Methods for Observing Microbial Biofilms Directly on Leaf Surfaces and Recovering Them for Isolation of Culturable Microorganisms

Epifluorescence microscopy, scanning electron microscopy, and confocal laser scanning microscopy were used to observe microbial biofilms directly on leaf surfaces. Biofilms were observed on leaves of all species sampled (spinach, lettuce, Chinese cabbage, celery, leeks, basil, parsley, and broad-leaved endive), although the epifluorescent images were clearest when pale green tissue or cuticle pieces were used. With these techniques, biofilms were observed that were about 20 (mu)m in depth and up to 1 mm in length and that contained copious exopolymeric matrices, diverse morphotypes of microorganisms, and debris. The epifluorescence techniques described here can be used to rapidly determine the abundance and localization of biofilms on leaves. An additional technique was developed to recover individual biofilms or portions of single biofilms from leaves and to disintegrate them for isolation of the culturable microorganisms they contained. Nineteen biofilms from broad-leaved endive, spinach, parsley, and olive leaves were thus isolated and characterized to illustrate the applications of this technique.     

Inactivation of Escherichia coli and Listeria monocytogenes on iceberg lettuce by dip wash treatments with organic acids

AIMS: To study and compare the efficacy of organic acids and chlorine dipping in inactivation of Escherichia coli and Listeria monocytogenes on fresh-cut iceberg lettuce. METHODS AND RESULTS: Fresh-cut iceberg lettuce leaves were inoculated with E. coli or L. monocytogenes. After inoculation, samples were stored at 4 degrees C for 24 h and dipped in organic acid or chlorine solutions for 2 and 5 min. E. coli and L. monocytogenes were enumerated on selective media. Treatment of fresh-cut iceberg lettuce with chlorine solution caused 1.0 and 2.0 log(10) CFU g(-1) reductions in the number of L. monocytogenes and E. coli, respectively. Maximum reduction for E. coli (about 2.0 log(10) CFU g(-1)) was obtained for samples dipped in lactic or citric acids while maximum reduction for L. monocytogenes (about 1.5 log(10) CFU g(-1)) was attained for samples dipped in lactic acid. CONCLUSIONS: Dipping of iceberg lettuce in 0.5% citric acid or 0.5% lactic acid solution for 2 min could be as effective as chlorine for reducing microbial populations on fresh-cut iceberg lettuce. SIGNIFICANCE AND IMPACT OF THE STUDY: Dipping in solutions containing organic acids is shown to be effective to reduce E. coli and L. monocytogenes on fresh-cut iceberg lettuce.     

Pseudomonads associated with midrib rot and soft rot of butterhead lettuce and endive

During the past ten years, bacterial soft rot and midrib rot of glasshouse-grown butterhead lettuce (Lactuca sativa L. var. capitata) and field-grown endive (Cichorium endivia L.) has become increasingly common in the region of Flanders, Belgium. Severe losses and reduced market quality caused by bacterial rot represent an important economical threat for the production sector. Symptoms of midrib rot are a brownish rot along the midrib of one or more inner leaves, often accompanied by soft rot of the leaf blade. Twenty-five symptomatic lettuce and endive samples were collected from commercial growers at different locations in Flanders. Isolations of dominant bacterial colony types on dilution plates from macerated diseased tissue extracts yielded 282 isolates. All isolates were characterized by colony morphology and fluorescence on pseudomonas agar F medium, oxidase reaction, and soft rot ability on detached chicory leaves. Whole-cell fatty acid methyl esters profile analyses identified the majority of isolates (85%) as belonging to the Gammaproteobacteria, which included members of the family Enterobacteriaceae (14%) and of the genera Pseudomonas (73%), Stenotrophomonas (9%), and Acinetobacter (3%). Predominant bacteria were a diverse group of fluorescent Pseudomonas species. They were further differentiated based on the non-host hypersensitive reaction on tobacco and the ability to rot potato slices into 4 phenotypic groups: HR-/P- (57 isolates), HR-/P+ (54 isolates), HR+/P (16 isolates) and HR+/P+ (35 isolates). Artificial inoculation of suspensions of HR-, pectolytic fluorescent pseudomonads in the leaf midrib of lettuce plants produced various symptoms of soft rot, but they did not readily cause symptoms upon spray inoculation. Fluorescent pseudomonads with phenotype HR+ were consistently isolated from typical dark midrib rot symptoms, and selected isolates reproduced the typical midrib rot symptoms when spray-inoculated onto healthy lettuce plants.     

