Jung-Whan Chon

Development of RNA aptamers for detection of Salmonella Enteritidis.

We developed and evaluated RNA aptamers to analyze their potential for use in detecting Salmonella Enteritidis. The selected aptamer was observed to specifically bind to Salmonella Enteritidis without any cross-reactivity to other Salmonella serovars. Thus, this study suggests that aptamers specific to Salmonella Enteritidis have a high potential for use in presumptive presumptive screening methods or alternative serotyping methods.

Toxin profile, antibiotic resistance, and phenotypic and molecular characterization of Bacillus cereus in Sunsik

Sunsik, a ready-to-eat food in Korea, is comprised of various agricultural and marine products, and has been an important concern in Bacillus cereus food poisoning. The aim of this study was to investigate the toxin profiles, genotypic and phenotypic patterns as well as antibiotic resistance of B. cereus strains isolated from Sunsik. A subtyping method known as automated repetitive sequence-based PCR system (DiversiLab™) was used to assess the intraspecific biodiversity of these isolates. Thirty-five B. cereus strains were isolated from 100 commercial Sunsik samples, all of which harbored at least 1 enterotoxin gene. The detection rates of nheABC, hblCDA, cytK, and entFM enterotoxin gene among all isolates were 97%, 86%, 77%, and 100%, respectively. Most strains also produced corresponding enterotoxins such as HBL (83%) and NHE (94%). One strain (2.9%) carried the emetic toxin genes, including ces and EM1, and was positive for the HEp-2 cell emetic toxin assay. Most strains were positive for various biochemical tests such as salicin hydrolysis (86%), starch fermentation (89%), hemolysis (89%), motility test (100%) and lecithinase hydrolysis (89%). All isolates were susceptible to most antibiotics although they were highly resistant to β-lactam antibiotics. By using the automated rep-PCR system, all isolates were successfully differentiated, indicating the diversity of B. cereus strains present in Sunsik.

Isolation and Characterization of Cronobacter from Desiccated Foods in Korea

A total of 115 desiccated food samples, including agricultural and marine products, were investigated for the presence of Cronobacter. Cronobacter species were characterized with biochemical tests. Antibiotic resistance was assessed with the disk diffusion method, and the molecular subtypes of Cronobacter isolates were identified using an automated repetitive sequence-based PCR (rep-PCR) system. A total of 18 (15.7%) Cronobacter strains were isolated from 115 desiccated food products. Fifteen Cronobacter isolates were C. sakazakii (13%), followed by 2 C. dublinensis (1.7%), and 1 C. universalis (0.9%). The most common antibiotic resistance of Cronobacter observed was against cephalothin (77.8%) followed by ampicillin (5.6%). With exception of 2 strains, all Cronobacter strains isolated from different sources were successfully differentiated by using the automated rep-PCR system, indicating that it can be used for the purpose of contamination or outbreak source tracking of the bacteria.

Comparison of 2 types of broths and 3 selective agars for the detection of Campylobacter species in whole-chicken carcass-rinse samples

In this study, we compared the effectiveness of 2 types of Bolton broths and 3 selective media for isolating Campylobacter spp. from naturally contaminated whole-chicken carcass-rinse samples. One hundred chickens were rinsed with buffered peptone water, and the rinses were added to 2× Bolton broth (with or without blood supplementation). The samples were incubated and then streaked onto Preston agar, modified cefoperazone charcoal deoxycholate agar (mCCDA), and Campy-Cefex agar, which was followed by incubation under microaerobic conditions. No statistical differences were observed (P > 0.05) in isolation rate and selectivity between the 2 types of Bolton broths. Among the 3 selective agars, Preston agar yielded a significantly (P < 0.05) better isolation rate and selectivity. The Campy-Cefex agar, which is recommended by many food authorities for its high quantitative detection ability, showed extensive contamination with competing microorganisms and exhibited the lowest isolation rate and selectivity.

Prevalence, phenotypic traits and molecular characterization of emetic toxin‐producing Bacillus cereus strains isolated from human stools in Korea

Aims:u2002 To investigate the prevalence and genotypic/phenotypic characters of emetic toxin‐producing Bacillus cereus strains isolated from sporadic food poisoning cases in Korea.

Improvement of modified charcoal-cefoperazone-deoxycholate agar by addition of potassium clavulanate for detecting Campylobacter spp. in chicken carcass rinse.

The presence of extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli (E. coli) in raw poultry is one of the most common factors that interfere with the isolation of Campylobacter by cefoperazone-based selective agar. The performance of modified charcoal-cefoperazone-deoxycholate agar (mCCDA) was improved by addition of an ESBL inhibitor, potassium clavulanate (0.5 mg/L). The ability of the supplemented medium (C-mCCDA) to detect Campylobacter species from chicken carcass rinse was compared with that of normal mCCDA. The isolation rate using C-mCCDA was significantly (p<0.05) higher compared with that using mCCDA (C-mCCDA, 67 out of 120; mCCDA, 38 out of 120). Furthermore, the selectivity of the C-mCCDA as assessed by comparing the number of contaminated plates (C-mCCDA, 44 out of 120; mCCDA, 110 out of 120) and growth index (C-mCCDA, 1.76; mCCDA, 2.79) of competing flora was also better (p<0.05) than that of mCCDA.

