Inhibitory effect of Cinnamomum cassia oil on non-O157 Shiga toxin-producing Escherichia coli
Introduction
Shiga toxin-producing Escherichia coli (STEC) are important foodborne pathogens associated with enteritis, hemolytic uremic syndrome and even death (Jacob et al., 2013). About 250 serotypes of E. coli have been reported to produce Shiga toxin, more than 100 of which have been associated with outbreaks and sporadic cases of diarrhea (Johnson, Thorpe, & Sears, 2006). E. coli O157:H7 is the most well-known pathogen among STEC, and other serotypes of STEC are called non-O157 STEC which have gained increasing awareness (Brooks et al., 2005). It has been estimated that non-O157 STEC cause more than 110,000 cases of illness annually throughout United States, which contributes to a larger portion of total STEC infections than E. coli O157:H7 in the United States (Scallan et al., 2011). Among all non-O157 STEC isolates from patients between 1983 and 2002, O26, O45, O103, O111, O121, and O145 accounted for 71%, so called “top six” non-O157 STECs (Brooks et al., 2005).
Contaminated food is the main vehicle for the transmission of STEC to humans (Erickson & Doyle, 2007), which include beef (Ju et al., 2012), apple cider (Schaffzin et al., 2012), sprouts (Buchholz et al., 2011), lettuce (Taylor et al., 2013) and others. Studies on the intervention of STEC in foods have been focused on O157. Currently far less attention has been paid to non-O157 STEC. One study assessed the efficacy of existing antimicrobial intervention methods used in the meat industry on non-O157 STEC and found that hot water was the most effective and followed by lactic acid wash (Kalchayanand et al., 2012). In another study, 30 s immersion of beef trimmings in 0.1% acidified sodium chlorite, 0.02% peroxyacetic acid, and 4% sodium metasilicate reduced bacterial population by 0.7–1.5 log CFU/cm2 (Geornaras et al., 2012). Due to the rising concern about chemical additives, demand for natural food additives has increased. As a result, increasing attempts have been made to use herbs, spices and essential oils to replace synthetic antimicrobial agents (Hong, Bae, Moon, & Lee, 2013). Cinnamon has been used as a spice for thousands of years. The most important cinnamon oils are extracted from Cinnamomum zeylanicum, Cinnamomum cassia and Cinnamomum camphora (Jayaprakasha & Rao, 2011). Both C. zeylanicum and C. cassia oils are rich in cinnamaldehyde, which has strong antimicrobial effect (Shan, Cai, Brooks, & Corke, 2007). Studies have shown that cinnamon oil has antibacterial effect on E. coli O157:H7 (Friedman et al., 2002, Smith-Palmer et al., 1998). There is no information about the effect of cinnamon oil on non-O157 STECs. Therefore, the aim of this study was to examine the inhibitory effect of C. cassia oil against the selected “top six” non-O157 STECs.
Section snippets
Essential oil and bacterial strains
C. cassia oil was purchased from Sigma/Aldrich (St. Louis, MO). The non-O157 STEC strains 97-3250 (E. coli O26:H11), DA-21 (E. coli O45:NM), MT#80 (E. coli O103:H2), RD8 (E. coli O111:H2), MT#2 (E. coli O121:H19); IH16 (E. coli O145:NT) were obtained from STEC center at Michigan State University. Bacteria were routinely grown in Luria Broth (LB) at 37 °C with aeration.
Gas chromatography-mass spectrometry (GC/MS) analysis of C. cassia oil
GC/MS analysis of C. cassia oil was performed on an HP6890 series GC system equipped with HP5973 mass selective detector
GC/MS analysis
The GC/MS analysis indicated that the trans-cinnamaldehyde (59.96%) was the main component in the C. cassia oil used in this study and O-methoxy-cinnamaldehyde came second (20.11%) (Fig. 1). The content of cinnamaldehyde along with O-methoxy-cinnamaldehyde and cinnamyl acetate reached 91.74%.
Disc diffusion assay for antibacterial activity of C. cassia oil
The antibacterial activities (diameter of inhibition zone) of C. cassia oil to O26:H11, O45:NM, O103:H2, O111:H2, O121:H19, and O145:NT are summarized in Fig. 2. As low as 5 μL of 4% (v/v) C. cassia oil per
Discussion
While several studies have reported the antibacterial effect of cinnamon on E. coli O157:H7 (Friedman et al., 2002, Smith-Palmer et al., 1998), its efficacy on non-O157 STEC has not been tested. The importance of non-O157 STECs had been under-appreciated until increasing outbreaks occur in recent years associated with non-O157 STECs (Bettelheim, 2007). In this study, the inhibitory effect of C. cassia oil on the CDC “top six” non-O157 was assessed.
In C. cassia oil, 11 compounds were identified.
Conclusion
C. cassia oil effectively inhibits the growth of non-O157 STECs at concentration as low as 0.025% (v/v). However, the inhibitory effects of C. cassia oil are strain and population dependent, with more efficacy against O121:H19, and O145:NT. Our data suggest that C. cassia oil has the potential to be used as a natural antibacterial agent to control non-O157 STECs in food industry.
Financial interest
The authors declare no competing financial interest.
Acknowledgments
We thank Drs. Shuming Zhang and Lee Deobald for their assistance in Gas chromatography-mass spectrometry (GC/MS) analysis. This activity was funded by an Emerging Research Issues Internal Competitive Grant from the Agricultural Research Center at Washington State University, College of Agricultural, Human, and Natural Resource Sciences.
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