamase production. Bacteria producing
β-lactamase can make hydrolytic inactivation of
β-lactam antibiotics, and common types are
blaTEM,
blaOXA, and
blaSHV. This experiment detected
blaTEM,
blaSHV, and
blaOXA2 in
Salmonella. The detection rate of the
blaTEM gene was estimated to be 21.62%, close to the values reported by Shitta (
31). One strain of
S. Enteritidis was found to carry
blaSHV and
blaOXA2. Thus, these strains have the potential for
β-lactam antibiotic resistance. According to the detection of drug resistance genes in this study,
blaTEM has a high carrying rate of
β-lactam antibiotic resistance genes, which should be taken into consideration.
In contrast, five strains of Salmonella were resistant to quinolones (the qnrA gene) with zero detection rate of qnrB, indicating that the resistance rate of Salmonella in Weifang People’s Hospital to quinolones was low, allowing to prioritize the treatment of Salmonella infection. Different serotypes of Salmonella have different carrying rates of antibiotic resistance genes. blaTEM and sul2 were the main drug resistance genes detected in S. Typhimurium, S. Typhi, and S. Paratyphi B. Appropriate antibiotics can be chosen based on distinct serotypes, allowing for more effective treatment of Salmonella infection.
5.1. Conclusions
The results revealed that Salmonella serotypes from Weifang People’s Hospital inpatients were widely spread. The detection rate of antibiotic-resistant genes and the carrying rate of SPI1-6 genes were high. Our findings necessitate strengthening the investigation of Salmonella molecular epidemiology and reducing the emergence of Salmonella antibiotic resistance.