Evaluation of a multiplex PCR assay for simultaneous identification of Salmonella sp., Salmonella enteritidis and Salmonella typhimurium from environmental swabs of poultry houses

A Multiplex PCR-based assay (m-PCR) with three sets of primers was developed for the detection of all serotypes of Salmonella enterica and the identification of Salmonella Enteritidis and Salmonella Typhimurium. This method was evaluated against a bacteriological method for the analysis of environmental swabs of poultry houses. Samples were preenriched in phosphate-buffered peptone water for 24 h and subjected to three different protocols prior to PCR: (i) an immunomagnetic separation using Dynabeads anti-Salmonella (Dynal); (ii) a DNA extraction procedure using the Instagene matrix; (iii) an additional step of culture on an MSRV medium. With protocols 1 and 2, eight positive results were found by PCR and 20 with the bacteriological method. Protocol 3 combining MSRV and PCR gave similar results to those obtained from bacteriological methods and allowed Salmonella detection within 2 days.     

Detection of Clavibacter michiganensis subsp. michiganensis in tomato seeds using immunomagnetic separation

The use of pathogen-free plant material is the main strategy for controlling bacterial canker of tomato caused by Clavibacter michiganensis subsp. michiganensis. However, detection and isolation of this pathogen from seeds before field or greenhouse cultivation is difficult when the bacterium is at low concentration and associated microbiota are present. Immunomagnetic separation (IMS), based on the use of immunomagnetic beads (IMBs) coated with specific antibodies, was used to capture C. michiganensis subsp. michiganensis cells, allowing removal of non-target bacteria from samples before plating on non-selective medium. Different concentrations of IMBs and of two antisera were tested, showing that IMS with 10(6)IMBs/ml coated with a polyclonal antiserum at 1/3200 dilution recovered more than 50% of target cells from initial inocula of 10(3) to 10(0)CFU/ml. Threshold detection was lower than 10CFU/ml even in seed extracts containing seed debris and high populations of non-target bacteria. The IMS permitted C. michiganensis subsp. michiganensis isolation from naturally infected seeds with higher sensitivity and faster than direct isolation on the semiselective medium currently used and could become a simple viable system for routinely testing tomato seed lots in phytosanitary diagnostic laboratories.     

基于免疫磁分离的三重荧光定量PCR检测食品中沙门氏菌、志贺氏菌和金黄色葡萄球菌

【目的】开发一种同时对食品中沙门氏菌、志贺氏菌和金黄色葡萄球菌快速、灵敏、准确的检测方法。【方法】利用特异性免疫磁球,在37°C条件下从250 m L猪肉增菌液体系中边富集边循环捕获目标菌。快速提取DNA后,利用特异性的引物与探针,对3种食源性致病菌进行三重荧光定量PCR检测。【结果】针对沙门氏菌、志贺氏菌和金黄色葡萄球菌的检测限分别达到2.0、6.8和9.6 CFU/g。方法总体灵敏度、特异性和准确度达到99.2%、100%及99.5%。对151份实际样品进行检测,与国标(GB/T 4789.4-2010、GB 4789.5-2012和GB/T4789.10-2010)方法的检测结果相比,金黄色葡萄球菌有一例阴性偏差。【结论】开发的基于免疫磁分离的三重荧光定量PCR方法,能够在8 h内完成对食品中3种致病菌检测,并且灵敏度高、特异性好、检测准确,可以作为快速应对此类食品安全突发事件的检测手段。     

Comparison of the MSRV method with an in-house conventional method for the detection of Salmonella in various high and low moisture foods

In this trial an in-house conventional method was compared with the MSRV method for Salmonella detection. Various high and low moisture foodstuffs (121 and 116 samples respectively), some either artificially or naturally contaminated, were examined.
Eleven different serotypes of Salmonella were used to inoculate samples and no significant difference was observed in the sensitivity of any of the media used. Significantly lower numbers of false-positive results were obtained with the MSRV agar when compared to MLCB agar.
This work suggests that the MSRV method as used here, could be used to replace conventional Salmonella detection methods for both high and low moisture foodstuffs.     

