Microbiological quality of ready-to-eat salads: an underestimated vehicle of bacteria and clinically relevant antibiotic resistance genes

The Hurdle Approach-A Holistic Concept for Controlling Food Safety Risks Associated With Pathogenic Bacterial Contamination of Leafy Green Vegetables. A Review

Consumers appreciate leafy green vegetables such as baby leaves for their convenience and wholesomeness and for adding a variety of tastes and colors to their plate. In Western cuisine, leafy green vegetables are usually eaten fresh and raw, with no step in the long chain from seed to consumption where potentially harmful microorganisms could be completely eliminated, e.g., through heating. A concerning trend in recent years is disease outbreaks caused by various leafy vegetable crops and one of the most important foodborne pathogens in this context is Shiga toxin-producing Escherichia coli (STEC). Other pathogens such as Salmonella, Shigella, Yersinia enterocolitica and Listeria monocytogenes should also be considered in disease risk analysis, as they have been implicated in outbreaks associated with leafy greens. These pathogens may enter the horticultural value network during primary production in field or greenhouse via irrigation, at harvest, during processing and distribution or in the home kitchen/restaurant. The hurdle approach involves combining several mitigating approaches, each of which is insufficient on its own, to control or even eliminate pathogens in food products. Since the food chain system for leafy green vegetables contains no absolute kill step for pathogens, use of hurdles at critical points could enable control of pathogens that pose a human health risk. Hurdles should be combined so as to decrease the risk due to pathogenic microbes and also to improve microbial stability, shelf-life, nutritional properties and sensory quality of leafy vegetables. The hurdle toolbox includes different options, such as physical, physiochemical and microbial hurdles. The goal for leafy green vegetables is multi-target preservation through intelligently applied hurdles. This review describes hurdles that could be used for leafy green vegetables and their biological basis, and identifies prospective hurdles that need attention in future research.

High microbial loads found in minimally-processed sliced mushrooms from Italian market

There is an increased consumer interest in minimally processed vegetables that has led to the development of products, such as pre-cut sliced mushrooms. Few data are available on the hygienic condition and the presence of foodborne pathogens in such products. Therefore, the current study aimed to evaluate the safety and hygienic characteristics of both ready-to-eat and ready-to-cook, pre-cut sliced mushrooms obtained from a local Italian market. For the evaluation of the hygienic condition, the aerobic mesophilic bacteria, aerobic psychrotrophic bacteria and Escherichia coli enumerations were performed. Salmonella spp., Listeria monocytogenes and Campylobacter spp. were considered in the assessment of the foodborne pathogens. High microbial loads were detected, including counts higher than 5 log CFU/g for E. coli and 6 log CFU/g for the other bacteria counts considered, but no pathogens were found. Ready-to-eat and ready-to-cook products differed only for aerobic mesophilic counts (7.87 and 8.26 log CFU/g, respectively, P=0.003). Strategies to enhance the hygienic level of the mushrooms, particularly the ready-to-eat products, are needed.

Draft Genome Sequence of Multidrug-Resistant Strain Citrobacter portucalensis MBTC-1222, Isolated from Uziza (Piper guineense) Leaves in Nigeria

In this work, we report the draft whole-genome sequence of the multiply antibiotic-resistant Citrobacter portucalensis strain MBTC-1222 isolated from the uziza leafy vegetable in Nigeria. Sequence analysis showed the assembled genome size to be 4,881,935 bp, containing 4,603 protein-coding genes, 131 pseudogenes, 7 rRNAs, 74 tRNAs, and 9 noncoding RNAs (ncRNAs).

Antimicrobial resistance and genetic diversity in ceftazidime non-susceptible bacterial pathogens from ready-to-eat street foods in three Taiwanese cities