Epidemiology of Pseudomonas cichorii,the Cause of Lettuce Midrib Rot

Bacterial midrib rot, caused by Pseudomonas cichorii, has become a serious threat to the production of greenhouse butterhead lettuce (Lactuca sativa L. var. capitata) in Belgium. Currently, there are no strategies for controlling this pathogen. Therefore, greenhouse experiments were conducted to obtain more knowledge about the epidemiology of P. cichorii on butterhead lettuce. Greenhouse butterhead lettuce becomes susceptible to lettuce midrib rot infections at head formation, and a single overhead irrigation with water containing 10² CFU/ml P. cichorii was sufficient to cause disease. The use of surface drip irrigation instead of overhead sprinkler irrigation significantly reduced midrib rot incidence in the greenhouse. P. cichorii isolates can be divided into subgroups based on BOX‐PCR genomic fingerprinting, with isolates belonging to subgroup C1 and C2 being more virulent than those of (or related to) subgroup C3. P. cichorii infections with distinct symptoms comparable to midrib rot have also been observed on field‐grown crisphead lettuce in California and Japan which, respectively, are referred to as ‘varnish spot’ or ‘tar’. We showed that symptom expression is strongly influenced by the lettuce cultivar group, irrespective of the P. cichorii isolate, resulting in varnish spot/tar on crisphead lettuce and midrib rot on butterhead or cutting group lettuce.     

Factors affecting the growth of Listeria monocytogenes on minimally processed fresh endive

The influence of various factors on the fate of Listeria monocytogenes on cut leaves of broad-leaved endive has been studied. Factors considered were temperature, characteristics of the leaves (age, quantity and quality of the epiphytic microflora) and characteristics of the L. monocytogenes inoculum (concentration, strain). The increases in numbers of L. monocytogenes were lower than those of the aerobic mesophilic microflora at 3°, 6°, 10° and 20°C. Doubling times of the populations of L. monocytogenes were in the same order of magnitude as those of aerobic bacteria at 10° and 20°C, but longer at 3° and 6°C. There were positive significant correlations between growth of L. monocytogenes and populations of aerobic bacteria, and between growth of L. monocytogenes and extent of spoilage on the leaves.
Of 225 bacteria isolated from the leaves, 84% were identified as fluorescent pseudomonads; there was no difference in the species isolated from leaves that showed a low growth of L. monocytogenes and leaves that showed a high growth of L. monocytogenes. Populations of L. monocytogenes increased faster during the first 2 and 4 d of storage at 10°C on leaves inoculated with 10–10³ cfu g^-1 than on leaves inoculated with about 10⁵ cfu g^-1, but the population reached after 7 d was lower. The behaviour of L. monocytogenes was similar among the three strains tested.     

Mild heat treatment of lettuce enhances growth of Listeria monocytogenes during subsequent storage at 5 degrees C or 15 degrees C