Differences in growth of Salmonella enterica and Escherichia coli O157:H7 on alfalfa sprouts

Sprout producers have recently been faced with several Salmonella enterica and Escherichia coli O157:H7 outbreaks. Many of the outbreaks have been traced to sprout seeds contaminated with low levels of human pathogens. Alfalfa seeds were inoculated with S. enterica and E. coli O157:H7 strains isolated from alfalfa seeds or other environmental sources and sprouted to examine growth of these human pathogens in association with sprouting seeds. S. enterica strains grew an average of 3.7 log(10) on sprouting seeds over 2 days, while E. coli O157:H7 strains grew significantly less, an average of 2.3 log(10). The initial S. enterica or E. coli O157:H7 inoculum dose and seed-sprouting temperature significantly affected the levels of both S. enterica and E. coli O157:H7 on the sprouts and in the irrigation water, while the frequency of irrigation water replacement affected only the levels of E. coli O157:H7. Colonization of sprouting alfalfa seeds by S. enterica serovar Newport and E. coli O157:H7 strains transformed with a plasmid encoding the green fluorescent protein was examined with fluorescence microscopy. Salmonella serovar Newport colonized both seed coats and sprout roots as aggregates, while E. coli O157:H7 colonized only sprout roots.     

Culture conditions influencing phytase production of Mitsuokella jalaludinii,a new bacterial species from the rumen of cattle

AIMS: To determine the effectiveness of combined treatments with chemicals, heat and ultrasound in killing or removing Salmonella and Escherichia coli O157:H7 on alfalfa seeds intended for sprout production. METHODS AND RESULTS: Alfalfa seeds inoculated with Salmonella or E. coli O157:H7 were treated with ultrasound (38.5-40.5 kHz) in solutions containing 1% Ca(OH)(2), 1% Tween 80, 1% Ca(OH)(2) plus 1% Tween 80, 160 microg ml(-1) Tsunami 200 and 0.5% Fit at 23 and 55 degrees C for 2 and 5 min. Highest reductions were in chemical solutions at 55 degrees C, but seed viability was also reduced compared with treatment at 23 degrees C. Inactivation of Salmonella and E. coli O157:H7 was generally enhanced by simultaneous treatments with ultrasound, chemicals and heat. CONCLUSIONS: Ultrasound treatment, in combination with chemicals and heat, had a modest enhancing effect on the effectiveness of chemicals in killing or removing pathogens on alfalfa seeds. Overall, treatment with 1% Ca(OH)(2) was most effective in killing Salmonella and E. coli O157:H7. SIGNIFICANCE AND IMPACT OF THE STUDY: The use of 1% Ca(OH)(2) instead of 20,000 microg ml(-1) chlorine, which is currently recommended as a sanitizer for seeds intended for sprout production in the US, should be considered. Ultrasound treatment of alfalfa seeds containing Salmonella or E. coli O157:H7, in combination with chemical treatment, contributes to achieving greater reductions in populations of these pathogens, thereby reducing the risk of contamination and the presence of pathogens in sprouts produced from these seeds.     

A comparison of sensitivity between direct plate culture, immunomagnetic separation and polymerase chain reaction for the isolation of enterohemorrhagic Escherichia coli O157]

Sensitivities of direct plate culture (DPC) method, immunomagnetic separation (IMS) method, and polymerase chain reaction (PCR) assay for successful detection Escherichia coli O157 in the food samples were compared. Three lots of minced beef and three lots of radish sprout, both of which were commercially retailed, were enriched with non-selective broth media at 36 degrees C for 6 h. After enrichment, the cultures of the minced beef and those of the radish sprout were found to have background microflora at ca.10(5)-10(7) CFU/ml and ca.10(8) CFU/ml, respectively. The cultures were then experimentally inoculated with E. coli O157 strains at various final concentrations ranging from ca.10 to 10(7) CFU/ml. The samples thus prepared were subjected to the above three methods to evaluate their detection limits. For the samples of minced beef, the detection limits of the DPC method was 10(2) CFU/ml whilst that of the IMS method was ca.10 CFU/ml. For the samples of radish sprout, the detection limits of the DPC method, the IMS method, and the PCR assay were ca.10(4) CFU/ml, ca.10(2) CFU/ml, and ca.10(6) CFU/ml, respectively. There results strongly suggest that the IMS method is most sensitive method for the detection of O157 from food samples among the methods currently available.     