Bacterial contamination of ready-to-eat (RTE) street foods is a major concern worldwide. Dissemination of antibiotic resistant pathogens from food is an emerging public-health threat. To investigate the prevalence of antibiotic resistance genes and ceftazidime resistance-associated efflux pumps in foodborne pathogens, 270 RTE street foods samples were collected in three densely populated Taiwanese cities. Among 70 ceftazidime non-susceptible isolates, 21 Stenotrophomonas maltophilia, 12 Pseudomonas spp., 22 Acinetobacter spp., and 15 Enterobacteriaceae isolates were identified. Phylogenetic analyses revealed high levels of genetic diversity between all of the different strains. Multi-drug resistance was observed in 86.4% (19/22) of Acinetobacter spp., 100% (12/12) of Pseudomonas spp., 71.4% (15/21) of S. maltophilia, and 93.3% (14/15) of Enterobacteriaceae. Of 70 ceftazidime non-susceptible isolates, 13 contained ESBLs or plasmid-mediated ampC genes and 23 contained ceftazidime resistance-associated efflux pumps, with Acinetobacter spp. identified as predominant isolate (69.6%; 16/23). AdeIJK pump RNA expression in Acinetobacter isolates was 1.9- to 2-fold higher in active efflux strains. Nine clinically resistant genes were detected: catIII and cmlA (chloramphenicol); aacC1, aacC2, aacC3, and aacC4 (gentamicin); tet(A), tet(C), and tet(D) (tetracycline). The scope and abundance of multidrug-resistant bacteria described in this report underscores the need for ongoing and/or expanded RTE monitoring and control measures.

Antimicrobial resistance in diarrheagenic Escherichia coli from ready-to-eat foods

Certain subgroups of Escherichia coli have congenital or acquired virulence properties that allow them to cause a wide spectrum of disease. The aim of this study was to investigate the occurrence of diarrheagenic E. coli strains in ready-to-eat (RTE) foods produced in institutional, commercial and hotel restaurants in Salvador, Brazil. The presence of virulent isolates and antimicrobial resistance were evaluated. Four hundred forty-six samples were collected and grouped into cereals and vegetables, meat-based preparations, cooked salads, raw salads, garnishes, soups and sauces, desserts and juices. E. coli were detected using the most probable number method, the presence of virulence factors in isolates was determined by polymerase chain reaction (PCR) assays, and antibiotic resistance was analyzed using the disc diffusion method. In total, 15 isolates (3.1%) of E. coli were recovered; raw salads had the highest detection rate, 1.4%, followed by cooked salads, 0.8%; meat-based preparations, 0.4%; and cereals and vegetables, 0.4%. PCR assays showed that none of the isolates had the virulence genes cnf1, cnf2, eae, sta, lt1, stx1, stx2 or cdtB. The isolates showed resistance to nine antibiotics of the 15 tested, and the highest levels of resistance were found for sulfamethoxazole/trimethoprim, tetracycline, ampicillin, and chloramphenicol (13.3% of isolates for each antibiotic). One isolate from cooked salad had plasmid-mediated multidrug resistance to tetracycline, trimethoprim/sulfamethoxazole, ampicillin and chloramphenicol. These results suggest that RTE foods, especially raw salads, can be reservoirs of E. coli and facilitate the spread of antibiotic resistance genes to the gastrointestinal microbiota of humans.

From Farms to Markets: Gram-Negative Bacteria Resistant to Third-Generation Cephalosporins in Fruits and Vegetables in a Region of North Africa

The role of food in human exposure to antimicrobial-resistant bacteria is a growing food safety issue. The contribution of fruits and vegetables eaten raw to this exposure is still unclear. The evaluation of contamination levels of fruits, vegetables and the agricultural environment by third-generation cephalosporin (3GC)-resistant Gram-negative bacteria was performed by analyzing 491 samples of fruits and vegetables collected from 5 markets and 7 farms in Bejaia area, north-eastern Mediterranean coast of Algeria. Ninety soil samples and 45 irrigation water samples were also sampled in farms in order to assess them as potential inoculum sources. All samples were investigated at the same time on ceftazidime-containing selective media for 3GC-resistant Gram-negative bacteria detection and on Hektoen media, for Salmonella spp. presence. The bacteria isolated (n = 30) from fruits and vegetables, soil and irrigation water collected in the farms were almost all non-fermenting bacterial species (Stenotrophomonas, Acinetobacter, Pseudomonas, Ochrobactrum) except one strain of Enterobacter cloacae and two strains of Citrobacter murliniae, isolated on one cucumber and two tomato samples in the same farm. Greater diversity in bacterial species and antimicrobial resistance profiles was observed at markets: Enterobacteriaceae (n = 41) were as strongly represented as non-fermenting bacteria (n = 37). Among Enterobacteriaceae, E. cloacae (n = 21), and Klebsiella pneumoniae (n = 13) were the most common isolates. Most of the K. pneumoniae isolates were extended-spectrum beta-lactamase (ESBL) producers (n = 11). No Salmonella spp. was recovered in any sample. This study showed that fruits and vegetables including those which may be eaten up raw constitute a reservoir of 3GC-resistant Gram-negative bacteria and multi-drug resistant-bacteria in general that can be transferred to humans through food. The general public should be informed of this hazard for health in order to encourage good domestic hygiene practices. In addition, further investigation is needed throughout the production chain to enrol professionals in actions to reduce this contamination.