AIMS: The objective of this study was to determine the influence of mild heat treatment, storage temperature and storage time on the survival and growth of Listeria monocytogenes inoculated onto cut iceberg lettuce leaves. METHODS AND RESULTS: Before or after inoculation with L. monocytogenes, cut iceberg lettuce leaves were dipped in water (20 or 50 degrees C) containing or not 20 mg l(-1) chlorine, for 90 s, then stored at 5 degrees C for up to 18 days or 15 degrees C for up to 7 days. The presence of 20 mg l(-1) chlorine in the treatment water did not significantly (alpha=0.05) affect populations of the pathogen, regardless of other test parameters. The population of L. monocytogenes on lettuce treated at 50 degrees C steadily increased throughout storage at 5 degrees C for up to 18 days. At day 10 and thereafter, populations were 1.7-2.3 log10 cfu g(-1) higher on lettuce treated at 50 degrees C after inoculation compared with untreated lettuce or lettuce treated at 20 degrees C, regardless of chlorine treatment. The population of L. monocytogenes increased rapidly on lettuce stored at 15 degrees C. At 2 and 4 days, significantly higher populations were detected on lettuce that had been treated at 50 degrees C, compared with respective samples that had been treated at 20 degrees C, regardless of inoculation before or after treatment, or the presence of 20 mg l(-1) chlorine in the treatment water. CONCLUSIONS: The results clearly demonstrated that mild heat treatment of cut lettuce leaves enhances the growth of L. monocytogenes during subsequent storage at 5 or 15 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: Mild heat treatment of cut lettuce may result in a prolonged shelf life as a result of delaying the development of brown discoloration. However, heat treatment also facilitates the growth of L. monocytogenes during storage at refrigeration temperature, thereby increasing the potential risk of causing listeriosis.     

Association analysis of bacterial leaf spot resistance and SNP markers derived from expressed sequence tags (ESTs) in lettuce (Lactuca sativa L.)

Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians, is a devastating disease of lettuce worldwide. Since there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A total of 179 lettuce genotypes consisting of 29 leaf, 15 crisphead, one stem, 21 romaine, and 113 butterhead types were evaluated for response to X. c. vitians. One source of high resistance and five sources of moderate resistance were identified with four being butterhead lettuce and two leaf lettuce. The population genetic structure based on 350 expressed-sequence-tag-derived single nucleotide polymorphism generated two clades: Clade I and Clade II. The butterhead type was genetically distinct from romaine and crisphead types, while the leaf type was found to frequently exchange genes with other types through the history of breeding. Association mapping identified one single nucleotide polymorphism (QGB19C20.yg-1-OP5) associated with disease severity in Q general linear model and Q + K mixed linear model. Two SNP markers (Contig15389-1-OP1 and Contig6039-19-OP1) were associated with the resistance in the leaf lettuce PI 358000-1 which had no disease symptoms. The marker QGB19C20.yg-1-OP5 is in linkage group 2, while both of Contig15389-1-OP1 and Contig6039-19-OP1 are in linkage group 4. The resistant lines and the associated SNPs should be useful to develop resistant cultivars to battle against the devastating disease in lettuce.     

New Fusarium wilts on vegetable crops in Italy

Three Fusarium wilts recently observed in Italy on lettuce (Lactuca sativa), wild (Diplo taxis spp.) and cultivated (Eruca sativa) rocket and lamb's lettuce (Valerianella olitoria) emerged as major production problems in Lumbardy (north-western Italy). Aspects of biology and epidemiology of the three diseases and some possibilities of disease management are discussed.     

Evidence of an antilisterial factor induced by wounding of iceberg lettuce tissues

AIMS: To examine the influence of wound-associated reactions in cut iceberg lettuce (Lactuca sativa L.) tissues on the fate of Listeria monocytogenes. METHODS AND RESULTS: Aqueous extracts prepared from shredded iceberg lettuce before and after storage in high oxygen permeability film were inoculated with L. monocytogenes. Listeria monocytogenes grew in extracts prepared from fresh lettuce. In contrast, inhibition ranging from arrested growth to a decline in cell viability was observed in extracts prepared from samples stored for 1-3 days. Similar behaviour was evident in lettuce shreds inoculated with 10(5) CFU g(-1)L. monocytogenes immediately after processing or after 3 days in storage. Heat treatment of the cut tissues at 47 degrees C for 3 min before storage diminished the inhibitory effect. CONCLUSIONS: The results provided evidence that an antilisterial factor or factors are released by wounded iceberg lettuce tissues. Antilisterial activity was mitigated by heat treatment of the lettuce. SIGNIFICANCE AND IMPACT OF STUDY: This study indicates that intrinsic factors associated with plant metabolism could play a significant role in the ecology of human pathogens in packaged horticultural products.     