Development of liposome immunoassay for salmonella spp. using immunomagnetic separation and immunoliposome

The ability to detect Salmonella spp. is essential in the prevention of foodborne illness. This study examined a Salmonella spp. detection method involving the application of immunomagnetic separation and immunoliposomes (IMS/IL) encapsulating sulforhodamine B (SRB), a fluorescent dye. A quantitative assay was conducted by measuring the fluorescence intensity of SRB that was produced from an immunomagnetic bead-Salmonella spp.-immunoliposome complex. The results indicated detection limits of 2.7x10(5) and 5.2x10(3) CFU/ml for Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) and Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium), respectivley. The signal/noise ratio was improved by using 4% skim milk as a wash solution rather than 2% BSA. In addition, higher fluorescence intensity was obtained by increasing the liposome size. Compared with the conventional plating method, which takes 3-4 days for the isolation and identification of Salmonella spp., the total assay time of 10 h only including 6 h of culture enrichment was necessary for the Salmonella detection by IMS/IL. These results indicate that the IMS/ IL has great potential as an alternative rapid method for Salmonella detection.     

Inactivation of Salmonella Typhimurium,Escherichia coli O157:H7 and Listeria monocytogenes on alfalfa seeds by the combination treatment of vacuumed hydrogen peroxide vapour and vacuumed dry heat

We evaluated the combined effects of vacuumed hydrogen peroxide vapour (VHPV) and vacuum-sealed dry heat (vacuum heat, VH) to inactivate food-borne pathogens (Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes) on alfalfa seeds. Alfalfa seeds inoculated with food-borne pathogens were sequentially treated initially with 1·0 ml of 0 or 30% VHPV for 1 min and later with dry heat (DH) or VH for 2 h, and the rate of seed germination was evaluated. The combination treatment decreased the populations of three food-borne pathogens below the limit of detection (1·0 log CFU per gram) on alfalfa seeds without decreasing germinability. The sequential treatment using VHPV and VH greatly reduced the total treatment time needed to inactivate pathogens on alfalfa seeds by more than 5 log CFU per gram. These results demonstrate that a combination of VHPV and VH has potentially employed as a new method for pasteurization of alfalfa seeds without affecting their germinability.     

Growth dynamics of Salmonella enterica strains on alfalfa sprouts and in waste seed irrigation water

Alfalfa sprouts and other seed sprouts have been implicated in numerous outbreaks of salmonellosis. The source of these epidemics appears to have been low-level contamination of seeds by Salmonella bacteria that developed into clinically significant populations during the seed germination process. To test the possibility that Salmonella enterica strains carry host range determinants that allow them to grow on alfalfa, strains isolated from alfalfa or other sources were surveyed for their ability to grow on germinating alfalfa seeds. An S. enterica serovar Cubana strain originally isolated from contaminated alfalfa sprouts multiplied most rapidly during the initial 24 h of the seed germination process. Germinating alfalfa seeds supported the multiplication of S. enterica cells prior to the emergence of the root radicle at 72 h. Thereafter, much lower rates of multiplication were apparent. The ability of S. enterica to grow on germinating alfalfa seeds was independent of the serovar, isolation source, or virulence of the strain. Isolates obtained from alfalfa attained population levels similar to those observed for strains isolated from contaminated meat products or stools. Each of the strains could be detected in the waste irrigation water, with populations being strongly correlated with those detected on the germinating alfalfa seeds. The S. enterica strains were capable of utilizing the waste irrigation water as a sole carbon and nitrogen source. S. enterica strains thus appear to grow saprophytically on soluble organics released from seeds during early phases of germination. The ability to detect S. enterica in the waste irrigation water early in the germination process indicates that this method may be used as a simple way to monitor the contamination of sprouts during commercial operations.     

Kinetics and strain specificity of rhizosphere and endophytic colonization by enteric bacteria on seedlings of Medicago sativa and Medicago truncatula