Prevalence, characterization, and antimicrobial susceptibility of Salmonella Gallinarum isolated from eggs produced in conventional or organic farms in South Korea

To determine the prevalence of Salmonella serotype Enteritidis in eggs in South Korea, we conducted a microbiological survey of commercially available eggs produced in conventional or organic farms during the period from 2010 to 2012. The contents of 7,000 raw shell eggs (6,000 of conventional and 1,000 of organic origin) were examined to evaluate the extent and type of Salmonella Enteritidis contamination. A total of 26 salmonellae (7.4% of all pooled samples) were isolated from 350 homogenized pools, each containing the contents from 20 eggs. An unexpected and particularly surprising finding was that all the Salmonella isolates were serotyped as Salmonella Gallinarum. Salmonella Gallinarum was more common in eggs from organic farms: 10 of 50 egg pools (20.0%) from organic and 16 of 300 egg pools (5.3%) from conventional farms tested positive for Salmonella Gallinarum. However, organic and conventional isolates showed similar antimicrobial susceptibilities. All the isolates and a vaccine strain, SG 9R, which has been widely used in South Korea, were further characterized using the automated repetitive sequence-based PCR (rep-PCR) system, DiversiLab, to ascertain the molecular subtypes and to identify differences from the vaccine strain. The rep-PCR identified 2 distinct clusters among the 26 Salmonella Gallinarum isolates with a greater than 96% similarity index. These were clearly differentiated from the vaccine strain, SG 9R, with which there was a less than 86% similarity index. We found there was low genetic heterogeneity among isolates within each cluster and were able to distinguish wild type strains from the live vaccine strain (SG 9R) using the DiversiLab system.

Development of a selective enrichment broth supplemented with bacteriological charcoal and a high concentration of polymyxin B for the detection of Campylobacter jejuni and Campylobacter coli in chicken carcass rinses.

A new Campylobacter-selective enrichment broth supplemented with bacteriological charcoal and a high concentration of polymyxin B was developed (charcoal-cefoperazone-polymyxin B-deoxycholate broth; CCPD broth). We compared the ability of CCPD broth to detect Campylobacter jejuni and Campylobacter coli in chicken carcass rinses to that of modified Bolton (mBolton) broth. Eighty whole chickens were purchased from retailers and rinsed with 400 mL buffered peptone water. The rinsed samples were enriched with 2× blood-free mBolton enrichment broth or 2× CCPD broth at 42 °C for 48 h and then streaked onto modified charcoal-cefoperazone-deoxycholate agar (mCCDA). The Campylobacter isolation rate was significantly higher in CCPD broth than in mBolton broth (CCPD broth, 61 out of 80; mBolton broth, 34 out of 80; p<0.05). Moreover, the selectivity of CCPD broth agar was also superior to that of mBolton broth when comparing the number of contaminated mCCDA plates (CCPD broth, 16 out of 80; mBolton broth, 58 out of 80; p<0.05) and the growth index of competing flora (CCPD broth, 1.4; mBolton broth, 2.9; p<0.05).

Evaluation of selective enrichment broths and chromogenic media for Salmonella detection in highly contaminated chicken carcasses

We evaluated the effectiveness of 2 selective enrichment broths, Rappaport-Vassiliadis Soy (RVS) and Muller-Kauffmann tetrathionate with novobiocin (MKTTn), for the isolation of Salmonella from chicken carcasses obtained from 3 different types of retail markets. We also compared a chromogenic agar, chromID Salmonella agar (SM-ID 2), with a classic plating medium, xylose lysine deoxycholate agar (XLD). Salmonella were isolated from 118 of the 180 samples (65.5%). Salmonella were detected in 105 samples (88%) plated on XLD and 111 samples (94%) plated on SM-ID 2 when RVS broth was used for enrichment, and 43 samples (36.4%) plated on XLD and 67 samples (56.8%) plated on SM-ID 2 when the MKTTn broth was used. The highest sensitivity was found in the RVS-XLD combination (0.99), followed by RVS-SM-ID 2 (0.97). The specificity of the RVS-SM-ID 2 combination was the highest (0.89), but that of the MKTTn-XLD combination was zero. The results of this study indicate that the selective enrichment broths had a great effect on the sensitivity and specificity of plating media, and our study confirms that the RVS broth is the most suitable enrichment for the investigation of Salmonella in chicken carcasses. This observation suggests that use of RVS broth for selective enrichment and SM-ID 2 for selective isolation may be the best combination to determine the presence of Salmonella in chicken carcasses.

Supplementation of Bolton broth with triclosan improves detection of Campylobacter jejuni and Campylobacter coli in chicken carcass rinse

We compared Bolton enrichment broth supplemented with antimicrobial triclosan (T-Bolton broth) and normal Bolton broth for the isolation of Campylobacter jejuni (C. jejuni) and Campylobacter coli (C. coli) from chicken carcass rinse. Whole chickens were rinsed with buffered peptone water prior to enrichment in normal Bolton broth or T-Bolton broth, followed by inoculation onto modified charcoal-cefoperazone-deoxycholate agar (mCCDA). Suspect colonies were confirmed by PCR. We observed a significantly higher number of C. jejuni or C. coli-positive samples in the T-Bolton broth (71.3%) than in the normal Bolton broth (27.5%) (p<0.05). Furthermore, the number of contaminated mCCDA plates was lower after enrichment in T-Bolton broth (3.8%) than in the normal Bolton broth (75%) (p<0.05), indicating that T-Bolton broth has higher selectivity. Finally, we identified extended-spectrum β-lactamase-producing Escherichia coli as the predominant competing flora in normal Bolton broth. In conclusion, the use of T-Bolton broth results in significant elimination of competing bacteria.