Detection of an arbuscular mycorrhizal fungus in roots of different plant species with the PCR.

PCR was used with the primer pair VANS1-NS21 to detect the arbuscular mycorrhizal fungus Glomus intraradices (commercial inoculum source) on roots of lettuce, zinnia, leek, pepper, and endive plants. The appropriate amplification product was obtained directly from roots without DNA extraction and purification.     

Population Sizes, Immigration, and Growth of Epiphytic Bacteria on Leaves of Different Ages and Positions of Field-Grown Endive (Cichorium endivia var. latifolia)

Total, fluorescent, and pectolytic epiphytic bacterial population sizes were quantified on leaves of different age groups of broad-leaved endive during field cultivation from leaf emergence until harvest. Greater bacterial population densities (log(inf10) CFU per square centimeter) were observed on outer leaves than on inner leaves of the plants throughout the growing season. These differences were statistically significant for total bacterial populations at all sampling times and were often significant for fluorescent and pectolytic bacterial populations. At harvest, a linear gradient of decreasing densities of epiphytic bacteria from outer (older) to inner (younger) leaves of the head was significant. Leaf age influenced the frequency distribution and variability of bacterial population sizes associated with leaves of broad-leaved endive. Total bacterial population sizes were greater at leaf emergence for leaves emerging during the second half of the cultivation period than for leaves emerging earlier. The size of fluorescent and pectolytic bacterial populations on newly emerged leaves increased throughout the season as plants aged. To assess the importance of plant age on bacterial immigration at leaf emergence, bacterial densities were quantified on leaves emerging simultaneously on plants of different ages. In two of the three experiments, greater bacterial population sizes were observed on leaves emerging on younger plants. This indicates that factors other than an increase in concentration of airborne bacteria can lead to increases in population sizes at leaf emergence as plants age in the field. Results of leaf pruning experiments suggested that adjacent leaves may act as a barrier for immigration of fluorescent bacteria on newly emerged leaves. Survival of an inoculated strain of Pseudomonas fluorescens on newly emerged leaves generally did not vary with the age of plants. However, these effects were not consistent among experiments, suggesting that interactions among micro- and macroenvironmental conditions, physiological condition of leaves, and accessibility of leaves to airborne bacteria are important in controlling epiphytic bacterial population sizes.     

Differentiation of nearly identical germplasm accessions by a combination of molecular and morphologic analyses

Polymerase chain reaction (PCR) based random amplified polymorphic DNA (RAPD) markers were used to study the extent of redundancy (duplication of genetic materials) within a genetic resources collection. Nine nearly phenotypically and identical accessions of butterhead lettuce (Lactuca sativa L.) were assayed for their genetic identities. A nonuniform, heterogeneous butterhead line and a crisphead cultivar were added for population comparison. PCR amplification using 13 oligonucleotide primers generated 93 polymorphic bands. The percentage of segregating bands was used to determine within-line variation; values ranged from 0.0 to 12.0%, except for the nonuniform line at 22.6%. Between-line similarity was measured using similarity coefficients and ranged from 0.919 to 0.985. The relationship between the crisphead accession and a composite of all butterhead accessions was 0.84. Selfed progeny of each line were measured for morphological uniformity. The variation obtained from these biological data was compared with variation detected at the DNA level and each was positively correlated. Results demonstrate that RAPD analyses may serve as a major source of information for separation of closely related accessions, especially when integrated with phenotypic measures.     