Raoultella spp.-clinical significance, infections and susceptibility to antibiotics

The genus Raoultella belongs to the family of Enterobacteriaceae. Raoultella spp. are Gram-negative, aerobic, non-motile rods. This genus can be distinguished from the genus Klebsiella, in that genus use histamine as the only source of carbon in the medium. Also, Raoultella grow at 4 °C and do not produce gas from lactose at 44.5 °C. Raoultella sp. is known to inhabit natural environments (water, soil, plants). The reservoir of Raoultella is the gastrointestinal tract and upper respiratory tract. Raoultella spp. are opportunistic bacteria, which usually cause infections of the biliary tract, pneumonia and bacteraemia in oncologic and with lower immunity patients. Raoultella planticola and Raoultella ornithinolytica are the most frequently encountered human pathogens among the genus Raoultella. In this review, the current knowledge on Raoultella infections is summarized.

Occurrence and Characterization of Enteropathogenic Escherichia coli (EPEC) in Retail Ready-to-Eat Foods in China

Enteropathogenic Escherichia coli (EPEC) is an important foodborne pathogen that potentially causes infant and adult diarrhea. The occurrence and characteristics of EPEC in retail ready-to-eat (RTE) foods have not been thoroughly investigated in China. This study aimed to investigate EPEC occurrence in retail RTE foods sold in the markets of China and to characterize the isolated EPEC by serotyping, virulence gene analyses, antibiotic susceptibility test, and molecular typing based on enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR). From May 2012 to April 2013, 459 RTE food samples were collected from retail markets in 24 cities of China. E. coli in general, and EPEC specifically, were detected in 144 (31.4%) and 39 (8.5%) samples, respectively. Cold vegetable in sauce was the food type most frequently contaminated with EPEC (18.6%). Of 39 EPEC isolates, 38 were atypical EPEC (eae+) and 1 was typical EPEC (eae+bfpA+) by multiplex PCR assays. The virulence genes espA, espB, tir, and iha were detected in 12, 9, 2, and 1 of 39 isolates, respectively, while genes toxB, etpD, katP, and saa were not detected. O-antigen serotyping results showed that among 28 typeable isolates, the most common serotype was O119, followed by O26, O111, and O128. Many isolates were resistant to tetracycline (64.1%; 25/39), ampicillin (48.7%; 19/39), and trimethoprim/sulfamethoxazole (48.7%; 19/39). ERIC-PCR indicated high genetic diversity in EPEC strains, which classified 42 strains (39 isolates and 3 reference strains) into 32 different profiles with a discrimination index of 0.981. The findings of this study highlight the need for close surveillance of the RTE foods at the level of production, packaging, and storage to minimize risks of foodborne disease.

Phylogeny and Comparative Genomics Unveil Independent Diversification Trajectories of qnrB and Genetic Platforms within Particular Citrobacter Species

To gain insights into the diversification trajectories of qnrB genes, a phylogenetic and comparative genomics analysis of these genes and their surrounding genetic sequences was performed. For this purpose, Citrobacter sp. isolates (n = 21) and genome or plasmid sequences (n = 56) available in public databases harboring complete or truncated qnrB genes were analyzed. Citrobacter species identification was performed by phylogenetic analysis of different genotypic markers. The clonal relatedness among isolates, the location of qnrB genes, and the genetic surroundings of qnrB genes were investigated by pulsed-field gel electrophoresis (PFGE), S1-/I-CeuI-PFGE and hybridization, and PCR mapping and sequencing, respectively. Identification of Citrobacter isolates was achieved using leuS and recN gene sequences, and isolates characterized in this study were diverse and harbored chromosomal qnrB genes. Phylogenetic analysis of all known qnrB genes revealed seven main clusters and two branches, with most of them included in two clusters. Specific platforms (comprising pspF and sapA and varying in synteny and/or identity of other genes and intergenic regions) were associated with each one of these qnrB clusters, and the reliable identification of all Citrobacter isolates revealed that each platform evolved in different recognizable (Citrobacter freundii, C. braakii, C. werkmanii, and C. pasteurii) and putatively new species. A high identity was observed between some of the platforms identified in the chromosome of Citrobacter spp. and in different plasmids of Enterobacteriaceae. Our data corroborate Citrobacter as the origin of qnrB and further suggest divergent evolution of closely related qnrB genes/platforms in particular Citrobacter spp., which were delineated using particular genotypic markers.