The presence of human-pathogenic, enteric bacteria on the surface and in the interior of raw produce is a significant health concern. Several aspects of the biology of the interaction between these bacteria and alfalfa (Medicago sativa) seedlings are addressed here. A collection of enteric bacteria associated with alfalfa sprout contaminations, along with Escherichia coli K-12, Salmonella enterica serotype Typhimurium strain ATCC 14028, and an endophyte of maize, Klebsiella pneumoniae 342, were labeled with green fluorescent protein, and their abilities to colonize the rhizosphere and the interior of the plant were compared. These strains differed widely in their endophytic colonization abilities, with K. pneumoniae 342 and E. coli K-12 being the best and worst colonizers, respectively. The abilities of the pathogens were between those of K. pneumoniae 342 and E. coli K-12. All Salmonella bacteria colonized the interiors of the seedlings in high numbers with an inoculum of 10(2) CFU, although infection characteristics were different for each strain. For most strains, a strong correlation between endophytic colonization and rhizosphere colonization was observed. These results show significant strain specificity for plant entry by these strains. Significant colonization of lateral root cracks was observed, suggesting that this may be the site of entry into the plant for these bacteria. At low inoculum levels, a symbiosis mutant of Medicago truncatula, dmi1, was colonized in higher numbers on the rhizosphere and in the interior by a Salmonella endophyte than was the wild-type host. Endophytic entry of M. truncatula appears to occur by a mechanism independent of the symbiotic infections by Sinorhizobium meliloti or mycorrhizal fungi.     

Evaluation of the efficacy of immunomagnetic separation for the detection of Clavibacter michiganensis subsp. michiganensis in tomato seeds

Aims: To evaluate the effectiveness of the optimized immunomagnetic separation (IMS)‐plating protocol in relation to other culture, serological and molecular techniques currently used for Clavibacter michiganensis subsp. michiganensis in seed‐testing laboratories. Methods and results: Bacterial suspensions, tomato seed extracts spiked with the pathogen and naturally infected seeds were IMS‐plated for the detection of C. m. subsp. michiganensis. These results were compared with plating on general (YPGA) and semiselective (mSCM) media, double‐antibody sandwich enzyme‐linked immunosorbent assay (DAS‐ELISA), immunofluorescent assay (IF) or polymerase chain reaction (PCR). Different seed lots and pathogen strains were also tested. IMS‐plating allowed the detection of less than 10 CFU ml^?1 of pathogen in all assayed samples. The mSCM medium provided positive results for 10 CFU ml^?1 in naturally infected seeds, but up to 14 days was necessary for the typical colonies of the target to be come visible. By serological techniques, 10³ and up to 10⁴ CFU ml^?1 were detected by IF and ELISA, respectively. DNA extraction was required to obtain positive results by PCR in seed extracts containing 10³ CFU ml^?1 or more. Conclusions: Among the evaluated methods, IMS‐plating provided the best results regarding sensitivity and specificity for C. m. subsp. michiganensis detection, allowing the recovery of viable bacteria from seed extracts. Significance and impact of the study: IMS‐plating increases isolation rates of C. m. subsp. michiganensis and could improve standard protocols currently used for routine analysis.     

Efficacy of chlorine and heat treatment in killing Salmonella stanley inoculated onto alfalfa seeds and growth and survival of the pathogen during sprouting and storage.

The efficacy of chlorine and hot water treatments in killing Salmonella stanley inoculated onto alfalfa seeds was determined. Treatment of seeds containing 10(2) to 10(3) CFU/g in 100-micrograms/ml active chlorine solution for 5 or 10 min caused a significant (P < or = 0.05) reduction in population, and treatment in 290-micrograms/ml chlorine solution resulted in a significant reduction compared with treatment in 100 micrograms of chlorine per ml. However, concentrations of chlorine of up to 1,010 micrograms/ml failed to result in further significant reductions. Treatment of seeds containing 10(1) to 10(2) CFU of S. stanley per g for 5 min in a solution containing 2,040 micrograms of chlorine per ml reduced the population to undetectable levels (< 1 CFU/g). Treatment of seeds in water for 5 or 10 min at 54 degrees C caused a significant reduction in the S. stanley population, and treatment at > or = 57 degrees C reduced populations to < or = 1 CFU/g. However, treatment at > or = 54 degrees C for 10 min caused a substantial reduction in viability of the seeds. Treatment at 57 or 60 degrees C for 5 min appears to be effective in killing S. stanley without substantially decreasing germinability of seeds. Storage of seeds for 8 to 9 weeks at 8 and 21 degrees C resulted in reductions in populations of S. stanley of about 1 log10 and 2 log10 CFU/g, respectively. The behavior of S. stanley on seeds during soaking germination, sprouting, and refrigerated storage of sprouts was determined. An initial population of 3.29 log10 CFU/g increased slightly during 6 h of soaking, by about 10(3) CFU/g during a 24-h germination period, and by an additional 10 CFU/g during a 72-h sprouting stage. A population of 10(7) CFU/g of mature alfalfa sprouts was detected throughout a subsequent 10-day storage period at 5 degrees C. These studies indicate that while populations of S. stanley can be greatly reduced, elimination of this organism from alfalfa seeds may not be reliably achieved with traditional disinfection procedures. If S. stanley is present on seeds at the initiation of the sprout production process, populations exceeding 10(7) CFU/g can develop and survive on mature sprouts exposed to handling practices used in commercial production and marketing.     