GROWTH OF EDIBLE CHLOROPHYLLOUS PLANT TISSUES IN VITRO

Hildebrandt, A. C, J. C. Wilmar, H. Johns, and A. J. Riker. (U. Wisconsin, Madison.) Growth of edible chlorophyllous plant tissues in vitro, Amer. Jour. Bot. 50(3): 248–254. Illus. 1963.—Plant callus cultures were attempted from roots, stems, leaves or excised embryos of 32 species of plants on a basal mineral salts–sucrose agar medium (T-medium), on T-medium + coconut milk + α-naphthaleneacetic acid + calcium pantothenate (C-medium) and on C-medium + 2,4-dichlorophenoxyacetic acid (D-medium). Embryos on T- or C-medium generally produced normal plants, while on D-medium, they often produced callus only. Fresh isolates of carrot, endive, lettuce, parsley, red kidney bean, and navy bean gave moderate to excellent callus on C-medium. Parsley and navy bean also produced excellent callus on D-medium. Strains of callus from potato, tomato, grape and rose also grew well on C- or D-medium. In the light, red pigmentation developed on rose, parsley, and grape callus. Chlorophyll formation was inhibited on D-medium, but on C-medium more or less chlorophyll was initiated in callus from carrot, endive, lettuce, pea, potato and certain rose varieties. Chlorophyll formation was also strong in endive callus on T-medium supplemented with casein hydrolysate, i-inositol and NAA. The amount and type of sugar used in C-medium influenced the amount of growth and were critical in relation to chlorophyll formation. Carrot tissues in constant light produced abundant chlorophyll and were still growing on media without added sugar after 6 weeks.     

Bacteriological quality of organically grown leaf lettuce in Norway

AIM: To investigate bacteriological quality in organically grown leaf lettuce, including the presence of selected pathogenic bacteria, and to obtain information about organic lettuce production, including fertilizing regimes. METHODS AND RESULTS: Altogether 179 samples of Norwegian organically grown lettuce were collected from 12 producers. Escherichia coli was isolated from 16 of the lettuce samples, but in 12 of these contamination was sufficiently low (<100 CFU g(-1)) that they would be considered to be of acceptable bacteriological quality. Escherichia coli O157 and Salmonella were not detected in any of the samples. Listeria monocytogenes serogroups 1 and 4 were isolated from two samples. CONCLUSIONS: Organic lettuce produced in Norway was generally of acceptable bacteriological quality, but the results show that contamination of organic lettuce with E. coli and L. monocytogenes do occasionally occur. SIGNIFICANCE AND IMPACT OF THE STUDY: These results suggest that organically grown lettuce may be contaminated with E. coli and L. monocytogenes during cultivation.     

A thorough study of a Paratylenchus sp. in glasshouse‐grown lettuce: Characterisation,population dynamics,host plants and damage threshold as keys to its integrated management

In glasshouses practising monoculture of butterhead lettuce in Belgium, high densities of pin nematodes (Paratylenchus spp.) are frequently associated with reduced plant growth. Growers currently apply chemical soil disinfestation measures to manage this problem, although stricter phytosanitary regulations are forcing a shift towards integrated management. Efficient implementation of such management requires knowledge about the factors influencing nematode population dynamics, and the damage threshold for lettuce. The nematode populations in five Belgian glasshouses were monitored for at least 1 year by frequently soil sampling at 0–30 cm and 30–60 cm depth. An undescribed species of Paratylenchus was identified in all glasshouses based on morphological and molecular features. High nematode densities (>20,000 (100 ml soil)^?1) occurred in winter and spring. Chemical soil disinfestation lowered these populations greatly, although up to 14% survived in the deeper soil layer. After soil steaming under negative pressure, no pin nematodes were found. After 2 months of black fallow pin nematode densities were reduced by 50%–76%. Lamb's lettuce, parsley and wild rocket were found to be poor hosts in a pot experiment, while reproduction factors (P_f/P_i) on lettuce cultivars varied between 1 and 3. In three experiments with butterhead lettuce ‘Cosmopolia’ in pots with a series of 9 or 10 densities of Paratylenchus sp. [up to 35,000 (100 ml soil)^?1], no damage to lettuce heads was observed. However, root weight and root quality were reduced, and the corresponding damage thresholds were rather low [1,754 and 362 Paratylenchus sp. (100 ml soil)^?1, respectively]. Management strategies such as crop rotation, soil disinfestation or fallow are recommended to avoid pin nematode population build‐up.     

