The occurrence and distribution characteristics of Cronobacter in diverse cereal kernels, flour, and flour-based products
Introduction
Cronobacter was accepted as a new bacterial genus in 2007 (Iversen et al., 2007). It consists of seven species including C. sakazakii, C. malonaticus, C. turicensis, C. muytjensii, C. dublinensis, C. universalis, and C. condimenti (Iversen et al., 2008; Joseph et al., 2012). This opportunistic foodborne pathogen causes severe systemic infections occasionally in neonates, such as meningitis, septicaemia, and enterocolitis, especially in low birth-weight or immunocompromised infants (Farmer, 2015). The mortality rate for resulting meningitis may be as high as 40%–80% (Nazarowec-White and Farber, 1997; Lai, 2001). The rates of Cronobacter infection was 0.66 cases per 100,000 population, with the highest rates occurred among persons >80 years of age (3.93 cases/100,000 population), followed by persons 70–79 years of age (2.11) and infants (1.81) (Patrick et al., 2014).
Cronobacter infections among infants have been epidemiologically linked to powdered infant formula (PIF) (Hunter and Bean, 2013; Yan et al., 2012). Cronobacter contamination of commercial PIF products is always an unavoidable problem (Fei et al., 2018; Parra-Flores et al., 2018). Recalls of PIF as well as cases of severe neonatal infection with Cronobacter have still occurred (Caubilla-Barron et al., 2007; Shi et al., 2018; https://sg.theasianparent.com/dumex-mamil-gold-infant-formula-recall). Actually, it is impossible to produce sterile PIF using current methods. In addition to PIF, this microorganism has been isolated from a wide spectrum of common foods and food ingredients (Berthold-Pluta et al., 2017; Brandao et al., 2017; Li et al., 2017; Vasconcellos et al., 2018). It has been confirmed that many Cronobacter isolates worldwide originates from plant sources (Chen et al., 2016; Sani and Odeyemi, 2015).
Although great research progress has been achieved, both the source and vehicle of transmission of Cronobacter are not always clear. It was previously showed that wheat flour is one likely reservoir and/or transmission route for Cronobacter because of the extremely high positive rate (100%) of this pathogen (Lou et al., 2014). Our research indicated that Cronobacter was present in different growth stages of rice and wheat, and Cronobacter detection rates in food samples notably changed with different processing methods. Moreover, Cronobacter in the domestic environment has been related to primary staple foods (Lou et al., 2019).
Several studies have reported that a number of cereal samples were tested positive for Cronobacter spp. (Brandao et al., 2017; Friedemann, 2007; Silva et al., 2019), while another study found that none of the 32 cereal and cereal product samples of were contaminated (Jaradat et al., 2009). This notable discrepancy suggests that Cronobacter might occur more frequently in certain kinds of cereals, which should be investigated in further detail. Therefore, the objective of this study was to know the positive rate of Cronobacter in a diversity of cereals, including their distribution and characteristics, to thus systematically enrich knowledge about the ecological niche of Cronobacter spp. in different cereals.
Section snippets
Food sample collection
In total, 594 cereal and cereal-related samples were collected for our investigation in two different stages. In the first stage, 467 cereal samples, including rice, wheat, and their related products, Job's tears, millet, buckwheat, sorghum, corn, and oat, were collected for testing the presence of Cronobacter. In the second stage, 127 related samples were collected for Cronobacter isolation, identification, and enumeration. All the samples were purchased from local markets or online shops from
Results and discussion
In the first stage, our evaluation of the presence of Cronobacter in a wide range of cereal products (N = 467) revealed that 54.0% (252/467) of the samples contained the bacterium (Table 1). Here, a triplex PCR targeting 16S rDNA, the ITS sequence and ompA gene was used as previously described (Lou et al., 2014) to achieve high accuracy of Cronobacter identification, and all of the 252 isolates tested positive for all three gene markers. There were 65 positive tests out of 153 samples (42.5%)
Conclusions
The main conclusion from this study is that the selected samples contained Cronobacter with a relatively high positive rate of 40.4%, and the majority of food categories (with the exception of infant nutrient rice flour) tested positive for Cronobacter. Our results also indicated that the processing of raw materials might be the critical source of these bacteria in different cereals. Moreover, cereal flour might pose a high risk for infection, and special care should be taken while preparing
Acknowledgements
This work was supported by the Project of National Natural Science Foundation of China (Grant number 81402682); the Project of Natural Science Foundation of Zhejiang Province, China (Grant number LY17D050006); The Health and Technology Program of Hangzhou, China (Grant number 2017A66).
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