Specific detection of Salmonella spp. by multiplex polymerase chain reaction.

Three sets of oligonucleotide primers were used in the polymerase chain reaction (PCR) assay to detect Salmonella species. phoP primers specific to the phoP/phoQ loci of coliform pathogenic bacteria such as Salmonella, Shigella, Escherichia coli, and Citrobacter species served as presumptive indicators of enteric bacteria. In addition to the phoP primers, the Hin and the H-1i primers, which targeted a 236-bp region of hin/H2 and a 173-bp region of the H-1i flagellin gene, respectively, were used. Both Hin and H-1i primers are specific to motile Salmonella species and are not present in Shigella, E. coli, or Citrobacter species. Thus, by multiplex PCR amplification, Salmonella species including Salmonella typhi, Salmonella typhimurium, Salmonella paratyphi A, and Salmonella enteritidis can be specifically detected. Optimal reaction conditions have been described to demonstrate this specific, sensitive detection of Salmonella species. By using agarose gel electrophoresis for detection of the PCR-amplified products, the sensitivity of detection was 10(2) CFU after 25 cycles of PCR and 1 (10(0)) CFU after a 50-cycle double PCR. The efficacy of these primers was demonstrated on environmental isolates which had previously been confirmed as Salmonella species by the use of conventional cultural techniques. In addition, positive amplifications resulted from Salmonella species in environmental samples including soil and water.     

Rapid concentration of bacteria using submicron magnetic anion exchangers for improving PCR-based multiplex pathogen detection

Rapid concentration of bacterial targets from dilute solutions to improve subsequent PCR detection is investigated in this study. Submicron (average size 500 nm) superparamagnetic anion-exchangers (SiMAG-DEAE) were used successfully to concentrate target bacteria from very dilute solutions. A mass-balance model predicted that for Escherichia coli, the extent of cell concentrating increases almost linearly with increasing sample/SiMAG volume ratio up to about 2000, accompanied by only a slight decrease in the capture efficiency (< 10%). Our experimental data generally support this analysis in that the SiMAG beads concentrated bacterial targets by two to three orders of magnitude using a sample/bead volume ratio of about 1000, and lowered the PCR detection limit to a level of 10² CFU/mL, from 10⁴ to 10⁵ CFU/mL without concentrating. Several target bacteria can be concentrated concurrently and detected via multiplex PCR, as illustrated using E. coli and Agrobacterium tumefaciens as model bacteria. Finally, concentration and detection of bacteria in fresh produce samples were demonstrated. The integration of submicron magnetic ion exchangers and PCR detection provides an appealing alternative to immunomagnetic separation/PCR in improving pathogen detection.     

免疫磁珠富集技术联合选择性培养基快速检测单增李斯特菌

单增李斯特菌是一种危害极大的食源性致病菌,建立快速及特异的检测方法对于食品安全监控尤为重要。文中联合免疫磁珠与选择性培养基对不同浓度(101~105CFU/mL)单增李斯特菌进行检测,并对3种李斯特菌、金黄色葡萄球菌及副溶血弧菌进行交叉试验;同时模拟食物污染,探索免疫磁珠-平板法检测样品的检测限以及该方法的最快检测时间。结果显示特异性免疫磁珠联合选择性平板法可检出浓度为103CFU/mL及以上的单增李斯特菌;牛奶样品仅需6 h增菌能被检出,检测限为0.7 CFU/mL。联合使用免疫磁珠富集技术与选择性培养基,能在30 h内完成对牛奶样品的检测,较国标法减少38 h以上,且具有同等的灵敏度。     

Detection of Salmonella enterica serovar Typhi (S. Typhi) by selective amplification of invA, viaB, fliC-d and prt genes by polymerase chain reaction in mutiplex format