Ribosomal Ribonucleic Acids of Chloroplastic and Mitochondrial Preparations

RNA prepared from fractions of chloroplasts and mitochondria sedimented at rates characteristic of ribosomal RNA. A predominance of the 18S species was frequently observed in preparations from chloroplasts from romaine lettuce and endive. The usual distribution, a preponderance of the 28S species, was observed in studies on tomato and spinach chloroplasts and mitochondria from mushroom and cauliflower. Comparisons of the base composition of RNA from organelles with their cytoplasmic ribosomal counterparts revealed that the 18S component from romaine lettuce chloroplast was different. A marginally significant difference was observed in the 28S particle from mushroom mitochondria preparations whereas distinct differences, reflected in all the bases, were seen when the 18S component of cauliflower mitochondria preparations was compared with cytoplasmic RNA.     

Nisin and ALTATM 2341 inhibit the growth of Listeria monocytogenes on smoked salmon packaged under vacuum or 100% CO2

The effects of nisin and ALTA 2341 on the growth of Listeria monocytogenes were assessed on smoked salmon packaged under vacuum or 100% CO2. Smoked salmon slices (pH 6.3) were inoculated with a cocktail of seven L. monocytogenes isolates at a level of approximately 2.5 log10 colony forming units (cfu) g-1. After inoculation, the surface of the smoked salmon slices was treated with either nisin (400 or 1250 IU g-1) or ALTA 2341 (0.1 or 1%). The smoked salmon was packaged and stored at 4 degrees C (28 d) or 10 degrees C (9 d). On untreated vacuum-packaged smoked salmon, L. monocytogenes grew by 3.8 log10 cfu g-1 at 4 degrees C and 5.1 log10 cfu g-1 at 10 degrees C. Growth was reduced on nisin- and ALTA 2341-treated vacuum-packaged smoked salmon. On the nisin-treated samples, L. monocytogenes increased by 2.5 (400 IU g-1) and 1.5 (1250 IU g-1) log10 cfu g-1 at 4 degrees C, and by 4.3 (400 IU g-1) and 2.7 (1250 IU g-1) log10 cfu g-1 at 10 degrees C. With the ALTA 2341-treated samples, L. monocytogenes increased by 2.8 (0.1%) or 1.6 (1.0%) log10 cfu g-1 at 4 degrees C, and 3.3 (0.1%) or 3.6 (1.0%) log10 cfu g-1 at 10 degrees C. The growth of L. monocytogenes was retarded by packaging the smoked salmon in 100% CO2. On untreated smoked salmon, only a 0.8 log10 cycle increase was observed at 10 degrees C. Under all the other conditions tested with 100% CO2, L. monocytogenes was detected but growth was prevented.     

Fate and Phytotoxicity of CeO2 Nanoparticles on Lettuce Cultured in the Potting Soil Environment

Cerium oxide nanoparticles (CeO₂ NPs) have been shown to have significant interactions in plants. Previous study reported the specific-species phytotoxicity of CeO₂ NPs by lettuce (Lactuca sativa), but their physiological impacts and vivo biotransformation are not yet well understood, especially in relative realistic environment. Butterhead lettuce were germinated and grown in potting soil for 30 days cultivation with treatments of 0, 50, 100, 1000 mg CeO₂ NPs per kg soil. Results showed that lettuce in 100 mg·kg^-1 treated groups grew significantly faster than others, but significantly increased nitrate content. The lower concentrations treatment had no impact on plant growth, compared with the control. However, the higher concentration treatment significantly deterred plant growth and biomass production. The stress response of lettuce plants, such as Superoxide dismutase (SOD), Peroxidase (POD), Malondialdehyde(MDA) activity was disrupted by 1000 mg·kg^-1 CeO₂ NPs treatment. In addition, the presence of Ce (III) in the roots of butterhead lettuce explained the reason of CeO₂ NPs phytotoxicity. These findings demonstrate CeO₂ NPs modification of nutritional quality, antioxidant defense system, the possible transfer into the food chain and biotransformation in vivo.     