Assessment of the Prevalence of Extended-Spectrum β-Lactamase-Producing Enterobacteriaceae in Ready-to-Eat Salads, Fresh-Cut Fruit, and Sprouts from the Swiss Market

Ready-to-eat (RTE) prepacked salads and fruit have been successfully marketed for the last decade in Switzerland and are increasingly important as a component of everyday diets. To determine whether extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae are present in RTE salads, fresh-cut fruit, and sprouts on the Swiss market, samples of 238 mixed and unmixed RTE produce from a large production plant and 23 sprout samples from two sprout farms were analyzed. Further, four samples from the production plant's recycled wash water, which is used for crop irrigation, were analyzed. Twelve (5%) of the 238 RTE products and one of the recycled wash water samples yielded ESBL-producing Enterobacteriaceae. Strain identification and PCR analysis of the blaESBL genes revealed Kluyvera ascorbata isolated from a tomato sample harboring a blaCTX-M-2-like gene; multidrug-resistant (MDR) Enterobacter cloacae detected in a chives sample imported from Spain harboring the clinically important bla(CTX-M-15) gene; and 10 Serratia spp. isolated from mixed salads (bla(FONA-2) and bla(FONA-2)-like genes were found in 6 [60%] and bla(FONA-4)-like and bla(FONA-5)-like genes were each found in 2 [20%] of the isolates). The recycled wash water sample tested positive for one extraintestinal pathogenic MDR Escherichia coli B2:ST131 harboring bla(CTX-M-27) and for one MDR E. coli A:ST88 containing bla(CTX-M-3). None of the sprout samples tested positive for ESBL-producing Enterobacteriaceae. Overall, the majority of the Enterobacteriaceae detected in Swiss RTE produce were environmental strains producing minor ESBLs. The detection of an isolate producing a clinically important ESBL in a single sample and of an international circulating pathogenic strain (B2:ST131) in recycled wash water highlights the importance of surveillance of fresh produce and of recycled wash water that will be reused for irrigation purposes.

Microbiological Quality of Ready-to-Eat Vegetables Collected in Mexico City: Occurrence of Aerobic-Mesophilic Bacteria, Fecal Coliforms, and Potentially Pathogenic Nontuberculous Mycobacteria

The aims of this study were to evaluate the microbiological quality and the occurrence of nontuberculous mycobacteria (NTM) in a variety of salads and sprouts from supermarkets and street vendors in Mexico City. Aerobic-mesophilic bacteria (AMB) were present in 100% of RTE-salads samples; 59% of samples were outside guidelines range (>5.17 log₁₀ CFU per g). Although fecal coliforms (FC) were present in 32% of samples, only 8% of them exceeded the permissible limit (100 MPN/g). Regarding the 100 RTE-sprouts, all samples were also positive for AMB and total coliforms (TC) and 69% for FC. Seven NTM species were recovered from 7 salad samples; they included three M. fortuitum, two M. chelonae, one M. mucogenicum, and one M. sp. Twelve RTE-sprouts samples harbored NTM, which were identified as M. porcinum (five), M. abscessus (two), M. gordonae (two), M. mucogenicum (two), and M. avium complex (one). Most RTE-salads and RTE-sprouts had unsatisfactory microbiological quality and some harbored NTM associated with illness. No correlation between the presence of coliforms and NTM was found. Overall, these results suggest that RTE-salads and RTE-sprouts might function as vehicles for NTM transmission in humans; hence, proper handling and treatment before consumption of such products might be recommendable.

Use of propolis in the sanitization of lettuce

The present study aimed to determine the effectiveness of propolis in reducing the microbial load in ready-to-eat (RTE) and fresh whole head (FWH) lettuces (Lactuca sativa L.) type Batavia. Two sanitizing solutions were employed: sodium hypochlorite (SH) and propolis (PS), during 15 and 30 min. Tap water (TW) was used as a control. Regarding the mean reduction on aerobic mesophiles, psychrotrophic and fecal coliforms, the SH and PS treatments showed the same pattern of variation. In all cases, PS was slightly more effective in the microbiological reduction in comparison with commercial SH. Reductions between two and three log cycles were obtained with PS on aerobic mesophiles and psychrotrophic counts. The information obtained in the present study can be used to evaluate the potential use of propolis as product for sanitizing other vegetables and for developing other food preservation technologies, with impact on human health.