AIMS: Development of a PCR assay that can target multiple genes for rapid detection of Salmonella enterica serovar Typhi (S. Typhi) from water and food samples. METHODS AND RESULTS: PCR primers for invasion, O, H and Vi antigen genes, invA, prt, fliC-d and viaB were designed and used for the rapid detection of S. Typhi by multiplex PCR. Internal amplification control, which co-amplified with prt primers, was also included in the assay. The results showed that all cultures of Salmonella were accurately identified by the assay with no nonspecific amplification in other cultures. The assay had 100% detection probability when a cell suspension of 10(4) CFU ml(-1) (500 CFU per reaction) was used. Salmonella Typhi bacteria were artificially inoculated in the water and food (milk and meat rinse) samples and detected by mPCR after overnight pre-enrichment in buffered peptone water. No Salmonella bacteria could be detected from water samples collected from the field by mPCR or standard culture method. CONCLUSIONS: The developed mPCR assay provides specific detection of S. Typhi. SIGNIFICANCE AND IMPACT OF THE STUDY: Rapid methods for detection of S. Typhi from complex environmental matrices are almost nonexistent. The mPCR assay reported in this study can be useful to identify S. Typhi bacteria in field environmental samples.     

Identification by a multiplex PCR-based assay of Salmonella Typhimurium and Salmonella Enteritidis strains from environmental swabs of poultry houses

A multiplex-PCR-based assay (m-PCR) was developed for the detection of Salmonella and for the identification of the two serotypes Enteritidis and Typhimurium. Three sets of primers selected from different genomic sequences amplified a 429 bp fragment specific for the genus Salmonella within a randomly cloned sequence, a 559 bp target specific for Salmonella Typhimurium within the fliC gene and a 312 bp fragment specific for Salmonella Enteritidis within the sefA gene. The m-PCR-based assay was used for detecting Salmonella from 1078 environmental swabs of poultry houses. Prior to PCR, these swabs were pre-enriched in phosphate-buffered peptone water for 18-20 h and then sub-cultured on a Modified Semi-solid Rappaport Vassiliadis medium (MSRV) for 18-20 h. The m-PCR combined with MSRV had a better sensitivity (95%) than the bacteriological method (92.5%). The MSRV-m-PCR assay and the bacteriological method had an agreement rate of 95.6%.     

Detection of Salmonella in food samples by the combination of immunomagnetic separation and PCR assay.

A combination of immunomagnetic separation and polymerase chain reaction (IMS-PCR) was used to detect Salmonella in food samples. Pre-enrichment of samples was combined with filtration through a membrane for the removal of food debris. The IMS-PCR assay combines selective extraction of bacteria by specific antibodies with primer specific PCR amplification that enables to detect Salmonella in non-fatty food samples in 24 h. In comparison with conventional cultural methods, the IMS-PCR is a rapid and specific method. Combined with filtration bags, it partially reduces the negative effects of the food matrix and allows the quick detection of Salmonella cells. The shortened protocols for Salmonella spp. detection described here can improve considerably current methodologies.     

Differences in attachment of Salmonella enterica serovars and Escherichia coli O157:H7 to alfalfa sprouts

Numerous Salmonella enterica and Escherichia coli O157:H7 outbreaks have been associated with contaminated sprouts. We examined how S. enterica serovars, E. coli serotypes, and nonpathogenic bacteria isolated from alfalfa sprouts grow on and adhere to alfalfa sprouts. Growth on and adherence to sprouts were not significantly different among different serovars of S. enterica, but all S. enterica serovars grew on and adhered to alfalfa sprouts significantly better than E. coli O157:H7. E. coli O157:H7 was essentially rinsed from alfalfa sprouts with repeated washing steps, while 1 to 2 log CFU of S. enterica remained attached per sprout. S. enterica Newport adhered to 3-day-old sprouts as well as Pantoea agglomerans and 10-fold more than Pseudomonas putida and Rahnella aquatilis, whereas the growth rates of all four strains throughout seed sprouting were similar. S. enterica Newport and plant-associated bacteria adhered 10- to 1,000-fold more than E. coli O157:H7; however, three of four other E. coli serotypes, isolated from cabbage roots exposed to sewage water following a spill, adhered to sprouts better than E. coli O157:H7 and as well as the Pseudomonas and Rahnella strains. Therefore, attachment to alfalfa sprouts among E. coli serotypes is variable, and nonpathogenic strains of E. coli to be used as surrogates for the study of pathogenic E. coli may be difficult to identify and should be selected carefully, with knowledge of the biology being examined.