Integrated Analysis of Growth and Light Interception in Winter Lettuce II. Differences between Cultivars

‘Integrated growth analysis’ (emphasizing aspectsof crop and plant structure) and ‘light conversion analysis’(stressing the efficiency of interception and photosyntheticconversion of light) have been used to investigate the wintergrowth of different cultivars of butterhead and crisphead typesof glasshouse lettuce. Measurements from ‘Ambassador’ (large-framed butterhead),‘Renate’ (medium-sized butterhead) and ‘Cristallo’(crisphead) were made, statistical progressions were fittedto the primary data and hence estimates of all the analyticalcomponents were derived. Curves for crop growth rate, like those for most other components,followed a generally similar pattern for all three cultivars.In integrated growth analysis, the biomass curve for Ambassadorlay above the curves for the other cultivars. The weight advantagewas initially 60 per cent and it persisted with only a smallreduction (to 40 per cent) until the final harvest. Relativegrowth rate varied little between cultivars because differencesobserved in leaf area ratio were complementary to those seenin net assimilation rate. In light conversion analysis, differences in light interceptingefficiency between cultivars were not statistically significant,though Ambassador attained full interception 4 days earlierthan Renate and 6 days earlier than Cristallo. Differences inlight utilizing efficiency were small and non-significant exceptduring the post-rosette stage when the value for Renate waslower than that of either Ambassador or Cristallo. Deviationsaround the fitted curves were correlated with fluctuations inthe light regime. An assessment is made of the utility and limitations of thetwo procedures. It is concluded that both approaches can assistin analyzing the rate of dry matter production in crops or plantstands. Integrated growth analysis is advantageous when theneed arises to treat individual and population-based attributessimultaneously, while light conversion analysis provides a meansof explicitly incorporating the primary environmental variableinfluencing growth. Lactuca sativa L., lettuce cultivars, growth analysis, crop growth rate, relative growth rate, light interception, light utilization     

Biocontrol of Listeria monocytogenes on fresh-cut produce by treatment with lytic bacteriophages and a bacteriocin

The fresh-cut produce industry has been the fastest-growing portion of the food retail market during the past 10 years, providing consumers with convenient and nutritious food. However, fresh-cut fruits and vegetables raise food safety concerns, because exposed tissue may be colonized more easily by pathogenic bacteria than intact produce. This is due to the higher availability of nutrients on cut surfaces and the greater potential for contamination because of the increased amount of handling. We found that applied Listeria monocytogenes populations survived and increased only slightly on fresh-cut Red Delicious apples stored at 10 degrees C but increased significantly on fresh-cut honeydew melons stored at 10 degrees C over 7 days. In addition, we examined the effect of lytic, L. monocytogenes-specific phages via two phage application methods, spraying and pipetting, on L. monocytogenes populations in artificially contaminated fresh-cut melons and apples. The phage mixture reduced L. monocytogenes populations by 2.0 to 4.6 log units over the control on honeydew melons. On apples, the reduction was below 0.4 log units. In combination with nisin (a bacteriocin), the phage mixture reduced L. monocytogenes populations by up to 5.7 log units on honeydew melon slices and by up to 2.3 log units on apple slices compared to the control. Nisin alone reduced L. monocytogenes populations by up to 3.2 log units on honeydew melon slices and by up to 2.0 log units on apple slices compared to the control. The phage titer was stable on melon slices, but declined rapidly on apple slices. The spray application of the phage and phage plus nisin reduced the bacterial numbers at least as much as the pipette application. The effectiveness of the phage treatment also depended on the initial concentration of L. monocytogenes.